125 research outputs found
The AMIGA sample of isolated galaxies. V. Quantification of the isolation
The AMIGA project aims to build a well defined and statistically significant
reference sample of isolated galaxies in order to estimate the environmental
effects on the formation and evolution of galaxies. The goal of this paper is
to provide a measure of the environment of the isolated galaxies in the AMIGA
sample, quantifying the influence of the candidate neighbours identified in our
previous work and their potential effects on the evolution of the primary
galaxies. Here we provide a quantification of the isolation degree of the
galaxies in this sample. Our starting sample is the Catalogue of Isolated
Galaxies (CIG). We used two parameters to estimate the influence exerted by the
neighbour galaxies on the CIG galaxy: the local number density of neighbour
galaxies and the tidal strength affecting the CIG galaxy. We show that both
parameters together provide a comprehensive picture of the environment. For
comparison, those parameters have also been derived for galaxies in denser
environments such as triplets, groups and clusters. The CIG galaxies show a
continuous spectrum of isolation, as quantified by the two parameters, from
very isolated to interacting. The fraction of CIG galaxies whose properties are
expected to be influenced by the environment is however low (159 out of 950
galaxies). The isolated parameters derived for the comparsion samples gave
higher values than for the CIG and we found clear differences for the average
values of the 4 samples considered, proving the sensitivity of these
parameters. The environment of the galaxies in the CIG has been characterised,
using two complementary parameters quantifying the isolation degree, the local
number density of the neighbour galaxies and the tidal forces affecting the
isolated galaxies. (Abridged)Comment: 10 pages, 12 figures, proposed for acceptance A&
Hubble Space Telescope images of submillimeter sources: large, irregular galaxies at high redshift
We present new Hubble Space Telescope STIS, high-resolution optical imaging
of a sample of 13 submillimeter (submm) luminous galaxies, for which the
optical emission has been pinpointed either through radio-1.4 GHz or millimeter
interferometry. We find a predominance of irregular and complex morphologies in
the sample, suggesting that mergers are likely common for submm galaxies. The
component separation in these objects are on average a factor two larger than
local galaxies with similarly high bolometric luminosities. The sizes and star
formation rates of the submm galaxies are consistent with the maximal star
formation rate densities of 20 Msun kpc^{-2} in local starburst galaxies
(Lehnert & Heckman 1996). We derive quantitative morphological information for
the optical galaxies hosting the submm emission; total and isophotal
magnitudes, Petrosian radius, effective radius, concentration, aspect ratio,
surface brightness, and asymmetry. We compare these morphological indices with
those of other galaxies lying within the same STIS images. Most strikingly, we
find ~70% of the submm galaxies to be extraordinarily large and elongated
relative to the field population, regardless of optical magnitude. Comparison
of the submm galaxy morphologies with those of optically selected galaxies at
z~2-3 reveal the submm galaxies to be a morphologically distinct population,
with generally larger sizes, higher concentrations and more prevalent
major-merger configurations.Comment: 16 pages, 6 figures, scheduled for ApJ, v599, Dec10, 2003. Minor
edits. For version with higher resolution figures, see
http://www.submm.caltech.edu/~schapman/ms_v3.ps.g
Morphological number-count and redshift distributions to I < 26 from the Hubble Deep Field: Implications for the evolution of Ellipticals, Spirals and Irregulars
We combine the photometric redshift data of Fernandez-Soto et al. (1997) with
the morphological data of Odewahn et al. (1996) for all galaxies with I < 26.0
detected in the Hubble Deep Field. From this combined catalog we generate the
morphological galaxy number-counts and corresponding redshift distributions and
compare these to the predictions of high normalization zero- and passive-
evolution models. From this comparison we conclude the following: (1) E/S0s are
seen in numbers and over a redshift range consistent with zero- or minimal
passive- evolution to I = 24. Beyond this limit fewer E/S0s are observed than
predicted implying a net negative evolutionary process --- luminosity dimming,
disassembly or masking by dust --- at I > 24. (2) Spiral galaxies are present
in numbers consistent with zero- evolution predictions to I = 22. Beyond this
magnitude some net- positive evolution is required. Although the number-counts
are consistent with the passive-evolution predictions to I=26.0 the redshift
distributions favor number AND luminosity evolution. (3) There is no obvious
explanation for the late-type/irregular class and this category requires
further subdivision. While a small fraction of the population lies at low
redshift (i.e. true irregulars), the majority lie at redshifts, 1 < z < 3. At z
> 1.5 mergers are frequent and, taken in conjunction with the absence of normal
spirals at z > 2, the logical inference is that they represent the progenitors
of normal spirals forming via hierarchical merging.Comment: Accepted for publication in ApJ Letters, colour plates available from
http://www.phys.unsw.edu.au/~spd/bib.htm
Data-Mining a Large Digital Sky Survey: From the Challenges to the Scientific Results
The analysis and an efficient scientific exploration of the Digital Palomar
Observatory Sky Survey (DPOSS) represents a major technical challenge. The
input data set consists of 3 Terabytes of pixel information, and contains a few
billion sources. We describe some of the specific scientific problems posed by
the data, including searches for distant quasars and clusters of galaxies, and
the data-mining techniques we are exploring in addressing them.
Machine-assisted discovery methods may become essential for the analysis of
such multi-Terabyte data sets. New and future approaches involve unsupervised
classification and clustering analysis in the Giga-object data space, including
various Bayesian techniques. In addition to the searches for known types of
objects in this data base, these techniques may also offer the possibility of
discovering previously unknown, rare types of astronomical objects.Comment: Invited paper, to appear in Applications of Digital Image Processing
XX, ed. A. Tescher, Proc. S.P.I.E. vol. 3164, in press; 10 pages, a
self-contained TeX file, and 3 separate postscript figure
The Axis Ratio Distribution of Local and Distant Galaxies
Surface photometry from 16 HST/WFPC2 fields in the I(F814W) filter is used to
derive the distribution of apparent axis ratios for galaxies in progressively
fainter magnitude intervals for I<25. We assess the systematic and accidental
errors in ellipticity measurements as a function of image resolution and
signal-to-noise ratio, and statistically correct for the effect of cosmological
surface brightness dimming on our isophotal measurements. The axis ratio
distribution for the local galaxy population was computed using logR
measurements for 1569 RC3 galaxies with Bt<13 mag. Nonparametric tests are used
to show that our distant samples, in the redshift range 0.1<z<1.5, are not
statistically different from the local sample. We present image montages of
galaxies selected randomly from different axis ratio and apparent magnitude
ranges and discuss the evolutionary consequences of the lack of a strong
difference between the ellipticity distributions in near and far data sets.Comment: LaTex, 35 pages, 8 figures, accepted for Dec97 A
The AMIGA sample of isolated galaxies. IV. A catalogue of neighbours around isolated galaxies
Studies of the effects of environment on galaxy properties and evolution
require well defined control samples. Such isolated galaxy samples have up to
now been small or poorly defined. The AMIGA project (Analysis of the
interstellar Medium of Isolated GAlaxies) represents an attempt to define a
statistically useful sample of the most isolated galaxies in the local (z <
0.05) Universe. A suitable large sample for the AMIGA project already exists,
the Catalogue of Isolated Galaxies (CIG, Karachentseva 1973; 1050 galaxies),
and we use this sample as a starting point to refine and perform a better
quantification of its isolation properties. Digitised POSS-I E images were
analysed out to a minimum projected radius R > 0.5 Mpc around 950 CIG galaxies
(those within Vr = 1500 km s-1 were excluded). We identified all galaxy
candidates in each field brighter than B = 17.5 with a high degree of
confidence using the LMORPHO software. We generated a catalogue of
approximately 54 000 potential neighbours (redshifts exist for 30% of this
sample). Six hundred sixty-six galaxies pass and two hundred eighty-four fail
the original CIG isolation criterion. The available redshift data confirm that
our catalogue involves a largely background population rather than physically
associated neighbours. We find that the exclusion of neighbours within a factor
of four in size around each CIG galaxy, employed in the original isolation
criterion, corresponds to Delta Vr ~ 18000 km s-1 indicating that it was a
conservative limit. Galaxies in the CIG have been found to show different
degrees of isolation. We conclude that a quantitative measure of this is
mandatory. It will be the subject of future work based on the catalogue of
neighbours obtained here.Comment: Accepted by A&A, 10 pages, 8 figures, 4 table
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