278 research outputs found
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
Circumnuclear stellar population, morphology and environment of Seyfert 2 galaxies: an evolutionary scenario
We investigate the relation between the characteristics of the circumnuclear
stellar population and both the galaxy morphology and the presence of close
companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present
unambiguous signatures of recent episodes of star formation within 300
pc from the nucleus. When we relate this property with the Hubble type of the
host galaxy, we find that the incidence of recent circumnuclear star formation
increases along the Hubble sequence, and seems to be larger than in non-Seyfert
galaxies for the early Hubble types S0 and Sa, but similar to that in
non-Seyfert galaxies for later Hubble types. Both in early-type and late-type
Seyferts, the presence of recent star-formation is related to the galaxy
morphology in the inner few kiloparsecs, as observed in HST images through the
filter F606W by Malkan et al., who has assigned a late ``inner Hubble type'' to
most Seyfert 2s with recent nuclear star-formation. This new classification is
due to the presence of dust lanes and spiral structures in the inner region.
The presence of recent star formation in Seyfert 2 nuclei is also related to
interactions: among the 13 galaxies of the sample with close companions or in
mergers, 9 have recent star formation in the nucleus. These correlations
between the presence of companions, inner morphology and the incidence of
recent star formation suggest an evolutionary scenario in which the interaction
is responsible for sending gas inwards which both feeds the AGN and triggers
star-formation. The starburst then fades with time and the composite Seyfert 2
+ Starburst nucleus evolves to a ``pure'' Seyfert 2 nucleus with an old stellar
population.Comment: 12 pages, 6 figures, 2 table
The difference between the narrow line region of Seyfert 1 and Seyfert 2 galaxies
This paper presents a comparative study of emission line ratios of the Narrow
Line Region (NLR) of Seyfert 1 and Seyfert 2 galaxies. It includes a literature
compilation of the emission line fluxes [OII]3727A, [NeIII]3869A, [OIII]5007A
and [NeV]3426A, as well as 60mum continuum flux, for a sample of 52 Seyfert 1's
and 68 Seyfert 2's. The distribution of the emission line ratios [OII]/[NeIII]
and [OII]/[NeV] shows that Seyfert 1's and Seyfert 2's are statistically
different, in the sense that Seyfert 1's have values smaller than those of
Seyfert 2's, indicating a higher excitation spectrum. These and other emission
line ratios are compared with sequences of models which combine different
proportions of matter and ionization bounded clouds and also sequences of
models which vary only the ionization parameter. This comparison shows that the
former models reproduce better the overall distribution of emission line
ratios, indicating that Seyfert 1's have a smaller number of ionization bounded
clouds than Seyfert 2's. This difference, together with other results available
in the literature, are interpreted from the point of view of four different
scenarios. The most likely scenario assumes that Seyfert 1's have NLR's smaller
than those of Seyfert 2's, possibly due to a preferential alignment of the
torus axis close to the host galaxy plane axis in Seyfert 1's.Comment: 13 pages, 9 postscript figures, 3 tables, accepted for publication in
the Astrophysical Journa
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