* Astronomy

Members Login
Post Info TOPIC: Quasars


L

Posts: 111969
Date:
RE: Quasars
Permalink  
 


Title: The most luminous quasars do not live in the most massive dark matter haloes at any redshift
Authors: N. Fanidakis, A. V. Maccio, C. M. Baugh, C. G. Lacey, C. S. Frenk

Quasars represent the brightest Active Galactic Nuclei (AGN) in the Universe and are thought to indicate the location of prodigiously growing Black Holes (BHs), with luminosities as high as 10^48 erg/sec. It is often expected though that such an extremely energetic process will take place in the most massive bound structures in the dark matter (DM) distribution. We show that in contrast to this expectation, in a galaxy formation model which includes AGN feedback, quasars are predicted to live in average DM halo environments with typical masses of a few times 10^12 solar masses. This fundamental prediction arises from the fact that quasar activity (i.e., BH accretion with luminosity greater than 10^46 erg/sec) is inhibited in DM haloes where AGN feedback operates. The galaxy hosts of quasars in our simulations are identified with over massive (in gas and stars) spheroidal galaxies, in which BH accretion is triggered via a galaxy merger or secular processes. We further show that the z=0 descendants of high redshift (z~6) QSOs span a wide range of morphologies, galaxy and halo masses. The z~6 BHs typically grow only by a modest factor by the present day. Remarkably, high redshift QSOs never inhabit the largest DM haloes at that time and their descendants are very seldom found in the most massive haloes at z=0. We also show that observationally it is very likely to find an enhancement in the abundance of galaxies around quasars at z~5. However, these enhancements are considerably weaker compared to the overdensities expected at the extreme peaks of the DM distribution. Thus, it is very unlikely that a quasar detected in the z\gtrsim5 Universe pinpoints the location of the progenitors of superclusters in the local Universe.

Read more (1061kb, PDF)



__________________


L

Posts: 111969
Date:
Permalink  
 

Title: The Discovery of Quasars
Authors: K. I. Kellermann

Although the extragalactic nature of quasars was discussed as early as 1960, it was rejected largely because of preconceived ideas about what appeared to be an unrealistically high radio and optical luminosity. Following the 1962 occultations of the strong radio source 3C 273 at Parkes, and the subsequent identification with an apparent stellar object, Maarten Schmidt recognised that the relatively simple hydrogen line Balmer series spectrum implied a redshift of 0.16 Successive radio and optical measurements quickly led to the identification of other quasars with increasingly large redshifts and the general, although for some decades not universal, acceptance of quasars as being by far the most distant and the most luminous objects in the Universe. Curiously, 3C 273, which is one of the strongest extragalactic sources in the sky, was first catalogued in 1959 and the magnitude 13 optical counterpart was observed at least as early as 1887. Since 1960, much fainter optical counterparts were being routinely identified using accurate radio interferometer positions, measured primarily at the Caltech Owens Valley Radio Observatory. However, 3C 273 eluded identification until the series of lunar occultation observations led by Cyril Hazard, although inexplicitly there was an earlier misidentification with a faint galaxy located about an arc minute away from the true position. Ironically, due to calculation error, the occultation position used by Schmidt to determine the redshift of 3C 273 was in error by 14 arcseconds, and a good occultation position was not derived until after Schmidt had obtained his 200 inch spectrum.

Read more (192kb, PDF)



__________________


L

Posts: 111969
Date:
Permalink  
 

Gone, with the Wind

The case of the missing quasar gas clouds has been solved by a worldwide team of astronomers, and the answer is blowin' in the wind.
Astronomers Nurten Filiz Ak and Niel Brandt of the Pennsylvania State University led the team, which announced their results in a paper published in today's issue of The Astrophysical Journal. The paper describes 19 distant quasars in which giant clouds of gas seemed to disappear in just a few years.

Read more



__________________


L

Posts: 111969
Date:
Permalink  
 

Spitzer, Hubble See Galaxy-Altering Quasars Ignite

NASA's Spitzer and Hubble Space Telescopes have caught sight of luminous quasars igniting after galaxies collide. Quasars are bright, energetic regions around giant, active black holes in galactic centers. 
The new observations shed light on a key early period of galactic evolution when quasars and their host galaxies begin to interact, but before the two have settled down after recent galactic smashups.

Read more



__________________


L

Posts: 111969
Date:
Permalink  
 

 Most Quasars Live on Snacks, Not Large Meals

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, Hubble and Spitzer, has found that 26 of the host galaxies bear no telltale 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, led by Kevin Schawinski of Yale University, New Haven, Conn., 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.

Read more



__________________


L

Posts: 111969
Date:
Permalink  
 

 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.

Read more



__________________


L

Posts: 111969
Date:
Permalink  
 

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.

Read more  (1920kb, PDF)



__________________


L

Posts: 111969
Date:
The first quasars
Permalink  
 


Title: Cold flows and the first quasars
Authors: Tiziana Di Matteo, Nishikanta Khandai, Colin DeGraf, Yu Feng, Rupert Croft, Julio Lopez, Volker Springel

Observations of the most distant bright quasars imply that billion solar mass supermassive black holes (SMBH) have to be assembled within the first eight hundred million years. Under our standard galaxy formation scenario such fast growth implies large gas densities providing sustained accretion at critical or supercritical rates onto an initial black hole seed. It has been a long standing question whether and how such high black hole accretion rates can be achieved and sustained at the centers of early galaxies. Here we use our new cosmological hydrodynamic simulation (MassiveBlack) covering a volume (0.75 \Gpc)^3 appropriate for studying the rare first quasars to show that steady high density cold gas flows responsible for assembling the first galaxies produce the high gas densities that lead to sustained critical accretion rates and hence rapid growth commensurate with the existence of ~10^9 solar mass black holes as early as z~7. We find that under these conditions quasar feedback is not effective at stopping the cold gas from penetrating the central regions and hence cannot quench the accretion until the host galaxy reaches M_halo > 10^{12} solar masses. This cold-flow driven scenario for the formation of quasars implies that they should be ubiquitous in galaxies in the early universe and that major (proto)galaxy mergers are not a requirement for efficient fuel supply and growth, particularly for the earliest SMBHs.

Read more  (4042kb, PDF)



__________________


L

Posts: 111969
Date:
RE: Quasars
Permalink  
 


'Monster' drives cosmic beacon

Astronomers have spied a monster black hole - the brightest object yet seen in the early Universe.
Detected by a UK telescope in Hawaii, the hole is seen as it was a mere 770 million years after the Big Bang.
This means its light has taken an astonishing 12.9 billion years to reach us here on Earth.

Read more



__________________


L

Posts: 111969
Date:
Permalink  
 

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.

Read more

__________________
1 2  >  Last»  | Page of 2  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard