1,992 research outputs found
Estimating Photometric Redshifts Using Support Vector Machines
We present a new approach to obtaining photometric redshifts using a kernel
learning technique called Support Vector Machines (SVMs). Unlike traditional
spectral energy distribution fitting, this technique requires a large and
representative training set. When one is available, however, it is likely to
produce results that are comparable to the best obtained using template fitting
and artificial neural networks. Additional photometric parameters such as
morphology, size and surface brightness can be easily incorporated. The
technique is demonstrated using samples of galaxies from the Sloan Digital Sky
Survey Data Release 2 and the hybrid galaxy formation code GalICS. The RMS
error in redshift estimation is for both samples. The strengths and
limitations of the technique are assessed.Comment: 10 pages, 3 figures, to appear in the PASP, minor typos fixed to make
consistent with published versio
Evidence for extended, obscured starbursts in submm galaxies
We compare high-resolution optical and radio imaging of 12 luminous submm
galaxies at z=2.2+/-0.2 observed with HST and the MERLIN and VLA at comparable
spatial resolution, 0.3" (2kpc). The radio emission traces the likely
far-infrared morphology of these dusty, luminous galaxies. In ~30% of the
sample the radio appears unresolved, suggesting that the emission is compact:
either an obscured AGN or nuclear starburst. However, in the majority, ~70%
(8/12), the radio emission is resolved by MERLIN/VLA on scales of ~1" (10 kpc).
For these galaxies the radio morphologies are broadly similar to their
restframe UV emission seen by HST. We discuss the probable mechanisms for the
extended emission and conclude that their luminous radio and submm emission
arises from a large, spatially-extended starburst. The median SFRs are
1700Mo/yr occuring within a ~40kpc^2 region, giving a star formation density of
45Mo/yr/kpc^2. Such vigorous and extended starbursts appear to be uniquely
associated with the submm population. A more detailed comparison of the
distribution of UV and radio emission shows that the broad similarities on
large scales are not carried through to smaller scales, where there is rarely a
one-to-one correspondance. We interpret this as resulting from highly
structured internal obscuration, suggesting that the vigorous activity is
producing wind-blown channels through the obscuration in these galaxies. If
correct this underlines the difficulty of using UV morphologies to understand
structural properties of this population and also may explain the surprising
frequency of Ly-alpha emission in their spectra. [Abridged]Comment: 7 pages, 3 figure
Far-Infrared Spectral Energy Distributions and Photometric Redshifts of Dusty Galaxies
We infer the large-scale source parameters of dusty galaxies from their
observed spectral energy distributions (SEDs) using the analytic radiative
transfer methodology presented in Chakrabarti & McKee (2005). For local
ultra-luminous infrared galaxies (ULIRGs), we show that the millimeter to
far-infrared (FIR) SEDs can be well fit using the standard dust opacity index
of 2 when self-consistent radiative transfer solutions are employed, indicating
that the cold dust in local ULIRGs can be described by a single grain model. We
develop a method for determining photometric redshifts of ULIRGs and sub-mm
galaxies from the millimeter-FIR SED; the resulting value of is typically
accurate to about 10%. As such, it is comparable to the accuracy of near-IR
photometric redshifts and provides a complementary means of deriving redshifts
from far-IR data, such as that from the upcoming . Since our analytic radiative transfer solution is developed for
homogeneous, spherically symmetric, centrally heated, dusty sources, it is
relevant for infrared bright galaxies that are primarily powered by compact
sources of luminosity that are embedded in a dusty envelope. We discuss how
deviations from spherical symmetry may affect the applicability of our
solution, and we contrast our self-consistent analytic solution with standard
approximations to demonstrate the main differences.Comment: 37 pages, 14 Figures, 3 Tables, submitted to ApJ. submitted to Ap
HST Astrometry of M4 and the Galactic Constant V_0/R_0
From multi-epoch WFPC2/HST observations we present astrometric measurements
of stars in the Galactic globular cluster M4 (NGC 6121) and in the
foreground/background. The presence of an extragalactic point source allows us
to determine the absolute proper motion of the cluster, and, through use of the
field stars in this region only 18 degree from the Galactic center, to measure
the difference between the Oort constants, A-B. We find: (mu_alpha cos dec,
mu_dec)_J2000 = (-13.21 +/- 0.35, -19.28 +/- 0.35) mas/yr, and A-B = V_0/R_0 =
27.6 +/- 1.7 km / s / kpc.Comment: 20 pages, 6 figures, A.J.__ACCEPTED__, 1 April, 2003, (...!
Distant red galaxies in the Hubble Ultra Deep Field
We take advantage of the Hubble Ultra Deep Field (UDF) data to study the
restframe optical and ultra violet (UV) morphologies of the novel population of
Distant Red Galaxies (DRGs). Six galaxies with J-Ks > 2.3 are found to Ks=21.5,
five of which have photometric redshifts z_phot > 2, corresponding to a surface
density of 0.9/arcmin^2. The surface brightness distributions of the z_phot > 2
galaxies are better represented by exponential disks than R^{1/4}-laws. Two of
the z_phot > 2 galaxies are extended, while three have compact morphologies.
The restframe optical morphology of the z_phot > 2 galaxies is quite different
from the restframe UV morphology: all the galaxies have red central components
which dominate in the NICMOS H_{160}-band images, and distinct off-center blue
features which show up in (and often dominate) the ACS images. The mean
measured effective radius of the z_phot > 2 galaxies is =1.9+/-1.4 kpc,
similar (within the errors) to the mean size of LBGs at similar redshifts. All
the DRGs are resolved in the ACS images, while four are resolved in the NICMOS
images. Two of the z_phot > 2 galaxies are bright X-ray sources and hence host
AGN. The diverse restframe optical and UV morphological properties of DRGs
derived here suggest that they have complex stellar populations, consisting of
both evolved populations that dominate the mass and the restframe optical
light, and younger populations, which show up as patches of star formation in
the restframe UV light; in many ways resembling the properties of normal local
galaxies. This interpretation is supported by fits to the broadband SEDs, which
for all five z_phot > 2 are best represented by models with extended star
formation histories and substantial amounts of dust.Comment: Accepted for publication in APJ
Galaxy Mergers at z>1 in the HUDF: Evidence for a Peak in the Major Merger Rate of Massive Galaxies
We present a measurement of the galaxy merger fraction and number density
from observations in the Hubble Ultra Deep Field for 0.5<z<2.5. We fit the
combination of broadband data and slitless spectroscopy of 1308 galaxies with
stellar population synthesis models to select merging systems based on a
stellar mass of >10^10 M_sol. When correcting for mass incompleteness, the
major merger fraction is not simply proportional to (1+z)^m, but appears to
peak at z_frac~=1.3+-0.4. From this merger fraction, we infer that ~42% of
massive galaxies have undergone a major merger since z~1. We show that the
major merger number density peaks at z_dens~1.2, which marks the epoch where
major merging of massive galaxies is most prevalent. This critical redshift is
comparable to the peak of the cosmic star formation rate density, and occurs
roughly 2.6 Gyr earlier in cosmic time than the peak in the number density of
X-ray selected active galactic nuclei. These observations support an indirect
evolutionary link between merging, starburst, and active galaxies.Comment: Accepted to ApJ. 7 pages, 6 figures, 1 table. Uses and includes
emulateapj.cls. In the initial submission, Figures 1 and 2 where switche
The OPTX Project I: The Flux and Redshift Catalogs for the CLANS, CLASXS, and CDF-N fields
We present the redshift catalogs for the X-ray sources detected in the
Chandra Deep Field North (CDF-N), the Chandra Large Area Synoptic X-ray Survey
(CLASXS), and the Chandra Lockman Area North Survey (CLANS). The catalogs for
the CDF-N and CLASXS fields include redshifts from previous work, while the
redshifts for the CLANS field are all new. For fluxes above 10^-14 ergs cm^-2
s^-1 (2-8 keV) we have redshifts for 76% of the sources. We extend the redshift
information for the full sample using photometric redshifts. The goal of the
OPTX Project is to use these three surveys, which are among the most
spectroscopically complete surveys to date, to analyze the effect of spectral
type on the shape and evolution of the X-ray luminosity functions and to
compare the optical spectral types with the X-ray spectral properties.
We also present the CLANS X-ray catalog. The nine ACIS-I fields cover a solid
angle of ~0.6 square degrees and reach fluxes of 7x10^-16 ergs cm^-2 s^-1
(0.5-2 keV) and 3.5x10^-15 ergs cm^-2 s^-1 (2-8 keV). We find a total of 761
X-ray point sources. Additionally, we present the optical and infrared
photometric catalog for the CLANS X-ray sources, as well as updated optical and
infrared photometric catalogs for the X-ray sources in the CLASXS and CDF-N
fields.
The CLANS and CLASXS surveys bridge the gap between the ultradeep pencil-beam
surveys, such as the CDFs, and the shallower, very large-area surveys. As a
result, they probe the X-ray sources that contribute the bulk of the 2-8 keV
X-ray background and cover the flux range of the observed break in the
logN-logS distribution. We construct differential number counts for each
individual field and for the full sample.Comment: Published in The Astrophysical Journal Supplement. 18 pages, 16
figures, 14 table
Tracing Galaxy Assembly: Tadpole Galaxies in the Hubble Ultra Deep Field
In the Hubble Ultra Deep Field (HUDF) an abundance of galaxies is seen with a
knot at one end plus an extended tail, resembling a tadpole. These "tadpole
galaxies" appear dynamically unrelaxed--presumably in an early merging
state--where tidal interactions likely created the distorted knot-plus-tail
morphology. Here we systematically select tadpole galaxies from the HUDF and
study their properties as a function of their photometric redshifts. In a
companion HUDF variability study, Cohen et al. (2005) revealed a total of 45
variable objects believed to be Active Galactic Nuclei (AGN). Here we show that
this faint AGN sample has no overlap with the tadpole galaxy sample, as
predicted by theoretical work. The tadpole morphology--combined with the lack
of overlap with the variable objects--supports the idea that these galaxies are
in the process of an early-stage merger event, i.e., at a stage that likely
precedes the "turn-on" of any AGN component and the onset of any point-source
variability.Comment: 7 pages, 4 figures. Accepted for publication by Astrophysical Journa
Lens or Binary? Chandra Observations of the Wide Separation Broad Absorption Line Quasar Pair UM425
We have obtained a 110 ksec Chandra ACIS-S exposure of UM425, a pair of QSOs
at z=1.47 separated by 6.5 arcsec, which show remarkably similar emission and
broad absorption line (BAL) profiles in the optical/UV. Our 5000 count X-ray
spectrum of UM425A (the brighter component) is well-fit with a power law
(photon spectral index Gamma=2.0) partially covered by a hydrogen column of
3.8x10^22 cm^-2. The underlying power-law slope for this object and for other
recent samples of BALQSOs is typical of radio-quiet quasars, lending credence
to the hypothesis that BALs exist in every quasar. Assuming the same Gamma for
the much fainter image of UM425B, we detect an obscuring column 5 times larger.
We search for evidence of an appropriately large lensing mass in our Chandra
image and find weak diffuse emission near the quasar pair, with an X-ray flux
typical of a group of galaxies at redshift z ~ 0.6. From our analysis of
archival HST WFPC2 and NICMOS images, we find no evidence for a luminous
lensing galaxy, but note a 3-sigma excess of galaxies in the UM425 field with
plausible magnitudes for a z=0.6 galaxy group. However, the associated X-ray
emission does not imply sufficient mass to produce the observed image
splitting. The lens scenario thus requires a dark (high M/L ratio) lens, or a
fortuitous configuration of masses along the line of sight. UM425 may instead
be a close binary pair of BALQSOs, which would boost arguments that
interactions and mergers increase nuclear activity and outflows.Comment: 13 pages, 9 figures, Accepted for publication in the Astrophysical
Journa
Discovery of a Galaxy Cluster in the Foreground of the Wide-Separation Quasar Pair UM425
We report the discovery of a cluster of galaxies in the field of UM425, a
pair of quasars separated by 6.5arcsec. Based on this finding, we revisit the
long-standing question of whether this quasar pair is a binary quasar or a
wide-separation lens. Previous work has shown that both quasars are at z=1.465
and show broad absorption lines. No evidence for a lensing galaxy has been
found between the quasars, but there were two hints of a foreground cluster:
diffuse X-ray emission observed with Chandra, and an excess of faint galaxies
observed with the Hubble Space Telescope. Here we show, via VLT spectroscopy,
that there is a spike in the redshift histogram of galaxies at z=0.77. We
estimate the chance of finding a random velocity structure of such significance
to be about 5%, and thereby interpret the diffuse X-ray emission as originating
from z=0.77, rather than the quasar redshift. The mass of the cluster, as
estimated from either the velocity dispersion of the z=0.77 galaxies or the
X-ray luminosity of the diffuse emission, would be consistent with the
theoretical mass required for gravitational lensing. The positional offset
between the X-ray centroid and the expected location of the mass centroid is
about 40kpc, which is not too different from offsets observed in lower redshift
clusters. However, UM425 would be an unusual gravitational lens, by virtue of
the absence of a bright primary lensing galaxy. Unless the mass-to-light ratio
of the galaxy is at least 80 times larger than usual, the lensing hypothesis
requires that the galaxy group or cluster plays a uniquely important role in
producing the observed deflections.
Based on observations performed with the Very Large Telescope at the European
Southern Observatory, Paranal, Chile.Comment: 12 pages, accepted by ApJ 2005, May 1
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