205 research outputs found
X-Ray Sources in the Star Forming Galaxies NGC 4194 and NGC 7541
We examine the X-ray point source population and 2-10 keV luminosity for two
galaxies with high star formation rates (SFRs), NGC 4194 and NGC 7541. The
X-ray point source luminosity function (XLF) for these two galaxies is
consistent with the XLF found by Grimm et al. (2003) for a sample of
star-forming galaxies. Combining our results with a sample of galaxies with
SFRs above 1 solar mass/year, we find that the number of X-ray point sources
above a luminosity of 2E38 erg/s is N = (1.8 +/- 0.4) SFR/(solar mass/year).
This number is lower than previously inferred by Grimm et al. based on a sample
of galaxies with lower SFRs. We find that the ratio of X-ray luminosity in the
2-10 keV band to SFR is L_X/(10E40 erg/s) = (0.37 +/- 0.08) SFR/(solar
mass/year). This value may serve as a calibration in attempts to use X-ray
luminosity to measure the SFR of galaxies at cosmological distances. The ratio
of mass accreted onto compact objects versus mass used to form stars is near
1E-6. This ratio may be useful in constraining population synthesis models of
X-ray binary formation in actively star forming systems.Comment: 12 pages, to appear in Ap
The mid-infrared emission of local Luminous Infrared Galaxies
In this paper we use the complementary imaging capabilities of Spitzer
(sensitivity) and Gemini-South/T-ReCS (spatial resolution) to study the mid-IR
properties of local (d < 75Mpc) LIRGs. The T-ReCS 8-10micron imaging
observations of LIRGs have allowed us to spatially resolve the nuclear emission
(star formation and/or AGN) and that of HII regions in the central 3-7kpc
regions of LIRGs. From the comparison of the 8micron/Pa-alpha ratios of the
integrated vs. resolved HII regions of LIRGs, we infer the existence of an
8micron diffuse component, not directly related to the ionizing stars, that can
be as luminous as that from the resolved HII regions. We conclude that although
the mid-IR integrated luminosity of galaxies undergoing dusty, intense star
formation is a good indicator of the star formation rate (SFR), the empirical
calibrations should be based on the integrated emission of nearby galaxies, not
that of HII regions alone. To this end we provide a calibration of the SFR in
terms of the integrated 24micron luminosity that can be used for distant dusty
galaxies.Comment: Invited talk at the First Light with the GTC Conference held in
Miami, June-July 2006, to appear in Revista Mexicana de Astronomi
Typical AGN at intermediate redshifts
We study the host galaxies and black holes of typical X-ray selected AGN at
intermediate redshifts (z~0.5-1.4). The AGN are selected such that their
spectral energy distributions are dominated by stellar emission, i.e., they
show a prominent 1.6micron bump thus minimizing the AGN emission contamination.
This AGN population comprises approximately 50% of the X-ray selected AGN at
these redshifts. AGN reside in the most massive galaxies at the redshifts
probed here, with characteristic stellar masses that are intermediate between
those of local type 2 AGN and high redshift (z~2) AGN. The inferred black hole
masses of typical AGN are similar to those of optically identified quasars at
similar redshifts. Since the AGN in our sample are much less luminous than
quasars, typical AGN have low Eddington ratios. This suggests that, at least at
intermediate redshifts, the cosmic AGN 'downsizing' is due to both a decrease
in the characteristic stellar mass of the host galaxies, and less efficient
accretion. Finally there is no strong evidence in AGN host galaxies for either
highly suppressed star formation, expected if AGN played a role in quenching
star formation, or elevated star formation when compared to mass selected
galaxies of similar stellar masses and redshifts.Comment: Conference proceedings of the meeting "Observational Evidence for
Black Holes" held in Calcutta, Feb 2008. Paper will be published by AI
Spitzer/IRS Mapping of Local Luminous Infrared Galaxies
We present results of our program Spitzer/IRS Mapping of local Luminous
Infrared Galaxies (LIRGs). The maps cover the central 20"x20" or 30"x 30"
regions of the galaxies, and use all four IRS modules to cover the full 5-38
microns spectral range. We have built spectral maps of the main mid-IR emission
lines, continuum and PAH features, and extracted 1D spectra for regions of
interest in each galaxy. The final goal is to fully characterize the mid-IR
properties of local LIRGs as a first step to understanding their more distant
counterparts.Comment: 4 pages, 5 figures. To appear in "Highlights of Spanish Astrophysics
V", Proceedings of the VIII Scientific Meeting of the Spanish Astronomical
Society (SEA) held in Santander, 7-11 July, 200
Near-infrared K-band Spectroscopic Investigation of Seyfert 2 Nuclei in the CfA and 12 Micron Samples
We present near-infrared K-band slit spectra of the nuclei of 25 Seyfert 2
galaxies in the CfA and 12 micron samples. The strength of the CO absorption
features at 2.3-2.4 micron produced by stars is measured in terms of a
spectroscopic CO index. A clear anti-correlation between the observed CO index
and the nuclear K-L color is present, suggesting that a featureless hot dust
continuum heated by an AGN contributes significantly to the observed K-band
fluxes in the nuclei of Seyfert 2 galaxies. After correction for this AGN
contribution, we estimate nuclear stellar K-band luminosities for all sources,
and CO indices for sources with modestly large observed CO indices. The
corrected CO indices for 10 (=40%) Seyfert 2 nuclei are found to be as high as
those observed in star-forming or elliptical (=spheroidal) galaxies. We combine
the K-band data with measurements of the L-band 3.3 micron polycyclic aromatic
hydrocarbon (PAH) emission feature, another powerful indicator for
star-formation, and find that the 3.3 micron PAH to K-band stellar luminosity
ratios are substantially smaller than those of starburst galaxies. Our results
suggest that the 3.3 micron PAH emission originates in the putative nuclear
starbursts in the dusty tori surrounding the AGNs, because of its high surface
brightness, whereas the K-band CO absorption features detected at the nuclei
are dominated by old bulge (=spheroid) stars, and thus may not be a powerful
indicator for the nuclear starbursts. We see no clear difference in the
strength of the CO absorption and PAH emission features between the CfA and 12
micron Seyfert 2s.Comment: 28 pages, 6 figures, accepted for publication in ApJ (10 October
2004, v614 issue
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