4,312 research outputs found
The Properties of Satellite Galaxies in External Systems. I. Morphology and Structural Parameters
We present the first results of an ongoing project to study the
morphological, kinematical, dynamical, and chemical properties of satellite
galaxies of external giant spiral galaxies. The sample of objects has been
selected from the catalogue by Zaritsky et al. (1997). The paper analyzes the
morphology and structural parameters of a subsample of 60 such objects. The
satellites span a great variety of morphologies and surface brightness
profiles. About two thirds of the sample are spirals and irregulars, the
remaining third being early-types. Some cases showing interaction between pairs
of satellites are presented and briefly discussed.Comment: Accepted for publication in Astrophys. Journal Supp. Se
Anisotropic Distribution of SDSS Satellite Galaxies: Planar (not Polar) Alignment
The distribution of satellite galaxies relative to isolated host galaxies in
the Sloan Digital Sky Survey (SDSS) is investigated. Host-satellite systems are
selected using three different methods, yielding samples of ~3300, ~1600, and
\~950 satellites. In the plane of the sky, the distributions of all three
samples show highly significant deviations from circular symmetry (> 99.99%, >
99.99%, and 99.79% confidence levels, respectively), and the degree of
anisotropy is a strong function of the projected radius, r_p, at which the
satellites are found. For r_p < 100 kpc, the SDSS satellites are aligned
preferentially with the major axes of the hosts. This is in stark contrast to
the Holmberg effect, in which satellites are aligned with the minor axes of
host galaxies. The degree of anisotropy in the distribution of the SDSS
satellites decreases with r_p and is consistent with an isotropic distribution
at of order the 1-sigma level for 250 kpc < r_p < 500 kpc.Comment: ApJ Letters (in press); Discussion section substantially revised,
SDSS DR3 included in the analysis, no significant changes to the result
A Hubble Space Telescope Snapshot Survey of Dynamically Close Galaxy Pairs in the CNOC2 Redshift Survey
We compare the structural properties of two classes of galaxies at
intermediate redshift: those in dynamically close galaxy pairs, and those which
are isolated. Both samples are selected from the CNOC2 Redshift Survey, and
have redshifts in the range 0.1 < z <0.6. Hubble Space Telescope WFPC2 images
were acquired as part of a snapshot survey, and were used to measure bulge
fraction and asymmetry for these galaxies. We find that paired and isolated
galaxies have identical distributions of bulge fractions. Conversely, we find
that paired galaxies are much more likely to be asymmetric (R_T+R_A >= 0.13)
than isolated galaxies. Assuming that half of these pairs are unlikely to be
close enough to merge, we estimate that 40% +/- 11% of merging galaxies are
asymmetric, compared with 9% +/- 3% of isolated galaxies. The difference is
even more striking for strongly asymmetric (R_T+R_A >= 0.16) galaxies: 25% +/-
8% for merging galaxies versus 1% +/- 1% for isolated galaxies. We find that
strongly asymmetric paired galaxies are very blue, with rest-frame B-R colors
close to 0.80, compared with a mean (B-R)_0 of 1.24 for all paired galaxies. In
addition, asymmetric galaxies in pairs have strong [OII]3727 emission lines. We
conclude that close to half of the galaxy pairs in our sample are in the
process of merging, and that most of these mergers are accompanied by triggered
star formation.Comment: Accepted for publication in the Astronomical Journal. 40 pages,
including 15 figures. For full resolution version, please see
http://www.trentu.ca/physics/dpatton/hstpairs
Dependence of Spiral Galaxy Distribution on Viewing Angle in RC3
The normalized inclination distributions are presented for the spiral
galaxies in RC3. The results show that, except for the bin of
-, in which the apparent minor isophotal diameters that
are used to obtain the inclinations, are affected by the central bulges, the
distributions for Sa, Sab, Scd and Sd are well consistent with the Monte-Carlo
simulation of random inclinations within 3-, and Sb and Sbc almost, but
Sc is different. One reason for the difference between the real distribution
and the Monte-Carlo simulation of Sc may be that some quite inclined spirals,
the arms of which are inherently loosely wound on the galactic plane and should
be classified to Sc galaxies, have been incorrectly classified to the earlier
ones, because the tightness of spiral arms which is one of the criteria of the
Hubble classification in RC3 is different between on the galactic plane and on
the tangent plane of the celestial sphere. Our result also implies that there
might exist biases in the luminosity functions of individual Hubble types if
spiral galaxies are only classified visually.Comment: 5 pages + 8 figures, LaTe
The Relationship Between Baryons and Dark Matter in Extended Galaxy Halos
The relationship between gas-rich galaxies and Ly-alpha absorbers is
addressed in this paper in the context of the baryonic content of galaxy halos.
Deep Arecibo HI observations are presented of two gas-rich spiral galaxies
within 125 kpc projected distance of a Ly-alpha absorber at a similar velocity.
The galaxies investigated are close to edge-on and the absorbers lie almost
along their major axes, allowing for a comparison of the Ly-alpha absorber
velocities with galactic rotation. This comparison is used to examine whether
the absorbers are diffuse gas rotating with the galaxies' halos, outflow
material from the galaxies, or intergalactic gas in the low redshift cosmic
web. The results indicate that if the gas resides in the galaxies' halos it is
not rotating with the system and possibly counter-rotating. In addition, simple
geometry indicates the gas was not ejected from the galaxies and there are no
gas-rich satellites detected down to 3.6 - 7.5 x 10^6 Msun, or remnants of
satellites to 5-6 x 10^{18} cm^{-2}. The gas could potentially be infalling
from large radii, but the velocities and distances are rather high compared to
the high velocity clouds around the Milky Way. The most likely explanation is
the galaxies and absorbers are not directly associated, despite the vicinity of
the spiral galaxies to the absorbers (58-77 kpc from the HI edge). The spiral
galaxies reside in a filament of intergalactic gas, and the gas detected by the
absorber has not yet come into equilibrium with the galaxy. These results also
indicate that the massive, extended dark matter halos of spiral galaxies do not
commonly have an associated diffuse baryonic component at large radii.Comment: Accepted by AJ, 33 pages preprint format, see
http://www.astro.lsa.umich.edu/~mputman/putman1.pdf for a higher resolution
versio
The Relationship Between Stellar Light Distributions of Galaxies and their Formation Histories
A major problem in extragalactic astronomy is the inability to distinguish in
a robust, physical, and model independent way how galaxy populations are
related to each other and to their formation histories. A similar, but
distinct, and also long standing question is whether the structural appearances
of galaxies, as seen through their stellar light distributions, contain enough
physical information to offer this classification. We argue through the use of
240 images of nearby galaxies that three model independent parameters measured
on a single galaxy image reveal its major ongoing and past formation modes, and
can be used as a robust classification system. These parameters quantitatively
measure: the concentration (C), asymmetry (A) and clumpiness (S) of a galaxy's
stellar light distribution. When combined into a three dimensional `CAS' volume
all major classes of galaxies in various phases of evolution are cleanly
distinguished. We argue that these three parameters correlate with important
modes of galaxy evolution: star formation and major merging activity. This is
argued through the strong correlation of Halpha equivalent width and broad band
colors with the clumpiness parameter, the uniquely large asymmetries of 66
galaxies undergoing mergers, and the correlation of bulge to total light
ratios, and stellar masses, with the concentration index. As an obvious goal is
to use this system at high redshifts to trace evolution, we demonstrate that
these parameters can be measured, within a reasonable and quantifiable
uncertainty, with available data out to z ~ 3 using the Hubble Space Telescope
GOODS ACS and Hubble Deep Field images.Comment: ApJS, in press, 30 pages, Figures 15 and 16 are in color. For a full
resolution version, please go to http://www.astro.caltech.edu/~cc/cas.p
Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228
We develop the ``simulated extinction method'' to measure average foreground
Galactic extinction from field galaxy number-counts and colors. The method
comprises simulating extinction in suitable reference fields by changing the
isophotal detection limit. This procedure takes into account selection effects,
in particular, the change in isophotal detection limit (and hence in isophotal
magnitude completeness limit) with extinction, and the galaxy color--magnitude
relation.
We present a first application of the method to the HST WFPC2 images of the
gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields,
including the HDF, are used as reference to measure the average extinction
towards the GRB in the F606W passband. From the counts, we derive an average
extinction of A_V = 0.5 mag, but the dispersion of 0.4 mag between the
estimates from the different reference fields is significantly larger than can
be accounted by Poisson plus clustering uncertainties. Although the counts
differ, the average colors of the field galaxies agree well. The extinction
implied by the average color difference between the GRB field and the reference
galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from
the four reference fields of only 0.1 mag. All our estimates are in good
agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and
with the extinction of 0.78\pm0.12 measured by Schlegel et al. from maps of
dust IR emission. However, the discrepancy between the widely varying counts
and the very stable colors in these high-latitude fields is worth
investigating.Comment: 14 pages, 2 figures; submitted to the Astrophysical Journa
The Anisotropic Distribution of Galactic Satellites
We present a study of the spatial distribution of subhalos in galactic dark
matter halos using dissipationless cosmological simulations of the concordance
LCDM model. We find that subhalos are distributed anisotropically and are
preferentially located along the major axes of the triaxial mass distributions
of their hosts. The Kolmogorov-Smirnov probability for drawing our simulated
subhalo sample from an isotropic distribution is P_KS \simeq 1.5 \times
10^{-4}. An isotropic distribution of subhalos is thus not the correct null
hypothesis for testing the CDM paradigm. The nearly planar distribution of
observed Milky Way (MW) satellites is marginally consistent (probability \simeq
0.02) with being drawn randomly from the subhalo distribution in our
simulations. Furthermore, if we select the subhalos likely to be luminous, we
find a distribution that is consistent with the observed MW satellites. In
fact, we show that subsamples of the subhalo population with a
centrally-concentrated radial distribution, similar to that of the MW dwarfs,
typically exhibit a comparable degree of planarity. We explore the origin of
the observed subhalo anisotropy and conclude that it is likely due to (1)
preferential accretion of subhalos along filaments, often closely aligned with
the major axis of the host halo, and (2) evolution of satellite orbits within
the prolate, triaxial potentials typical of CDM halos. Agreement between
predictions and observations requires the major axis of the outer dark matter
halo of the Milky Way to be nearly perpendicular to the disk. We discuss
possible observational tests of such disk-halo alignment with current large
galaxy surveys.Comment: 14 pages (including appendix), 9 figures. Accepted for Publication in
ApJ. Minor changes to reflect referee's comment
Loose Groups of Galaxies in the Las Campanas Redshift Survey
A ``friends-of-friends'' percolation algorithm has been used to extract a
catalogue of dn/n = 80 density enhancements (groups) from the six slices of the
Las Campanas Redshift Survey (LCRS). The full catalogue contains 1495 groups
and includes 35% of the LCRS galaxy sample. A clean sample of 394 groups has
been derived by culling groups from the full sample which either are too close
to a slice edge, have a crossing time greater than a Hubble time, have a
corrected velocity dispersion of zero, or contain a 55-arcsec ``orphan'' (a
galaxy with a mock redshift which was excluded from the original LCRS redshift
catalogue due to its proximity to another galaxy -- i.e., within 55 arcsec).
Median properties derived from the clean sample include: line-of-sight velocity
dispersion sigma_los = 164km/s, crossing time t_cr = 0.10/H_0, harmonic radius
R_h = 0.58/h Mpc, pairwise separation R_p = 0.64/h Mpc, virial mass M_vir =
(1.90x10^13)/h M_sun, total group R-band luminosity L_tot = (1.30x10^11)/h^2
L_sun, and R-band mass-to-light ratio M/L = 171h M_sun/L_sun; the median number
of observed members in a group is 3.Comment: 32 pages of text, 27 figures, 7 tables. Figures 1, 4, 6, 7, and 8 are
in gif format. Tables 1 and 3 are in plain ASCII format (in paper source) and
are also available at http://www-sdss.fnal.gov:8000/~dtucker/LCLG . Accepted
for publication in the September 2000 issue of ApJ
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