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