Was 49b: An Overmassive AGN in a Merging Dwarf Galaxy?

We present a combined morphological and X-ray analysis of Was 49, an isolated, dual AGN system notable for the presence of a dominant AGN Was 49b in the disk of the primary galaxy Was 49a, at a projected radial distance of 8 kpc from the nucleus. Using X-ray data from Chandra, NuSTAR, and Swift, we find that this AGN has a bolometric luminosity of L_bol ~ 2 x 10^45 erg/s, with a black hole mass of M_BH=1.3^{+2.9}_{-0.9} x 10^8 M_Sol. Despite its large mass, our analysis of optical data from the Discovery Channel Telescope shows that the supermassive black hole is hosted by a stellar counterpart with a mass of only 5.6^{+4.9}_{-2.6} x 10^9 M_Sol, making the SMBH potentially larger than expected from SMBH-galaxy scaling relations, and the stellar counterpart exhibits a morphology that is consistent with dwarf elliptical galaxies. Our analysis of the system in the r and K bands indicates that Was 49 is a minor merger, with a mass ratio of Was 49a to Was 49b between 1:7 and 1:15. This is in contrast with findings that the most luminous merger-triggered AGNs are found in major mergers, and that minor mergers predominantly enhance AGN activity in the primary galaxy.Comment: 11 pages, 6 figures, accepted for publication in Ap

Estimating the Ages of Bars: Implications for the Bar-AGN-Star Formation Connection

In an effort to elevate to higher grounds our understanding on the impact of the formation and evolution of bars in the formation and evolution of galaxies, we have developed a diagnostic tool to distinguish between recently formed and evolved bars. Our method was applied in the study of a sample of 14 galaxies and revealed that, apparently, AGN activity tends to appear in galaxies which have young bars rather than evolved bars. This suggests that the time scale for the fueling of AGN by bars is short, and may help to explain, for instance, why there is not a clear correlation between the presence of bars and AGN in galaxies.Comment: 4 pages including 2 figures and 1 table; contributed talk to appear in the proceedings of the IAU Symp. 222, The Interplay among Black Holes, Stars and ISM in Galactic Nuclei, Th. Storchi Bergmann, L.C. Ho, H.R. Schmitt, eds., held in Gramado, March 200

The Stellar Content of Active Galaxies

We present results of a long-slit spectroscopic study of 39 active and 3 normal galaxies. Stellar absorption features, continuum colors and their radial variations are analyzed in an effort to characterize the stellar population in these galaxies and detect the presence of a featureless continuum underlying the starlight spectral component. Spatial variations of the equivalent widths of conspicuous absorption lines and continuum colors are detected in most galaxies. Star-forming rings, in particular, leave clear fingerprints in the equivalent widths and color profiles. We find that the stellar populations in the inner regions of active galaxies present a variety of characteristics, and cannot be represented by a single starlight template. Dilution of the stellar lines by an underlying featureless continuum is detected in most broad-lined objects, but little or no dilution is found for the most of the 20 type 2 Seyferts in the sample. Color gradients are also ubiquitous. In particular, all but one of the observed Seyfert 2s are redder at the nucleus than in its immediate vicinity. Possible consequences of these findings are outlined.Comment: MNRAS, accepted. Uses mn and epsf.sty. [29 pages, 3 jpg + 44 ps figures]. also available at http://www.fsc.ufsc.br/~ci

The Nature of the Optical Light in Seyfert 2 Galaxies with Polarized Continuum

We investigate the nature of the optical continuum and stellar population in the central kpc of the Seyfert 2s Mrk 348, Mrk 573, NGC 1358 and Mrk 1210 using long-slit spectra obtained along the radio or extended emission axis. These galaxies are known to have polarized continuum-including polarized broad lines in Mrk 348 and Mrk 1210--and previous studies indicate featureless continuum (FC) contributions in the 20-50% range at 5500 A. Nevertheless, our measurements of the equivalent widths of absorption lines and continuum ratios as a function of distance from the nuclei show no dilution of the lines nor bluening of the spectrum, as expected if a blue FC was present at the nucleus in the above proportions. We investigate one possibility to account for this effect: that the stellar population at the nucleus is the same as that from the surrounding bulge and dominates the nuclear light. A spectral analysis confirms that this hypothesis works for Mrk 348, NGC 1358 and Mrk 1210, for which we find stellar contributions at the nucleus larger than 90% at all wavelengths. We find that a larger stellar population contribution to the nuclear spectra can play the role of the ``second FC'' source inferred from previous studies. Stellar population synthesis shows that the nuclear regions of Mrk 348 and Mrk 1210 have important contributions of young to intermediate age stars (0--100 Myr), not present in templates of elliptical galaxies. In the case of Mrk 1210, this is further confirmed by the detection of a ``Wolf-Rayet feature'' in the nuclear emission-line spectrum.Comment: ApJ, accepted. Uses aaspp4.sty. [22 pages

The Frequency of Active and Quiescent Galaxies with Companions: Implications for the Feeding of the Nucleus

We analyze the idea that nuclear activity, either AGN or star formation, can be triggered by interactions, studying the percentage of active, HII and quiescent galaxies with companions. Our sample was selected from the Palomar survey, and avoids selection biases faced by previous studies. The comparison between the local galaxy density distributions showed that in most cases there is no statistically significant difference among galaxies of different activity types. The comparison of the percentage of galaxies with nearby companions showed that there is a higher percentage of LINERs, transition, and absorption line galaxies with companions than Seyferts and HII galaxies. However, we find that when we consider only galaxies of similar morphological types (ellipticals or spirals), there is no difference in the percentage of galaxies with companions among different activity types, indicating that the former result was due to the morphology-density effect. Also, only small differences are found when we consider galaxies with similar Halpha luminosities. The comparison between HII galaxies of different Halpha luminosities shows that there is a significantly higher percentage of galaxies with companions among the higher luminosity HII galaxies, indicating that interactions increase the amount of circumnuclear star formation, in agreement with previous results. The fact that we find that galaxies of different activity types have the same percentage of companions, suggests that interactions between galaxies is not a necessary condition to trigger the nuclear activity in AGNs. We compare our results with previous ones and discuss their implications. (abridged)Comment: 30 pages, including 6 figures and 3 tables. To appear in The Astronomical Journal, November issu