87 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
A survey of the properties of early-type galaxies
A compilation of the properties of elliptical and early disk galaxies was completed. In addition to material from the literature, such as Infrared Astronomy Satellite (IRAS) fluxes, the compilation includes recent measurements of HI and CO, as well as a review of the x ray properties by Forman and Jones. The data are used to evaluate the gas content of early systems and to search for correlations with x ray emission. The interstellar medium in early-type galaxies is generally dominated by hot interstellar gas (T approx. 10 to the 7th power K; c.f. the review by Fabbiano 1989 and references therein). In addition, a significant fraction of these galaxies show infrared emission (Knapp, et al., 1989), optical emission lines, and visible dust. Sensitive studies in HI and CO of a number of these galaxies have been completed recently, resulting in several detections, particularly of the later types. Researchers wish to understand the connection among these different forms of the interstellar medium, and to examine the theoretical picture of the fate of the hot gas. To do so, they compiled observations of several forms of interstellar matter for a well-defined sample of early-type galaxies. Here they present a statistical analysis of this data base and discuss the implications of the results
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
Cosmic Filaments in Superclusters
Large-scale structure calculations show that modest overdensity filaments
will connect clusters of galaxies and these filaments are reservoirs of
baryons, mainly in gaseous form. To determine whether such filaments exist, we
have examined the UV absorption line properties of three AGNs projected behind
possible filaments in superclusters of galaxies; the AGNs lie within 3 Mpc of
the centerlines of loci connecting clusters. All three lines of sight show
absorption in Ly\alpha, Ly\beta, or/and OVI at redshifts within about 1300 km/s
of the nearby galaxy clusters that would define the closest filaments. For one
AGN, the absorption line redshifts are close to the emission line redshift of
the AGN, so we cannot rule out self-absorption for this object. These
absorption line associations with superclusters are unlikely to have occurred
by chance, a result consistent with the presence of cosmic filaments within
superclusters.Comment: 16 pages, 7 eps figures, Accepted for publication in Ap
The Closest Damped Lyman Alpha System
A difficulty of studying damped Lyman alpha systems is that they are distant,
so one knows little about the interstellar medium of the galaxy. Here we report
upon a damped Lyman alpha system in the nearby galaxy NGC 4203, which is so
close (v_helio = 1117 km/s) and bright (B_o = 11.62) that its HI disk has been
mapped. The absorption lines are detected against Ton 1480, which lies only
1.9' (12 h_50 kpc) from the center of NGC 4203. Observations were obtained with
the Faint Object Spectrograph on HST (G270H grating) over the 2222-3277
Angstrom region with 200 km/s resolution. Low ionization lines of Fe, Mn, and
Mg were detected, leading to metallicities of -2.29, -2.4, which
are typical of other damped Lyman alpha systems, but well below the stellar
metallicity of this type of galaxy. Most notably, the velocity of the lines is
1160 +- 10 km/s, which is identical to the HI rotational velocity of 1170 km/s
at that location in NGC 4203, supporting the view that these absorption line
systems can be associated with the rotating disks of galaxies. In addition, the
line widths of the Mg lines give an upper limit to the velocity dispersion of
167 km/s, to the 99% confidence level.Comment: 4 pages LaTeX, including 1 figure and 1 table, uses emulateapj.sty.
Accepted for publication by Astrophysical Journal Letter
On the Lack of a Soft X-Ray Excess from Clusters of Galaxies
A soft X-ray excess has been claimed to exist in and around a number of
galaxy clusters and this emission has been attributed to the warm-hot
intergalactic medium that may constitute most of the baryons in the local
universe. We have re-examined a study of the XMM-Newton observations on this
topic by Kaastra et al. (2003) and find that the X-ray excess (or deficit)
depends upon Galactic latitude and appears to be most closely related to the
surface brightness of the 1/4 keV emission, which is largely due to emission
from the Local hot bubble and the halo of the Milky Way. We suggest that the
presence of the soft X-ray excess is due to incorrect subtraction of the soft
X-ray background. An analysis is performed where we choose a 1/4 keV background
that is similar to the background near the cluster (and for similar HI column).
We find that the soft X-ray excess largely disappears using our background
subtraction and conclude that these soft X-ray excesses are not associated with
the target clusters. We also show that the detections of "redshifted" O VII
lines claimed by Kaastra et al. (2003) are correlated with solar system charge
exchange emission suggesting that they are not extragalactic either.Comment: 8 pages, 6 figures, accepted for publication in Ap
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