166 research outputs found
Using oblique decision trees for the morphological classification of galaxies
We discuss the application of a class of machine learning algorithms known as decision trees to the process of galactic classification. In particular, we explore the application of oblique decision trees induced with different impurity measures to the problem of classifying galactic morphology data provided by Storrie-Lombardi et al.(1992). Our results are compared to those obtained by a neural network classifier created by Storrie-Lombardi et al, and we show that the two methodologies are comparable. We conclude with a demonstration that the original data can be easily classified into less well-defined categories
The Morphologically Divided Redshift Distribution of Faint Galaxies
We have constructed a morphologically divided redshift distribution of faint
field galaxies using a statistically unbiased sample of 196 galaxies brighter
than I = 21.5 for which detailed morphological information (from the Hubble
Space Telescope) as well as ground-based spectroscopic redshifts are available.
Galaxies are classified into 3 rough morphological types according to their
visual appearance (E/S0s, Spirals, Sdm/dE/Irr/Pec's), and redshift
distributions are constructed for each type. The most striking feature is the
abundance of low to moderate redshift Sdm/dE/Irr/Pec's at I < 19.5. This
confirms that the faint end slope of the luminosity function (LF) is steep
(alpha < -1.4) for these objects. We also find that Sdm/dE/Irr/Pec's are fairly
abundant at moderate redshifts, and this can be explained by strong luminosity
evolution. However, the normalization factor (or the number density) of the LF
of Sdm/dE/Irr/Pec's is not much higher than that of the local LF of
Sdm/dE/Irr/Pec's. Furthermore, as we go to fainter magnitudes, the abundance of
moderate to high redshift Irr/Pec's increases considerably. This cannot be
explained by strong luminosity evolution of the dwarf galaxy populations alone:
these Irr/Pec's are probably the progenitors of present day ellipticals and
spiral galaxies which are undergoing rapid star formation or merging with their
neighbors. On the other hand, the redshift distributions of E/S0s and spirals
are fairly consistent those expected from passive luminosity evolution, and are
only in slight disagreement with the non-evolving model.Comment: 11 pages, 4 figures (published in ApJ
Luminosity Functions of Elliptical Galaxies at z < 1.2
The luminosity functions of E/S0 galaxies are constructed in 3 different
redshift bins (0.2 < z < 0.55, 0.55 < z < 0.8, 0.8 < z < 1.2), using the data
from the Hubble Space Telescope Medium Deep Survey (HST MDS) and other HST
surveys. These independent luminosity functions show the brightening in the
luminosity of E/S0s by about 0.5~1.0 magnitude at z~1, and no sign of
significant number evolution.
This is the first direct measurement of the luminosity evolution of E/S0
galaxies, and our results support the hypothesis of a high redshift of
formation (z > 1) for elliptical galaxies, together with weak evolution of the
major merger rate at z < 1.Comment: To be published in ApJ Letters, 4 pages, AAS Latex, 4 figures, and 2
table
The Top Ten List of Gravitational Lens Candidates from the HST Medium Deep Survey
A total of 10 good candidates for gravitational lensing have been discovered
in the WFPC2 images from the HST Medium Deep Survey (MDS) and archival primary
observations. These candidate lenses are unique HST discoveries, i.e. they are
faint systems with sub-arcsecond separations between the lensing objects and
the lensed source images. Most of them are difficult objects for ground-based
spectroscopic confirmation or for measurement of the lens and source redshifts.
Seven are ``strong lens'' candidates which appear to have multiple images of
the source. Three are cases where the single image of the source galaxy has
been significantly distorted into an arc. The first two quadruply lensed
candidates were reported in Ratnatunga et al 1995 (ApJL, 453, L5) We report on
the subsequent eight candidates and describe them with simple models based on
the assumption of singular isothermal potentials. Residuals from the simple
models for some of the candidates indicate that a more complex model for the
potential will probably be required to explain the full structural detail of
the observations once they are confirmed to be lenses. We also discuss the
effective survey area which was searched for these candidate lens objects.Comment: 26 pages including 12 figures and 10 tables. AJ Vol. 117, No.
Compact Nuclei in Galaxies at Moderate Redshift:II. Their Nature and Implications for the AGN Luminosity Function
This study explores the space density and properties of active galaxies to
z=0.8. We have investigated the frequency and nature of unresolved nuclei in
galaxies at moderate redshift as indicators of nuclear activity such as Active
Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged
galaxies with multi-component models using maximum likelihood estimate
techniques to determine the best model fit. We select those galaxies requiring
an unresolved point-source component in the galaxy nucleus, in addition to a
disk and/or bulge component, to adequately model the galaxy light. We have
searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies
containing compact nuclei. In our survey of 1033 galaxies, the fraction
containing an unresolved nuclear component greater than 5% of the total galaxy
light is 9+/-1% corrected for incompleteness. In this second of two papers in
this series, we discuss the nature of the compact nuclei and their hosts.
We present the upper limit luminosity function (LF) for low-luminosity AGN
(LLAGN) in two redshift bins to z=0.8. Mild number density evolution is
detected for nuclei at -18 -16
and this flatness, combined with the increase in number density, is
inconsistent with pure luminosity evolution. Based on the amount of density
evolution observed for these objects, we find that almost all present-day
spiral galaxies could have hosted a LLAGN at some point in their lives. We also
comment on the likely contribution of these compact nuclei to the soft X-ray
background.Comment: 50 pages, 14 figures, to appear in ApJ, April 199
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