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TOPIC: Distant Galaxies
Blobrana


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RE: Distant Galaxies
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Distant carbon atoms betray very early enrichment in the early universe
Robert A. Simcoe of the MIT-Kavli Centre for Astrophysics and Space Research has found new evidence of intensive heavy-element enrichment in intergalactic gas, occurring less than 1 billion years (Gyr) after the Big Bang. His discovery of a high concentration of carbon atoms in the spectra of two very distant quasars implies that vigorous star formation was already underway in some neighbourhoods when the universe was only 7% of its present age.
Charting the evolution of the carbon abundance is like taking a core sample of the early universe's stellar content. Since carbon nuclei are created through fusion in stellar cores, one would expect their abundance to decline at early times, approaching the formation epoch of the first stars. Simcoe's new observations show that the level of triply ionised carbon (i.e. carbon which has been stripped of three electrons, usually denoted as C IV) remains surprisingly constant as far as redshift z ~ 6. In other words, there is no evidence for a downturn in the integrated C IV abundance, even at the most distant and earliest epochs where it has now been measured.

fig1_001

Simcoe conducted infrared observations of the C IV spectral regions in two high redshift quasars-SDSS1306+0356 (zem = 6.002) and SDSS1030+0524 (zem = 6.272)-using the Gemini Near Infra Red Spectrograph (GNIRS) at Gemini South (Figure 1). Infrared spectrographs are essential for studying C IV absorption at z > 5, because the relevant transitions move from the optical to the infrared spectral domain. For this reason, GNIRS is a uniquely suited instrument for exploring the z ~ 6 universe and beyond.

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-- Edited by Blobrana at 01:17, 2006-11-02

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Blobrana


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Resolving the Hearts of Luminous Infrared Galaxies
An international team of researchers used Gemini mid-infrared images to show that a sample of nearby Luminous InfraRed Galaxies (LIRGs) represent a major contribution to the obscured star formation rate (SFR) density at redshift z=1, when the universe was half of its present age.

The data were obtained with the mid-infrared imager/spectrograph T-ReCS on Gemini South. In Figure 1 the diffraction-limited images reveal that at high-spatial resolution, approximately 0.3 arcsecond at the observed wavelengths of 8 and 10 microns, the morphology of the mid-infrared emission in the nuclear regions of these galaxies is strikingly similar to that of the ionised gas (Paschen-alpha) revealed by Hubble Space Telescope (HST) NICMOS observations at near-infrared wavelengths. The observed mid-infrared emission of these LIRGs generally consists of bright nuclear emission, multiple circumnuclear and/or extranuclear H II regions, and diffuse emission. The stellar emission, traced by the NICMOS near-infrared continuum images, is more extended and does not show such a close resemblance with the hot dust emission traced by the T-ReCS images.

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Title: High spatial resolution T-ReCS mid-infrared imaging of Luminous Infrared Galaxies
Authors: A. Alonso-Herrero (1), L. Colina (1), C. Packham (2), T. Diaz-Santos (1), G. H. Rieke (3), J. Radomski (4), C. M. Telesco (2) ((1) DAMIR, IEM, CSIC, Spain (2) University of Floriday (3) University of Arizona, (4) Gemini Observatory)

We present diffraction-limited (FWHM ~ 0.3arcsec) Gemini/T-ReCS mid-infrared (MIR: N-band or narrow-band at 8.7micron) imaging of four Luminous Infrared Galaxies (LIRGs) drawn from a representative local sample. The MIR emission in the central few kpc is strikingly similar to that traced by Pa-alpha, and generally consists of bright nuclear emission and several compact circumnuclear and/or extranuclear HII regions. The central MIR emission is dominated by these powerful HII regions, consistent with the majority of AGN in this local sample of LIRGs contributing a minor part of the MIR emission. The luminous circumnuclear HII regions detected in LIRGs follow the extrapolation of the 8micron vs. Pa-alpha relation found for M51 HII knots. The integrated central 3-7kpc of galaxies, however, present elevated 8micron/Pa-alpha ratios with respect to individual HII regions, similar to the integrated values for star-forming galaxies. Our results show that the diffuse 8micron emission, not directly related to the ionising stellar population, can be as luminous as that from the resolved HII regions. Therefore, calibrations of the star formation rate for distant galaxies should be based on the integrated 8micron emission of nearby galaxies, not that of the HII regions alone.

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-- Edited by Blobrana at 00:59, 2006-11-02

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Blobrana


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Posts: 131433
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Galaxy 964 and Galaxy 1417
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Astronomers have discovered two of the most distant galaxies ever seen, when the Universe was only 700 million years old. The galaxies were first discovered as part of the Hubble Space Telescope’s Deep Field Survey, which looked into the distant Universe. Astronomers then did follow-on observations with the Spitzer Space Telescope to confirm their distance and age. The galaxies are thought to be between 50-300 million years old, and have only 1% of the mass of our own Milky Way.
Astronomers have taken amazing pictures of two of the most distant galaxies ever seen. The ultradeep images, taken at infrared wavelengths, confirm for the first time that these celestial cherubs are real. The researchers are now able to weigh galaxies and determine their age at earlier times than ever before, providing important clues about the evolutionary origins of galaxies like our Milky Way. The work appears in the October 1 issue of Astrophysical Journal Letters.

Galaxy 964 and 1417
Galaxy 1417 and Galaxy 964
These images show a pair of galaxies at redshifts 7-7.5, when the universe was only 700 million years old, or only 5% of its current age. The color images are a combination of an exposure in the optical i-band filter (0.8 micron; here in red), the optical z-band filter (0.9 micron; here in pink), and a combined near-infrared J+H band filter (1.1-1.6 micron; here in blue). Because of their extreme distance, the sources are only visible in the near-infrared, and therefore appear red in this image.
Credit Ivo Labbé and Rychard Bouwens

Carnegie Fellow Ivo Labbé, along with Rychard Bouwens and Garth Illingworth of the UCO/Lick Observatory at the University of California, Santa Cruz, and Marijn Franx of the Leiden Observatory, examined galaxies in the Hubble Ultra Deep Field (HUDF) using the sensitive Infrared Array Camera (IRAC) aboard NASA’s Spitzer Space Telescope. The HUDF, scanned by NASA’s Hubble Space Telescope in late 2003, remains the deepest view ever taken at visible and near-infrared wavelengths.
The two galaxies are seen when the universe was just a baby—700 million years after the Big Bang, or five percent of the universe’s current age. They belong to a precious small sample of similarly ancient galaxies, discovered two years ago by Bouwens, Illingworth, and Franx and analysed in-depth in Nature last month. The relative deficit of such far-away luminous sources indicates that this early period is when galaxies were rapidly building up from a very small number of stars to the massive galaxies we see at later times.
Because there are so few of them, verifying the existence of the distant galaxies and measuring their properties is of paramount importance. The new mid-infrared observations from Spitzer proved essential for Labbé’s team, as the analyses were not possible from the Hubble data alone.

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Blobrana


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Reconstructing the Cosmic Evolution of Quasars
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Title: Reconstructing the Cosmic Evolution of Quasars from the Age Distribution of Local Early-Type Galaxies
Authors: Zoltan Haiman (Columbia), Raul Jimenez (UPenn), Mariangela Bernardi (UPenn)

We use the spectra of 22,000 nearby early-type galaxies from the Sloan Digital Sky Survey (SDSS) to determine the age distribution of these galaxies as a function of their velocity dispersion sigma_v in the range 100 km/s < sigma_v < 280 km/s. We then combine the inferred age-distributions with the local abundance of spheroids, including early-type galaxies and late-type bulges, to predict the evolution of the quasar luminosity function (LF) in the redshift range 0 < z < 6. We make the following simple assumptions: (i) the formation of stars in each galaxy, at the epoch identified with the mean mass-weighted stellar age, is accompanied by the prompt assembly of the nuclear supermassive black hole (SMBH); (ii) the mass of the SMBH obeys the M_bh-sigma_v correlation observed in nearby galaxies; (iii) the SMBH radiates at a fraction f_Edd of the Eddington limit for a fixed duration t_Q, and is identified as a luminous quasar during this epoch, (iv) the intrinsic dispersions in the Eddington ratio and the M_bh-sigma_v relation produce a combined scatter of Delta(log L_Q) around the mean logarithmic quasar luminosity <log L_Q> at fixed sigma_v. These assumptions require that the SMBH remnants of quasars with bolometric luminosity below L_bol=10^12.5 f_Edd L_sun reside predominantly in bulges of late type galaxies. We find that evolution of the observed quasar LF can be fit over the entire redshift range in this simple model, 0 < z < 6 with the choices of Delta(log L_Q)=0.6-0.9, t_Q= (6-8)x10^7 yr, and < f_Edd >=0.3-0.5. We find no evidence that any of the model parameters evolves with redshift, supporting the strong connection between the formation of stars and nuclear SMBHs in spheroids.

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Blobrana


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The Clustering of Galaxy Groups
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Title: The Clustering of Galaxy Groups: Dependence on Mass and Other Properties
Authors: Andreas A. Berlind, Eyal Kazin, Michael R. Blanton, Sebastian Pueblas, Roman Scoccimarro, David W. Hogg

We investigate the clustering of galaxy groups and clusters in the SDSS using the Berlind et al. (2006) group sample, which is designed to identify galaxy systems that each occupy a single dark matter halo. We estimate group masses from their abundances, and measure their relative large-scale bias as a function of mass. Our measurements are in agreement with the theoretical halo bias function, given a standard LCDM cosmological model, and they tend to favour a low value of the power spectrum amplitude sigma_8. We search for a residual dependence of clustering on other group properties at fixed mass, and find the strongest signal for central galaxy colour in high mass groups. Massive groups with less red central galaxies are more biased on large scales than similar mass groups with redder central galaxies. We show that this effect is unlikely to be caused by errors in our mass estimates, and is most likely observational evidence of recent theoretical findings that halo bias depends on a ''second parameter'' other than mass, such as age or concentration. To compare with the data, we study the bias of massive halos in N-body simulations and quantify the strength of the relation between halo bias and concentration at fixed mass. In addition to confirming a non-trivial prediction of the LCDM cosmological model, these results have important implications for the role that environment plays in shaping galaxy properties.

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Blobrana


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INSIDE A QUASAR
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ASTRONOMERS SEE INSIDE A QUASAR FOR THE FIRST TIME

For the first time, astronomers have looked inside quasars -- the brightest objects in the universe -- and have seen evidence of black holes.
The study lends further confirmation to what scientists have long suspected -- that quasars are made up of super-massive black holes and the super-heated disks of material that are spiralling into them.

The astronomers studied the variability of both the X-rays and visible light coming from the quasars and compared those measurements to calculate the size of the accretion disk in each. They used a computer program that Kochanek created especially for such calculations, and ran it on a 48-processor computer cluster. Calculations for each quasar took about a week to complete.

The two quasars they studied are named RXJ1131-1231 and Q2237+0305, and there's nothing special about them, Kochanek said, except that they were both gravitationally lensed. He and his group are currently studying 20 such lensed quasars, and they'd like to eventually gather X-ray data on all of them.

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Blobrana


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Two New Gravitationally Lensed Double Quasars
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Title: Two New Gravitationally Lensed Double Quasars from the Sloan Digital Sky Survey
Authors: Naohisa Inada, Masamune Oguri, Robert H. Becker, Richard L. White, Issha Kayo, Christopher S. Kochanek, Patrick B. Hall, Donald P. Schneider, Donald G. York, Gordon T. Richards

We report the discoveries of the two-image gravitationally lensed quasars, SDSS J0746+4403 and SDSS J1406+6126, selected from the Sloan Digital Sky Survey (SDSS). SDSS J0746+4403, which will be included in our lens sample for statistics and cosmology, has a source redshift of zs=2.00, an estimated lens redshift of zl~0.3, and an image separation of 1.08". SDSS J1406+6126 has a source redshift of zs=2.13, a spectroscopically measured lens redshift of zl=0.27, and an image separation of 1.98".
We find that the two quasar images of SDSS J1406+6126 have different intervening MgII absorption strengths, which are suggestive of large variations of absorbers on kpc scales. The positions and fluxes of both the lensed quasar systems are easily reproduced by simple mass models with reasonable parameter values. These objects bring to 18 the number of lensed quasars that have been discovered from the SDSS data.

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-- Edited by Blobrana at 10:52, 2006-09-27

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Blobrana


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Suppressed star formation
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A GNIRS (Gemini South) study identifies a surprising class of distant massive galaxies with strongly suppressed star formation.

An international team led by Mariska Kriek of Leiden Observatory (Holland) and Yale University has found that 45% of a small sample (20) of massive high-redshift galaxies exhibit very low or no star formation activity. The existence of "red and dead" massive galaxies at a time when the universe was between 1/4 to a 1/3 of its current age is surprising.

During the past few years our knowledge of galaxy formation evolution has grown significantly. Several recent papers indicate that most of the stars in high-mass galaxies were formed at a higher redshift (earlier in the history of the universe) than those of low mass galaxies. This newly published work puts a new twist on the growing evidence that most massive galaxies formed at an extremely early epoch in the universe.
Kriek's team used the Near Infrared Spectrograph (GNIRS) at Gemini South to study twenty galaxies having redshifts that ranged between 2.0 < z < 2.7. They found a surprising number of galaxies (9) with no detected emission lines. The galaxies were selected from a sample of galaxies with K magnitudes brighter than 19.7, ensuring adequate signal-to-noise ratio in the near-infrared spectra. The selected galaxies are relatively massive, ranging between 0.9 - 4.6 x 10^11 Msun.

The authors used the equivalent width of the Balmer Hα line to derive the ratio of current to past star formation. Several of the galaxies correspond to very low Hα equivalent widths (WHα), implying extremely low star formation rates. Galaxies of this sample show a star formation rate varying by a factor of 100 between the lowest and most active ones. Some show no activity at all. Both the Hα measurements and the stellar continuum modelling imply that star formation in these galaxies has been strongly suppressed.
It is suggested that feedback mechanisms like supernova or active galactic nuclei (AGN)-driven mass loss could produce the dead massive galaxies seen in this work. Injection of huge amounts of mechanical energy and momentum over a relatively short period may remove a huge fraction of the galaxy's gas in a short time and heat the remaining interstellar medium, making it stable against gravitational collapse. Furthermore, some of the line-emission in the sample may not be due to star formation. The high (NII)/Hα in some galaxies suggest the possibility of emission produce by AGN activity. Such activity could also clear the gas from the dense central regions of these galaxies.

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Title: SPECTROSCOPIC IDENTIFICATION OF MASSIVE GALAXIES AT Z ~ 2.3 WITH STRONGLY SUPPRESSED STAR FORMATION
Authors: Mariska Kriek, Pieter G. van Dokkum, Marijn Franx, Ryan Quadri, Eric Gawiser, David Herrera,Garth D. Illingworth, Ivo Labb´e, Paulina Lira, Danilo Marchesini, Hans-Walter Rix, Gregory Rudnick, Edward N. Taylor, Sune Toft, C. Megan Urry, and Stijn Wuyts

stellasuppressed3
The equivalent width of Hα vs. the specific SFR derived from our model fits to the spectra. Filled red circles are galaxies with no detected Hα emission and purple squares are emission line galaxies in our sample. Yellow crosses are UV-selected galaxies by Erb et al. (2006a,b,c). DRGs are indicated by open green diamonds. Upper limits for H and the specific SFR are 2σ. Expected relations between WH and specific SFR are derived from the Bruzual & Charlot (2003) and Kennicutt (1998) models and drawn for a τ100Myr (gray dotted line) and a τ1000Myr (gray dashed line) model for the first 3 Gyr. Both Hα measurements and the independent SED modelling demonstrate that in 9 out of 20 galaxies in our sample the star formation has been strongly suppressed.

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Blobrana


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RE: Distant Galaxies
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Astronomers analysing two of the deepest views of the cosmos made with the Hubble Space Telescope have uncovered a gold mine of galaxies, more than 500 that existed less than a billion years after the Big Bang. This sample in the constellation Fornax represents the most comprehensive compilation of galaxies in the early universe, researchers said. The discovery is scientifically invaluable for understanding the origin of galaxies, considering that just a decade ago early galaxy formation was largely uncharted territory. Astronomers had not seen even one galaxy that existed when the universe was a billion years old, so finding 500 in a Hubble survey is a significant leap forward for cosmologists. This Hubble Space Telescope image shows 28 of the more than 500 young galaxies the researchers uncovered in their analysis of two Hubble surveys.


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Position (J2000): R.A. 3h 32m 40s.0 Dec. -27° 48' 00"

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Blobrana


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Title: Clustering of the IR Background Light with Spitzer: Contribution from Resolved Sources
Authors: Ian Sullivan (Caltech), Asantha Cooray (UC Irvine), Ranga-Ram Chary (Caltech), James J. Bock (JPL), Mark Brodwin (JPL), Michael J. I. Brown (Princeton), Arjun Dey (NOAO), Mark Dickinson (NOAO), Peter Eisenhardt (JPL), Henry C. Ferguson (STSCI), Mauro Giavalisco (STSCI), Brian Keating (UCSD), Andrew Lange (Caltech), Bahram Mobasher (STSCI), William T. Reach (Caltech), Daniel Stern (JPL), Edward L. Wright (UCLA)

We describe the angular power spectrum of resolved sources at 3.6 microns (L-band) in Spitzer imaging data of the GOODS HDF-N, the GOODS CDF-S, and the NDWFS Bootes field in several source magnitude bins. We also measure angular power spectra of resolved sources in the Bootes field at KS and J-bands using ground-based IR imaging data. In the three bands, J, KS, and L, we detect the clustering of galaxies on top of the shot-noise power spectrum at multipoles between ell ~ 10^2 and 10^5. The angular power spectra range from the large, linear scales to small, non-linear scales of galaxy clustering, and in some magnitude ranges, show departure from a power-law clustering spectrum. We consider a halo model to describe clustering measurements and to establish the halo occupation number parameters of IR bright galaxies at redshifts around one. We also extend our clustering results and completeness-corrected faint source number counts in GOODS fields to understand the underlying nature of unresolved sources responsible for IR background (IRB) anisotropies that were detected in deep Spitzer images. While these unresolved fluctuations were measured at sub-arcminute angular scales, if a high-redshift diffuse component associated with first galaxies exists in the IRB, then it's clustering properties are best studied with shallow, wide-field images that allow a measurement of the clustering spectrum from a few degrees to arcminute angular scales.

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