* Astronomy

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RE: Quasars

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Most Quasars Live on Snacks, Not Large Meals

Black holes in the early universe needed a few snacks rather than one giant meal to fuel their quasars and help them grow, a new study shows. Quasars are the brilliant beacons of light that are powered by black holes feasting on captured material, and in the process, heating some of the matter to millions of degrees. The brightest quasars reside in galaxies distorted by collisions with other galaxies. These encounters send lots of gas and dust into the gravitational whirlpool of hungry black holes. Now, however, astronomers are uncovering an underlying population of fainter quasars that thrive in normal-looking spiral galaxies. They are triggered by black holes snacking on such tasty treats as a batch of gas or the occasional small satellite galaxy.
A census of 30 quasar host galaxies conducted with two of NASA's premier observatories, the Hubble Space Telescope and Spitzer Space Telescope, has found that 26 of the host galaxies bear no tell-tale signs of collisions with neighbours, such as distorted shapes. Only one galaxy in the sample shows evidence of an interaction with another galaxy. The galaxies existed roughly 8 billion to 12 billion years ago, during a peak epoch of black-hole growth. The study bolsters evidence that the growth of most massive black holes in the early universe was fuelled by small, long-term events rather than dramatic short-term major mergers.
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Title: The Colour Variability of Quasars
Authors: Kasper B. Schmidt, Hans-Walter Rix, Joseph C. Shields, Matthias Knecht, David W. Hogg, Dan Maoz, Jo Bovy

We quantify quasar colour-variability using an unprecedented variability database - ugriz photometry of 9093 quasars from SDSS Stripe 82, observed over 8 years at ~60 epochs each. We confirm previous reports that quasars become bluer when brightening. We find a redshift dependence of this blueing in a given set of bands (e.g. g and r), but show that it is the result of the flux contribution from less-variable or decayed emission lines in the different SDSS bands at different redshifts. After correcting for this effect, quasar colour-variability is remarkably uniform, and independent not only of redshift, but also of quasar luminosity and black hole mass. The colour variations of individual quasars, as they vary in brightness on year timescales, are much more pronounced than the ranges in colour seen in samples of quasars across many orders of magnitude in luminosity. This indicates distinct physical mechanisms behind quasar variability and the observed range of quasar luminosities at a given black hole mass - quasar variations cannot be explained by changes in the mean accretion rate. We do find some dependence of the colour variability on the characteristics of the flux variations themselves, with fast, low-amplitude, brightness variations producing more colour variability. The observed behaviour could arise if quasar variability results from flares or ephemeral hot spots in an accretion disc.

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The first quasars

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RE: Quasars

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Quasars fingered for cosmic climate change

Climate change doesn't just happen on Earth. Billions of years ago, a heatwave struck the universe, leaving its imprint in the light from distant galaxies.
George Becker of the University of Cambridge and colleagues studied the light coming from galaxies at different times in the universe's history. Dark lines in the spectra mark where certain wavelengths have been absorbed by clouds of gas as the light travels to Earth. The hotter the gas, the more blurred these lines become.
About 12 billion years ago, the gas warmed from 8000 to 15,000 kelvin, probably due to heating from quasars, objects powered by giant black holes, the team will report in Monthly Notices of the Royal Astronomical Society.
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Mauna Kea scopes aid study of quasars

Astronomers at Mauna Kea's Keck Observatory have unlocked a powerful new tool to study the brightest and most mysterious bodies in the night sky.
Scientists from the California Institute of Technology and Switzerland's Ecole Polytechnique Federale de Lausanne announced Thursday they have found a way to use bent light from distant celestial bodies to determine the size of galaxies that house quasars, large bodies likely created by light escaping from high-energy black holes at the galaxies' center.
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Title: Witnessing the Birth of a Quasar
Authors: Takamitsu Tanaka, Zoltán Haiman, Kristen Menou
(Version v3)

The coalescence of a supermassive black hole binary (SMBHB) is thought to be accompanied by an electromagnetic (EM) afterglow, produced by the viscous infall of the surrounding circumbinary gas disk after the merger. It has been proposed that once the merger has been detected in gravitational waves (GWs) by LISA, follow-up EM searches for this afterglow can help identify the EM counterpart of the LISA source. Here we study whether the afterglows may be sufficiently bright and numerous to be detectable in EM surveys alone. The viscous afterglow, which lasts for years to decades for SMBHBs in LISA's sensitivity window, is characterized by rapid increases in both the bolometric luminosity and in the spectral hardness of the source. If quasar activity is triggered by the same major galaxy mergers that produce SMBHBs, then the afterglow could be interpreted as a signature of the birth of a quasar. Using an idealized model for the post-merger viscous spreading of the circumbinary disk and the resulting light curve, and using the observed luminosity function of quasars as a proxy for the SMBHB merger rate, we delineate the survey requirements for identifying such birthing quasars. If circumbinary disks have a high disk surface density and viscosity, an all-sky soft X-ray survey with a sensitivity of ~<3x10^-14 erg s^-1 cm^-2 and a time resolution of ~months could identify dozens of birthing quasars with sustained brightening rates of >10%/yr. If >1% of the X-ray emission is reprocessed into optical frequencies, birthing quasars could also be identified in optical transient surveys such as the LSST. Distinguishing a birthing quasar from other variable sources may be facilitated by the monotonic hardening of its spectrum, but will likely remain challenging. This reinforces the notion that joint EM-plus-GW observations offer the best prospects for identifying the EM signatures of SMBHB mergers.

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Research clears dust on quasars theory

University of Hawaii astrophysicist Ezequiel Treister and colleagues have identified a large number of quasars obscured by dust up to 11 billion light-years away, likely formed when the universe was in its infancy.
The quasars -- intense sources of X-rays and visible light -- were created when intergalactic collisions triggered the growth of supermassive black holes in the centres of galaxies.
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Title: Mass Functions of the Active Black Holes in Distant Quasars from the Large Bright Quasar Survey, the Bright Quasar Survey, and the Colour-Selected Sample of the SDSS Fall Equatorial Stripe
Authors: M. Vestergaard (1), Patrick S. Osmer (2) ((1) Tufts University, (2) The Ohio State University)

We present mass functions of distant actively accreting supermassive black holes residing in luminous quasars discovered in the Large Bright Quasar Survey, the Bright Quasar Survey, and the Fall Equatorial Stripe of the Sloan Digital Sky Survey (SDSS). The quasars cover a wide range of redshifts (0 <~ z <= 5) and were subject to different selection criteria and flux density limits. These samples are thus complementary and can help us gain additional insight on the true underlying black hole mass distribution, free from selection effects and mass estimation errors through future studies. We see evidence that the active z~4 black hole population is somewhat different than that at lower z. In particular, there is a sharp increase in the space density of the detected active black holes (M_BH >~ 10^8 Msun) between redshifts ~4 and ~2.5. Also, the z~4 SDSS quasar mass function has a somewhat flatter high mass-end slope, beta = -1.75 ± 0.56, compared to the mass functions based on quasars below z of 3, which display typical slopes of beta =~ -3.3; the latter are consistent with the mass functions at similar redshifts based on the SDSS Data Release 3 quasar catalogue presented by Vestergaard et al. We see clear evidence of cosmic downsizing in the comoving space density distribution of active black holes in the LBQS sample alone. In forthcoming papers, further analysis, comparison, and discussion of these mass functions will be made with other existing black hole mass functions, notably that based on the SDSS DR3 quasar catalogue. We present the relationships used to estimate the black hole mass based on the MgII emission line; the relations are calibrated to the Hbeta and CIV relations by means of several thousand high quality SDSS spectra. Mass estimates of the individual black holes of these samples are also presented.

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