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TOPIC: Distant Galaxies
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RE: Distant Galaxies
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rgal_aas07-560
An illustration of the ORELSE results.
Credit ORELSE team

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Blobrana


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Large galaxy clusters are the universe's metropolises, and for years many astronomers have focused their attention on the crowded "downtowns." However, a new map of some of the largest ancient galactic cities shows that much of the "action" is happening in the cosmic suburbs.
An announcement on this result will be made today (May 28) at the 210th meeting of the American Astronomical Society, in Honolulu, Hawaii. Drs. Gordon Squires, Mark Lacy, and Jason Surace of the Spitzer Science Centre, Pasadena, California are co-investigators on the project.

"The most interesting thing that we've found so far is the incredible amount of activity occurring in galactic suburbia. We see unusually large numbers of galaxies with high star formation rates, producing over 100 new suns per year, and with active central supermassive black holes" - Dr. Lori Lubin, of the University of California at Davis, who is the principal investigator of the Observations of Redshift Evolution in Large Scale Environments (ORELSE) Survey.

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In just a short amount of time, NASA's Spitzer Space Telescope has bagged thousands of previously unknown dwarf galaxies in a giant cluster of galaxies.
Despite their diminutive sizes, dwarf galaxies play a crucial role in cosmic evolution. Astronomers think they were the first galaxies to form, and they provided the building blocks for larger galaxies. They are by far the most numerous galaxies in our Universe, and are an important tracer of the large-scale structure of the cosmos. Computer simulations of cosmic evolution suggest that high-density regions of the universe, such as giant clusters, should contain significantly more dwarf galaxies than astronomers have observed to date.

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Blobrana


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Title: A Compact Cluster of Massive Red Galaxies at a Redshift of 1.51
Authors: P. J. McCarthy, H. Yan (1), R. G. Abraham, E. Mentuch (2), K. Glazebrook (3), L. Yan (4), H.-W. Chen (5), S. E. Persson (1), P. Nair (2), S. Savaglio (6), D. Crampton (7), S. Juneau (8), D. Le Borgne (9), R. G. Carlberg (2), R. O. Marzke (10), I. Jorgensen (11), K. Roth (11), R. Murowinski (2) ((1) Carnegie, (2) Toronto, (3) Swinburn, (4) SSC, (5) Chicago, (6) MPE, (7) HIA, (8) Arizona, (9) Saclay, (10) SFSU, (11) Gemini)

We describe a compact cluster of massive red galaxies at z=1.51 discovered in one of the Gemini Deep Deep Survey (GDDS) fields. Deep imaging with the Near Infrared Camera and Multi Object Spectrometer (NICMOS) on the Hubble Space Telescope reveals a high density of galaxies with red optical to near-IR colours surrounding a galaxy with a spectroscopic redshift of 1.51. Mid-IR imaging with Infrared Array Camera (IRAC) on the Spitzer Space telescope shows that these galaxies have spectral energy distributions that peak between 3.6 and 4.5 microns. Fits to 12-band photometry reveal 12 or more galaxies with spectral shapes consistent with z = 1.51. Most are within ~170 co-moving kpc of the GDDS galaxy. Deep F814W images with the Advanced Camera for Surveys (ACS) on HST reveal that these galaxies are a mix of early-type galaxies, disk galaxies and close pairs. The total stellar mass enclosed within a sphere of 170 kpc in radius is > 8E+11 solar masses. The colours of the most massive galaxies are close to those expected from passive evolution of simple stellar populations (SSP) formed at much higher redshifts. We suggest that several of these galaxies will merge to form a single, very massive galaxy by the present day. This system may represent an example of a short-lived dense group or cluster core typical of the progenitors of massive clusters in the present day and suggests the red sequence was in place in over-dense regions at early times.

GDDS-12-5869
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Near-infrared HST imaging reveals a large number of faint galaxies around the central luminous galaxies clustering around GDDS-12-5869

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Blobrana


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Title: Cosmic ray confinement in fossil cluster bubbles
Authors: M. Ruszkowski (1), T.A. Ensslin (1), M. Bruggen (2), M.C. Begelman (3), E. Churazov (1) ((1) MPA, (2) Jacobs University, (3) JILA)

Most cool core clusters of galaxies possess active galactic nuclei (AGN) in their centres. These AGN inflate buoyant bubbles containing non-thermal radio emitting particles. If such bubbles efficiently confine cosmic rays (CR) then this could explain ''radio ghosts'' seen far from cluster centres. We simulate the diffusion of cosmic rays from buoyant bubbles inflated by AGN. Our simulations include the effects of the anisotropic particle diffusion introduced by magnetic fields. Our models are consistent with the X-ray morphology of AGN bubbles, with disruption being suppressed by the magnetic draping effect. We conclude that for such magnetic field topologies, a substantial fraction of cosmic rays can be confined inside the bubbles on buoyant rise timescales even when the parallel diffusivity coefficient is very large. For isotropic diffusion at a comparable level, cosmic rays would leak out of the bubbles too rapidly to be consistent with radio observations. Thus, the long confinement times associated with the magnetic suppression of CR diffusion can explain the presence of radio ghosts. We show that the partial escape of cosmic rays is mostly confined to the wake of the rising bubbles, and speculate that this effect could: (1) account for the excitation of the H\alpha filaments trailing behind the bubbles in the Perseus cluster, (2) inject entropy into the metal enriched material being lifted by the bubbles and, thus, help to displace it permanently from the cluster centre and (3) produce observable \gamma-rays via the interaction of the diffusing cosmic rays with the thermal intracluster medium (ICM).

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Blobrana


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Massive galaxies
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 An Australian team, led by Sarah Brough of the Swinburne University of Technology used the Gemini South Telescope to find that the most massive galaxies evolve through a variety of mechanisms which are dependent on the mass of their cluster environment. These observations are not consistent with any single model of galaxy formation, presenting a challenge for theorists.
Brightest cluster galaxies (BCGs) are the most massive galaxies known. They are generally red, elliptical type galaxies with remarkably similar brightnesses that are only found near the centres of galaxy clusters. To understand how galaxies form and evolve it is vitally important to understand the formation of these massive galaxies. The uniformity of BCGs suggests that they have experienced similar evolutionary histories and their position at the centres of clusters suggests that their histories must be connected to their environment.
Current models of galaxy formation predict that the stars in these most massive galaxies should have formed more than 12 Billion years ago, while the galaxies themselves should have undergone many dry mergers (without star formation) with other galaxies since that time to build up the very large galaxies we observe today. The models also predict that galaxies in less massive clusters are at an earlier stage in their evolution, so they should have undergone fewer mergers.

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Blobrana


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Brightest cluster and group galaxies
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Title: Spatially resolved kinematics and stellar populations of brightest cluster and group galaxies
Authors: S. Brough (Swinburne University), R. Proctor (Swin), D. Forbes (Swin), W. Couch (Swin), C. Collins (Liverpool John Moores), D. Burke (CfA), R. Mann (ROE)

We present an examination of the kinematics and stellar populations of a sample of 3 Brightest Group Galaxies (BGGs) and 3 Brightest Cluster Galaxies (BCGs) in X-ray groups and clusters. We have obtained high signal-to-noise Gemini/GMOS (Gemini South Multi-Object Spectrograph) long-slit spectra of these galaxies and use Lick indices to determine ages, metallicities and alpha-element abundance ratios out to at least their effective radii. We find that the BGGs and BCGs have very uniform masses, central ages and central metallicities. Examining the radial dependence of their stellar populations, we find no significant velocity dispersion, age, or alpha-enhancement gradients. However, we find a wide range of metallicity gradients, suggesting a variety of formation mechanisms. The range of metallicity gradients observed is surprising given the homogeneous environment these galaxies probe and their uniform central stellar populations. However, our results are inconsistent with any single model of galaxy formation and emphasise the need for more theoretical understanding of both the origins of metallicity gradients and galaxy formation itself. We postulate two possible physical causes for the different formation mechanisms.

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Blobrana


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RE: Distant Galaxies
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Title: Lyman-break galaxies at z~5 - I. First significant stellar mass assembly in galaxies that are not simply z~3 LBGs at higher redshift
Authors: Aprajita Verma, Matthew D. Lehnert, Natascha M. F¨orster Schreiber, Malcolm N. Bremer, Laura Douglas

We determine the ensemble properties of z~5 Lyman break galaxies (LBGs) selected as V-band dropouts to iAB <26.3 in the Chandra Deep Field South using their rest-frame UV-to-visible spectral energy distributions. By matching the selection and performing the same analysis that has been used for z~3 samples, we show clear differences in the ensemble properties of two samples of LBGs which are separated by 1Gyr in lookback time. We find that z~5 LBGs are typically much younger (<100Myr) and have lower stellar masses (~10^9 Solar masses) than their z~3 counterparts (which are typically ~few×10^10 Solar masses and ~320Myr old). The difference in mass is significant even when considering the presence of an older, underlying population in both samples. Such young and moderately massive systems dominate the luminous z~5 LBG population (&70 per cent), whereas they comprise .30 per cent of LBG samples at z~3. This result, which we demonstrate is robust under all reasonable modelling assumptions, shows a clear change in the properties of the luminous LBGs between z~5 and z~3. These young and moderately massive z~5 LBGs appear to be experiencing their first (few) generations of large-scale star formation and are accumulating their first significant stellar mass. Their dominance in luminous LBG samples suggests that z~5 witnesses a period of wide-spread, recent galaxy formation. As such, z~5 LBGs are the likely progenitors of the spheroidal components of present-day massive galaxies.
This is supported by their high stellar mass surface densities, and is consistent with their core phase-space densities, as well as the ages of stars in the bulge of our Galaxy and other massive systems. With implied formation redshifts of z~6 - 7, these luminous z~5 LBGs could have only contributed to the UV photon budget at the end of reionisation. However, their high star formation rates per unit area suggest these systems host outflows or winds that enrich the intra- and inter-galactic media with metals, as has been established for z~3 LBGs. Their estimated young ages are consistent with inefficient metal-mixing on galaxy-wide scales. Therefore these galaxies may contain a significant fraction of metal-free stars as has been previously proposed for z~3 LBGs (Jimenez & Haiman 2006).

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Blobrana


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A team of UK, French and German astronomers have discovered that the majority of the most distant galaxies so far identified are very young, undergoing their first extremely vigorous bursts of star formation. This discovery allows the astronomers to study the first important stages in the formation of the kind of galaxies we see in the Universe today.
One of the scientists involved in the study, Dr Malcolm Bremer of the University of Bristol will present the team's findings in his talk on Tuesday 17 April at the Royal Astronomical Society National Astronomy Meeting in Preston. Full details of the study will soon appear as a paper in the journal Monthly Notices of the Royal Astronomical Society.

z5lbgs
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Credit University of Bristol

The light that we see from these galaxies was emitted when the Universe was about 10 per cent of its present age (or just over a billion years old). They are forming stars at a very high rate (up to a hundred times the rate at which our own Galaxy, the Milky Way, is currently forming stars). The duration of these intense star formation events is short astronomically-speaking, comparable to the time it would take for a star to cross one of these galaxies (a few tens of millions of years). This indicates that we are seeing in these galaxies one of their first major star formation events, and are therefore watching the earliest stages of galaxy formation in the young Universe.
The team of astronomers discovered that the galaxies have a very high density of stars, the like of which is seen in only the centres of the most massive galaxies today. The stars that are forming in these young galaxies will end up in the biggest galaxies seen in the Universe today. Previous analysis of the light emitted by massive galaxies close to our own indirectly suggested that most stars in these galaxies formed just 1-2 billion years after the Big Bang. The new results give direct evidence for this, the observed galaxies are captured in the first major phases of their star formation.

Source: RAS

Title: Lyman-break galaxies at z~5 -I. First significant stellar mass assembly in galaxies that are not simply z~3 LBGs at higher redshift
Authors: Verma, A, Lehnert, M.D., Foerster-Schreiber, N., Bremer, M.N., Douglas, L.

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Blobrana


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Extended Groth Strip International Survey
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The universe is a seemingly never-ending area of study for astronomers, but UC Berkeley researchers are well on their way to unlocking the mysteries in the night sky and some of the large-scale structures formed over the last nine billion years.
Researchers at UC Berkeley and Lawrence Berkeley National Laboratory are developing a colour chart of the galaxies clustered in a small area of the sky in an attempt to better understand the history of our universe.

"The goal was to study the universe as it was when it was about half as old as it is at present. Weve gotten such fabulous datait just blows your socks off" - Marc Davis, UC Berkeley astronomy professor.

The five-year international project, titled the All-wavelength Extended Groth Strip International Survey, focuses on a specific area of the sky called the Extended Groth Strip, which in the night sky is about the size of two full moons.

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