26,144 research outputs found
Structured Sparse Modelling with Hierarchical GP
In this paper a new Bayesian model for sparse linear regression with a
spatio-temporal structure is proposed. It incorporates the structural
assumptions based on a hierarchical Gaussian process prior for spike and slab
coefficients. We design an inference algorithm based on Expectation Propagation
and evaluate the model over the real data.Comment: SPARS 201
NICMOS Imaging of a Damped Lyman-alpha Absorber at z=1.89 toward LBQS 1210+1731 : Constraints on Size and Star Formation Rate
We report results of a high-resolution imaging search (in rest frame
H- and optical continuum) for the galaxy associated with the damped
Lyman- (DLA) absorber at toward the quasar
LBQS 1210+1731, using HST/NICMOS. After PSF subtraction, a feature is seen in
both the broad-band and narrow-band images, at a projected separation of
0.25\arcsec from the quasar. If associated with the DLA, the object would be
kpc in size with a flux of Jy in
the F160W filter, implying a luminosity at {\AA} in
the rest frame of L at ,
for . However, no significant H- emission is seen,
suggesting a low star formation rate (SFR) (3 upper limit of 4.0
M yr), or very high dust obscuration.
Alternatively, the object may be associated with the host galaxy of the quasar.
H-band images obtained with the NICMOS camera 2 coronagraph show a much fainter
structure kpc in size and containing four knots of
continuum emission, located 0.7\arcsec away from the quasar. We have probed
regions far closer to the quasar sight-line than in most previous studies of
high-redshift intervening DLAs. The two objects we report mark the closest
detected high-redshift DLA candidates yet to any quasar sight line. If the
features in our images are associated with the DLA, they suggest faint,
compact, somewhat clumpy objects rather than large, well-formed proto-galactic
disks or spheroids.Comment: 52 pages of text, 19 figures, To be published in Astrophysical
Journal (accepted Dec. 8, 1999
The Ks-band Luminosity and Stellar Mass Functions of Galaxies in z~1 Clusters
We present the near-infrared (Ks-band) luminosity function of galaxies in two
z~1 cluster candidates, 3C336 and Q1335+28. A third cluster, 3C289, was
observed but found to be contaminated by a foreground system. Our wide field
imaging data reach to Ks=20.5 (5sigma), corresponding to ~M*+2.7 with respect
to the passive evolution. The near-infrared luminosity traces the stellar mass
of a galaxy due to its small sensitivity to the recent star formation history.
Thus the luminosity function can be transformed to the stellar mass function of
galaxies using the colours with only a small correction (factor<2) for
the effects of on-going star formation. The derived stellar mass function spans
a wide range in mass from ~3 x 10^{11}Msun down to ~6 x 10^{9}Msun (set by the
magnitude limit). The form of the mass function is very similar to lower
redshift counterparts such as that from 2MASS/LCRS clusters (Balogh et al.
2001) and the z=0.31 clusters (Barger et al. 1998). This indicates little
evolution of galaxy masses from z=1 to the present-day. Combined with colour
data that suggest star formation is completed early (z>>1) in the cluster core,
it seems that the galaxy formation processes (both star formation and mass
assembly) are strongly accerelated in dense environments and has been largely
completed by z=1. We investigate whether the epoch of mass assembly of massive
cluster galaxies is earlier than that predicted by the hierarchical galaxy
formation models. These models predict the increase of characteristic mass by
more than factor ~3 between z=1 and the present day. This seems incompatible
with our data.Comment: 12 pages, including 12 figures, uses mn.sty and epsf.sty. Accepted
for publication in MNRAS Main Journa
A Search for the Damped Ly-alpha Absorber at z=1.86 toward QSO 1244+3443 with NICMOS
We have carried out a high-resolution imaging search for the galaxy
associated with the damped Lyman-alpha (DLA) absorber at z=1.859 toward the
z_{em}=2.48 quasar QSO 1244+3443, using the HST and the NICMOS. Images were
obtained in the broad filter F160W and the narrow filter F187N with camera 2 on
NICMOS with the goal of detecting the rest-frame optical continuum and the
H-alpha line emission from the DLA. After PSF subtraction, two weak features
are seen at projected separations of 0.16-0.24" from the quasar. Parts of these
features may be associated with the DLA absorber, although we cannot completely
rule out that they could be artifacts of the point spread function (PSF). If
associated with the DLA, the objects would be ~1-2 h_{70}^{-1} kpc in size with
integrated flux densities of 2.5 and 3.3 mu Jy in the F160W filter, implying
luminosities at lambda_{central}=5600 A in the DLA rest frame of 4.4-5.9 x
10^{9} h_{70}^{-2} L_{solar} at z=1.86, for q0=0.5. However, no significant
H-alpha line emission is seen from these objects, suggesting low star formation
rates (SFRs). Our 3 sigma upper limit on the SFR in the DLA is 1.3 h_{70}^{-2}
M_{solar}/yr for q0 = 0.5 (2.4 h_{70}^{-2} M_{solar} yr^{-1} for q0 = 0.1).
This together with our earlier result for LBQS 1210+1731 mark a significant
improvement over previous constraints on the star formation rates of DLAs. A
combination of low SFR and some dust extinction is likely to be responsible for
the lack of H-alpha emission. Alternatively, the objects, may be associated
with the quasar host galaxy. In any case, our observations suggest that the DLA
is not a large bright proto-disk, but a compact object or a low-surface
brightness galaxy. If the two features are PSF artifacts then the constraints
on DLA properties are even more severe.Comment: 28 pages, 9 figures. Accepted for publication in The Astrophysical
Journal. Figures are given at a slightly lower resolution here, to decrease
file sizes. The higher resolution versions can be found in the Ap
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