367 research outputs found
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
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
[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
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
Optical/near-infrared selection of red QSOs: Evidence for steep extinction curves towards galactic centers?
We present the results of a search for red QSOs using a selection based on
optical imaging from SDSS and near-infrared imaging from UKIDSS. For a sample
of 58 candidates 46 (79%) are confirmed to be QSOs. The QSOs are predominantly
dust-reddened except a handul at redshifts z>3.5. The dust is most likely
located in the QSO host galaxies. 4 (7%) of the candidates turned out to be
late-type stars, and another 4 (7%) are compact galaxies. The remaining 4
objects we could not identify. In terms of their optical spectra the QSOs are
similar to the QSOs selected in the FIRST-2MASS red Quasar survey except they
are on average fainter, more distant and only two are detected in the FIRST
survey. We estimate the amount of extinction using the SDSS QSO template
reddened by SMC-like dust. It is possible to get a good match to the observed
(restframe ultraviolet) spectra, but for nearly all the reddened QSOs it is not
possible to match the near-IR photometry from UKIDSS. The likely reasons are
that the SDSS QSO template is too red at optical wavelengths due to
contaminating host galaxy light and that the assumed SMC extinction curve is
too shallow. Our survey has demonstrated that selection of QSOs based on
near-IR photometry is an efficent way to select QSOs, including reddened QSOs,
with only small contamination from late-type stars and compact galaxies. This
will be useful with ongoing and future wide-field near-IR surveys such as the
VISTA and EUCLID surveys. [Abridged]Comment: 74 pages, 6 figures. Accepted for for publication in ApJ
A study of distant Ly-alpha emitters in overdense regions
Recently, we conducted a Very Large Telescope (VLT) large program to search
for forming clusters by looking for overdensities of Ly-alpha emitters around
high redshift radio galaxies. In total seven proto-clusters were discovered,
including a proto-cluster around the radio galaxy MRC 0316--257 at z ~ 3.13.
This structure has an excess of Ly-alpha emitters by a factor of 3 as compared
to the field, and the derived mass is 2-5 x 10^14 M_sun. The Ly-alpha emitters
in the proto-cluster are on average bluer than Lyman Break Galaxies (LBGs).
Also, the galaxies are faint (sub L_*) and small (half light radii < 1.7 kpc,
which is smaller than the average size of LBGs). This might indicate that, at
least a fraction of, Ly-alpha emitters could be young (~ 10^6 yr), nearly
dust-free, forming galaxies.Comment: 6 pages, 2 figures. To appear in the proceedings of the Venice
conference "Multiwavelength Mapping of Galaxy Formation and Evolution",
October 13-16, 200
Serendipitous discovery of a projected pair of QSOs separated by 4.5 arcsec on the sky
We present the serendipitous discovery of a projected pair of quasi-stellar
objects (QSOs) with an angular separation of arcsec. The
redshifts of the two QSOs are widely different: one, our programme target, is a
QSO with a spectrum consistent with being a narrow line Seyfert 1 AGN at
. For this target we detect Lyman-, \ion{C}{4}, and
\ion{C}{3]}. The other QSO, which by chance was included on the spectroscopic
slit, is a Type 1 QSO at a redshift of , for which we detect
\ion{C}{4}, \ion{C}{3]} and \ion{Mg}{2}. We compare this system to previously
detected projected QSO pairs and find that only about a dozen previously known
pairs have smaller angular separation.Comment: 4 pages, 3 figures. Accepted for publication in A
Discovery of three z>6.5 quasars in the VISTA Kilo-degree Infrared Galaxy (VIKING) survey
Studying quasars at the highest redshifts can constrain models of galaxy and
black hole formation, and it also probes the intergalactic medium in the early
universe. Optical surveys have to date discovered more than 60 quasars up to
z~6.4, a limit set by the use of the z-band and CCD detectors. Only one z>6.4
quasar has been discovered, namely the z=7.08 quasar ULAS J1120+0641, using
near-infrared imaging. Here we report the discovery of three new z>6.4 quasars
in 332 square degrees of the Visible and Infrared Survey Telescope for
Astronomy Kilo-degree Infrared Galaxy (VIKING) survey, thus extending the
number from 1 to 4. The newly discovered quasars have redshifts of z=6.60,
6.75, and 6.89. The absolute magnitudes are between -26.0 and -25.5, 0.6-1.1
mag fainter than ULAS J1120+0641. Near-infrared spectroscopy revealed the MgII
emission line in all three objects. The quasars are powered by black holes with
masses of ~(1-2)x10^9 M_sun. In our probed redshift range of 6.44<z<7.44 we can
set a lower limit on the space density of supermassive black holes of
\rho(M_BH>10^9 M_sun) > 1.1x10^(-9) Mpc^(-3). The discovery of three quasars in
our survey area is consistent with the z=6 quasar luminosity function when
extrapolated to z~7. We do not find evidence for a steeper decline in the space
density of quasars with increasing redshift from z=6 to z=7.Comment: 14 pages, 9 figures. Published in Ap
Are we seeing accretion flows in a 250kpc-sized Ly-alpha halo at z=3?
Using MUSE on the ESO-VLT, we obtained a 4 hour exposure of the z=3.12 radio
galaxy MRC0316-257. We detect features down to ~10^-19 erg/s/cm^2/arcsec^2 with
the highest surface brightness regions reaching more than a factor of 100
higher. We find Ly-alpha emission out to ~250 kpc in projection from the active
galactic nucleus (AGN). The emission shows arc-like morphologies arising at
150-250 kpc from the nucleus in projection with the connected filamentary
structures reaching down into the circum-nuclear region. The most distant arc
is offset by 700 km/s relative to circum-nuclear HeII 1640 emission, which we
assume to be at the systemic velocity. As we probe emission closer to the
nucleus, the filamentary emission narrows in projection on the sky, the
relative velocity decreases to ~250 km/s, and line full-width at half maximum
range from 300-700 km/s. From UV line ratios, the emission on scales of 10s of
kpc from the nucleus along a wide angle in the direction of the radio jets is
clearly excited by the radio jets and ionizing radiation of the AGN. Assuming
ionization equilibrium, the more extended emission outside of the axis of the
jet direction would require 100% or more illumination to explain the observed
surface brightness. High speed (>300 km/s) shocks into rare gas would provide
sufficiently high surface brightness. We discuss the possibility that the arcs
of Ly-alpha emission represent accretion shocks and the filamentary emission
represent gas flows into the halo, and compare our results with gas accretion
simulations.Comment: 4 pages, 2 figures, 1 table, A&A letters accepte
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