704 research outputs found
The galaxy environment of a QSO at z ~ 5.7
High-redshift quasars are believed to reside in massive halos in the early
universe and should therefore be located in fields with overdensities of
galaxies, which are thought to evolve into galaxy clusters seen in the local
universe. However, despite many efforts, the relationship between galaxy
overdensities and z~6 quasars is ambiguous. This can possibly be attributed to
the difficulty of finding galaxies with accurate redshifts in the vicinity of
z~6 quasars. So far, overdensity searches around z~6 quasars have been based on
studies of Lyman break galaxies (LBGs), which probe a redshift range of Delta z
~ 1. This range is large enough to select galaxies that may not be physically
related to the quasar. We use deep narrow- and broadband imaging to study the
environment of the z=5.72 quasar ULAS J0203+0012. The redshift range probed by
our narrow-band selection of Lyman alpha emitters (LAEs) is Delta z ~ 0.1,
which is significantly narrower than the LBG searches. This is the first time
that LAEs were searched for near a z~6 quasar, in an effort to provide clues
about the environments of quasars at the end of the epoch of reionization. We
find no enhancement of LAEs in the surroundings of ULAS J0203+0012 in
comparison with blank fields. We explore different explanations and
interpretations for this non-detection of a galaxy overdensity, including that
(1) the strong ionization from the quasar may prevent galaxy formation in its
immediate vicinity and (2) high-redshift quasars may not reside in the center
of the most massive dark matter halos.Comment: text updated to match published versio
Discovery of a high-z protocluster with tunable filters: the case of 6C0140+326 at z=4.4
We present the first results obtained using a tunable narrowband filter in
the search for high-z protoclusters. Using the recently commissioned red
tunable filter on the Gran Telescopio Canarias we have searched for Lya
emitters in a 75 arcmin^2 field centered on the z=4.413 radio galaxy
6C0140+326. With three different wavelength tunings we find a total of 27
unique candidate Lya emitters. The availability of three different wavelength
tunings allows us to make estimates of the redshifts for each of the objects.
It also allows us to separate a possible protocluster from structure in the
immediate foreground. This division shows that the foreground region contains
significantly fewer Lya emitters. Also, the spatial distribution of the objects
in the protocluster field deviates from a random distribution at the 2.5 sigma
level. The observed redshift distribution of the emitters is different from the
expected distribution of a blank field at the ~3 sigma level, with the Lya
emitters concentrated near the radio galaxy at z>4.38. The 6C0140+326 field is
denser by a factor of 9+/-5 than a blank field, and the number density of Lya
emitters close to the radio galaxy is similar to that of the z~4.1 protocluster
around TNJ1338-1942. We thus conclude that there is an overdensity of Lya
emitters around the radio galaxy 6C0140+326. This is one of few known
overdensities at such a high redshift.Comment: 10 pages, 6 figures, accepted for publication in MNRA
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
Overdensities of 24um Sources in the Vicinities of High-Redshift Radio Galaxies
We present a statistical study of the environments of 63 high-redshift radio
galaxies (HzRGs) between redshifts 1<z<5.2, using the 24um, waveband of the
MIPS instrument aboard the Spitzer Space Telescope. Using a counts-in-cell
analysis, a statistically significant source overdensity is found in 1.75arcmin
radius circular cells centred on the HzRGs when compared to reference fields.
We report an average overdensity of delta (= {N}_{targets} / {N}_{reference}) =
2.2 +/- 1.2 at a flux density cut of f24um=0.3mJy. This result implies that
HzRGs are likely to lie in protoclusters of active and star-forming galaxies at
high redshift. Over 95% of our targeted HzRGs lie in higher than average
density fields. Further, 20 (32%) of our selected fields are found to be
overdense to at least a 3sigma significance, of which 9 are newly identified
protocluster candidates. We observe a weak correlation between redshift and
24um, source density, and discuss the populations being probed at different
redshifts. In our uniformly selected sample, which was designed to cover two
orders of magnitude in radio luminosity throughout z=1-4, we find that the
24um, source density does not depend on radio luminosity. We also compare this
result with recent work describing IRAC source overdensities around the same
HzRGs and find correlations between the results.Comment: 10 pages, 7 figures, 2 tables, accepted for publication in A&
A search for high redshift clusters associated with radio galaxies at 2 < z < 4
High redshift radio galaxies are amongst the most massive galaxies in the
early Universe and have properties expected from central galaxies in forming
clusters. We are carrying out an observational programme on the VLT to find and
study galaxy proto clusters around radio galaxies at redshifts 2 < z < 4.
First, we use narrow band imaging to select candidate galaxies which show
excess Lyman alpha emission at redshifts similar to the central radio galaxy.
Then, we use multi object spectroscopy to confirm the redshifts of these
candidates and measure the velocity dispersion of the cluster members. Our goal
is to observe a sample of about 10 targets and investigate galaxy overdensities
as a function of redshift. Here, we report on the current progress of the
programme and show some preliminary results which include the discovery of a
structure of galaxies at redshift 4.1.Comment: 4 pages, 3 figures, Sesto conference proceeding 'Probing cosmic
evolution with galaxy clusters
400 pc Imaging of a Massive Quasar Host Galaxy at a Redshift of 6.6
We report high spatial resolution (~0.076", 410pc) Atacama Large
Millimeter/submillimeter Array imaging of the dust continuum and the ionized
carbon line [CII] in a luminous quasar host galaxy at z=6.6, 800 million years
after the big bang. Based on previous studies, this galaxy hosts a ~1x10^9
M_sun black hole and has a star-formation rate of ~1500 M_sun/yr. The
unprecedented high resolution of the observations reveals a complex morphology
of gas within 3kpc of the accreting central black hole. The gas has a high
velocity dispersion with little ordered motion along the line of sight, as
would be expected from gas accretion that has yet to settle in a disk. In
addition, we find the presence of [CII] cavities in the gas distribution (with
diameters of ~0.5kpc), offset from the central black hole. This unique
distribution and kinematics cannot be explained by a simple model. Plausible
scenarios are that the gas is located in a truncated or warped disk, or the
holes are created by interactions with nearby galaxies or due to energy
injection into the gas. In the latter case, the energy required to form the
cavities must originate from the central active galactic nucleus, as the
required energy far exceeds the energy output expected from supernovae. This
energy input into the gas, however, does not inhibit the high rate of
star-formation. Both star-formation and black hole activity could have been
triggered by interactions with satellite galaxies; our data reveal three
additional companions detected in [CII] emission around the quasar.Comment: Published in ApJ Letter
The visibility of gamma-ray burst afterglows in dusty star-forming regions
Recent observations of the environments of gamma-ray bursts (GRBs) favour
massive stars as their progenitors, which are likely to be surrounded by gas
and dust. The visibility of the optical and UV emission of a GRB are expected
to depend on the characteristics of both the dust and the GRB emission itself.
A reasonable distribution of surrounding dust is capable of absorbing all the
optical and UV emission of the optical flash and afterglow of a GRB, unless the
optical flash has a peak isotropic luminosity L_peak > 10^49 erg/s. This means
that dark bursts should exist and these bursts will have to be studied at
infrared rather than optical wavelengths. In this paper details will be given
about the infrared GRB dust emission. The reprocessed dust emission peaks at a
rest-frame wavelength of about 8 micron. Forthcoming space telescopes, in
particular the IRAC camera aboard the Space Infrared Telescope Facility could
detect this emission out to a redshift of about 2. However, an accurate
position of the GRB afterglow must be provided for this emission to be
identified, because the light curve of the reprocessed dust emission does not
vary on time-scales less than several years.Comment: 11 pages, 6 figures, minor changes, matches version published in
MNRA
[OII] emitters in the GOODS field at z~1.85: a homogeneous measure of evolving star formation
We present the results of a deep, near-infrared, narrow band imaging survey
at a central wavelength of 1.062 microns (FWHM=0.01 microns) in the GOODS-South
field using the ESO VLT instrument, HAWK-I. The data are used to carry out the
highest redshift search for [OII]3727 emission line galaxies to date. The
images reach an emission line flux limit (5 sigma) of 1.5 x 10^-17 erg cm^-2
s^-1, additionally making the survey the deepest of its kind at high redshift.
In this paper we identify a sample of [OII]3727 emission line objects at
redshift z~1.85 in a co-moving volume of ~4100 Mpc^3. Objects are selected
using an observed equivalent width (EW_obs) threshold of EW_obs = 50 angstroms.
The sample is used to derive the space density and constrain the luminosity
function of [OII] emitters at z=1.85. We find that the space density of objects
with observed [OII] luminosities in the range log(L_[OII]) > 41.74 erg s^-1 is
log(rho)=-2.45+/-0.14 Mpc^-3, a factor of 2 greater than the observed space
density of [OII] emitters reported at z~1.4. After accounting for completeness
and assuming an internal extinction correction of A_Halpha=1 mag (equivalent to
A_[OII]=1.87), we report a star formation rate density of rho* ~0.38+/-0.06
Msun yr^-1 Mpc^-3. We independently derive the dust extinction of the sample
using 24 micron fluxes and find a mean extinction of A_[OII]=0.98+/-0.11
magnitudes (A_Halpha=0.52). This is significantly lower than the A_Halpha=1
(A[OII]=1.86) mag value widely used in the literature. Finally we incorporate
this improved extinction correction into the star formation rate density
measurement and report rho*~0.24+/-0.06 Msun yr^-1 Mpc^-3.Comment: 11 pages, 10 figures, accepted for publication in MNRA
The compact, ∼1 kpc host galaxy of a quasar at a redshift of 7.1
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [C ii] fine-structure line and the underlying far-infrared (FIR) dust continuum emission in J1120+0641, the most distant quasar currently known (). We also present observations targeting the CO(2–1), CO(7–6), and [C i] 369 μm lines in the same source obtained at the Very Large Array and Plateau de Bure Interferometer. We find a [C ii] line flux of Jy and a continuum flux density of mJy beam−1, consistent with previous unresolved measurements. No other source is detected in continuum or [C ii] emission in the field covered by ALMA (~ 25''). At the resolution of our ALMA observations (0farcs23, or 1.2 kpc, a factor of ~70 smaller beam area compared to previous measurements), we find that the majority of the emission is very compact: a high fraction (~80%) of the total line and continuum flux is associated with a region 1–1.5 kpc in diameter. The remaining ~20% of the emission is distributed over a larger area with radius lesssim4 kpc. The [C ii] emission does not exhibit ordered motion on kiloparsec scales: applying the virial theorem yields an upper limit on the dynamical mass of the host galaxy of , only ~20 × higher than the central black hole (BH). The other targeted lines (CO(2–1), CO(7–6), and [C i]) are not detected, but the limits of the line ratios with respect to the [C ii] emission imply that the heating in the quasar host is dominated by star formation, and not by the accreting BH. The star formation rate (SFR) implied by the FIR continuum is 105–340 , with a resulting SFR surface density of ~100–350 kpc−2, well below the value for Eddington-accretion-limited star formation
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