66 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
Clustering of i-dropout galaxies at z=6 in GOODS and the UDF
We measured the angular clustering at z~6 from a large sample of i-dropout
galaxies (293 with z<27.5 from GOODS and 95 with z<29.0 from the UDF). Our
largest and most complete subsample (having L>0.5L*) shows the presence of
clustering at 94% significance. For this sample we derive a (co-moving)
correlation length of r_0=4.5^{+2.1}_{-3.2} h_{72}^{-1} Mpc and bias
b=4.1^{+1.5}_{-2.6}, using an accurate model for the redshift distribution. No
clustering could be detected in the much deeper but significantly smaller UDF,
yielding b<4.4 (1 sigma). We compare our findings to Lyman break galaxies at
z=3-5 at a fixed luminosity. Our best estimate of the bias parameter implies
that i-dropouts are hosted by dark matter halos having masses of ~10^11 M_sun,
similar to that of V-dropouts at z~5. We evaluate a recent claim that at z>5
star formation might have occurred more efficiently compared to that at z=3-4.
This may provide an explanation for the very mild evolution observed in the UV
luminosity density between z=6 and z=3. Although our results are consistent
with such a scenario, the errors are too large to find conclusive evidence for
this.Comment: minor changes to match published versio
Evidence for Elevated X-ray Emission in Local Lyman Break Galaxy Analogs
In this paper, we study the relationship between the 2-10 keV X-ray
luminosity (L_X), assumed to originate from X-ray binaries (XRBs), and star
formation rate (SFR) in UV-selected z<0.1 Lyman break analogs (LBAs). We
present Chandra observations for four new GALEX-selected LBAs. Including
previously studied LBAs, Haro 11 and VV 114, we find that LBAs demonstrate
L_X/SFR ratios that are elevated by ~1.5sigma compared to local galaxies,
similar to the ratios found for stacked LBGs in the early Universe (z>2). We
show that these LBAs are unlikely to harbor AGN, based on their optical and
X-ray spectra and the spatial distribution of the X-rays in three spatially
extended cases. We expect that high-mass X-ray binaries (HMXBs) dominate the
X-ray emission in these galaxies, based on their high specific SFRs
(sSFRs=SFR/M* > 10^{-9}/yr), which suggest the prevalence of young stellar
populations. Since both LBAs and LBGs have lower dust attenuations and
metallicities compared to similar samples of more typical local galaxies, we
investigate the effects of dust extinction and metallicity on the L_X/SFR for
the broader population of galaxies with high sSFRs (>10^{-10}/yr). The
estimated dust extinctions (corresponding to column densities of
N_H<10^{22}/cm^2) are expected to have insignificant effects on observed
L_X/SFR ratio for the majority of galaxy samples. We find that the observed
relationship between L_X/SFR and metallicity appears consistent with
theoretical expectations from X-ray binary population synthesis models.
Therefore, we conclude that lower metallicities, related to more luminous HMXBs
such as ultraluminous X-ray sources (ULXs), drive the elevated L_X/SFR observed
in our sample of z<0.1 LBAs. The relatively metal-poor, active mode of star
formation in LBAs and distant z>2 LBGs may yield higher total HMXB luminosity
than found in typical galaxies in the local Universe.Comment: 11 pages, 7 figures, Submitted to ApJ (references updated in v2
Discovery of Protoclusters at z∼3.7 and 4.9: Embedded in Primordial Superclusters
We have carried out follow-up spectroscopy on three overdense regions of -
and -dropout galaxies in the Canada-France-Hawaii Telescope Legacy Survey
Deep Fields, finding two new protoclusters at , 3.721 and a possible
protocluster at . The protocluster overlaps with a
previously identified protocluster at . The redshift separation
between these two protoclusters is , which is slightly larger
than the size of typical protoclusters. Therefore, if they are not the
progenitors of a halo, they would grow into
closely-located independent halos like a supercluster. The other protocluster
at is also surrounded by smaller galaxy groups. These systems
including protoclusters and neighboring groups are regarded as the early phase
of superclusters. We quantify the spatial distribution of member galaxies of
the protoclusters at and 3.721 by fitting triaxial ellipsoids,
finding a tentative difference: one has a pancake-like shape while the other is
filamentary. This could indicate that these two protoclusters are in different
stages of formation. We investigate the relation between redshift and the
velocity dispersion of protoclusters, including other protoclusters from the
literature, in order to compare their dynamical states. Although there is no
significant systematic trend in the velocity dispersions of protoclusters with
redshift, the distribution is skewed to higher velocity dispersion over the
redshift range of . This could be interpreted as two phases of
cluster formation, one dominated by the steady accretion of galaxies, and the
other by the merging between group-size halos, perhaps depending on the
surrounding large-scale environments.Comment: Accepted for publication in ApJ, 24 pages, 12 figures, 5 table
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