5,304 research outputs found
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
Tidally-Triggered Star Formation in Close Pairs of Galaxies
We analyze new optical spectra of a sample of 502 galaxies in close pairs and
n-tuples, separated by <= 50/h kpc. We extracted the sample objectively from
the CfA2 redshift survey, without regard to the surroundings of the tight
systems. We probe the relationship between star formation and the dynamics of
the systems of galaxies. The equivalent widths of H\alpha (EW(H\alpha) and
other emission lines anti-correlate strongly with pair spatial separation
(\Delta D) and velocity separation. We use the measured EW(H\alpha) and the
starburst models of Leitherer et al. to estimate the time since the most recent
burst of star for- mation began for each galaxy. In the absence of a large
contribution from an old stellar population to the continuum around H\alpha,
the observed \Delta D -- EW(H\alpha) correlation signifies that starbursts with
larger separations on the sky are, on average, older. By matching the dynamical
timescale to the burst timescale, we show that the data support a simple
picture in which a close pass initiates a starburst; EW(H\alpha) decreases with
time as the pair separation increases, accounting for the anti-correlation.
This picture leads to a method for measuring the duration and the initial mass
function of interaction-induced starbursts: our data are compatible with the
starburst and orbit models in many respects, as long as the starburst lasts
longer than \sim10^8 years and the delay between the close pass and the
initiation of the starburst is less than a few \times 10^7 years. If there is
no large contribution from an old stellar population to the continuum around
H\alpha the Miller-Scalo and cutoff (M <= 30 M_\sun) Salpeter initial mass
functions fit the data much better than a standard Salpeter IMF. (Abridged.)Comment: 43 pages, 22 figures, to appear in the ApJ; we correct an error which
had minor effects on numerical values in the pape
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 Geneva-Copenhagen Survey of the Solar Neighbourhood
We report on a new survey of metallicities, ages, and Galactic orbits for a complete, magnitude-limited, and kinematically unbiased all-sky sample of 16 682 nearby F- and G-dwarfs. Our ⌠63 000 new, accurate radial velocities for nearly 13 500 of the stars, combined with Hipparcos parallaxes and Tycho-2 proper motions, complete the kinematic data for 14 139 stars and allow us to identify most of the binary stars in the sample. Isochrone ages have been determined whenever reliable results are possible, with particular attention to realistic error estimates. Among the basic properties of the Galactic disk that can be reinvestigated from our data are the metallicity distribution of G-dwarfs and the age-metallicity and age-velocity relations of the solar neighbourhood. We confirm the lack of metal-poor G-dwarfs relative to classical model predictions (the 'G-dwarf problem'), the near-constancy of the mean metallicity since the formation of the thin disk, and the appearance of the kinematic signature of the thick disk ⌠10 Gyr ag
An HST/WFPC2 Snapshot Survey of 2MASS-Selected Red QSOs
Using simple infrared color selection, 2MASS has found a large number of red,
previously unidentified, radio-quiet QSOs. Although missed by UV/optical
surveys, the 2MASS QSOs have K-band luminosities that are comparable to
"classical" QSOs. This suggests the possible discovery of a previously
predicted large population of dust-obscured radio-quiet QSOs. We present the
results of an imaging survey of 29 2MASS QSOs observed with WFPC2 onboard the
Hubble Space Telescope. I-band images, which benefit from the relative
faintness of the nuclei at optical wavelengths, are used to characterize the
host galaxies, measure the nuclear contribution to the total observed I-band
emission, and to survey the surrounding environments. The 2MASS QSOs are found
to lie in galaxies with a variety of morphologies, luminosities, and dynamical
states, not unlike those hosting radio-quiet PG QSOs. Our analysis suggests
that the extraordinary red colors of the 2MASS QSOs are caused by extinction of
an otherwise typical QSO spectrum due to dust near the nucleus.Comment: 23 pages including 9 figures and 7 tables, accepted for publication
in ApJ, higher resolution HST images at:
http://shapley.as.arizona.edu/~amarble/papers/twomq
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
HST/STIS Imaging of the Host Galaxy of GRB980425/SN1998bw
We present HST/STIS observations of ESO 184-G82, the host galaxy of the
gamma-ray burst GRB 980425 associated with the peculiar Type Ic supernova
SN1998bw. ESO 184-G82 is found to be an actively star forming SBc sub-luminous
galaxy. We detect an object consistent with being a point source within the
astrometric uncertainty of 0.018 arcseconds of the position of the supernova.
The object is located inside a star-forming region and is at least one
magnitude brighter than expected for the supernova based on a simple
radioactive decay model. This implies either a significant flattening of the
light curve or a contribution from an underlying star cluster.Comment: 12 pages, 2 figures, AASTeX v5.02 accepted for publication in ApJ
Letter
Transformation of Morphology and Luminosity Classes of the SDSS Galaxies
We present a unified picture on the evolution of galaxy luminosity and
morphology. Galaxy morphology is found to depend critically on the local
environment set up by the nearest neighbor galaxy in addition to luminosity and
the large scale density. When a galaxy is located farther than the virial
radius from its closest neighbor, the probability for the galaxy to have an
early morphological type is an increasing function only of luminosity and the
local density due to the nearest neighbor (). The tide produced by the
nearest neighbor is thought to be responsible for the morphology transformation
toward the early type at these separations. When the separation is less than
the virial radius, i.e. when , its morphology
depends also on the neighbor's morphology and the large-scale background
density over a few Mpc scales () in addition to luminosity and
. The early type probability keeps increasing as increases if
its neighbor is an early type. But the probability decreases as
increases when the neighbor is a late type. The cold gas streaming from the
late type neighbor can be the reason for the morphology transformation toward
late type. The overall early-type fraction increases as increases
when . This can be attributed to the hot halo gas
of the neighbor which is confined by the pressure of the ambient medium held by
the background mass. We have also found that galaxy luminosity depends on
, and that the isolated bright galaxies are more likely to be recent
merger products. We propose a scenario that a series of morphology and
luminosity transformation occur through distant interactions and mergers, which
results in the morphology--luminosity--local density relation.Comment: 14 pages, 7 figures, for higher resolution figures download PDF file
at http://astro.kias.re.kr/docs/trans.pdf ; references added and typos in
section 3.2 corrected; Final version accepted for publication in Ap
Fabry Perot Halpha Observations of the Barred Spiral NGC 3367
We report the gross properties of the velocity field of the barred spiral
galaxy NGC 3367. The following values were found: inclination with respect to
the plane of the sky, i=30 deg; position angle (PA) of receding semi major axis
PA=51 and systemic velocity V(sys)=3032 km/s. Large velocity dispersion are
observed of upt o 120 km/s in the nuclear region, of up to 70 km/s near the
eastern bright sources just beyond the edge of the stellar bar where three
spiral arms seem to start and in the western bright sources at about 10 kpc.
Deviations from normal circular velocities are observed from all the disk but
mainly from the semi circle formed by the string of south western Halpha
sources. An estimate of the dynamical mass is M(dyn)=2x10^11 Msolar.Comment: Accepted to be published in May 2001 issue in the A.J. 19 pages, 7
figure
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
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