55 research outputs found
How JWST can measure First Light, Reionization and Galaxy Assembly
We summarize the design and performance of the James Webb Space Telescope
that is to be launched to an L2 orbit in 2011, and how it is designed, in
particular, to study the epochs of First Light, Reionization and Galaxy
Assembly.Comment: 12 pages, Latex2e requires 'elsart' (included), 7 Postscript figures.
To appear in the Proceedings of the UC Irvine Workshop on "First Light and
Reionization: Theoretical Study and Experimental Detection of the First
Luminous Sources", eds. A. Cooray & E. Barton (New Astron. Rev., 2005). A
full-resolution PDF version is available at www.asu.edu/clas/hst/www/jwst/ .
Revision includes minor corrections and legible labels in Fig.
Evidence for a correlation between the sizes of quiescent galaxies and local environment to z ~ 2
We present evidence for a strong relationship between galaxy size and
environment for the quiescent population in the redshift range 1 < z < 2.
Environments were measured using projected galaxy overdensities on a scale of
400 kpc, as determined from ~ 96,000 K-band selected galaxies from the UKIDSS
Ultra Deep Survey (UDS). Sizes were determined from ground-based K-band
imaging, calibrated using space-based CANDELS HST observations in the centre of
the UDS field, with photometric redshifts and stellar masses derived from
11-band photometric fitting. From the resulting size-mass relation, we confirm
that quiescent galaxies at a given stellar mass were typically ~ 50 % smaller
at z ~ 1.4 compared to the present day. At a given epoch, however, we find that
passive galaxies in denser environments are on average significantly larger at
a given stellar mass. The most massive quiescent galaxies (M_stellar > 2 x
10^11 M_sun) at z > 1 are typically 50 % larger in the highest density
environments compared to those in the lowest density environments. Using Monte
Carlo simulations, we reject the null hypothesis that the size-mass relation is
independent of environment at a significance > 4.8 sigma for the redshift range
1 < z < 2. In contrast, the evidence for a relationship between size and
environment is much weaker for star-forming galaxies.Comment: Accepted for publication in MNRAS. 16 pages, 11 figures, 6 table
Detailed Decomposition of Galaxy Images. II. Beyond Axisymmetric Models
We present a two-dimensional (2-D) fitting algorithm (GALFIT, Version 3) with
new capabilities to study the structural components of galaxies and other
astronomical objects in digital images. Our technique improves on previous 2-D
fitting algorithms by allowing for irregular, curved, logarithmic and power-law
spirals, ring and truncated shapes in otherwise traditional parametric
functions like the Sersic, Moffat, King, Ferrer, etc., profiles. One can mix
and match these new shape features freely, with or without constraints, apply
them to an arbitrary number of model components and of numerous profile types,
so as to produce realistic-looking galaxy model images. Yet, despite the
potential for extreme complexity, the meaning of the key parameters like the
Sersic index, effective radius or luminosity remain intuitive and essentially
unchanged. The new features have an interesting potential for use to quantify
the degree of asymmetry of galaxies, to quantify low surface brightness tidal
features beneath and beyond luminous galaxies, to allow more realistic
decompositions of galaxy subcomponents in the presence of strong rings and
spiral arms, and to enable ways to gauge the uncertainties when decomposing
galaxy subcomponents. We illustrate these new features by way of several case
studies that display various levels of complexity.Comment: 41 pages, 22 figures, AJ accepted. Minor changes. Full resolution
version of this paper is available at:
http://users.obs.carnegiescience.edu/peng/work/galfit/galfit3.pd
The XMM Cluster Survey: a massive galaxy cluster at z = 1.45
We report the discovery of XMMXCS J2215.9-1738, a massive galaxy cluster at z=1.45, which was found in the XMM Cluster Survey. The cluster candidate was initially identified as an extended X-ray source in archival XMM data. Optical spectroscopy shows that six galaxies within a ~60" diameter region lie at z=1.45+/-0.01. Model fits to the X-ray spectra of the extended emission yield kT=7.4+2.7-1.8 keV (90% confidence); if there is an undetected central X-ray point source, then kT=6.5+2.6-1.8 keV. The bolometric X-ray luminosity is LX=4.4+0.8-0.6C 1044 ergs s-1 over a 2 Mpc radial region. The measured TX, which is the highest for any known cluster at z>1, suggests that this cluster is relatively massive for such a high redshift. The redshift of XMMXCS J2215.9-1738 is the highest currently known for a spectroscopically confirmed cluster of galaxies
Tidal dwarf galaxies in the nearby Universe
We present a statistical observational study of the tidal dwarf (TD)
population in the nearby Universe, by exploiting a large, homogeneous catalogue
of galaxy mergers compiled from the SDSS. 95% of TD-producing mergers involve
two spiral progenitors, while most remaining systems have at least one spiral
progenitor. The fraction of TD-producing mergers where both parents are
early-type galaxies is <2%, suggesting that TDs are unlikely to form in such
mergers. The bulk of TD-producing systems inhabit a field environment and have
mass ratios greater than 1:7 (the median value is 1:2.5). TDs forming at the
tidal-tail tips are ~4 times more massive than those forming at the base of the
tails. TDs have stellar masses that are less than 10% of the stellar masses of
their parents and typically lie within 15 optical half-light radii of their
parent galaxies. The TD population is typically bluer than the parents, with a
median offset of ~0.3 mag in the (g-r) colour and the TD colours are not
affected by the presence of AGN activity in their parents. An analysis of their
star formation histories indicates that TDs contain both newly formed stars
(with a median age of ~30 Myr) and old stars drawn from the parent disks, each
component probably contributing roughly equally to their stellar mass. Thus,
TDs are not formed purely through gas condensation in tidal tails but host a
significant component of old stars from the parent disks. Finally, an analysis
of the TD contribution to the local dwarf-to-massive galaxy ratio indicates
that ~6% of dwarfs in nearby clusters may have a tidal origin, if TD production
rates in nearby mergers are representative of those in the high-redshift
Universe. Even if TD production rates at high redshift were several factors
higher, it seems unlikely that the entire dwarf galaxy population today is a
result of merger activity over the lifetime of the Universe.Comment: MNRAS in pres
Observations of the initial formation and evolution of spiral galaxies at 1 < z < 3 in the CANDELS fields
Many aspects concerning the formation of spiral and disc galaxies remain unresolved, despite their discovery and detailed study over the past 150 years. As such, we present the results of an observational search for proto-spiral galaxies and their earliest formation, including the discovery of a significant population of spiral-like and clumpy galaxies at z > 1 in deep Hubble Space Telescope CANDELS imaging. We carry out a detailed analysis of this population, characterizing their number density evolution, masses, star formation rates and sizes. Overall, we find a surprisingly high overall number density of massive M* > 1010 M⊙ spiral-like galaxies (including clumpy spirals) at z > 1 of 0.18 per arcmin−2. We measure and characterise the decline in the number of these systems at higher redshift using simulations to correct for redshift effects in identifications, finding that the true fraction of spiral-like galaxies grows at lower redshifts as ∼ (1 + z)−1.1. This is such that the absolute numbers of spirals increases by a factor of ∼10 between z = 2.5 and z = 0.5. We also demonstrate that these spiral-like systems have large sizes at z > 2, and high star formation rates, above the main-sequence, These galaxies represent a major mode of galaxy formation in the early universe, perhaps driven by the spiral structure itself. We finally discuss the origin of these systems, including their likely formation through gas accretion and minor mergers, but conclude that major mergers are an unlikely cause
Galaxy Zoo 1 : Data Release of Morphological Classifications for nearly 900,000 galaxies
Morphology is a powerful indicator of a galaxy's dynamical and merger
history. It is strongly correlated with many physical parameters, including
mass, star formation history and the distribution of mass. The Galaxy Zoo
project collected simple morphological classifications of nearly 900,000
galaxies drawn from the Sloan Digital Sky Survey, contributed by hundreds of
thousands of volunteers. This large number of classifications allows us to
exclude classifier error, and measure the influence of subtle biases inherent
in morphological classification. This paper presents the data collected by the
project, alongside measures of classification accuracy and bias. The data are
now publicly available and full catalogues can be downloaded in electronic
format from http://data.galaxyzoo.org.Comment: Accepted by MNRAS, 14 pages. Updated to match final version; problem
with table 7 header fixed. Full tables available at http://data.galaxyzoo.or
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