Disentangling AGN and Star Formation in Soft X-rays

We have explored the interplay of star formation and AGN activity in soft X-rays (0.5-2 keV) in two samples of Seyfert 2 galaxies (Sy2s). Using a combination of low resolution CCD spectra from Chandra and XMM-Newton, we modeled the soft emission of 34 Sy2s using power law and thermal models. For the 11 sources with high signal-to-noise Chandra imaging of the diffuse host galaxy emission, we estimate the luminosity due to star formation by removing the AGN, fitting the residual emission. The AGN and star formation contributions to the soft X-ray luminosity (i.e. L$_{x,AGN}$ and L$_{x,SF}$) for the remaining 24 Sy2s were estimated from the power law and thermal luminosities derived from spectral fitting. These luminosities were scaled based on a template derived from XSINGS analysis of normal star forming galaxies. To account for errors in the luminosities derived from spectral fitting and the spread in the scaling factor, we estimated L$_{x,AGN}$ and L$_{x,SF}$ from Monte Carlo simulations. These simulated luminosities agree with L$_{x,AGN}$ and L$_{x,SF}$ derived from Chandra imaging analysis within a 3\sigma\ confidence level. Using the infrared [NeII]12.8\mu m and [OIV]26\mu m lines as a proxy of star formation and AGN activity, respectively, we independently disentangle the contributions of these two processes to the total soft X-ray emission. This decomposition generally agrees with L$_{x,SF}$ and L$_{x,AGN}$ at the 3\sigma\ level. In the absence of resolvable nuclear emission, our decomposition method provides a reasonable estimate of emission due to star formation in galaxies hosting type 2 AGN.Comment: accepted for publication in ApJ; 34 pages, 9 tables, 4 figure

Evidence for Black Hole Growth in Local Analogs to Lyman Break Galaxies

We have used XMM-Newton to observe six Lyman Break Analogs (LBAs): members of the rare population of local galaxies that have properties that are very similar to distant Lyman Break Galaxies. Our six targets were specifically selected because they have optical emission-line properties that are intermediate between starbursts and Type 2 (obscured) AGN. Our new X-ray data provide an important diagnostic of the presence of an AGN. We find X-ray luminosities of order 10^{42} erg/s and ratios of X-ray to far-IR luminosities that are higher than values in pure starburst galaxies by factors ranging from ~ 3 to 30. This strongly suggests the presence of an AGN in at least some of the galaxies. The ratios of the luminosities of the hard (2-10 keV) X-ray to [O III]\lambda 5007 emission-line are low by about an order-of-magnitude compared to Type 1 AGN, but are consistent with the broad range seen in Type 2 AGN. Either the AGN hard X-rays are significantly obscured or the [O III] emission is dominated by the starburst. We searched for an iron emission line at ~ 6.4 keV, which is a key feature of obscured AGN, but only detected emission at the ~ 2\sigma level. Finally, we find that the ratios of the mid-infrared (24\mu m) continuum to [O III]\lambda 5007 luminosities in these LBAs are higher than the values for Type 2 AGN by an average of 0.8 dex. Combining all these clues, we conclude that an AGN is likely to be present, but that the bolometric luminosity is produced primarily by an intense starburst. If these black holes are radiating at the Eddington limit, their masses would lie in the range of 10^5 to 10^6 M_{sun}. These objects may offer ideal local laboratories to investigate the processes by which black holes grew in the early universe.Comment: Accepted for publication in Ap

Using R-W1 to Uncover Obscured AGN in X-ray Surveys

X-ray surveys provide perhaps the most efficient method for discovering AGN across the Universe, though a major challenge is the time investment involved in following-up X-ray sources to identify them. To make the best use of limited resources, we explored the efficacy of m_r (optical) - m_3.4-micron (mid-infrared), hereafter R-W1, as a color diagnostic to hone in on signatures of obscured black hole growth in wide-area X-ray surveys which would otherwise be missed in optical and infrared surveys. We calibrated this color on the 16.5 deg^2 release of the Stripe 82X survey, finding that most X-ray emitting stars lie along a well-defined locus in R-W1 v. R-K color space and most AGN hosted in optically normal galaxies beyond z>0.5 have red R-W1 colors (R-W1 > 4 Vega). Our results will be useful for crafting target lists for current and future wide-area X-ray surveys to follow-up the most promising obscured AGN candidates

NGC 4388: A Test Case for Relativistic Disk Reflection and Fe K Fluorescence Features

We present a new analysis of the Suzaku X-ray spectrum of the Compton-thin Seyfert 2 galaxy NGC 4388. The spectrum above $\sim$2 keV can be described by a remarkably simple and rather mundane model, consisting of a uniform, neutral spherical distribution of matter, with a radial column density of $2.58 \pm 0.02 \times 10^{23}$ cm$^{-2}$, and an Fe abundance of $1.102^{+0.024}_{-0.021}$ relative to solar. The model does not require any phenomenological adjustments to self-consistently account for the low-energy extinction, the Fe K$\alpha$ and Fe K$\beta$ fluorescent emission lines, the Fe K edge, and the Compton-scattered continuum from the obscuring material. The spherical geometry is not a unique description, however, and the self-consistent, solar abundance MYTORUS model, applied with toroidal and non-toroidal geometries, gives equally good descriptions of the data. In all cases, the key features of the spectrum are so tightly locked together that for a wide range of parameters, a relativistic disk-reflection component contributes no more than $\sim$2% to the net spectrum in the 2-20 keV band. We show that the commonly invoked explanations for weak X-ray reflection features, namely a truncated and/or very highly ionized disk, do not work for NGC 4388. If relativistically-broadened Fe K$\alpha$ lines and reflection are ubiquitous in Seyfert 1 galaxies, they should also be ubiquitous in Compton-thin Seyfert 2 galaxies. The case of NGC 4388 shows the need for similar studies of more Compton-thin AGN to ascertain whether this is true.Comment: MNRAS accepted. 21 pages, 7 figures, 4 tables; Appendix with historical notes and 1 table. This version: Corrected minor typo and affiliatio

Suprathermal electron isotropy in high-beta solar wind and its role in heat flux dropouts

[1] Time variations in plasma beta and a parameter which measures isotropy in suprathermal electron pitch angle distributions show a remarkably close correspondence throughout the solar wind. The finding implies that high-beta plasma, with its multiple magnetic holes and sharp field and plasma gradients, is conducive to electron pitch-angle scattering, which reduces heat flux from the Sun without field-line disconnection. Thus the finding impacts our understanding of signatures we use to determine magnetic topology in the heliosphere

A complete census of AGN and their hosts from optical surveys?

Large optical surveys provide an unprecedented census of galaxies in the local Universe, forming an invaluable framework into which more detailed studies of objects can be placed. But how useful are optical surveys for understanding the co-evolution of black holes and galaxies, given their limited wavelength coverage, selection criteria, and depth? In this conference paper I present work-in-progress comparing optical and mid-IR diagnostics of three "unusual" low redshift populations (luminous Seyferts, dusty Balmer-strong AGN, ULIRGs) with a set of ordinary star-forming galaxies from the SDSS. I address the questions: How well do the mid-infrared and optical diagnostics of star formation and AGN strength agree? To what extent do optical surveys allow us to include extreme, dusty, morphologically disturbed galaxies in our "complete" census of black hole-galaxy co-evolution?Comment: Proceedings of contributed talk at "Co-Evolution of Central Black Holes and Galaxies" (eds. B.M. Peterson, R.S. Somerville, and T. Storchi-Bergmann), IAU symposium 267, August 2009. 6 pages, 5 figure