Closer to home, the planet Venus shows large amounts of evidence of volcanic activity. Despite being shrouded under a thick layer of cloud, spacecraft have been able to map the surface of our nearest neighbour using radar, leading to the realisation that much of the planet's surface is comparatively young, suggesting that at some point in the recent past the planet underwent a complete resurfacing. However the question remains whether Venus is currently a geologically active planet.... Read more »
Smrekar, S., Stofan, E., Mueller, N., Treiman, A., Elkins-Tanton, L., Helbert, J., Piccioni, G., & Drossart, P. (2010) Recent Hotspot Volcanism on Venus from VIRTIS Emissivity Data. Science, 328(5978), 605-608. DOI: 10.1126/science.1186785
Most known extrasolar planets are massive gas giants orbiting close to their parent stars. If one of these planets happens to pass directly between us and its parent star during its orbit, then sensitive spectroscopy can be used to determine the chemical make-up of its atmosphere. Models of such atmospheres predict which gases should be present and in what relative abundances, based on physical conditions such as the temperature. Recent infra red observations carried out with the have provided the first details of the atmospheric composition of a so-called hot Neptune.The planet, known as , orbits an M-type dwarf star in the constellation of Leo. It is similar to Neptune in size, but orbits its parent star in just 2.6 days. Previous observations of the planet showed that its surface temperature was estimated to be 712 K, higher than predicted due to stellar heating alone, and the new observations () suggest that its atmosphere may not be in equilibrium. The team, led by Kevin Stevenson at the University of Central Florida, observed the planet's day side as it passed around the far side of the star and examined the infra red spectrum for various chemical signatures. What they found was a high abundance of carbon monoxide and a deficiency of methane compared to predictions from atmospheric models at this temperature for an atmosphere thought to be dominated by hydrogen. In an atmosphere such as this, methane (one carbon atom and four hydrogen atoms) should be the main carbon-bearing molecule, but the observations show the actual abundance is less than that predicted by a factor of seven thousand. The large amount of absorption due to carbon monoxide is also unexpected, the results suggesting that the atmosphere may not be in thermochemical equilibrium.One alternative explanation considered by the authors is that the atmosphere may not be dominated by hydrogen, but this is unlikely given the dominance of hydrogen in planet forming disks. Another possibility is that vertical mixing within the atmosphere may dredge up carbon monoxide from lower, hotter parts of the atmosphere, although the authors point out that, in order to explain the observed abundances, the amount of mixing would have to be large. These new data will provide useful information for future atmospheric modeling.This blog post is a news story from the , aired in the edition.... Read more »
Stevenson, K., Harrington, J., Nymeyer, S., Madhusudhan, N., Seager, S., Bowman, W., Hardy, R., Deming, D., Rauscher, E., & Lust, N. (2010) Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b. Nature, 464(7292), 1161-1164. DOI: 10.1038/nature09013
Many stars vary in brightness, sometimes due to changes within the star itself such as novae or Cepheid variables, others because of external factors. One is , an F-type supergiant in the constellation of Auriga, located at an estimated distance of 625 parsecs (2,100 light years). Since its variable nature was discovered in the 1820s, the star has been seen to fade in brightness every 27.1 years. During these eighteen-month-long eclipses, the brightness of the star fades to around 50 per cent of its normal magnitude. While the variability of the system has been well-studied, the exact physical nature of the eclipsing companion is less certain as it has remained undetected, and many models have been put forward to explain the unusual nature of the system. Observations of epsion Aurigae show that the star and its darker companion have a similar mass which, until recently, was thought to be around 15 times the mass of the Sun. have shown that the supergiant star has a much lower mass of between two and three solar masses, and that the companion may be a single B5V-type star embedded within a disk of opaque material.Now, using the CHARA interferometer, an array of infrared telescopes located on Mount Wilson in California, a team led by Brian Kloppenborg from the University of Denver have . This is the first time a spatially resolved observation of an eclipsing binary has been made. Their observations show that the eclipsing object is an opaque disk of dust, tilted to our line of sight by an estimated 84 degrees. From the motion of the disk between two observations carried out in November and December 2009, the team infer that the companion object is more massive than the visible F-type supergiant. Assuming the B-type star within the disk has a typical mass of 5.9 solar masses, the researchers calculate a mass of 3.6 solar masses for the F-type supergiant. They also calculate that if the disk is composed entirely of dust, then its mass is less than 10 per cent of the Earth's.While the nature of the disk is now clearer, there are still several unanswered questions which remain. The model that best fits the data is of a geometrically thin disk tilted to our line of sight, rather than a thick disk seen edge on. However, the fact that it is opaque suggests that its nature is more like a debris disk than a dusty accretion disk around a young stellar object. The tilted disk model also predicts a central hole which should cause a mid-eclipse brightening of the F-type star. Observers the world-over will continue to monitor the system during the eclipse, and the data should help build up a profile of the disk and constrain the evolutionary history of the system.This blog post is a news story from the , aired in the edition.... Read more »
Kloppenborg, B., Stencel, R., Monnier, J., Schaefer, G., Zhao, M., Baron, F., McAlister, H., ten Brummelaar, T., Che, X., Farrington, C.... (2010) Infrared images of the transiting disk in the ε Aurigae system. Nature, 464(7290), 870-872. DOI: 10.1038/nature08968
The first set of results from the Herschel Space Telescope have been flooding out over the past couple of weeks*, so it’s about time they got a mention here. Rather than rehashing one of the many press releases, I thought I’d focus on an interesting result that I doubt will get much attention – the [...]... Read more »
Amongst all the excitement over the first results from Herschel, it’s easy to forget about its comparatively tiny American cousin Spitzer. Launched in 2003 with its 3 instruments IRAC, IRS and MIPS, Spitzer covers the infrared wavelengths from around 3 to 150 microns – a region that from Earth is either totally inaccessible or severely [...]... Read more »
James M. De Buizer, & William D. Vacca. (2010) Direct Spectroscopic Identification of the Origin of 'Green Fuzzy' Emission in Star Forming Regions. accepted in ApJ. arXiv: 1005.2209v1
C. J. Cyganowski, B. A. Whitney, E. Holden, E. Braden, C. L. Brogan, E. Churchwell, R. Indebetouw, D. F. Watson, B. L. Babler, R. Benjamin.... (2008) A Catalog of Extended Green Objects (EGOs) in the GLIMPSE Survey: A new sample of massive young stellar object outflow candidates. Astronomical Journal, 136(6), 2391-2412. arXiv: 0810.0530v1
Mathematicians have a concept, Omega, that is defined as something so huge that any attempt to define it actually defines something smaller. In a similar vein I reckon that any attempt to describe the ingenuity of the Antikythera Mechanism actually ends up describing something less ingenious instead. More research on the device has been published [...]... Read more »
Evans, J., Carman, C.C., & Thorndike, A.S. (2010) Solar Anomaly and Planetary Displays in the Antikythera Mechanism. Journal for the History of Astronomy, 41(1), 1-39. info:/
The internet has been abuzz this week with news of a recently discovered massive star being booted from its home cluster. I have the good fortune to know one of the scientists involved in this discovery and have asked him to give us an insight in what went in to this find. So without further [...]... Read more »
The object known simply as 3C 279 is rather distinctive for several reasons, in spite of the rather unassuming name. For one thing it's an active galaxy – that is, it has a supermassive black hole at its center, and that black hole is sucking in surrounding matter at a rate high enough to generate as much energy as all stars the in the galaxy where it resides combined. Only about 1% of visible galaxies are active galaxies like 3C 279.But that's not all. 3C 279 is also a radio galaxy, a subset of only about 10% of active galaxies that also feature strong radio-frequency emissions. Such strong emissions are generally thought to be produced by a violent outflow of matter from the vicinity of the black hole in the form of narrow jets. The flow is so violent that matter in the jets reaches velocities close to the velocity of light.And if that's not enough, one of the jets of 3C 279 is pointed almost straight at us. Only a few percent of active radio galaxies are oriented that way, by chance. Because we're looking essentially straight into the most active part of the object, with basically no dust or gas to obscure the view, 3C 279 appears especially luminous – the term for such an object is "blazar".Although its jet is aimed right at us, there's nothing to be particularly concerned about, since 3C 279 has a redshift of z=0.536, which means it's actually about 6.5 billion light years away.3C 279I just wrote at some length about active galaxies, here, in some detail, so you might like to review that if you need to refresh your memory on many basics of the subject. There may be some aspects of the present discussion that will make more sense in light of that.Even though 3C 279 came to the attention of astronomers over 40 years ago, because of its unusual apparent brightness and radio emissions, it is not an especially powerful active galaxy, as those things go. The central black hole is estimated to have a mass around 6×108 M⊙, somewhat short of 109 M⊙ that is typical of the largest quasars.The peak velocity of matter in the jet of 3C 279 has been inferred to be about 99.8% of the speed of light, which is "relativistic" by anyone's definition. In other words, this velocity is v=0.998c. It's customary to express this velocity as β = v/c = 0.998. The inference is based on apparent (but not real) "superluminal" (faster than light) motion of jet-related material. This is a common phenomenon seen in active galaxy jets that are nearly parallel to our line of sight. A related quantity, the Lorentz factor, is defined as γ = (1-β2)-1/2, so in this case γ ≈ 16. That'll play a role in an important calculation later.Since astronomers have been interested in 3C 279 for over 40 years, it's been studied a lot, although that's been difficult, because of its rather large distance. Radio galaxies like this produce electromagnetic emissions all the way from radio frequencies on up to gamma rays – spanning 11 or 12 orders of magnitude in photon energy, from under .001 eV to over 100 Mev. Many of those frequency ranges can be observed only from instruments in space, so until recently it hasn't been possible to observe a single object continuously for long periods of time in many bands. This has now been done for the blazar 3C 279 – and perhaps by chance something rather interesting showed up, which could only have been observed in an active galaxy whose jet is nearly parallel to our line of sight.A change in the optical polarization associated with a γ-ray flare in the blazar 3C 279It is widely accepted that strong and variable radiation detected over all accessible energy bands in a number of active galaxies arises from a relativistic, Doppler-boosted jet pointing close to our line of sight. The size of the emitting zone and the location of this region relative to the central supermassive black hole are, however, poorly known, with estimates ranging from light-hours to a light-year or more. Here we report the coincidence of a gamma (γ)-ray flare with a dramatic change of optical polarization angle. This provides evidence for co-spatiality of optical and γ-ray emission regions and indicates a highly ordered jet magnetic field. The results also require a non-axisymmetric structure of the emission zone, implying a curved trajectory for the emitting material within the jet, with the dissipation region located at a considerable distance from the black hole, at about 105 gravitational radii.The main thing that this paper reports is an "event", evidently some sort of disturbance affecting the jet, manifested in the spectrum of 3C 329. The event was most pronounced in the γ-ray part of the spectrum, which – in this object – is the dominant and also the most variable part. The γ-ray flux of 3C 279 can vary over an order of magnitude, and at one point – the beginning of the event – the flux increased rapidly from an already elevated level to its maximum value, then dropped a little more slowly, over a span of 20 days, to its minimum.Other parts of the spectrum were also affected, but not so dramatically. Flux in ultraviolet, optical, and near infrared bands also decreased from somewhat elevated levels during the same 20 days, though there was no spike up at the start. There was, however, little change in the X-ray and radio bands during this period.There was one additional dramatic change in the same period. The percentage of polarization in optical emissions (blue) dropped from 30-40% down to 10% before recovering at the end of the period. And at the same time, the direction of polarization changed smoothly over the 20 days by about 180°.Since our line of sight is nearly parallel (to within about 2°) to the jet, it is difficult to distinguish where in 3C 279 different emissions originate. Strong γ-ray emissions are typically associated with jets (when present), but a key question that the paper examines concerns what part of the jet the dramatic changes in γ-ray flux could have been associated with. Was it relatively close to the black hole, or much farther out? It's not too surprising that there was little change in X-ray flux, since that's normally associated in active galaxies with the "corona", which is symmetrically distributed in a region with a radius of a few hundred light years around the black hole. Radio emissions, however, do generally originate from the jets, but often from "lobes" at very large distances from the center. In this case it would seem that the source of radio emissions had little to do with the "event".On the other hand, since distinct changes in ultraviolet, optical, and infrared flux – as well as the dramatic change in polarization – occurred at exactly the same time as the γ-ray "event", it's natural to suppose that whatever caused the disturbance affected a part of the jet where these emissions originated.So what can be said about the size and (perhaps) location of the disturbance? The key fact is that the event was observed to last 20 days. However, since we're dealing with matter moving at a relativistic velocity, it doesn't at all follow that the disturbance affected only a portion of the jet about 20 light days in extent. It's not hard to calculate the "actual" size of the disturbance, but it does take a little work.Suppose we let r0 be the distance along the jet from the central black hole at which the disturbance began, and r1 be the distance at which the state of the jet has returned to "normal". Then r1 >r0. The distance d = r1 - r0 is what we want to compute. If v is the average velocity of matter in the jet when the event occurred, then we have already noted v = 0.998c, so that β =v/c = 0.998. Note that d is the distance in our reference frame. The time (in our frame) it takes light to travel that distance is d/c. If we assume that the matter within the jet that's subject to the disturbance is moving with velocity v, then the time it takes the leading edge of that matter to go ... Read more »
Abdo, A., Ackermann, M., Ajello, M., Axelsson, M., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Baughman, B., Bechtol, K.... (2010) A change in the optical polarization associated with a γ-ray flare in the blazar 3C 279. Nature, 463(7283), 919-923. DOI: 10.1038/nature08841
Via Jennifer Ouellette on Twitter, I ran across a Discovery News story touting a recent arxiv preprint claiming to see variation in the fine-structure constant. It's a basically OK story, but garbles a few details, so I thought it would be worth giving it the ResearchBlogging treatment, in the now-traditional Q&A format.
What did they do? The paper looks at some spectral lines in radio emission from a moderately distant galaxy with the poetic name "PKS1413+135." These lines are produced by OH molecules in interstellar gas clouds, and the frequencies they see suggest that there may have been changes in some dimensionless constants during the not quite three billion years since the light was emitted.
Dimensionless constants? What are those? Most of the things we tend to think of as fundamental constants-- particle masses, Planck's constant, and that sort of thing-- are numbers with units. The only way to really measure one of these things, though, is by measuring it relative to one or more of the others. As a result, people who think about hard-core particle physics and cosmology and that sort of thing prefer to talk about "dimensionless constants," which are ratios of these things arranged so that all the units cancel. The most famous is the "fine structure constant" α which is the ratio of the electron charge squared to Planck's Constant times the speed of light, helpfully shown in an image lifted from Wikipedia. Other dimensionless constants of interest are the ratio of the proton and electron masses, and the "gyromagnetic ratio" which relates the spin of a fundamental particle to its magnetic moment. These ratios are the things that really matter if you want to look for changes in the fundamental constants.
Changes in the fundamental constants? Aren't they, you know, constant? You'd like to think that, but they don't necessarily have to be. And a lot of the tricks particle theorists pull when they're trying to explain fundamental forces end up giving you fundamental "constants" that change in time. This is something that you can look for experimentally, and that's what the current paper is about.
How do you do that? It's not like you can get a three billion year old proton and weigh it, can you? No, but you can look at the light emitted by really old atoms and molecules. The frequencies at which atoms and molecules emit light depend on the exact values of those dimensionless constants, so if the constants change, then the frequencies change.
How can you tell, though? Doesn't the Doppler shift from the expanding universe shift all the frequencies we see, anyway? The trick is to compare the light emitted by different transitions in the same atoms or molecules. Some states will shift up in frequency with a change in the fine structure constant (for example), while other states will shift down. Doppler shifts due to the motion of the universe or objects in it will always go in the same direction, depending on the velocity of the source. If you look at the relative frequencies of light from these different states, then, you can take out the Doppler shift, and still see if there's been a change in the relative frequencies.
Read the rest of this post... | Read the comments on this post...... Read more »
Nissim Kanekar, Jayaram N. Chengalur, & Tapasi Ghosh. (2010) Probing fundamental constant evolution with redshifted conjugate-satellite OH lines. Astrophysical Journal Letters. arXiv: 1004.5383v1
Dark matter is like the Rome of astronomy, all observations lead to dark matter. The problem is that physicists and astronomers, don't know what it actually is. The observations which support dark matter come from many different independent observations, so it is not just some observational error. The observations which corroborate the dark matter paradigm make for a fantastic discussion, but for right now I would like to focus on explanations for what dark matter may be. Specifically, what kind of particles are dark matter?Dark Matter is not like any particle beforeThe most attractive candidate theory for dark matter would be simple, it would be motivated from fundamental particle physics, and it would make testable predictions for future observations. Whatever dark matter particles are we know that dark matter does not interact with photons, is electrically neutral, is highly non-relativistic, and it is dissipationless; so basically that means dark matter is boring. To drive at what dark matter really is I would first ask, can even begin to understand the observations without tying ourselves to any specific models for dark matter's nature? I do not know. I am merely going to outline the fundamentals of a plausible dark matter model and address how it confronts observations. There are dark matter particles theories invoked from supersymmtery, universal extra dimensions, and branes; the most common dark matter candidate which I will focus on here is the so called weakly interacting massive particle (WIMP). If dark matter is a WIMP, then we still have only narrowed down what kind of particle it is to a zoo of particles. If we want a pure motivation for an unknown dark matter particle we could go back to the early 1930's when Enrico Fermi developed a theory of beta decay that involved the neutrino and necessitated a new mass scale in nature; this mass scale introduces new fundamental particles that would interact weakly with regular matter and presto, WIMPs. It turns out that WIMPs, particularly supersymmetric neutralinos, are a compelling fit given the current data, but as more precises observations accumulate the theory faces scrutiny. There was a plague of rumors of an affirmative dark matter detection a year ago which were of course unfounded (it took much restraint not to throw in my lot with the speculations). Today dark matter looms as a unsolved problem because its existence is not in question, only its origin.A plausible theory for dark matterDark matter particles zoom right through matter without interacting, similarly to neutrinos, but unlike neutrinos dark matter particles are very massive. The gravitational potential of the largest structures in the universe, like galaxies and galaxy clusters, is dominated by dark matter. Dark matter is everywhere (yes, even here on Earth dark matter is present and it does have some gravitational effect, but it is infinitely more feeble compared to the Earth or the Sun), but dark matter doesn't clump, it doesn't form dark molecules, and it doesn't form dark galaxies. Regular matter that you or I are made of (known as baryonic matter) is gravitationally attracted to dark matter (non-baryonic matter), but dark matter dominates because there is so much of it. It is hard to notice dark matter because it only interacts with regular matter through gravity for the most part and gravity is the weakest of forces. Dark matter remains like giant clouds in which a galaxy or a cluster of galaxies resides within. Dark matter is different from regular matter because regular matter can be detected through electromagnetic waves which dark matter shouldn't produce, unless dark matter self annihilates.If dark matter is thermal weakly interacting massive particles (WIMPs) then it may produce observable signals when it self annihilates. A popular model for the WIMP which would self annihilate is the neutralino (see figure at right). Self annihilation is exactly what it sounds like, like two identical but opposite forces meeting, the result is an explosion of energy and particles (the interaction conserves energy, momentum, and other quantum numbers). WIMP self annihilations into positrons and electrons could be detected by cosmic ray detectors. These self annihilations and the observable signal would be rare though. If they were common we would see a lot more signal. In the early universe these annihilations would have been much more common such that if there was a primordial abundance of dark matter it would have self annihilated to be consistent with the much lower density of dark matter we observe today (to wrap your head around this take my contrived analogy of a king who has many identical twin sons all born at once, in time they might murder each other until there was enough land for the remaining sons to all have just enough). Mathematically this is stated that the density of dark matter today, Ω, is proportional to the inverse of the particle cross section times relative velocity at freeze-out, σν, (think of this as probability the particles would interact). Today dark matter is said to have frozen out at its current density because on average a dark matter particle will travel the entire distance across the universe (this distance changes also invoking the Hubble scaling h) before interacting with another dark matter particle:This is known as the WIMP miracle. Particle physics independently predicts a particle with the right density to be the dark matter that astronomers observe.Evidence for WIMPsRecent observations from experiments and collaborations including ATIC, Fermi/GLAST, PAMELA, and others have observed an excess over the expected background of cosmic ray positrons (I am lumping these experiments together, but they actually detect subtly different kinds of particles and found different kinds of anomalies; for example PAMELA detected an upturn in the positron fraction e+/(e++e-) from 10-100 Gev while ATIC detected and excess in the total e++e- count at energies of 300-800 Gev). The reality is that anomalies are not unexpected given the uncertainty in astrophysical foregrounds which vary with energy.Cosmic rays are charged particles (like positrons, e+,and electrons, e-) fired like bullets moving close to the speed of light at random throughout the universe and may be created by any of your favorite high energy astrophysical sources like magnetars, super massive black holes, supernovae, and so forth. Cosmic ray particles (any very fast moving charged particles) can interact with the galactic magnetic field, the interstellar medium, or the interstellar radiation field. These interactions generally cause the particles to lose energy and that energy is emitted in the form of observable photons. Thus dark matter will leave an astronomical signature not only in the form of cosmic rays from direct annihilation into positrons or electrons as discussed above, but also in the form of scattering of photons. The reason for all this talk about cosmic rays is that now we can see that the dark matter models explaining the PAMELA, ATIC, and Fermi results would also produce other observables, for example, an excess of gamma ray photons from the galactic center (they would be focused from the galactic center because that is where the dark matter would be densest and annihilating most rapidly) at energies 100 GeV and more. Another signal that may come from dark matter was seen in observations made by the WMAP satellite which detected residual microwave emission from the galactic center known as the 'WMAP haze'.You can't just ask a particle where it came from. The electrons and positrons of WIMP annihilation are cosmic rays with an exotic origin, but because there may be an undetected 'local' pulsar producing the cosmic ray anomalies being detected we... Read more »
Abdo, A., Ackermann, M., Ajello, M., Atwood, W., Baldini, L., Ballet, J., Barbiellini, G., Bastieri, D., Baughman, B., Bechtol, K.... (2010) Spectrum of the Isotropic Diffuse Gamma-Ray Emission Derived from First-Year Fermi Large Area Telescope Data. Physical Review Letters, 104(10). DOI: 10.1103/PhysRevLett.104.101101
Feng, J., Kaplinghat, M., & Yu, H. (2010) Halo-Shape and Relic-Density Exclusions of Sommerfeld-Enhanced Dark Matter Explanations of Cosmic Ray Excesses. Physical Review Letters, 104(15). DOI: 10.1103/PhysRevLett.104.151301
For over a decade, through the ingenious tracking of stellar orbits in the galactic centre, we’ve known that a supermassive black hole weighing the equivalent of several million solar masses is lurking at the centre of our galaxy. But this discovery, while offering us the tantalising opportunity to study these enigmatic objects in our own [...]... Read more »
Gabriele Ponti, Regis Terrier, Andrea Goldwurm, Guillaume Belanger, & Guillaume Trap. (2010) Discovery of a superluminal Fe K echo at the Galactic Center: The glorious past of Sgr A* preserved by molecular clouds. ApJ . arXiv: 1003.2001v1
Sunyaev, R., & Churazov, E. (1998) Equivalent width, shape and proper motion of the iron fluorescent line emission from molecular clouds as an indicator of the illuminating source X-ray flux history. Monthly Notices of the Royal Astronomical Society, 297(4), 1279-1291. DOI: 10.1046/j.1365-8711.1998.01684.x
Most galaxies have a supermassive black hole in their center – sometimes even more than one. These black holes can have masses up to ten billion solar masses (1010 M⊙) or more. One of the largest known examples is part of a binary system, and it weighs in at 1.8×1010 M⊙ – see here, here, or here. (There are exceptions, such as the nearby M33, which apparently does not have a central black hole of mass more than 3000 M⊙.)All black holes gravitationally attract any nearby matter, because of their high mass, which is generally ≥ 1 M⊙ even for comparatively tiny stellar mass black holes. Such matter does not necessarily fall directly into the black hole, but instead can go into orbit around the black hole. If there is enough matter close to the black hole, and if it is pulled in rapidly enough, the results can be a spectacular light show, such as one might see (if one could see simultaneously at all wavelengths from very high radio frequencies to X-rays) in the Centaurus A galaxy, about 13 million light years away:This is a view of the whole galaxy – you can see that the central area, which contains the black hole, is unusually bright, and there are jets extending more than the radius of the galaxy itself in both directions perpendicular to the galactic plane. Centaurus A is an example of an "active galaxy", and it shows the impressive effects produced by the central black hole of such an object. (For more about Centaurus A and this image, see here, here. Another image: here.)The innermost region of an active galaxy, which is the interesting part, is called an "active galactic nucleus" (AGN). This is a general term for a number of puzzling astronomical objects that were noticed at first on account of their unusually vigorous output of energy, but whose similarities were not immediately recognized. AGNs were eventually deduced to be (in almost all cases) just relatively ordinary galaxies with massive central black holes that appear to be responsible for liberating at least as much energy as all the stars in the remainder of the galaxy.Although most galaxies seem to have a supermassive black hole in their center, behavior of AGNs is rather unusual, and AGNs are somewhat rare in the nearby universe, but more common at large distances – hence at an earlier time in the universe. AGN behavior requires not only a supermassive black hole, but also a substantial amount of surrounding interstellar gas that fuels their energy output. AGNs are rather profligate in using their fuel, so presumably most of the available interstellar gas close to a central black hole is consumed over a relatively short period of time (compared to the age of the universe). Consequently, in most nearby galaxies the fuel was used up long ago. The Milky Way has a relatively small central black hole associated with the radio source named Sagittarius A* (Sgr A*). The black hole has a mass of ~4×106 M⊙. Sgr A* is not nearly luminous enough to be considered an AGN, so evidently either there is not now enough nearby interstellar gas, or perhaps the black hole is just too small to have ever attracted enough.How luminous does a galactic nucleus need to be in order to qualify as an AGN? It's really a question of how bright the nucleus is compared to the rest of the galaxy. Messier 77, also known as NGC 1068, is the first galaxy now considered to have an AGN that came to special attention. In 1908 E. A. Fath obtained its spectrum and found it had unusually strong emission lines. (This was at a time when it was still assumed that nebulae were simply fuzzy objects inside our own galaxy.) V. N. Slipher later obtained a better spectrum and noted that the width of the lines implied high velocities – hundreds of kilometers per second. NGC 1068 is fairly nearby – 47 million light years away – and rather large, with a diameter of 170,000 light years (compared to the Milky Way's diameter of 100,000 light years). NGC 1068 is still under active study at this time – see here.Finally in 1943 Carl Seyfert recognized that NGC 1068 was similar to a number of other galaxies that formed a distinct class, based on the nature of their spectra and because their innermost regions were as bright as the entire rest of the galaxy. This concentration of luminosity in the center was not only exceptional, but it was quite unlikely to be physically possible for a sufficient number of stars to be located in such a small volume of space. Naturally, galaxies of this sort became known as Seyfert galaxies. Other peculiarities of Seyfert galaxies were eventually recognized as well. For example, their spectra contain broad, strong emission lines of hydrogen, helium, nitrogen, and oxygen. This in turn implied that the emitting material had to be in rapid motion in order to produce Doppler broadening of the emission lines. And this in turn implied that a large amount of mass needed to be concentrated in a small volume to account for such high velocities. The characteristics of high central luminosity and broad emission lines tended to occur together enough to justify recognizing Seyfert galaxies as a distinct class, which made up about 1% of nearby spiral galaxies.About 15 years after Seyfert galaxies were discovered, another peculiar type of astrophysical object was noticed – quasars, or, as they were sometimes known, "quasi-stellar-objects". These came first to attention as strong sources of radio emission, in early radio telescope surveys, such as the original Third Cambridge Catalogue of Radio Sources. The strong radio signal was somewhat mysterious, since electromagnetic radiation at radio frequencies (up to 100 GHz at the high end) is normally emitted only by rather cold matter (under about 2 degrees above absolute zero).Many of these sources ("radio galaxies") were eventually identified with optically visible objects, many of which had already been cataloged as unremarkable stars. But when they came under attention as radio sources and their optical spectra were obtained, their high redshift implied that they needed to have an astonishingly high intrinsic luminosity – too luminous to be explainable simply as very large galaxies containing even the brightest possible stars. Phenomena that are seemingly impossible to explain in terms of familiar examples tend to draw a lot of attention – and this is true, in some respects, of quasars even today. A lot of the mystery is now explainable in terms of active galaxies that are powered by supermassive black holes, and we'll get more into that in a little bit. But there are also some AGN features, such as the jets that a few AGNs expel at relativistic velocities, which are still not well understood.Although there was still controversy at the time whether redshift could be used as a reliable gauge of distance, if the conventional redshift interpretation (Hubble expansion) was assumed, then quasars would need to be (what was considered at the time, ca. 1960) extremely distant. One of the earliest-recognized quasars, 3C 273, which is the quasar with the largest apparent magnitude and is visible through amateur telescopes, has z=0.158, corresponding to an optical distance of ~2.4 billion light years. 3C 273 was therefore intrinsically far brighter than any star, about 100 times as bright as an entire spiral galaxy. 3C 273 is sometimes considered to be ... Read more »
Evans, D., Reeves, J., Hardcastle, M., Kraft, R., Lee, J., & Virani, S. (2010) THE HARD X-RAY VIEW OF REFLECTION, ABSORPTION, AND THE DISK-JET CONNECTION IN THE RADIO-LOUD AGN 3C 33. The Astrophysical Journal, 710(1), 859-868. DOI: 10.1088/0004-637X/710/1/859
In September of 2008, I was sitting at the controls of the McDonald Observatory 2.1-meter Struve Telescope. I was there to help some of my colleagues who work on a team associated with NASA's Kepler Mission. This team, the Kepler Astroseismic Science Consortium, isn't on the lookout for planets. They are studying the stars themselves, looking for variations in the light from stars caused by sound waves in the star. The study of these sound waves, known as asteroseismology, allows astronomers to probe the interior structure of a star, just like geologists use seismic waves from earthquakes to study the interior of the Earth.
My specific goal at the telescope was to look for pulsating white dwarfs. While around a hundred pulsating white dwarfs are known in the sky, none are known in the patch of sky where the Kepler mission stares. I had a long list of candidates, some that I chose, and some that our European collaborators selected. One of the stars selected by Viennese astronomer Gerald Handler and his collaborators, the star in the center of the picture above, had been observed with the Telescopio Nazionale Galileo in the Canary Islands, and looked "interesting".
So, my job was to follow up on the interesting star. I pointed the telescope and started taking pictures, measuring how bright the star was every 10 seconds. The pulsating white dwarfs we look for vary their brightness in a very regular fashion, getting brighter and fainter by a few percent every few minutes. At first, this star looked like it was doing exactly that, and then, in a space of just a couple of minutes, the star got a whopping 20% brighter. I could actually see on the computer screen that the star had brightened relative to the other stars (this is very rare; we need computers to tell us about the few percent variations).
I knew then that this was not a pulsating white dwarf, and I suspected it was a cataclysmic variable, or CV, where a white dwarf is pulling material off of another star. I suspected this because CVs often show this behavior of getting suddenly brighter and fainter. The rest of that night, and some of the following night, I saw this variability of the star. I notified our European contacts, who looked with a telescope in Bologna and verified the variability. At this point I also started to blog about it, but was then asked not to say much. If we had found something new, we wanted to make sure we got the data we needed to publish a paper and not be scooped by someone else.
Our next step was to make sure that we had actually discovered something new, so we scoured journal articles, online databases, and various catalogs, and while we found the star listed in some basic catalogs, nobody had ever studied it or determined that the star was a CV. What we needed, though, was a spectrum of the object. Spectroscopy, the analysis of the different colors of light coming from an object, allows us to identify uniquely each object, as well as garner a lot of additional information.
None of us had time on a telescope with a spectrograph before the star went behind the sun for the winter. But two of my friends and colleagues were using the MMT and its spectrograph later that month, so I asked them if they might look at it. The first of them didn't have much time but grabbed a quick spectrum, which he misinterpreted as a "normal" white dwarf star, so he went on with his program. That's fine, these things happen. But we got lucky with my other colleague, Patrick Dufour (who has made some very interesting discoveries on his own). When he got on the telescope, the wind was coming out of the east so fast that he could not safely point the telescope any direction but west, and all of his targets were in the east. Our star was in the west. So, Patrick very kindly pointed the telescope west and took about 90 minutes of spectral data. (He could just as easily closed the dome and gone to bed, and we would have been none the wiser).
Patrick's high-quality data proved that this star is a cataclysmic variable, and even allowed us to start to classify what kind of CV it is. CVs come in many flavors, so one of the most basic bits of analysis for a new CV is to fit it into the existing classification scheme, if possible. So, we managed to narrow down its class somewhat, though there is still more work that needs to be done. This star is definitely a nova-like variable of the UX Uma class, and it may belong to the SW Sextantis subclass. (To compare to a topic that may be more familiar, this is like saying we found a creature that we know is a mammal, we know is a primate, and we know is a great ape, but we're not yet sure if it is a gorilla or a chimpanzee.) To make a better classification, we need better data. But since this star is outside my area of expertise, we published the information that we have so other people can go and dig deeper.
To me, the exciting thing about this discovery is that the CV is in the field of view of the Kepler mission (I think four or five others are known in the Kepler field). Kepler is just a phenomenal instrument, and its ability to stare at an object for months on end and to obtain very high precision measurements mean that people who study CVs can try new avenues of analysis that have never been done before. This is illustrated by the recent discovery by Kepler of Doppler boosting, a brightening of light from an object due to its motion toward you (like a Doppler shift). This boosting effect is very tiny for the majority of objects in the Universe, but Kepler can measure it. In fact, Kepler has already looked at this system for about a month, and those data should be available for download very soon.
So, in short, we found an interesting cataclysmic variable in the same region of the sky that the Kepler mission is looking. We don't have more than a basic description, and some of the important parameters and classification of the system remain to be done. However, since I only dabble in the field of cataclysmic variables, I probably won't be doing much more with this star. Some of my collaborators may well be working on it, and because we've published it other researchers can feel free to follow up on it, too. All of them can do a better and more efficient job studying this star than I can as a newbie. So, friends, have at it!
Kurtis A. Williams, Domitilla de Martino, Roberto Silvotti, Ivan Bruni, Patrick Dufour, Thomas S. Riecken, Martin Kronberg, Anjum Mukadam, & G. Handler (2010). Discovery of a Nova-Like Cataclysmic Variable in the Kepler Mission
Field The Astronomical Journal, in press, (arXiv: 1004.3743v1)... Read more »
Kurtis A. Williams, Domitilla de Martino, Roberto Silvotti, Ivan Bruni, Patrick Dufour, Thomas S. Riecken, Martin Kronberg, Anjum Mukadam, & G. Handler. (2010) Discovery of a Nova-Like Cataclysmic Variable in the Kepler Mission Field. The Astronomical Journal. arXiv: 1004.3743v1
I read 3 papers! Three beautiful, science-packed, revolutionary and mind-blowing papers! Ok, maybe not – but trust me, after being so busy with things like measuring redshifts, fixing codes and mountains of admin/conference organising, almost any science is pure beauty for the old brain.
But one of these papers did tap into something I’m quite interested in, and it’s related to the Cosmic Microwave Background (CMB)....... Read more »
Today, it’s pretty much common knowledge that time doesn’t behave the same way across space. The shape of the space-time fabric around massive objects like stars and planets actually slows its passage, and since the universe is constantly expanding, events seem to unfold slower the farther back in time we look relative to our planet’s [...]... Read more »
Video credit: University of Michigan / Boston University / Cosmovision
We went looking for a small black hole in our neighborhood, maybe a few hundred light-years away, and instead we found a supermassive black hole nearly 7 billion light-years away. Sometimes astronomy can be that way...
Back in February, my colleagues and I were looking at white dwarfs with the Keck I telescope in Hawaii. Before our second night started, the astronomers at the neighboring telescope (Keck II, my telescope's twin) came over and asked if we had time to look at an object for them, because their telescope had the wrong camera for the observations they needed. We had the time, and agreed.
Their target was discovered by the Fermi Gamma-ray Space Telescope, a space-based telescope that looks at gamma rays, the most energetic type of light in the Universe. On February 1, the Large Area Telescope on Fermi, a telescope that maps the entire sky in gamma rays every three hours, had detected a new source of gamma rays. This new source was long-lived, meaning it was not one of the enigmatic gamma-ray bursts that come and go in a few seconds. Since gamma rays are made by very energetic processes, the Fermi scientists knew that they were seeing something very exciting.
The new gamma ray source that Fermi saw was right in the band
of the Milky Way as seen from Earth, in the middle of the constellation Cassiopeia.
Because most of the stars in our own galaxy appear to be in the band of
the Milky Way, Fermi scientists thought we must be seeing something in our own galaxy. And since gamma rays have to come from very energetic things, we suspected it may be some kind of black hole in the Milky Way. But while the dust and gas in the galaxy can act like a screen and block many kinds of light, gamma rays can travel right through. So, if there is a distant galaxy that just happens to appear lined up with our Milky Way, we might be seeing something millions of times further away. We needed more information.
The natural place to start was to look at pictures of this point of sky in other wavelengths of light. Some of the astronomers therefore went and looked in X-rays, some in radio waves, and some in optical (visible) light. And there was something there in all three of those types of light! In fact, in visible light there are two faint little dots. This isn't surprising -- there are so many stars in the Milky Way, you are bound to see one or more stars no matter where you look. One of these two points of light was the source of the gamma-rays, and one was probably just a normal star.
This is where my white dwarf collaborators and I came into the picture, as we just happened to be sitting in front of the telescope at the right time. We were asked to get a visible-light spectrum of both sources. Spectra, the splitting of light into its component colors, are like fingerprints. Each type of object has its own unique spectrum. Was the Keck telescope, one of the most powerful astronomical tools on the planet, able to tell us what these objects were? We pointed the Keck telescope and its spectrometer at the two stars, recorded the data, and sent it to the gamma ray astronomers to analyze.
One of the faint spots was an ordinary star. Ordinary stars don't produce gamma rays, so this first source was the wrong one. The second spectrum was something very interesting: a quasar. Quasars are ginormous black holes located at the centers of galaxies. As the material near the black hole falls in, it heats up and releases the visible and X-ray light that we were seeing. We had our gamma-ray source, but it wasn't nearby. It was 7 billion light years away, and by chance happened to be lined up with the band of our Milky Way.
Fermi has seen many gamma-ray sources like this one. When gas is falling into a black hole, it travels faster and faster, often colliding with other gas that is also falling in. The energy from all of this heat is enough to create jets, pencil-thin beams of matter that stream away from the black hole at nearly the speed of light (see the video at the top of this post for an artist's conception of these jets). If these jets happen to be pointed at the Earth, we are seeing the most energetic part of the whole system, which means lots and lots of gamma rays. The amount of gamma rays produced varies as the amount of energy in the jets changes. Seven billion years later, these gamma rays arrive at the Earth, where we just happen to be looking with a gamma-ray telescope.
Some of the astronomers were a little disappointed. Giant black holes are really cool, but there are thousands of them known. Fermi has discovered many new types of gamma-ray sources in our own galaxy, and we were hoping this might be yet another unexpected discovery. Instead, we had the chance alignment of a very, very distant galaxy with the stars of our own galaxy.
My part in this whole adventure was small. I took the optical light spectrum, and I wrote up a paragraph describing how I did that. Twenty-one other astronomers were involved in the rest of the project, gathering data and analyzing from all kinds of telescope, and each contributing our little niche of expertise to a bigger problem that none of us would have solved individually.
Our paper on this discovery is still going through the review process, where another scientist is checking over what we did to look for any mistakes or problems with our logic. Here's hoping she agrees with our analysis!
J. Vandenbroucke, R. Buehler, M. Ajello, K. Bechtol, A. Bellini, M. Bolte, C. C. Cheung, F. Civano, D. Donato, L. Fuhrmann, S. Funk, S. E. Healey, A. B. Hill, C. Knigge, G. M. Madejski, R. W. Romani, M. Santander-García, M. S. Shaw, D. Steeghs, M. A. P. Torres, A. Van Etten, & K. A. Williams (2010). Discovery of a GeV blazar shining through the Galactic plane Astrophysical Journal Letters arXiv: 1004.1413v1... Read more »
J. Vandenbroucke, R. Buehler, M. Ajello, K. Bechtol, A. Bellini, M. Bolte, C. C. Cheung, F. Civano, D. Donato, L. Fuhrmann.... (2010) Discovery of a GeV blazar shining through the Galactic plane. Astrophysical Journal Letters. arXiv: 1004.1413v1
Paolo Salucci has a bone to pick with the community. The Trieste-based astronomer is fed up with his colleagues’ misconceptions about galaxy rotation curves and has decided to Do Something About It. In his short paper posted to astro-ph last Friday, he describes the experiment he’s set up to convince the world that galaxy rotation [...]... Read more »
Paolo Salucci. (2010) Can Social Networks help the progress of Astrophysics and Cosmology? An experiment in the field of Galaxy Kinematics. Arxiv. arXiv: 1004.1190v1
Since a few weeks some PhD students and postdocs have been organising astro-ph coffee meetings three times a week, where the youngsters in the department can sit together and chat about recent papers. The advantage of having these meetings for only students and postdocs is that we can admit to our utter ignorance about stuff [...]... Read more »
Kloppenborg, B., Stencel, R., Monnier, J., Schaefer, G., Zhao, M., Baron, F., McAlister, H., ten Brummelaar, T., Che, X., Farrington, C.... (2010) Infrared images of the transiting disk in the ε Aurigae system. Nature, 464(7290), 870-872. DOI: 10.1038/nature08968
D. W. Hoard, S. B. Howell, & R. E. Stencel. (2010) Taming the Invisible Monster: System Parameter Constraints for Epsilon Aurigae from the Far-Ultraviolet to the Mid-Infrared. ApJ accepted. arXiv: 1003.3694v1
by Jon Voisey in Angry Astronomer
A long time ago, I trashed a Creationist article on Stellar Evolution. It was a fun time. I got one of the most ignorant trolls I've yet had on this site and even Phil Plait came over to gawk and point at the giant logical gaps. One of the claims that the author had made was that, because we shouldn't see young stars in very close quarters to black holes in the centers of galaxies (the nearest 1 parsec), and we do, that all of stellar evolution must be wrong and thus, God did it.Mollishka from A Geocentric View (which sadly hasn't been updated in a year) pointed out that the only galaxies for which we could have sufficient resolution to even discuss the stellar populations of the innermost parsec, would be our own and Andromeda. So trying to draw grand conclusions from a tiny number of cases, as the Creationist was trying to do was just idiotic. Compile that with the completely unrelated conclusion that it meant that God did it, and you get normal Creationist mentality.Meanwhile, the problem of young stars in the inner portions of galaxies was still an unsolved problem. But instead of tossing their hands up and declaring it impossible a la Behe1. But while Creationists buried their heads in the sand, real scientists continued to try to figure out what was going on.A new paper looks to have some promising answers. The real trick in the whole problem is getting gas in close enough to the black hole to form young stars without chucking it in to be eaten.The new paper ran simulations and found that due to gravitational instabilities that effected stars and gas differently, galaxies with supermassive black holes like M31's would form disks of each separately and tilted with respect to one another. The interplay between these disks allows the stellar disk to yank some of the gas out and send it towards the black hole. There, "[s]ome of the gas turns into stars, those stars are excited into the m = 1 mode, allowing the perturbation to efficiently propagate inwards to ~0.1pc."Booya.New stars. Real science.1 - "As scientists, we yearn to understand how this magnificent mechanism came to be, but the complexity of the system dooms all Darwinian explanations to frustration. Sisyphus himself would pity us."~Michael Behe: Darwin's Black BoxPhilip F. Hopkins, & Eliot Quataert (2010). The Nuclear Stellar Disk in Andromeda: A Fossil from the Era of Black Hole Growth MNRAS arXiv: 1002.1079v2... Read more »
Philip F. Hopkins, & Eliot Quataert. (2010) The Nuclear Stellar Disk in Andromeda: A Fossil from the Era of Black Hole Growth. MNRAS. arXiv: 1002.1079v2
Thunderstorms are epic demonstrations of nature that can be quite fascinating when they aren't terrifying. The study of thunderstorms, in particular lightning, is of obvious practical interest, but also there is also a purely aesthetic and amusing aspect to them.... Read more »
Siingh, D., Singh, A., Patel, R., Singh, R., Singh, R., Veenadhari, B., & Mukherjee, M. (2009) Thunderstorms, Lightning, Sprites and Magnetospheric Whistler-Mode Radio Waves. Surveys in Geophysics, 29(6), 499-551. DOI: 10.1007/s10712-008-9053-z
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