2,360 research outputs found
Dissipative transformation of non-nucleated dwarf galaxies into nucleated systems
Recent photometric observations by the {\it Hubble Space Telescope (HST)}
have revealed the physical properties of stellar galactic nuclei in nucleated
dwarf galaxies in the Virgo cluster of galaxies. In order to elucidate the
formation processes of nucleated dwarfs, we numerically investigate gas
dynamics, star formation, and chemical evolution within the central 1 kpc of
gas disks embedded within the galactic stellar components of non-nucleated
dwarfs. We find that high density, compact stellar systems can be formed in the
central regions of dwarfs as a result of dissipative, repeated merging of
massive stellar and gaseous clumps developed from nuclear gaseous spiral arms
as a result of local gravitational instability. The central stellar components
are found to have stellar masses which are typically 5% of their host dwarfs
and show very flattened shapes, rotational kinematics, and central velocity
dispersions significantly smaller than those of their host dwarfs. We also find
that more massive dwarfs can develop more massive, more metal-rich, and higher
density stellar systems in their central regions, because star formation and
chemical enrichment proceed more efficiently owing to the less dramatic
suppression of star formation by supernovae feedback effects in more massive
dwarfs. Based on these results, we suggest that gas-rich, non-nucleated dwarfs
can be transformed into nucleated ones as a result of dissipative gas dynamics
in their central regions. We discuss the origin of the observed correlations
between physical properties of stellar galactic nuclei and those of their host
galaxies.Comment: 13 pages, 4 figures (1 color), ApJL in pres
Resistance and Propulsion Test Results on Two Cb=0.60 Merchant Hull Geosims
http://deepblue.lib.umich.edu/bitstream/2027.42/96607/1/39015087358712.pd
Cluster Galaxy Evolution from a New Sample of Galaxy Clusters at 0.3 < z < 0.9
(Abridged) We analyze photometry and spectroscopy of a sample of 63 clusters
at 0.3<z<0.9 drawn from the Las Campanas Distant Cluster Survey to empirically
constrain models of cluster galaxy evolution. Specifically, by combining data
on our clusters with those from the literature we parametrize the redshift
dependence of 1) M*_I in the observed frame; 2) the V-I color of the E/S0 red
sequence in the observed frames; and 3) the I-K' color of the E/S0 red sequence
in the observed frame. Using the peak surface brightness of the cluster
detection, S, as a proxy for cluster mass, we find no correlation between S and
M* or the location of the red envelope in V-I. We suggest that these
observations can be explained with a model in which luminous early type
galaxies (or more precisely, the progenitors of current day luminous early type
galaxies) form the bulk of their stellar populations at high redshift (>~ 5)
and in which many of these galaxies, if not all, accrete mass either in the
form of evolved stellar populations or gas that causes only a short term
episode of star formation at lower redshifts (1.5 < z < 2). Our data are too
crude to reach conclusions regarding the evolutionary state of any particular
cluster or to investigate whether the morphological evolution of galaxies
matches the simple scenario we discuss, but the statistical nature of this
study suggests that the observed evolutionary trends are universal in massive
clusters.Comment: 35 pages, accepted for publication in Ap
Hot melt adhesive attachment pad
A hot melt adhesive attachment pad for releasably securing distinct elements together is described which is particularly useful in the construction industry or a spatial vacuum environment. The attachment pad consists primarily of a cloth selectively impregnated with a charge of hot melt adhesive, a thermo-foil heater, and a thermo-cooler. These components are securely mounted in a mounting assembly. In operation, the operator activates the heating cycle transforming the hot melt adhesive to a substantially liquid state, positions the pad against the attachment surface, and activates the cooling cycle solidifying the adhesive and forming a strong, releasable bond
Spatially Resolved Spectroscopy of the E+A Galaxies in the z=0.32 Cluster AC114
We present spatially resolved intermediate resolution spectroscopy of a
sample of twelve E+A galaxies in the z=0.32 rich galaxy cluster AC 114,
obtained with the FLAMES multi-integral field unit system on the European
Southern Observatory's VLT. Previous integrated spectroscopy of all these
galaxies by Couch & Sharples (1987) had shown them to have strong Balmer line
absorption and an absence of [OII 3727] emission -- the defining
characteristics of the``E+A'' spectral signature, indicative of an abrupt halt
to a recent episode of quite vigorous star formation. We have used our spectral
data to determine the radial variation in the strength of Hdelta absorption in
these galaxies and hence map out the distribution of this recently formed
stellar population. Such information provides important clues as to what
physical event might have been responsible for this quite dramatic change in
star formation activity in these galaxies' recent past. We find a diversity of
behaviour amongst these galaxies in terms of the radial variation in Hdelta
absorption: Four galaxies show little Hdelta absorption across their entire
extent; it would appear they were misidentified as E+A galaxies in the earlier
integrated spectroscopic studies. The remainder show strong Hdelta absorption,
with a gradient that is either negative (Hdelta equivalent width decreasing
with radius), flat, or positive. By comparing with numerical simulations we
suggest that the first of these different types of radial behaviour provides
evidence for a merger/interaction origin, whereas the latter two types of
behaviour are more consistent with the truncation of star formation in normal
disk galaxies. It would seem therefore that more than one physical mechanism is
responsible for E+A formation in the same environment.Comment: 15 pages, 10 figures, accepted MNRA
Spectroscopic evolution of dusty starburst galaxies
By using a one-zone chemical and spectrophotometric evolution model of a disk
galaxy undergoing a dusty starburst, we investigate, numerically, the optical
spectroscopic properties in order to explore galaxy evolution in distant
clusters. We adopt an assumption that the degree of dust extinction
(represented by ) depends on the ages of starburst populations in such a
way that younger stars have larger (originally referred to as selective
dust extinction by Poggianti & Wu 2000). In particular, we investigate how the
time evolution of the equivalent widths of [OII]3727 and H is
controlled by the adopted age dependence. This leads to three main results: (1)
If a young stellar population (with the age of yr) is more
heavily obscured by dust than an old one ( yr), the galaxy can show
an ``e(a)'' spectrum characterized by strong H absorption and
relatively modest [OII] emission. (2) A dusty starburst galaxy with an e(a)
spectrum can evolve into a poststarburst galaxy with an a+k (or k+a) spectrum
0.2 Gyr after the starburst and then into a passive one with a k-type spectrum
1 Gyr after the starburst. This result clearly demonstrates an evolutionary
link between galaxies with different spectral classes (i.e., e(b), e(a), a+k,
k+a, and k). (3) A dusty starburst galaxy can show an a+k or k+a spectrum even
in the dusty starburst phase if the age-dependence of dust extinction is rather
weak, i.e., if young starburst populations with different ages (
yr) are uniformly obscured by dust.Comment: 27 pages 12 figures,2001,ApJ,in pres
An investigation of the existence of a surface water layer on aircraft radomes during simulated flight in heavy precipitation
An experiment to investigate the electromagnetic attenuation effects of an impacting water spray on an aircraft weather radome was conducted in Langley's 4 X 7 m. wind tunnel equipped with a water spray system. Results indicate no significant liquid water film formed at the stagnation point of the radome under the test conditions. However, a water sheath was observed standing away from the radome surface, which could possibly have significant attenuation properties of its own. Due to the lack of fidelity in modeling both the natural environment with the tunnel apparatus and the water sheath, it is recommended that further studies be undertaken to better define the water distribution in the vicinity of the radome and measure its effect on weather radar performance
Evidence for the Evolution of Young Early-Type Galaxies in the GOODS/CDF-S Field
We have developed an efficient photometric technique for identifying young
early-type galaxy candidates using a combination of photometric redshifts,
spectral-type classification, and optical/near-infrared colors. Applying our
technique to the GOODS HST/ACS and VLT/ISAAC data we have selected a complete
and homogeneous sample of young elliptical candidates among early-type field
galaxies. The distribution of structural parameters for these candidates shows
that their selection, which is based on early spectral types, is fully
consistent with early morphological types. We investigate the evolution of
their luminosities and colors as a function of redshift and galaxy mass and
find evidence for an increasing starburst mass fraction in these young
early-type galaxy candidates at higher redshifts, which we interpret in terms
of massive field galaxies experiencing more massive/intense starbursts at
higher redshifts. Moreover, we find indications for a systematically larger
young elliptical fraction among sub-L*/2 early-type galaxies compared to their
brighter counterparts. The total fraction among the field early-type galaxies
increases with redshift, irrespective of galaxy luminosity. Our results are
most consistent with galaxy formation scenarios in which stars in massive
early-type field galaxies are assembled earlier than in their low-mass
counterparts.Comment: 11 pages, 10 figures, accepted for publication in A
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