93 research outputs found
Compact X-ray Sources in Nearby Galaxy Nuclei
We have found compact, near-nuclear X-ray sources in 21 (54\%) of a complete
sample of 39 nearby face-on spiral and elliptical galaxies with available ROSAT
HRI data. ROSAT X-ray luminosities (0.2 2.4 keV) of these compact X-ray
sources are 1010 erg~s. The mean displacement
between the location of the compact X-ray source and the optical photometric
center of the galaxy is 390 pc. ASCA spectra of six of the 21 galaxies
show the presence of a hard component with relatively steep (
2.5) spectral slope. A multicolor disk blackbody plus power-law model fits the
data from the spiral galaxies well, suggesting that the X-ray objects in these
galaxies may be similar to a black hole candidate (BHC) in its soft (high)
state. ASCA data from the elliptical galaxies indicate that hot (kT
0.7 keV) gas dominates the emission. The fact that the spectral slope of the
spiral galaxy sources is steeper than in normal type 1 active galactic nuclei
(AGNs) and that relatively low absorbing columns (N 10
cm) were found to the power-law component indicates that these objects
are somehow geometrically and/or physically different from AGNs in normal
active galaxies. The X-ray sources in the spiral galaxies may be BHCs,
low-luminosity AGNs, or possibly X-ray luminous supernovae. We estimate the
black hole masses of the X-ray sources in the spiral galaxies (if they are BHCs
or AGNs) to be 1010 M. The X-ray sources in the
elliptical galaxies may be BHCs, AGNs or young X-ray supernova also.Comment: 4 pages, TeX, two postscript figures, to be published in proceedings
of 32nd COSPAR Session E1.2 (1998 July 15-17 Nagoya) "The AGN-Normal Galaxy
Connection
Status of guidance in the large secondary schools of Massachusetts
Thesis (M.A.)--Boston University, 1935. This item was digitized by the Internet Archive
The Radio Emission from the Ultra-Luminous Far-Infrared Galaxy NGC 6240
We present new radio observations of the ``prototypical'' ultra-luminous far-
infrared galaxy NGC~6240, obtained using the VLA at 20~cm in `B' configuration
and at 3.6~cm in `A' configuration. These data, along with those from four
previous VLA observations, are used to perform a comprehensive study of the
radio emission from NGC~6240. Approximately 70\% (~3 x 10 W~Hz)
of the total radio power at 20 cm originates from the nuclear region ( <~ 1.5
kpc), of which half is emitted by two unresolved (\alphaS_{\nu} \propto \nu^{-\alpha}). The
supernova rate required to power the diffuse component is consistent with that
predicted by the stellar evolution models of Rieke \etal (1985). If the radio
emission from the two compact cores is powered by supernova remnants, then
either the remnants overlap and form hot bubbles in the cores, or they are very
young ( <~ 100 yr). Nearly all of the other 30\% of the total radio power comes
from an ``arm-like'' region extending westward from the nuclear region. The
western arm emission has a steep spectrum (\alpha^{-14}$ erg/s/cm2 in the 2-10 keV band. No significant radio
emission is detected from or near the possible ultra-massive ``dark core''
hypothesized by Bland-Hawthorn, Wilson \& Tully (1991).Comment: 36 pages (text and tables) as an uuencoded compressed postscript file
(figures available upon request), accepted for the ApJ (20 Nov issue), STScI
preprint no. ?? -- May 199
Modeling and Simulation of a Long-Wave Infrared Polarimetric Sensor for Space Object Detection and Characterization
Long-Wave Infrared (LWIR, wavelength \u3e 8 um) polarimetric measurements can be used to characterize space objects. A simulation of a sensor for collection of LWIR polarimetric signatures of space objects has been assembled using two software packages: MATLAB, and FRED. A statistical approach developed for unresolved visible light polarimetric observations of GEO satellites has been adapted for unresolved LWIR polarimetric observations of LEO satellites, showing both that well-known objects can be recognized and anomalies--for example, a major change in shape due to the presence in the scene of another object--can be detected. Though the satellites are effectively point sources, the aggregate polarization values across many measurements can be used to differentiate objects of different shape and material composition
Old and Young X-ray Point Source Populations in Nearby Galaxies
We analyzed 1441 Chandra X-ray point sources in 32 nearby galaxies. The total
point-source X-ray luminosity L_XP is well correlated with B, K, and FIR+UV
luminosities of spiral host galaxies, and with the B and K luminosities for
ellipticals. This suggests an intimate connection between L_XP and both the old
and young stellar populations, for which K and FIR+UV luminosities are proxies
for the galaxy mass M and star-formation rate SFR. We derive proportionality
constants 1.3E29 erg/s/Msol and 0.7E39 erg/s/(Msol/yr), which can be used to
estimate the old and young components from M and SFR, respectively. The
cumulative X-ray luminosity functions for the point sources have quite
different slopes for the spirals (gamma ~= 0.5-0.8) and ellipticals (gamma ~=
1.4), implying *the most luminous point sources dominate L_XP* for the spirals.
Most of the point sources have X-ray colors that are consistent with either
LMXBs or Ultraluminous X-ray sources (ULXs a.k.a. IXOs) and we rule out
classical HMXBs (e.g. neutron-star X-ray pulsars) as contributing much to L_XP.
Thus, for spirals, the ULXs dominate L_XP. We estimate that >~20% of all ULXs
found in spirals originate from the older (pop II) stellar populations,
indicating that many of the ULXs that have been found in spiral galaxies are in
fact pop II ULXs, like those in elliptical galaxies. The linear dependence of
L_XP on the SFR argues for either a steepening in the X-ray luminosity function
of the young (pop I) X-ray source population at L_X >~10^(38.5-39) erg/s, or a
decreasing efficiency for producing all types of young X-ray point sources as
the galaxy SFR increases.Comment: 33 pages AASTEX, ApJ accepted. Please download full version with
figures from http://www.pha.jhu.edu/~colbert/chps_accepted.p
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