558 research outputs found
Discovery of a Radio-Selected z ~ 6 Quasar
We present the discovery of only the second radio-selected, z ~ 6 quasar. We
identified SDSS J222843.54+011032.2 (z=5.95) by matching the optical detections
of the deep Sloan Digital Sky Survey (SDSS) Stripe 82 with their radio
counterparts in the Stripe82 VLA Survey. We also matched the
Canadian-France-Hawaiian Telescope Legacy Survey Wide (CFHTLS Wide) with the
Faint Images of the Radio Sky at Twenty-cm (FIRST) survey but have yet to find
any z ~ 6 quasars in this survey area. The discovered quasar is
optically-faint, z = 22.3 and M_{1450} ~ -24.5, but radio-bright, with a flux
density of f = 0.31mJy and a radio-loudness of R ~ 1100 (where
R = f_{5GHz}/f_{2500}). The i-z color of the discovered quasar places it
outside the color selection criteria for existing optical surveys. We conclude
by discussing the need for deeper wide-area radio surveys in the context of
high-redshift quasars.Comment: 20 pages, 6 figures, and ApJ accepte
A Survey of z~6 Quasars in the SDSS Deep Stripe. II. Discovery of Six Quasars at z_{AB}>21
We present the discovery of six new quasars at z~6 selected from the Sloan
Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by
repeatedly scanning a stripe along the celestial equator. The six quasars are
about two magnitudes fainter than the luminous z~6 quasars found in the SDSS
main survey and one magnitude fainter than the quasars reported in Paper I
(Jiang et al. 2008). Four of them comprise a complete flux-limited sample at
21<z_AB<21.8 over an effective area of 195 deg^2. The other two quasars are
fainter than z_AB=22 and are not part of the complete sample. The quasar
luminosity function at z~6 is well described as a single power law
\Phi(L_{1450}) \propto L_{1450}^{\beta} over the luminosity range
-28<M_{1450}<-25. The best-fitting slope \beta varies from -2.6 to -3.1,
depending on the quasar samples used, with a statistical error of 0.3-0.4.
About 40% of the quasars discovered in the SDSS southern survey have very
narrow Lya emission lines, which may indicate small black hole masses and high
Eddington luminosity ratios, and therefore short black hole growth time scales
for these faint quasars at early epochs.Comment: Accepted for publication in A
Direct evidence of the receding `torus' around central nuclei of powerful radio sources
The presence of obscuring material (or a dusty `torus') in active galactic
nuclei (AGN) is central to the unification model for AGN. Two models, the
multi-population model for radio sources and the receding torus model, are
capable of describing observational properties of powerful radio galaxies and
radio quasars. Here, I study the changes of the opening angle of the obscuring
torus with radio luminosity at 151 MHz, [O III] emission-line luminosity and
cosmic epoch aiming to discriminate between two working models. An analytical
expression relating the half opening angle of the torus to the mean projected
linear sizes of double radio galaxies and quasars is derived. The sizes of
powerful double radio sources taken from the combined sample of 3CRR, 6CE and
7CR complete samples are used to estimate the torus opening angle. I found a
statistically significant correlation between the half opening angle of the
torus and [O III] emission-line luminosity. The opening angle increases from 20
to 60 degrees with increasing [O III] emission-line luminosity. This
correlation is interpreted as direct evidence of the receding torus around
central engines of powerful double radio sources.Comment: 8 pages, 6 figures, after substantial revision accepted by A&
Constraining Quasar and IGM Properties Through Bubble Detection in Redshifted 21-cm Maps
The infrared detection of a z>7 quasar has opened up a new window to directly
probe the IGM during the epoch of reionization. In this paper we theoretically
consider the possibility of detecting the ionized bubble around a z=8 quasar
using targeted redshifted 21-cm observations with the GMRT. The apparent shape
and size of the ionized bubble, as seen by a distant observer, depends on the
parameters \dot{N}_{phs}/C, x_HI/C and \tau_Q where \dot{N}_{phs}, \tau_Q, x_HI
and C are respectively the photon emission rate, age of the quasar, the neutral
fraction and clumping factor of the IGM.Here we have analytically estimated the
shape and size of a quasar's ionized bubble assuming an uniform IGM and
ignoring other ionizing sources besides the quasar, and used this as a template
for matched filter bubble search with the GMRT visibility data. We have assumed
that \dot{N}_{phs} is known from the infrared spectrum and C from theoretical
considerations, which gives us two free parameters x_HI and \tau_Q for bubble.
Considering 1,000 hr of observation, we find that there is a reasonably large
region of parameter space where a 3\sigma detection is possible. We also find
that it will be possible to place lower limits on x_HI and \tau_Q with this
observation. Deeper follow up observations can place upper limits on \tau_Q and
x_HI. Value of C affect the estimation of x_HI but the estimation of \tau_Q
remains unaffected.We have used a semi-numerical technique to simulate the
apparent shape and size of quasar ionized bubbles considering the presence of
other ionizing sources and inhomogeneities in the IGM. The presence of other
sources increase the size of the quasar bubble, leading to underestimation of
x_HI. Clustering of other ionizing sources around the quasar can produce severe
distortions in bubble's shape. However, this does not severely affect parameter
estimation in the bubbles that are large.Comment: 18 pages, 16 figures, 3 tables. Minor change in text. Accepted for
publication in MNRA
The ultracool-field dwarf luminosity-function and space density from the Canada-France Brown Dwarf Survey
Context. Thanks to recent and ongoing large scale surveys, hundreds of brown
dwarfs have been discovered in the last decade. The Canada-France Brown Dwarf
Survey is a wide-field survey for cool brown dwarfs conducted with the MegaCam
camera on the Canada-France-Hawaii Telescope telescope. Aims. Our objectives
are to find ultracool brown dwarfs and to constrain the field brown-dwarf
luminosity function and the mass function from a large and homogeneous sample
of L and T dwarfs. Methods. We identify candidates in CFHT/MegaCam i' and z'
images and follow them up with pointed near infrared (NIR) imaging on several
telescopes. Halfway through our survey we found ~50 T dwarfs and ~170 L or
ultra cool M dwarfs drawn from a larger sample of 1400 candidates with typical
ultracool dwarfs i' - z' colours, found in 780 square degrees. Results. We have
currently completed the NIR follow-up on a large part of the survey for all
candidates from mid-L dwarfs down to the latest T dwarfs known with utracool
dwarfs' colours. This allows us to draw on a complete and well defined sample
of 102 ultracool dwarfs to investigate the luminosity function and space
density of field dwarfs. Conclusions. We found the density of late L5 to T0
dwarfs to be 2.0pm0.8 x 10-3 objects pc-3, the density of T0.5 to T5.5 dwarfs
to be 1.4pm0.3 x 10-3 objects pc-3, and the density of T6 to T8 dwarfs to be
5.3pm3.1 x 10-3 objects pc-3 . We found that these results agree better with a
flat substellar mass function. Three latest dwarfs at the boundary between T
and Y dwarfs give the high density 8.3p9.0m5.1 x 10-3 objects pc-3. Although
the uncertainties are very large this suggests that many brown dwarfs should be
found in this late spectral type range, as expected from the cooling of brown
dwarfs, whatever their mass, down to very low temperature.Comment: Accepted for publication in A&
Constraints on the faint end of the quasar luminosity function at z~5 in the COSMOS field
We present the result of our low-luminosity quasar survey in the redshift
range of 4.5 < z < 5.5 in the COSMOS field. Using the COSMOS photometric
catalog, we selected 15 quasar candidates with 22 < i' < 24 at z~5, that are ~
3 mag fainter than the SDSS quasars in the same redshift range. We obtained
optical spectra for 14 of the 15 candidates using FOCAS on the Subaru Telescope
and did not identify any low-luminosity type-1 quasars at z~5 while a
low-luminosity type-2 quasar at z~5.07 was discovered. In order to constrain
the faint end of the quasar luminosity function at z~5, we calculated the
1sigma confidence upper limits of the space density of type-1 quasars. As a
result, the 1sigma confidence upper limits on the quasar space density are Phi<
1.33*10^{-7} Mpc^{-3} mag^{-1} for -24.52 < M_{1450} < -23.52 and Phi<
2.88*10^{-7} Mpc^{-3} mag^{-1} for -23.52 < M_{1450} < -22.52. The inferred
1sigma confidence upper limits of the space density are then used to provide
constrains on the faint-end slope and the break absolute magnitude of the
quasar luminosity function at z~5. We find that the quasar space density
decreases gradually as a function of redshift at low luminosity (M_{1450} ~
-23), being similar to the trend found for quasars with high luminosity
(M_{1450}<-26). This result is consistent with the so-called downsizing
evolution of quasars seen at lower redshifts.Comment: 8 pages, 9 figures, 1 table, accepted for publication in Ap
Extending the Canada-France brown Dwarfs Survey to the near-infrared: first ultracool brown dwarfs from CFBDSIR
We present the first results of the ongoing Canada-France Brown Dwarfs
Survey-InfraRed, hereafter CFBDSIR, a Near InfraRed extension to the optical
wide-field survey CFBDS. Our final objectives are to constrain ultracool
atmosphere physics by finding a statistically significant sample of objects
cooler than 650K and to explore the ultracool brown dwarf mass function
building on a well defined sample of such objects. Candidates are identified in
CFHT/WIRCam J and CFHT/MegaCam z' images using optimised psf-fitting, and we
follow them up with pointed near infrared imaging with SOFI at NTT. We finally
obtain low resolution spectroscopy of the coolest candidates to characterise
their atmospheric physics. We have so far analysed and followed up all
candidates on the first 66 square degrees of the 335 square degrees survey. We
identified 55 T-dwarfs candidates with z'-J > 3:5 and have confirmed six of
them as T-dwarfs, including 3 that are strong later-than-T8 candidates, based
on their far-red and NIR colours. We also present here the NIR spectra of one
of these ultracool dwarfs, CFBDSIR1458+1013 which confirms it as one of the
coolest brown dwarf known, possibly in the 550-600K temperature range. From the
completed survey we expect to discover 10 to 15 dwarfs later than T8, more than
doubling the known number of such objects. This will enable detailed studies of
their extreme atmospheric properties and provide a stronger statistical base
for studies of their luminosity function.Comment: A&A, Accepte
The Evolution of the Stellar Hosts of Radio Galaxies
We present new near-infrared images of z>0.8 radio galaxies from the
flux-limited 7C-III sample of radio sources for which we have recently obtained
almost complete spectroscopic redshifts. The 7C objects have radio luminosities
about 20 times fainter than 3C radio galaxies at a given redshift. The absolute
magnitudes of the underlying host galaxies and their scale sizes are only
weakly dependent on radio luminosity. Radio galaxy hosts at z~2 are
significantly brighter than the hosts of radio-quiet quasars at similar
redshifts and the model AGN hosts of Kauffmann & Haehnelt (2000). There is no
evidence for strong evolution in scale size, which shows a large scatter at all
redshifts. The hosts brighten significantly with redshift, consistent with the
passive evolution of a stellar population that formed at z>~3. This scenario is
consistent with studies of host galaxy morphology and submillimeter continuum
emission, both of which show strong evolution at z>~2.5. The lack of a strong
``redshift cutoff'' in the radio luminosity function to z>4 suggests that the
formation epoch of the radio galaxy host population lasts >~1Gyr from z>~5 to
z~3. We suggest these facts are best explained by models in which the most
massive galaxies and their associated AGN form early due to high baryon
densities in the centres of their dark matter haloes.Comment: To appear in A
The ultracool eld dwarfs luminosity function from the Canada-France Brown Dwarf Survey
The Canada-France Brown Dwarf Survey is a wide eld survey for cool brown
dwarfs conducted with the MegaCam camera on the CFHT telescope. Our objectives
are to nd ultracool brown dwarfs and to constrain the eld brown dwarf mass
function from a large and homogeneous sample of L and T dwarfs. We identify
candidates in CFHT/Megacam i' and z' images and follow them up with pointed NIR
imaging on several telescopes. Our survey has to date found 50 T dwarfs
candidates and 170 L or late M dwarf candidates drawn from a larger sample of
1300 candidates with typical ultracool dwarfs i'-z' colours, found in 900
square degrees. We currently have completed the NIR follow-up on a large part
of the survey for all candidates from the latest T dwarfs known to the late L
color range. This allows us to build on a complete and well de ned sample of
ultracool dwarfs to investigate the luminosity function of eld L and T dwarfs.Comment: Cool Stars XV conference. to appear in proceedings of Cool Stars XV
Conferenc
Characterizing the Mid-IR Extragalactic Sky with WISE and SDSS
The Wide-field Infrared Survey Explorer (WISE) has completed its all-sky
survey at 3.4-22 micron. We merge the WISE data with optical SDSS data and
provide a phenomenological characterization of mid-IR, extragalactic sources.
WISE is most sensitive at 3.4micron(W1) and least at 22micron(W4). The W1 band
probes massive early-type galaxies out to z\gtrsim1. This is more distant than
SDSS identified early-type galaxies, consistent with the fact that 28% of
3.4micron sources have faint or no r-band counterparts (r>22.2). In contrast,
92-95% of 12 and 22micron sources have SDSS optical counterparts with r<22.2.
WISE 3.4micron detects 89.8% of the entire SDSS QSO catalog at SNR(W1)>7, but
only 18.9% at 22micron with SNR(W4)>5. We show that WISE colors alone are
effective in isolating stars (or local early-type galaxies), star-forming
galaxies and strong AGN/QSOs at z<3. We highlight three major applications of
WISE colors: (1) Selection of strong AGN/QSOs at z0.8 and
W2<15.2 criteria, producing a census of this population. The surface density of
these strong AGN/QSO candidates is 67.5+-0.14/deg^2. (2) Selection of
dust-obscured, type-2 AGN/QSO candidates. We show that WISE W1-W2>0.8, W2<15.2
combined with r-W2>6 (Vega) colors can be used to identify type-2 AGN
candidates. The fraction of these type-2 AGN candidates is 1/3rd of all WISE
color-selected AGNs. (3) Selection of ULIRGs at z\sim2 with extremely red
colors, r-W4>14 or well-detected 22micron sources lacking detections in the 3.4
and 4.6micron bands. The surface density of z~2 r-W4>14 ULIRGs is
0.9+-0.07/deg^2 at SNR(W4)>5 (flux(W4)>=2.5mJy), which is consistent with that
inferred from smaller area Spitzer surveys. Optical spectroscopy of a small
number of these high-redshift ULIRGs confirms our selection, and reveals a
possible trend that optically fainter or r-W4 redder candidates are at higher
redshifts.Comment: 41 pages, 20 figures, Accepted for publication by the Astronomical
Journa
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