803 research outputs found
Evidence for Black Hole Growth in Local Analogs to Lyman Break Galaxies
We have used XMM-Newton to observe six Lyman Break Analogs (LBAs): members of
the rare population of local galaxies that have properties that are very
similar to distant Lyman Break Galaxies. Our six targets were specifically
selected because they have optical emission-line properties that are
intermediate between starbursts and Type 2 (obscured) AGN. Our new X-ray data
provide an important diagnostic of the presence of an AGN. We find X-ray
luminosities of order 10^{42} erg/s and ratios of X-ray to far-IR luminosities
that are higher than values in pure starburst galaxies by factors ranging from
~ 3 to 30. This strongly suggests the presence of an AGN in at least some of
the galaxies. The ratios of the luminosities of the hard (2-10 keV) X-ray to [O
III]\lambda 5007 emission-line are low by about an order-of-magnitude compared
to Type 1 AGN, but are consistent with the broad range seen in Type 2 AGN.
Either the AGN hard X-rays are significantly obscured or the [O III] emission
is dominated by the starburst. We searched for an iron emission line at ~ 6.4
keV, which is a key feature of obscured AGN, but only detected emission at the
~ 2\sigma level. Finally, we find that the ratios of the mid-infrared (24\mu m)
continuum to [O III]\lambda 5007 luminosities in these LBAs are higher than the
values for Type 2 AGN by an average of 0.8 dex. Combining all these clues, we
conclude that an AGN is likely to be present, but that the bolometric
luminosity is produced primarily by an intense starburst. If these black holes
are radiating at the Eddington limit, their masses would lie in the range of
10^5 to 10^6 M_{sun}. These objects may offer ideal local laboratories to
investigate the processes by which black holes grew in the early universe.Comment: Accepted for publication in Ap
Ancient Light from Young Cosmic Cities: Physical and Observational Signatures of Galaxy Proto-Clusters
For a complete picture of galaxy cluster formation, it is important that we
start probing the early epoch of z~2-7 during which clusters and their galaxies
first began to form. Because the study of these so-called "proto-clusters" is
currently limited by small number statistics, widely varying selection
techniques and assumptions, we have performed a systematic study of cluster
formation utilizing cosmological simulations. We use the Millennium Simulations
to track the evolution of dark matter and galaxies in ~3,000 clusters from the
earliest times to z=0. We define an effective radius R_e for proto-clusters and
characterize their growth in size and mass. We show that the progenitor regions
of galaxy clusters (M>10^14 M_sun/h) can already be identified at least up to
z~5, provided that the galaxy overdensities, delta_gal, are measured on a
sufficiently large scale (R_e~5-10 cMpc). We present the overdensities in
matter, DM halos, and galaxies as functions of present-day cluster mass,
redshift, bias, and window size that can be used to interpret the structures
found in real surveys. We derive the probability that a structure having a
delta_gal, defined by a set of observational selection criteria, is indeed a
proto-cluster, and show how their z=0 masses can already be estimated long
before virialization. Galaxy overdensity profiles as a function of radius are
presented. We further show how their projected surface overdensities decrease
as the uncertainties in redshift measurements increase. We provide a table of
proto-cluster candidates selected from the literature, and discuss their
properties in the light of our simulations predictions. This work provides the
general framework that will allow us to extend the study of cluster formation
out to much higher redshifts using the large number of proto-clusters that are
expected to be discovered in, e.g., the upcoming HETDEX and HSC surveys.Comment: 16 pages, 13 figures, 5 tables; Published in Ap
Discovery of a large number of candidate proto-clusters traced by ~15 Mpc-scale galaxy overdensities in COSMOS
To demonstrate the feasibility of studying the epoch of massive galaxy
cluster formation in a more systematic manner using current and future galaxy
surveys, we report the discovery of a large sample of proto-cluster candidates
in the 1.62 deg^2 COSMOS/UltraVISTA field traced by optical/IR selected
galaxies using photometric redshifts. By comparing properly smoothed 3D galaxy
density maps of the observations and a set of matched simulations incorporating
the dominant observational effects (galaxy selection and photometric redshift
uncertainties), we first confirm that the observed ~15 comoving Mpc scale
galaxy clustering is consistent with LCDM models. Using further the relation
between high-z overdensity and the present day cluster mass calibrated in these
matched simulations, we found 36 candidate structures at 1.6<z<3.1, showing
overdensities consistent with the progenitors of M_z=0 ~10^15 M_sun clusters.
Taking into account the significant upward scattering of lower mass structures,
the probabilities for the candidates to have at least M_z=0 ~10^14 M_sun are
~70%. For each structure, about 15%-40% of photometric galaxy candidates are
expected to be true proto-cluster members that will merge into a cluster-scale
halo by z=0. With solely photometric redshifts, we successfully rediscover two
spectroscopically confirmed structures in this field, suggesting that our
algorithm is robust. This work generates a large sample of uniformly-selected
proto-cluster candidates, providing rich targets for spectroscopic follow-up
and subsequent studies of cluster formation. Meanwhile, it demonstrates the
potential for probing early cluster formation with upcoming redshift surveys
such as the Hobby-Eberly Telescope Dark Energy Experiment and the Subaru Prime
Focus Spectrograph survey.Comment: 6 pages, 3 figures, 1 table; Accepted for publication in ApJ
Dust Attenuation in UV-selected Starbursts at High Redshift and their Local Counterparts: Implications for the Cosmic Star Formation Rate Density
We present a new analysis of the dust obscuration in starburst galaxies at
low and high redshift. This study is motivated by our unique sample of the most
extreme UV-selected starburst galaxies in the nearby universe (z<0.3), found to
be good analogs of high-redshift Lyman Break Galaxies (LBGs) in most of their
physical properties. We find that the dust properties of the Lyman Break
Analogs (LBAs) are consistent with the relation derived previously by Meurer et
al. (M99) that is commonly used to dust-correct star formation rate
measurements at a very wide range of redshifts. We directly compare our results
with high redshift samples (LBGs, BzK, and sub-mm galaxies at z=2-3) having IR
data either from Spitzer or Herschel. The attenuation in typical LBGs at z=2-3
and LBAs is very similar. Because LBAs are much better analogs to LBGs compared
to previous local star-forming samples, including M99, the practice of
dust-correcting the SFRs of high redshift galaxies based on the local
calibration is now placed on a much more solid ground. We illustrate the
importance of this result by showing how the locally calibrated relation
between UV measurements and extinction is used to estimate the integrated,
dust-corrected star formation rate density at z=2-6.Comment: Accepted for publication in ApJ Letters (6 pages, 4 figures, 1 table
On the oxygen and nitrogen chemical abundances and the evolution of the "green pea" galaxies
We have investigated the oxygen and nitrogen chemical abundances in extremely
compact star-forming galaxies with redshifts between 0.11-0.35, popularly
referred to as "green peas". Direct and strong-line methods sensitive to the
N/O ratio applied to their SDSS spectra reveals that these systems are genuine
metal-poor galaxies, with mean oxygen abundances 20% solar. At a given
metallicity these galaxies display systematically large N/O ratios compared to
normal galaxies, which can explain the strong difference between our
metallicities measurements and previous ones. While their N/O ratios follow the
relation with stellar mass of local star-forming galaxies in the SDSS, we find
that the mass--metallicity relation of the "green peas" is offset \ga0.3 dex
to lower metallicities. We argue that recent interaction-induced inflow of gas,
possibly coupled with a selective metal-rich gas loss, driven by supernova
winds, may explain our findings and the known galaxy properties, namely high
specific star formation rates, extreme compactness, and disturbed optical
morphologies. The "green pea" galaxy properties seem to be not common in the
nearby Universe, suggesting a short and extreme stage of their evolution.
Therefore, these galaxies may allow us to study in great detail many processes,
such as starburst activity and chemical enrichment, under physical conditions
approaching those in galaxies at higher redshifts.Comment: 8 pages, 3 Figures, ApJ Letters in press. Proof version with
correction
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