19 research outputs found
The Calibration and Data Products of the Galaxy Evolution Explorer
We describe the calibration status and data products pertaining to the GR2
and GR3 data releases of the Galaxy Evolution Explorer (GALEX). These releases
have identical pipeline calibrations that are significantly improved over the
GR1 data release. GALEX continues to survey the sky in the Far Ultraviolet
(FUV, ~154 nm) and Near Ultraviolet (NUV, ~232 nm) bands, providing
simultaneous imaging with a pair of photon counting, microchannel plate, delay
line readout detectors. These 1.25 degree field-of-view detectors are
well-suited to ultraviolet observations because of their excellent red
rejection and negligible background. A dithered mode of observing and photon
list output pose complex requirements on the data processing pipeline,
entangling detector calibrations and aspect reconstruction algorithms. Recent
improvements have achieved photometric repeatability of 0.05 and 0.03 mAB in
the FUV and NUV, respectively. We have detected a long term drift of order 1%
FUV and 6% NUV over the mission. Astrometric precision is of order 0.5" RMS in
both bands. In this paper we provide the GALEX user with a broad overview of
the calibration issues likely to be confronted in the current release.
Improvements are likely as the GALEX mission continues into an extended phase
with a healthy instrument, no consumables, and increased opportunities for
guest investigations.Comment: Accepted to the ApJS (a special GALEX issue
Ongoing Formation of Bulges and Black Holes in the Local Universe: New Insights from GALEX
We analyze a volume-limited sample of massive bulge-dominated galaxies with
data from both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer
(GALEX) satellite. The galaxies have central velocity dispersions greater than
100 km/s and stellar surface mass densities that lie above the value where
galaxies transition from actively star forming to passive systems. The sample
is limited to redshifts 0.03<z<0.07. At these distances, the SDSS spectra
sample the light from the bulge-dominated central regions of the galaxies. The
GALEX NUV data provide high sensitivity to low rates of global star formation
in these systems. Our sample of bulge-dominated galaxies exhibits a much larger
dispersion in NUV-r colour than in optical g-r colour. Nearly all of the
galaxies with bluer NUV-r colours are AGN. Both GALEX images and SDSS colour
profiles demonstrate that the excess UV light is associated with an extended
disk. We find that galaxies with red outer regions almost never have a young
bulge or a strong AGN. Galaxies with blue outer regions have bulges and black
holes that span a wide range in age and accretion rate. Galaxies with young
bulges and strongly accreting black holes almost always have blue outer disks.
Our suggested scenario is one in which the source of gas that builds the bulge
and black hole is a low mass reservoir of cold gas in the disk.The presence of
this gas is a necessary, but not sufficient condition for bulge and black hole
growth. Some mechanism must transport this gas inwards in a time variable way.
As the gas in the disk is converted into stars, the galaxies will turn red, but
further gas infall can bring them back into the blue NUV-r sequence.(Abridged)Comment: 34 pages, 16 figures. Accepted for the GALEX special issue of ApJ
Clustering Properties of restframe UV selected galaxies I: the correlation length derived from GALEX data in the local Universe
We present the first measurements of the angular correlation function of
galaxies selected in the far (1530 A) and near (2310 A) Ultraviolet from the
GALEX survey fields overlapping SDSS DR5 in low galactic extinction regions.
The area used covers 120 sqdeg (GALEX - MIS) down to magnitude AB = 22,
yielding a total of 100,000 galaxies. The mean correlation length is ~ 3.7 \pm
0.6 Mpc and no significant trend is seen for this value as a function of the
limiting apparent magnitude or between the GALEX bands. This estimate is close
to that found from samples of blue galaxies in the local universe selected in
the visible, and similar to that derived at z ~ 3 for LBGs with similar rest
frame selection criteria. This result supports models that predict anti-biasing
of star forming galaxies at low redshift, and brings an additional clue to the
downsizing of star formation at z<1.Comment: Accepted for publication in GALEX Special ApJs, December 200
Clustering Properties of restframe UV selected galaxies II: Migration of Star Formation sites with cosmic time from GALEX and CFHTLS
We analyze the clustering properties of ultraviolet selected galaxies by
using GALEX-SDSS data at z<0.6 and CFHTLS deep u' imaging at z=1. These
datasets provide a unique basis at z< 1 which can be directly compared with
high redshift samples built with similar selection criteria. We discuss the
dependence of the correlation function parameters (r0, delta) on the
ultraviolet luminosity as well as the linear bias evolution. We find that the
bias parameter shows a gradual decline from high (b > 2) to low redshift (b ~
0.79^{+0.1}_{-0.08}). When accounting for the fraction of the star formation
activity enclosed in the different samples, our results suggest that the bulk
of star formation migrated from high mass dark matter halos at z>2 (10^12 <
M_min < 10^13 M_sun, located in high density regions), to less massive halos at
low redshift (M_min < 10^12 M_sun, located in low density regions). This result
extends the ``downsizing'' picture (shift of the star formation activity from
high stellar mass systems at high z to low stellar mass at low z) to the dark
matter distribution.Comment: Accepted for Publication in the Special GALEX Ap. J. Supplement,
December 2007 Version with full resolution fig1 available at
http://taltos.pha.jhu.edu/~sebastien/papers/Galex_p2.ps.g
Statistical Properties of the GALEX/SDSS matched source catalogs, and classification of the UV sources
We use the Galaxy Evolution Explorer (GALEX) Medium and All-Sky-Imaging
Survey (MIS & AIS) data from the first public data release (GR1), matched to
the Sloan Digital Sky Survey (SDSS) DR3 catalog, to perform source
classification. The GALEX surveys provide photometry in far- and near-UV bands
and the SDSS in five optical bands (u,g,r,i,z). The GR1/DR3 overlapping areas
are 363[83]deg^2 for the GALEX AIS[MIS], for sources within the 0.5deg central
area of the GALEX fields. Our sample covers mostly |b|>30deg galactic
latitudes. We present statistical properties of the GALEX/SDSS matched sources
catalog, containing >2x10^6 objects detected in at least one UV band. We
classify the matched sources by comparing the seven-band photometry to model
colors constructed for different classes of astrophysical objects. For sources
with photometric errors <0.3 mag, the corresponding typical AB-magnitude limits
are m_FUV~21.5, m_NUV~22.5 for AIS, and m_FUV~24, m_NUV~24.5 for MIS. At AIS
depth, the number of Galactic and extragalactic objects are comparable, but the
latter predominate in the MIS. Based on our stellar models, we estimate the
GALEX surveys detect hot White Dwarfs throughout the Milky Way halo (down to a
radius of 0.04 R_sun at MIS depth), providing an unprecedented improvement in
the Galactic WD census. Their observed surface density is consistent with Milky
Way model predictions. We also select low-redshift QSO candidates, extending
the known QSO samples to lower magnitudes, and providing candidates for
detailed z~1 follow-up investigations. SDSS optical spectra available for a
large subsample confirm the classification for the photometrically selected
candidates with 97% purity for single hot stars, ~45%(AIS)/31%(MIS) for
binaries containing a hot star and a cooler companion, and about 85% for QSOs.Comment: 33 pages, 11 figures, accepted for the GALEX special issue of ApJS.
For a version with full resolution figures see
http://dolomiti.pha.jhu.edu/publgoto.htm
The UV-Optical Color Dependence of Galaxy Clustering in the Local Universe
We measure the UV-optical color dependence of galaxy clustering in the local
universe. Using the clean separation of the red and blue sequences made
possible by the NUV - r color-magnitude diagram, we segregate the galaxies into
red, blue and intermediate "green" classes. We explore the clustering as a
function of this segregation by removing the dependence on luminosity and by
excluding edge-on galaxies as a means of a non-model dependent veto of highly
extincted galaxies. We find that \xi (r_p, \pi) for both red and green galaxies
shows strong redshift space distortion on small scales -- the "finger-of-God"
effect, with green galaxies having a lower amplitude than is seen for the red
sequence, and the blue sequence showing almost no distortion. On large scales,
\xi (r_p, \pi) for all three samples show the effect of large-scale streaming
from coherent infall. On scales 1 Mpc/h < r_p < 10 Mpc/h, the projected
auto-correlation function w_p(r_p) for red and green galaxies fits a power-law
with slope \gamma ~ 1.93 and amplitude r_0 ~ 7.5 and 5.3, compared with \gamma
~ 1.75 and r_0 ~ 3.9 Mpc/h for blue sequence galaxies. Compared to the
clustering of a fiducial L* galaxy, the red, green, and blue have a relative
bias of 1.5, 1.1, and 0.9 respectively. The w_p(r_p) for blue galaxies display
an increase in convexity at ~ 1 Mpc/h, with an excess of large scale
clustering. Our results suggest that the majority of blue galaxies are likely
central galaxies in less massive halos, while red and green galaxies have
larger satellite fractions, and preferentially reside in virialized structures.
If blue sequence galaxies migrate to the red sequence via processes like
mergers or quenching that take them through the green valley, such a
transformation may be accompanied by a change in environment in addition to any
change in luminosity and color.Comment: accepted by MNRA
The UV-Optical Color Magnitude Diagram II: Physical Properties and Morphological Evolution On and Off of a Star-Forming Sequence
We use the UV-optical color magnitude diagram in combination with
spectroscopic and photometric measurements derived from the SDSS spectroscopic
sample to measure the distribution of galaxies in the local universe (z<0.25)
and their physical properties as a function of specific star formation rate
(SSFR) and stellar mass. Throughout this study our emphasis is on the
properties of galaxies on and off of a local "star-forming sequence." We
discuss how the physical characteristics of galaxies along this sequence are
related to scaling relations typically derived for galaxies of different
morphological types. We find, among other trends that our measure of the star
formation rate surface density is nearly constant along this sequence. We
discuss this result and implications for galaxies at higher redshift. For the
first time, we report on measurements of the local UV luminosity function
versus galaxy structural parameters as well as inclination. We also split our
sample into disk-dominated and bulge-dominated subsamples using the i-band
Sersic index and find that disk-dominated galaxies occupy a very tight locus in
SSFR vs. stellar mass space while bulge-dominated galaxies display a much
larger spread of SSFR at fixed stellar mass. A significant fraction of galaxies
with SSFR and SF surface density above those on the "star-forming sequence" are
bulge-dominated. We can use our derived distribution functions to ask whether a
significant fraction of these galaxies may be experiencing a final episode of
star formation (possibly induced by a merger or other burst), soon to be
quenched, by determining whether this population can explain the growth rate of
the non-star-forming galaxies on the "red sequence." (Abridged)Comment: 30 pages, 28 figures, scheduled to appear as part of the GALEX
Special Ap.J.Suppl., December, 2007 (29 papers