2,185 research outputs found
The Ages of Elliptical Galaxies from Infrared Spectral Energy Distributions
The mean ages of early-type galaxies obtained from the analysis of optical
spectra, give a mean age of 8 Gyr at z = 0, with 40% being younger than 6 Gyr.
Independent age determinations are possible by using infrared spectra (5-21
microns), which we have obtained with the Infrared Spectrograph on the Spitzer
Observatory. This age indicator is based on the collective mass loss rate of
stars, where mass loss from AGB stars produces a silicate emission feature at
9-12 microns. This feature decreases more rapidly than the shorter wavelength
continuum as a stellar population ages, providing an age indicator. From
observations of 30 nearby early-type galaxies, 29 show a spectral energy
distribution dominated by stars and one has significant emission from the ISM
and is excluded. The infrared age indicators for the 29 galaxies show them all
to be old, with a mean age of about 10 Gyr and a standard deviation of only a
few Gyr. This is consistent with the ages inferred from the values of M/L_B,
but is inconsistent with the ages derived from the optical line indices, which
can be much younger. All of these age indicators are luminosity-weighted and
should be correlated, even if multiple-age components are considered. The
inconsistency indicates that there is a significant problem with either the
infrared and the M/L_B ages, which agree, or with the ages inferred from the
optical absorption lines.Comment: Accepted for publication in Ap
Deep Optical Observations of Compact Groups of Galaxies
Compact groups of galaxies appear to be extremely dense, making them likely
sites of intense galaxy interaction, while their small populations make them
relatively simple to analyze. In order to search for optical interaction
tracers such as diffuse light and galaxy tidal features in Hickson compact
groups (HCGs), we carried out deep photometry in three filters on a sample of
HCGs with observations. Using a modeling procedure to subtract the
light of bright early-type galaxies, we found shell systems and extended
envelopes around many, but not all, of those galaxies. Only one group in our
sample, HCG 94, has diffuse light in the group potential (with a luminosity of
7 L); the other groups do not contain more than 1/3 L in diffuse light.
With the exception of HCG 94 (which is the most X-ray--luminous HCG), we found
no correlation between the presence of shells or other tidal features and the
X-ray luminosity of a group. Better predictors of detectable group X-ray
emission are a low spiral fraction and belonging to a larger galaxy
condensation---neither of which are correlated with optical disturbances in the
group galaxies. Two elliptical galaxies that are extremely optically luminous
but X-ray--faint are found to have shells and very complex color structures.
This is likely due to recent infall of gas-rich material into the galaxies,
which would produce both the disruption of stellar orbits and a significant
amount of star formation.Comment: 24 pages, to appear in October 1995 Astronomical Journal; postscript
text and figures (low resolution scans, tar'ed and compressed) available at
ftp://astro.lsa.umich.edu/pub/get/pildis
Numerical modeling of the interstellar medium in galactic disks
We have been developing detailed hydrodynamic models of the global interstellar medium in the hope of understanding the mass and volume occupied by various phases, as well as their structure and kinematics. In our model, the gas is modeled by one fluid while representative Pop 1 stars are modeled by a second fluid. The two fluids are coupled in that the gas forms into stars at a rate given by a Schmidt law while stellar mass loss returns matter into the gas phase (on a time scale of 100 Myr). Also, the stars heat the gas through stellar winds and the gas cools through optically thin radiation. The time behavior of these two fluids is studied in two spatial dimensions with the Eulerian finite difference numerical hydrodynamic code Zen. The two spatial dimensions are along the plane of a disk (x, total length of 2 kpc) and perpendicular to the disk (z, total height of +/- 15 kpc) and a galactic gravitational field in the z direction, typical of that at the solar circle, is imposed upon the simulation; self-gravity and rotation are absent. For the boundary conditions, outflow is permitted at the top and bottom of the grid (z = +/- 15 kpc) while periodic boundary conditions are imposed upon left and right sides of the grid. As initial conditions, we assumed a gaseous distribution like that seen for the H1 by earlier researchers, although the results are insensitive to the initial conditions. We have run simulations in which the heating due to stars, parameterized as a stellar wind velocity, a, is varied from low (a = 150 km/s), to intermediate (a = 300 km/s), to high (a = 600 km/s). Since the intermediate case is roughly equivalent to the Galactic energy injection rate from supernovae, this summary will concentrate on results from this simulation
The Search for Million Degree Gas Through The NVII Hyperfine Line
Gas in the million degree range occurs in a variety of astronomical
environments, and it may be the main component of the elusive missing baryons
at low redshift. The NVII ion is found in this material and it has a hyperfine
spin-flip transition with a rest frequency of 53.042 GHz, which can be observed
for z > 0.1, when it is shifted into a suitably transparent radio band. We used
the 42-48 GHz spectrometer on the Green Bank Telescope to search for both
emission and absorption from this NVII transmission. For absorption studies,
3C273, 3C 279, 3C 345, and 4C+39.25 were observed but no feature were seen
above the 5 sigma level. For emission line studies, we observed Abell 1835,
Abell 2390 and the star-forming galaxy PKS 1345+12, but no features were seen
exceeding 5 sigma. We examine whether the strongest emission feature, in Abell
2390 (3.7 sigma), and the strongest absorption feature, toward 4C+39.25 (3.8
sigma), might be expected from theoretical models. The emission feature would
require ~1E10 Msolar of 1E6 K gas, which is inconsistent with X-ray limits for
the O VII Kalpha line, so it is unlikely to be real. The NVII absorption
feature requires a NVII column of 6E16 cm^-2, higher than model predictions by
at least an order of magnitude, which makes it inconsistent with model
expectations. The individual observations were less than 1 hr in length, so for
lengthy observations, we show that NVII absorption line observations can begin
to be useful in in the search for hot intergalactic gas.Comment: 27 total pages; 16 figures; Accepted for publication in The
Astrophysical Journa
Simultaneous observations of active galactic nuclei with IUE
The IUE observations of four active nuclei were coordinated with radio, infrared, and X-ray measurements to obtain simultaneous determinations of their continuous spectra. The results for the BL Lac objects 0735+178 and I Zw 187 indicate sufficient UV and X-ray fluxes to ionize any gas. Comparison of the X-ray measurements with the extrapolated optical UV continuum show a definite X-ray excess for I Zw 187 but none for the other BL Lac object
Small-Scale structure in the Galactic ISM: Implications for Galaxy Cluster Studies
Observations of extragalactic objects need to be corrected for Galactic
absorption and this is often accomplished by using the measured 21 cm HI
column. However, within the beam of the radio telescope there are variations in
the HI column that can have important effects in interpreting absorption line
studies and X-ray spectra at the softest energies. We examine the HI and
DIRBE/IRAS data for lines of sight out of the Galaxy, which show evidence for
HI variations in of up to a factor of three in 1 degree fields. Column density
enhancements would preferentially absorb soft X-rays in spatially extended
objects and we find evidence for this effect in the ROSAT PSPC observations of
two bright clusters of galaxies, Abell 119 and Abell 2142.
For clusters of galaxies, the failure to include column density fluctuations
will lead to systematically incorrect fits to the X-ray data in the sense that
there will appear to be a very soft X-ray excess. This may be one cause of the
soft X-ray excess in clusters, since the magnitude of the effect is comparable
to the observed values.Comment: 16 pages, 9 figures, to appear in the Astrophysical Journal, vol. 597
(1 Nov 2003
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