17 research outputs found
A Quantitative Evaluation of the Galaxy Component of COSMOS and APM Catalogs
We have carried out an independent quantitative evaluation of the galaxy
component of the "COSMOS/UKST Southern Sky Object Catalogue" (SSC) and the
"APM/UKST J Catalogue" (APM). Using CCD observations our results corroborate
the accuracy of the photometry of both catalogs, which have an overall
dispersion of about 0.2 mag in the range 17 <= b_J <= 21.5. The SSC presents
externally calibrated galaxy magnitudes that follow a linear relation, while
the APM instrumental magnitudes of galaxies, only internally calibrated by the
use of stellar profiles, require second-order corrections. The completeness of
both catalogs in a general field falls rapidly fainter than b_J = 20.0, being
slightly better for APM. The 90% completeness level of the SSC is reached
between b_J = 19.5 and 20.0, while for APM this happens between b_J = 20.5 and
21.0. Both SSC and APM are found to be less complete in a galaxy cluster field.
Galaxies misclassified as stars in the SSC receive an incorrect magnitude
because the stellar ones take saturation into account besides using a different
calibration curve. In both cases, the misclassified galaxies show a large
diversity of colors that range from typical colors of early-types to those of
blue star-forming galaxies. A possible explanation for this effect is that it
results from the combination of low sampling resolutions with properties of the
image classifier for objects with characteristic sizes close to the
instrumental resolution. We find that the overall contamination by stars
misclassified as galaxies is < 5% to b_J = 20.5, as originally estimated for
both catalogs. Although our results come from small areas of the sky, they are
extracted from two different plates and are based on the comparison with two
independent datasets.Comment: 14 pages of text and tables, 8 figures; to be published in the
Astronomical Journal; for a single postscript version file see
ftp://danw.on.br/outgoing/caretta/caretta.p
Mid-infrared selection of AGN
Since a large fraction of active galactic nuclei (AGN) is missed in common
UV-excess surveys and is even hard to find in radio, near-IR and X-ray surveys,
we have used a new AGN selection technique which is expected to be not affected
by extinction. Within the scientific verification of the ISOCAM Parallel Survey
at 6.7 micron we have discovered objects with exceptional mid-infrared (MIR)
emission. They are essentially not detected on IRAS-ADDSCANs and only very few
of them show up in the NVSS and FIRST radio surveys. Various colour criteria of
the 6.7 micron data with 2MASS and optical wavebands show that the sources
reach more extreme IR colours than the sources in the Hubble Deep Field-South
and the ELAIS survey. The comparison with known object types suggests that we
have found AGN with a pronounced MIR emission, probably due to circum-nuclear
dust. First results from optical spectroscopy of ten candidates corroborate
this interpretation showing four AGN, two reddened LINER and four extremely
reddened emission-line galaxies with MIR/FIR flux ratios higher than for known
pure starburst galaxies. The results will make a significant contribution to
the debate on the entire AGN population.Comment: 4 pages, 2 figures, accepted for publication as Letter in Astronomy &
Astrophysic
The Aquarius Superclusters - I. Identification of Clusters and Superclusters
We study the distribution of galaxies and galaxy clusters in a 10^deg x 6^deg
field in the Aquarius region. In addition to 63 clusters in the literature, we
have found 39 new candidate clusters using a matched-filter technique and a
counts-in-cells analysis. From redshift measurements of galaxies in the
direction of these cluster candidates, we present new mean redshifts for 31
previously unobserved clusters, while improved mean redshifts are presented for
35 other systems. About 45% of the projected density enhancements are due to
the superposition of clusters and/or groups of galaxies along the line of
sight, but we could confirm for 72% of the cases that the candidates are real
physical associations similar to the ones classified as rich galaxy clusters.
On the other hand, the contamination due to galaxies not belonging to any
concentration or located only in small groups along the line of sight is ~ 10%.
Using a percolation radius of 10 h^{-1} Mpc (spatial density contrast of about
10), we detect two superclusters of galaxies in Aquarius, at z = 0.086 and at z
= 0.112, respectively with 5 and 14 clusters. The latter supercluster may
represent a space overdensity of about 160 times the average cluster density as
measured from the Abell et al. (1989) cluster catalog, and is possibly
connected to a 40 h^{-1} Mpc filament from z ~ 0.11 to 0.14.Comment: LateX text (21 pages) and 12 (ps/eps/gif) figures; figures 5a, 5b and
6 are not included in the main LateX text; to be published in the
Astronomical Journal, March issu
Galaxy Zoo: Dust in Spirals
We investigate the effect of dust on spiral galaxies by measuring the
inclination-dependence of optical colours for 24,276 well-resolved SDSS
galaxies visually classified in Galaxy Zoo. We find clear trends of reddening
with inclination which imply a total extinction from face-on to edge-on of 0.7,
0.6, 0.5 and 0.4 magnitudes for the ugri passbands. We split the sample into
"bulgy" (early-type) and "disky" (late-type) spirals using the SDSS fracdeV (or
f_DeV) parameter and show that the average face-on colour of "bulgy" spirals is
redder than the average edge-on colour of "disky" spirals. This shows that the
observed optical colour of a spiral galaxy is determined almost equally by the
spiral type (via the bulge-disk ratio and stellar populations), and reddening
due to dust. We find that both luminosity and spiral type affect the total
amount of extinction, with "disky" spirals at M_r ~ -21.5 mags having the most
reddening. This decrease of reddening for the most luminous spirals has not
been observed before and may be related to their lower levels of recent star
formation. We compare our results with the latest dust attenuation models of
Tuffs et al. We find that the model reproduces the observed trends reasonably
well but overpredicts the amount of u-band attenuation in edge-on galaxies. We
end by discussing the effects of dust on large galaxy surveys and emphasize
that these effects will become important as we push to higher precision
measurements of galaxy properties and their clustering.Comment: MNRAS in press. 25 pages, 22 figures (including an abstract comparing
GZ classifications with common automated methods for selecting disk/early
type galaxies in SDSS data). v2 corrects typos found in proof
The Luminosity Functions of the Galaxy Cluster MS1054-0321 at z=0.83 based on ACS Photometry
We present new measurements of the galaxy luminosity function (LF) and its
dependence on local galaxy density, color, morphology, and clustocentric radius
for the massive z=0.83 cluster MS1054-0321. Our analyses are based on imaging
performed with the ACS onboard the HST in the F606W, F775W and F850LP passbands
and extensive spectroscopic data obtained with the Keck LRIS. Our main results
are based on a spectroscopically selected sample of 143 cluster members with
morphological classifications derived from the ACS observations. Our three
primary findings are (1) the faint-end slope of the LF is steepest in the
bluest filter, (2) the LF in the inner part of the cluster (or highest density
regions) has a flatter faint-end slope, and (3) the fraction of early-type
galaxies is higher at the bright end of the LF, and gradually decreases toward
fainter magnitudes. These characteristics are consistent with those in local
galaxy clusters, indicating that, at least in massive clusters, the common
characteristics of cluster LFs are established at z=0.83. We also find a 2sigma
deficit of intrinsically faint, red galaxies (i-z>0.5, Mi>-19) in this cluster.
This trend may suggest that faint, red galaxies (which are common in z<0.1 rich
clusters) have not yet been created in this cluster at z=0.83. The
giant-to-dwarf ratio in MS1054-0321 starts to increase inwards of the virial
radius or when Sigma>30 Mpc^-2, coinciding with the environment where the
galaxy star formation rate and the morphology-density relation start to appear.
(abridged)Comment: ApJ in press, references update
Radial distribution of dust, stars, gas, and star-formation rate in DustPedia face-on galaxies
Aims. The purpose of this work is the characterization of the radial distribution of dust, stars, gas, and star-formation rate (SFR) in a sub-sample of 18 face-on spiral galaxies extracted from the DustPedia sample.
Methods. This study is performed by exploiting the multi-wavelength DustPedia database, from ultraviolet (UV) to sub-millimeter bands, in addition to molecular (12CO) and atomic (Hi) gas maps and metallicity abundance information available in the literature. We fitted the surface-brightness profiles of the tracers of dust and stars, the mass surface-density profiles of dust, stars, molecular gas, and total gas, and the SFR surface-density profiles with an exponential curve and derived their scale-lengths. We also developed a method to solve for the CO-to-H2 conversion factor (αCO) per galaxy by using dust- and gas-mass profiles.
Results. Although each galaxy has its own peculiar behavior, we identified a common trend of the exponential scale-lengths versus wavelength. On average, the scale-lengths normalized to the B-band 25 mag/arcsec2 radius decrease from UV to 70 ÎŒm, from 0.4 to 0.2, and then increase back up to ~0.3 at 500 microns. The main result is that, on average, the dust-mass surface-density scale-length is about 1.8 times the stellar one derived from IRAC data and the 3.6 ÎŒm surface brightness, and close to that in the UV. We found a mild dependence of the scale-lengths on the Hubble stage T: the scale-lengths of the Herschel bands and the 3.6 ÎŒm scale-length tend to increase from earlier to later types, the scale-length at 70 ÎŒm tends to be smaller than that at longer sub-mm wavelength with ratios between longer sub-mm wavelengths and 70 ÎŒm that decrease with increasing T. The scale-length ratio of SFR and stars shows a weak increasing trend towards later types. Our αCO determinations are in the range (0.3â9) Mâ pc-2 (K km s-1)-1, almost invariant by using a fixed dust-to-gas ratio mass (DGR) or a DGR depending on metallicity gradient