1,034 research outputs found
On the Gas Surrounding High Redshift Galaxy Clusters
Francis & Hewett (1993) identified two 10-Mpc scale regions of the high
redshift universe that were seemingly very overdense in neutral hydrogen.
Subsequent observations showed that at least one of these gas-rich regions
enveloped a cluster of galaxies at redshift 2.38. We present improved
observations of the three background QSOs with sightlines passing within a few
Mpc of this cluster of galaxies. All three QSOs show strong neutral hydrogen
absorption at the cluster redshift, suggesting that this cluster (and perhaps
all high redshift clusters) may be surrounded by a ~5 Mpc scale region
containing ~ 10^12 solar masses of neutral gas.
If most high redshift clusters are surrounded by such regions, we show that
the gas must be in the form of many small ( 0.03 cm^-3)
clouds, each of mass < 10^6 solar masses. These clouds are themselves probably
gathered into > 20 kpc sized clumps, which may be galaxy halos or
protogalaxies.
If this gas exists, it will be partially photoionised by the UV background.
We predict the diffuse Ly-alpha flux from this photoionisation, and place
observational limits on its intensity.Comment: 19 pages, 5 figures. Accepted for publication in PAS
Witnessing the formation of a brightest cluster galaxy at z>2
We present deep observations taken with the HST Advanced Camera for Surveys
of the central massive galaxy in a forming cluster at z=2.2. The galaxy hosting
the powerful radio source MRC 1138-262 is associated with one of the most
extensive merger systems known in the early universe. Our HST/ACS image shows
many star-forming galaxies merging within a ~200 kpc region that emits both
diffuse line emission and continuum in the rest-frame UV. Because this galaxy
lives in an overdense environment, it represents a rare view of a brightest
cluster galaxy in formation at z>2 which may serve as a testbed for predictions
of massive cluster galaxy formation.Comment: Contribution to the proceedings of "The Fate of Gas in Galaxies",
Dwingeloo, July 2006, with 2 colour figures. To appear in New Astronomy
Reviews, Vol. 51 (2007), eds. Morganti, Oosterloo, Villar-Martin & van Gorko
Evolution in the Escape Fraction of Ionizing Photons and the Decline in Strong Lya Emission from z>6 Galaxies
The rapid decline in the number of strong Lyman Alpha (Lya) emitting galaxies
at z > 6 provides evidence for neutral hydrogen in the IGM, but is difficult to
explain with plausible models for reionization. We demonstrate that the
observed reduction in Lya flux from galaxies at z > 6 can be explained by
evolution in the escape fraction of ionizing photons, f_esc. We find that the
median observed drop in the fraction of galaxies showing strong Lya emission,
as well as the observed evolution of the Lya luminosity function both follow
from a small increase in f_esc of Delta f_esc ~ 0.1 from f_esc ~ 0.6 at z ~ 6.
This high escape fraction may be at odds with current constraints on the
ionising photon escape fraction, which favor smaller values of f_esc < 20%.
However, models that invoke a redshift evolution of f_ esc that is consistent
with these constraints can suppress the z~7 Lya flux to the observed level, if
they also include a small evolution in global neutral fraction of Delta x_HI ~
0.2. Thus, an evolving escape fraction of ionising photons can be a plausible
part of the explanation for evolution in the Lya emission of high redshift
galaxies. More generally, our analysis also shows that the drop in the Lya
fraction is quantitatively consistent with the observed evolution in the Lya
luminosity functions of Lya Emitters.Comment: 9 pages, 3 figures. Accepted for publication in MNRAS. Added
references & brief discussion on line shape evolutio
On the X-ray emission of z~2 radio galaxies: IC scattering of the CMB & no evidence for fully-formed potential wells
We present the results of 20 ksec Chandra observations for each of 5 radio
galaxies in the redshift range 2.0 < z < 2.6. For 4 of the 5 targets we detect
unresolved X-ray components coincident with the radio nuclei. From spectral
analysis of one of the cores and comparison to the empirical radio to X-ray
luminosity ratio correlation, we find that obscuring material (n(HI)~10^22
cm^-2) may be surrounding the nuclei.
We detect X-ray emission coincident with the radio hotspots or lobes in 4 of
the 5 targets, which can be explained by Inverse-Compton (IC) scattering of CMB
photons. The magnetic field strengths of ~100-200 muG that we derive agree with
the equipartition magnetic field strengths. The relative ease with which the
lobe X-ray emission is detected is a consequence of the (1+z)^4 increase in the
energy density of the CMB. An HST image of one of the sources shows that the
X-ray emission could also be produced by a reservoir of hot, shocked gas, as
evidenced by a bright, optical bow-shock.
By stacking our data we created a deep, 100 ksec exposure to search for
diffuse X-ray emission from intra-cluster gas. We detect no diffuse emission
and derive upper limits of ~1e+44 erg/s, thereby ruling out a virialized
structure of cluster-size scale at z~2.
The average number of soft X-ray sources in our fields is consistent with the
number density of AGN in the Chandra Deep Fields. Their angular distribution
shows no evidence for large-scale structure associated with the radio galaxies.Comment: 17 pages, 9 figures (3 color figs), 6 tables, Accepted for
Publication in Astronomy & Astrophysic
The Spiderweb galaxy: a forming massive cluster galaxy at z~2
We present a deep image of the radio galaxy MRC 1138-262 taken with the
Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to
have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized
structure, identified with a protocluster. The morphology shown on the new deep
HST/ACS image is reminiscent of a spider's web. More than 10 individual clumpy
features are observed, apparently star-forming satellite galaxies in the
process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago.
There is an extended emission component, implying that star formation was
occurring over a 50 times 40 kpc region at a rate of more than 100 M_sun/yr. A
striking feature of the newly named ``Spiderweb galaxy'' is the presence of
several faint linear galaxies within the merging structure. The dense
environments and fast galaxy motions at the centres of protoclusters may
stimulate the formation of these structures, which dominate the faint resolved
galaxy populations in the Hubble Ultra Deep Field. The new image provides a
unique testbed for simulations of forming dominant cluster galaxies.Comment: 5 pages, 2 figures (reduced to grayscale); ApJ Letter
Proto-clusters in the Lambda CDM Universe
We compare the highly clustered populations of very high redshift galaxies
with proto-clusters identified numerically in a standard CDM universe
() simulation. We evolve 256^3 dark matter
particles in a comoving box of side 150h^{-1}Mpc. By the present day there are
63 cluster sized objects of mass in excess of 10^{14}h^{-1}Mo in this box. We
trace these clusters back to higher redshift finding that their progenitors at
z=4--5 are extended regions of typically 20--40 Mpc (comoving) in size, with
dark halos of mass in excess of 10^{12}h^{-1}Mo and are overdense by typically
1.3--13 times the cosmological mean density. Comparison with the observation of
Lyman alpha emitting (LAEs) galaxies at z=4.86 and at z=4.1 indicates that the
observed excess clustering is consistent with that expected for a proto-cluster
region if LAEs typically correspond to massive dark halos of more than
10^{12}h^{-1}Mo. We give a brief discussion on the relation between high
redshift concentration of massive dark halos and present day rich clusters of
galaxies.Comment: 4 pages, 5 figures, Accepted for publication in ApJ Letter
Can the intergalactic medium cause a rapid drop in Lyman alpha emission at z>6?
The large cross-section of the Lyman alpha (Lya) line makes it a sensitive
probe of the ionization state of the intergalactic medium (IGM). Here we
present the most complete study to date of the IGM Lya opacity, and its
application to the redshift evolution of the 'Lya fraction', i.e. the fraction
of color-selected galaxies with a detectable Lya emission line. We use a tiered
approach, which combines large-scale semi-numeric simulations of reionization
with moderate-scale hydrodynamic simulations of the ionized IGM. This allows us
to simultaneously account for evolution in both: (i) the opacity from an
incomplete (patchy) reionization, parameterized by the filling factor of
ionized regions, Q_HII; and (ii) the opacity from self-shielded systems in the
ionized IGM, parameterized by the average photo-ionization rate inside HII
regions, \Gamma. In contrast to recent empirical models, attenuation from
patchy reionization has a unimodal distribution along different sightlines,
while attenuation from self-shielded systems is more bimodal. We quantify the
average IGM transmission in our (Q_HII, \Gamma) parameter space, which can
easily be used to interpret new data sets. Using current observations, we
predict that the Lya fraction cannot drop by more than a factor of ~2 with IGM
attenuation alone, even for HII filling factors as low as Q_HII>0.1. Larger
changes in the Lya fraction could result from a co-evolution with galaxy
properties. Marginalizing over \Gamma, we find that current observations
constrain Q_HII < 0.6 at z=7 [68% confidence level (C.L.)]. However, all of our
parameter space is consistent with observations at 95% C.L., highlighting the
need for larger observational samples at z >= 6.Comment: 12 pages, 10 figures, MNRAS submitte
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