204 research outputs found
IPC two-color analysis of x ray galaxy clusters
The mass distributions were determined of several clusters of galaxies by using X ray surface brightness data from the Einstein Observatory Imaging Proportional Counter (IPC). Determining cluster mass distributions is important for constraining the nature of the dark matter which dominates the mass of galaxies, galaxy clusters, and the Universe. Galaxy clusters are permeated with hot gas in hydrostatic equilibrium with the gravitational potentials of the clusters. Cluster mass distributions can be determined from x ray observations of cluster gas by using the equation of hydrostatic equilibrium and knowledge of the density and temperature structure of the gas. The x ray surface brightness at some distance from the cluster is the result of the volume x ray emissivity being integrated along the line of sight in the cluster
Non-isothermal X-ray Emitting Gas in Clusters of Galaxies
We have analyzed X-ray spectra from six galaxy clusters which contain cooling
flows: A85, A478, A1795, A2142, A2147, & A2199. The X-ray spectra were taken
with the HEAO1-A2 Medium and High Energy Detectors and the Einstein Solid State
Spectrometer. For each cluster, we simultaneously fit the spectra from these
three detectors with models incorporating one or more emission components
comprised of either thermal or cooling flow models. Five of the clusters (all
but A2142) are better fit by a multi-component model (a cooling flow plus one
or two thermal components or a two thermal component model) than by isothermal
models. In four of the clusters (A85, A1795, A2147, & A2199), we find evidence
for cool gas outside of the canonical cooling flow region. These latter four
clusters can be characterized by three temperature components: a temperature
inversion in the central region, a hotter region with an emission-weighted
temperature which is higher than that of an isothermal model fit to the entire
cluster, and a cooler region with an emission-weighted temperature of ~1 keV.
The cool component outside the cooling flow region has a large minimum emission
measure which we attribute, in part, to diffuse cool gas in the outer cluster
atmosphere. If at least some of the cool exterior gas is virialized, this would
imply a radially decreasing temperature profile. Together with the density
profiles we have found, this leads to a baryon fraction in gas which increases
with radius and is larger than that for an isothermal cluster atmosphere.
Consequently, if clusters of galaxies trace the mass distribution in the
Universe, the gas mass fraction we have calculated for an isothermal gas (which
is ~15%) together with the nominal galaxy contribution (~5%) gives a baryon
fraction of ~20%. Using the upper limit to the baryon density derived from BigComment: gzipped tar file of 26 PostScript pages, including 2 figures, 7
tables. Also available at
http://www.astr.ua.edu/preprints/white/INDEX_READ_ME_1st.htm
Accounting for the dispersion in the x ray properties of early-type galaxies
The x ray luminosities of early-type galaxies are correlated with their optical (e.g., blue) luminosities (L sub X approx. L sub B exp 1.6), but the x ray luminosities exhibit considerable scatter for a given optical luminosity L sub B. This dispersion in x ray luminosity is much greater than the dispersion of other properties of early-type galaxies (for a given L sub B), such as luminosity scale-length, velocity dispersion, color, and metallicity. Here, researchers consider several possible sources for the dispersion in x ray luminosity. Some of the scatter in x ray luminosity may result from stellar population variations between galaxies with similar L sub B. Since the x ray emitting gas is from accumulated stellar mass loss, the L sub X dispersion may be due to variations in integrated stellar mass loss rates. Another possible cause of the L sub X dispersion may be variations in the amount of cool material in the galaxies; cool gas may act as an energy sink for the hot gas. Infrared emission may be used to trace such cool material, so researchers look for a correlation between the infrared emission and the x ray emission of early-type galaxies at fixed L sub B. Velocity dispersion variations between galaxies of similar L sub B may also contribute to the L sub X dispersion. The most likely a priori source of the dispersion in L sub X is probably the varying amount of ram-pressure stripping in a range of galaxy environments. The hot gaseous halos of early-type galaxies can be stripped in encounters with other galaxies or with ambient cluster gas if the intracluster gas is sufficiently dense. Researchers find that the most likely cause of dispersion in the x ray properties of early type galaxies is probably the ram-pressure stripping of gaseous halos from galaxies. For a sample of 81 early-type galaxies with x ray luminosities or upper limits derived from Einstein Observatory observations (CFT) researchers calculated the cumulative distribution of angular distances between the x ray sample members and bright galaxies from the Revised Shapley - Ames catalog. Collectively, galaxies with low x ray luminosities (for a given L sub B) tend to be in denser environments than galaxies with higher x ray luminosities
X-ray Binaries and Globular Clusters in Elliptical Galaxies
The X-ray emission from normal elliptical galaxies has two major components:
soft emission from diffuse gas and harder emission from populations of
accreting (low-mass) stellar X-ray binaries (LMXB). If LMXB populations are
tied to the field stellar populations in galaxies, their total X-ray
luminosities should be proportional to the optical luminosities of galaxies.
However, recent ASCA and Chandra X-ray observations show that the global
luminosities of LMXB components in ellipticals exhibit significant scatter at a
given optical luminosity. This scatter may reflect a range of evolutionary
stages among LMXB populations in ellipticals of different ages. If so, the
ratio of the global LMXB X-ray luminosity to the galactic optical luminosity,
L_LMXB/L_opt, may be used to determine when the bulk of stars were formed in
individual ellipticals. To test this, we compare variations in L_LMXB/L_opt for
LMXB populations in ellipticals to optically-derived estimates of stellar ages
in the same galaxies. We find no correlation, implying that L_LMXB/L_opt
variations are not good age indicators for ellipticals. Alternatively, LMXBs
may be formed primarily in globular clusters (through stellar tidal
interactions), rather than in the stellar fields of galaxies. Since elliptical
galaxies exhibit a wide range of globular cluster populations for a given
galaxian luminosity, this may induce a dispersion in the LMXB populations of
ellipticals with similar optical luminosities. Indeed, we find that
L_LMXB/L_opt ratios for LMXB populations are strongly correlated with the
specific globular cluster frequencies in elliptical galaxies. This suggests
that most LMXBs were formed in globular clusters.Comment: 5 pages, emulateapj5 style, 2 embedded EPS figures, to appear in ApJ
Letter
Cooling Flow Spectra in Ginga Galaxy Clusters
The primary focus of this research project has been a joint analysis of Ginga LAC and Einstein SSS X-ray spectra of the hot gas in galaxy clusters with cooling flows is reported. We studied four clusters (A496, A1795, A2142 & A2199) and found their central temperatures to be cooler than in the exterior, which is expected from their having cooling flows. More interestingly, we found central metal abundance enhancements in two of the clusters, A496 and A2142. We have been assessing whether the abundance gradients (or lack thereof) in intracluster gas is correlated with galaxy morphological gradients in the host clusters. In rich, dense galaxy clusters, elliptical and SO galaxies are generally found in the cluster cores, while spiral galaxies are found in the outskirts. If the metals observed in clusters came from proto-ellipticals and proto-S0s blowing winds, then the metal distribution in intracluster gas may still reflect the distribution of their former host galaxies. In a research project which was inspired by the success of the Ginga LAC/Einstein SSS work, we analyzed X-ray spectra from the HEAO-A2 MED and the Einstein SSS to look for temperature gradients in cluster gas. The HEAO-A2 MED was also a non-imaging detector with a large field of view compared to the SSS, so we used the differing fields of view of the two instruments to extract spatial information. We found some evidence of cool gas in the outskirts of clusters, which may indicate that the nominally isothermal mass density distributions in these clusters are steepening in the outer parts of these clusters
Seeing Galaxies Through Thick & Thin. III. HST Imaging of the Dust in Backlit Spiral Galaxies
We present analysis of WFPC2 imaging of two spiral galaxies partially backlit
by E/S0 systems in the pairs AM1316-241 and AM0500-620, and the spiral
foreground system in NGC 1275. Images in B and I are used to determine the
reddening curve of in these systems. The spiral component of AM1316-241 shows
dust strongly concentrated in discrete arms, with a reddening law very close to
the Milky Way mean. The dust distribution is scale-free between about 100 pc
and the arm scale. The spiral in AM0500-620 shows dust concentrated in arms and
interarm spurs, with measurable interarm extinction as well. Although its dust
properties are less well-determined, we find evidence for a steeper extinction
law here. The shape of the reddening law suggests that, at least in AM1316-241,
we have resolved most of the dust structure. In AM0500-620, the slope of the
fractal perimeter-scale relation steepens systematically from low to high
extinction. In AM1316-241, we cannot determine a unique fractal dimension from
the defining area-perimeter relation, so the projected dust distribution is
best defined as fractal-like. In neither galaxy do we see regions even on
single-pixel scales in spiral arms with AB > 2.5. The measurements in NGC 1275
are compromised by our lack of independent knowledge of the foreground system's
light distribution, but masked sampling of the absorption suggests an effective
reddening curve much flatter than the Milky Way mean (perhaps indicating that
the foreground system has been affected by immersion in the hot intracluster
gas).Comment: Astronomical Journal, in press. 13 figures. Full-size PostScript
figures available at http://www.astr.ua.edu/preprints/kee
X-Ray Emission from M32: X-Ray Binaries or a micro-AGN?
We have analysed archival {\it ROSAT} PSPC data for M32 in order to study the
x-ray emission from this nearest elliptical galaxy. We fit spectra from three
long exposures with Raymond-Smith, thermal bremsstrahlung, and power-law
models. All models give excellent fits. The thermal fits have kT4 keV,
the Raymond-Smith iron abundance is Solar, the power-law
fit has =1.60.1, and all fits have consistent with the
Galactic column. The source is centered on M32 to an accuracy of 9, and
unresolved at 27 FWHM (90 pc). M32 is x-ray variable by a factor of
3--5 on timescales of a decade down to minutes, with evidence for a possible
period of 1.3 days.
There are two plausible interpretations for these results: 1) Emission due to
low-mass x-ray binaries; 2) Emission due to accretion onto a massive central
black hole. Both of these possibilities are supported by arguments based on
previous studies of M32 and other old stellar systems; the {\it ROSAT} PSPC
data do not allow us to unambiguously choose between them. Observations with
the {\it ROSAT} HRI and with {\it ASCA} are required to determine which of
these two very different physical models is correct.Comment: 9 pages, 5 PostScript figures, uses AASTeX style files, Accepted for
publication in Astrophysical Journal Letter
Seeing Galaxies Through Thick and Thin: II. Direct Measures of Extinction in Spiral Disks Through Spectroscopy of Overlapping Galaxies
We use slit spectroscopy of overlapping pairs of galaxies to directly
determine the extinction in disks of foreground spiral galaxies. The Doppler
shifts of pair members are determined via cross-correlation and their relative
correlation amplitudes are used to separate their contributions to the combined
spectra in regions of overlap. This spectroscopic approach is less subject to
stringent symmetry constraints than our previous purely photometric analyses.
Extinctions of foreground members were obtained for 6 of the candidates in our
sample of 18 mostly spiral/spiral pairs, when the signal to noise and velocity
difference were suitable. In agreement with our previous imaging results, we
find that the extinction in interarm regions is very modest, typically A_B=0.1
mag (corrected to face on), while spiral arms exhibit higher extinctions of 0.3
mag.Comment: 14 figures, 3 tables, Accepted to Astrophysical Journa
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