1,454 research outputs found
Examining the Role of Environment in a Comprehensive Sample of Compact Groups
(Abridged) Compact groups, with their high number densities, small velocity
dispersions, and an interstellar medium that has not been fully processed,
provide a local analog to conditions of galaxy interactions in the earlier
universe. The frequent and prolonged gravitational encounters that occur in
compact groups affect the evolution of the constituent galaxies in a myriad of
ways, for example gas processing and star formation. Recently, a statistically
significant "gap" has been discovered mid-infrared IRAC colorspace of compact
group galaxies. This gap is not seen in field samples and is a new example of
how the compact group environment may affect the evolution of member galaxies.
In order to investigate the origin and nature of this gap, we have compiled a
sample of 49 compact groups. We find that a statistically significant deficit
of galaxies in this gap region of IRAC colorspace is persistant in this sample,
lending support to the hypothesis that the compact group environment inhibits
moderate SSFRs. We note a curvature in the colorspace distribution, which is
fully consistent with increasing dust temperature as the activity in a galaxy
increases. This full sample of 49 compact groups allows us to subdivide the
data according to physical properties of the groups. An analysis of these
subsamples indicates that neither projected physical diameter nor density show
a trend in colorspace within the values represented by this sample. We
hypothesize that the apparent lack of a trend is due to the relatively small
range of properties in this sample. Thus, the relative influence of stochastic
effects becomes dominant. We analyze spectral energy distributions of member
galaxies as a function of their location in colorspace and find that galaxies
in different regions of MIR colorspace contain dust with varying temperatures
and/or PAH emission.Comment: 24 pages, 13 figures. Accepted for publication in The Astronomical
Journa
Impact of Reionization on the Stellar Populations of Nearby Dwarf Galaxies
Cold dark matter models for galaxy formation predict that low-mass systems
will be the first sites of star formation. As these objects have shallow
gravitational potential wells, the subsequent growth of their stellar
populations may be halted by heating and gas loss due to reionization. This
effect has been suggested to have profoundly influenced properties of
present-day dwarf galaxies, including their stellar populations and even
survival as visible galaxies. In this Letter we draw on results from
quantitative studies of Local Group dwarf galaxy star formation histories,
especially for Milky Way satellites, to show that no clear signature exists for
a widespread evolutionary impact from reionization. All nearby dwarf galaxies
studied in sufficient detail contain ancient populations indistinguishable in
age from the oldest Galactic globular clusters. Ancient star formation activity
proceeded over several Gyr, and some dwarf spheroidal galaxies even experienced
fairly continuous star formation until just a few Gyr ago. Despite their
uniformly low masses, their star formation histories differ considerably. The
evolutionary histories of nearby dwarf galaxies appear to reflect influences
from a variety of local processes rather than a dominant effect from
reionization.Comment: Accepted by The Astrophysical Journal Letters. 5 pages, one figur
Some Like It Hot: Linking Diffuse X-ray Luminosity, Baryonic Mass, and Star Formation Rate in Compact Groups of Galaxies
We present an analysis of the diffuse X-ray emission in 19 compact groups of
galaxies (CGs) observed with Chandra. The hottest, most X-ray luminous CGs
agree well with the galaxy cluster X-ray scaling relations in and
, even in CGs where the hot gas is associated with only the
brightest galaxy. Using Spitzer photometry, we compute stellar masses and
classify HCGs 19, 22, 40, and 42 and RSCGs 32, 44, and 86 as fossil groups
using a new definition for fossil systems that includes a broader range of
masses. We find that CGs with total stellar and HI masses
M are often X-ray luminous, while lower-mass CGs only sometimes exhibit
faint, localized X-ray emission. Additionally, we compare the diffuse X-ray
luminosity against both the total UV and 24 m star formation rates of each
CG and optical colors of the most massive galaxy in each of the CGs. The most
X-ray luminous CGs have the lowest star formation rates, likely because there
is no cold gas available for star formation, either because the majority of the
baryons in these CGs are in stars or the X-ray halo, or due to gas stripping
from the galaxies in CGs with hot halos. Finally, the optical colors that trace
recent star formation histories of the most massive group galaxies do not
correlate with the X-ray luminosities of the CGs, indicating that perhaps the
current state of the X-ray halos is independent of the recent history of
stellar mass assembly in the most massive galaxies.Comment: 20 pages, 7 figures, accepted for publication in Ap
A 2:1 Co-Crystal of Hydroquinone and 3,5-Bis(2-pyridyl)-1,2,4-triazole
The title compound, 2C₁₂H₉N5₅.C₆H₆O₂, exhibits a three-dimensional hydrogen-bonded network of N-H...N, C-H...N, O-H...N, C-H...O, C-H...π and π...π interactions
Hubble Space Telescope Imaging of the Circumstellar Nebulosity of T Tauri
Short-exposure Planetary Camera images of T Tauri have been obtained using broadband filters spanning the wavelength range 0.55-0.80 μm. The optically visible star lies very close to an arc of reflection nebulosity. The arc's northern arm extends approximately 5" from the star, while its southwestern arm appears brighter and extends only 2". The arc shows an approximate symmetry along an axis toward the west-northwest, the direction of Hind's Nebula and the blueshifted molecular outflow. The morphology of the reflected light is similar to models of scattered light within an illuminated, axisymmetric outflow cavity in a circumbinary envelope, viewed ≈ 45° from the outflow axis. However, our model images do not successfully account for the amount of limb brightening that is seen. No optical counterpart to the infrared companion is seen to a limiting magnitude of V = 19.6, which suggests A_V > 7 mag toward this source. There is no evidence for an optical tertiary, to a limiting ΔV = 5.1 mag fainter than the primary, at the position where such an object has been previously reported
Structure property relationships in halogenated aromatic amides and imides
The effect of halogens (X) and pyridine N atom substitution patterns on molecular structure and conformation is analyzed and discussed herein. Several series of 3 x 3 isomer grids (Scheme 1; Figs 1-3) of halo-N-(pyridyl)benzamides (Xxx) (C12H9N2OX, x = para-/meta-/ortho-) and their corresponding imides (Fig. 4) have been evaluated and correlated in terms of their structural relationships
Stellar Populations at the Center of IC 1613
We have observed the center of the Local Group dwarf irregular galaxy IC 1613
with WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W
filters. We find a dominant old stellar population (aged ~7 Gyr), identifiable
by the strong red giant branch (RGB) and red clump populations. From the (V-I)
color of the RGB, we estimate a mean metallicity of the intermediate-age
stellar population [Fe/H] = -1.38 +/- 0.31. We confirm a distance of 715 +/- 40
kpc using the I-magnitude of the RGB tip. The main-sequence luminosity function
down to I ~25 provides evidence for a roughly constant SFR of approximately
0.00035 solar masses per year across the WFPC2 field of view (0.22 square kpc)
during the past 250-350 Myr. Structure in the blue loop luminosity function
implies that the SFR was ~50% higher 400-900 Myr ago than today. The mean heavy
element abundance of these young stars is 1/10th solar. The best explanation
for a red spur on the main-sequence at I = 24.7 is the blue horizontal branch
component of a very old stellar population at the center of IC 1613. We have
also imaged a broader area of IC 1613 using the 3.5-meter WIYN telescope under
excellent seeing conditions. The AGB-star luminosity function is consistent
with a period of continuous star formation over at least the age range 2-10
Gyr. We present an approximate age-metallicity relation for IC 1613, which
appears similar to that of the Small Magellanic Cloud. We compare the Hess
diagram of IC 1613 to similar data for three other Local Group dwarf galaxies,
and find that it most closely resembles the nearby, transition-type dwarf
galaxy Pegasus (DDO 216).Comment: To appear in the September 1999 Astronomical Journal. LaTeX, uses
AASTeX v4.0, emulateapj style file, 19 pages, 12 postscript figures, 2
tables. 5 of the figures available separately via the WW
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