494 research outputs found
Starburst-AGN Connection: A Lesson from High-z Powerful Radio Galaxies
Powerful radio galaxies at high redshift are highly useful in studies of
early evolution of AGN-hosting galaxies because their observed optical and near
infrared light are dominated by their stellar population rather than the
nonthermal continuum emitted by the central engine of AGNs. In addition, the
presence of AGN activity in them implies that a supermassive black hole has
been already made in their nuclei. These properties allow us to investigate a
possible starburst-AGN connection in early universe and then provide some
crucial hints for the formation mechanism of supermassive black holes. Taking
observational properties of high-z powerful radio galaxies into account, we
discuss a possible formation mechanism of supermassive black holes in their
nuclei.Comment: 7 pages, 1 figure, To be published in the proceedings of
"Stellar-Mass, Intermediate-Mass, and Supermassive Black Holes
Circumnuclear ionized gas in starburst nuclei
In order to study kinematical properties of starburst nuclei (SBNs), researchers made high-resolution spectroscopy of fifteen SBNs in the H alpha region using an intensified Reticon system attached to the coude focus of the 188-cm reflector at the Okayama Astrophysical Observatory. The instrumental resolution is 21 km s(-1) Full Width Half Maximum (FWHM) at lambda sub H alpha. As for the archetypical SBN, Mrk 538 (=NGC 7714), researchers present high-resolution emission line profiles of several species of ions such as (OIII), (NII), (SII), and (OII). Main results and conclusions are summarized. It has been known that emission-line profiles of SBNs are symmetrical and narrow. However, this high-resolution spectroscopy shows that the observed emission-line profiles of the SBNs have the following asymmetrical patterns; blueward, redward, and double-peaked. It is known that such features have been observed for narrow line regions (NLRs) of active galactic nuclei (AGNs). There is no remarkable correlation between the asymmetry index and the reddening indicator such as a Balmer decrement. Thus the line asymmetry is not attributed to inhomogeneous obscuration in the emitting regions. The observed FWHMs of the H alpha emission lines cover a range from 85 km s(-1) to 318 km s(-1) and are slightly larger than those of (NII) lambda 6584A emission except for the double-peaked SBNs. The FWHMs of H alpha emission show a good correlation with sin i (i is an inclination angle of galaxy). This correlation means that the FWHMs of the SBNs suffer significantly from rotational broadening. Mrk 52 is an anomalous SBN because it has narrow emission line widths for its high inclination angle (cf. Taniguchi 1987). From the above correlation, it is estimated that the intrinsic (i.e., rotation free) FWHMs of H alpha emission are about 50 km s(-1)
Dusty Tori of Seyfert Nuclei Probed by the Water Vapor Maser Emission: How Large are the Dusty Tori ?
Geometrical and physical properties of dusty tori of Seyfert nuclei probed by
the water vapor maser emission at 22 GHz are discussed. We assume that the
dusty torus has a simple cylindrical form and the maser emission can be
detected only when we observe the torus from almost edge-on views. The observed
low frequency of occurrence of the water vapor maser emission (less than 10
percent) suggests that the torus is a vertically thin cylinder whose outer
radius between a few pc and ~ 10 pc. However, the observed masing regions are
concentrated in the inner 1 pc regions of the torus. This property can be
explained by that only the inner a few pc regions have physical conditions
enough to cause the maser emission; the temperature is as high as several
hundred K and the density is as high as ~ 10^(10) cm^(-3).Comment: To appear in the Astrophysical Journal (Letters
Why Does NGC 1068 Have a More Powerful Active Galactic Nucleus than NGC 4258 ?
The nuclear gas kinematics probed by water vapor maser emission has shown
that two nearby active galaxies, NGC 1068 and NGC 4258, have a supermassive
object in their nuclei and their masses are nearly comparable; a few 10^7 solar
masses. Nevertheless, the activity of the central engine of NGC 1068 is more
powerful by two orders of magnitude than that of NGC 4258. Since it is
generally considered that the huge luminosities of active galactic nuclei are
attributed to the mass accretion onto a supermassive black hole, the above
observational results suggest that the accretion rate in NGC 1068 is much
higher than that in NGC 4258. Comparing the kinematical properties of the
accreting molecular clouds between NGC 1068 and NGC 4258, we find possible
evidence for dynamical gas accretion in NGC 1068, which may be responsible for
the more powerful central engine in this galaxy.Comment: 9 Latex (AASTeX) pages, 2 PostScript figures. To appear in
Publications of the Astronomical Society of Japa
Wing galaxies: A formation mechanism of the clumpy irregular galaxy Markarian 297
In order to contribute to an understanding of collision-induced starburst activities, the authors present a detailed case study on the starburst galaxy Markarian 297 (= NGC 6052 = Arp 209; hereafter Mrk 297). This galaxy is classified as a clumpy irregular galaxy (CIG) according to its morphological properties (cf. Heidmann, 1987). Two major clumps and many small clumps are observed in the entire region of Mrk 297 (Hecquet, Coupinot, and Maucherat 1987). The overall morphology of Mrk 297 is highly chaotic and thus it seems difficult to determine possible orbits of galaxy-galaxy collision. However, the authors have serendipitously found a possible orbit during a course of numerical simulations for a radial-penetration collision between galaxies. The radial-penetration collision means that an intruder penetrates a target galaxy radially passing by its nucleus. This kind of collision is known to explain a formation mechanism of ripples around disk galaxies (Wallin and Struck-Marcell 1988). Here, the authors show that the radial-penetration collision between galaxies successfully explains both overall morphological and kinematical properties of Mrk 297. The authors made two kinds of numerical simulations for Mrk 297. One is N-body (1x10(exp 4) particles) simulations in which effects of self gravity of the stellar disk are taken into account. These simulations are used to study detailed morphological feature of Mrk 297. The response of gas clouds are also investigated in order to estimate star formation rates in such collisions. The other is test-particle simulations, which are utilized to obtain a rough picture of Mrk 297 and to analyze the velocity field of Mrk 297. The techniques of the numerical simulations are the same as those in Noguchi (1988) and Noguchi and Ishibashi (1986). In the present model, an intruding galaxy with the same mass of a target galaxy moves on a rectilinear orbit which passes the center of the target
Seyfert-Type Dependences of Narrow Emission-Line Ratios and Physical Properties of High-Ionization Nuclear Emission-Line Regions in Seyfert Galaxies
In order to examine how narrow emission-line flux ratios depend on the
Seyfert type, we compiled various narrow emission-line flux ratios of 355
Seyfert galaxies from the literature. We present in this paper that the
intensity of the high-ionization emission lines, [Fe VII]6087, [Fe X]6374 and
[Ne V]3426, tend to be stronger in Seyfert 1 galaxies than in Seyfert 2
galaxies. In addition to these lines, [O III]4363 and [Ne III]3869, whose
ionization potentials are not high (< 100 eV), but whose critical densities are
significantly high (> 10^7 cm^-3), also exhibit the same tendency. On the other
hand, the emission-line flux ratios among low-ionization emission lines do not
show such a tendency. We point out that the most plausible interpretation of
these results is that the high-ionization emission lines arise mainly from
highly-ionized, dense gas clouds, which are located very close to nuclei, and
thus can be hidden by dusty tori. To examine the physical properties of these
highly-ionized dense gas clouds, photoionization model calculations were
performed. As a result, we find that the hydrogen density and the ionization
parameter of these highly-ionized dense gas clouds are constrained to be n_H >
10^6 cm^-3 and U > 10^-2, respectively. These lower limits are almost
independent both from the metallicity of gas clouds and from the spectral
energy distribution of the nuclear ionizing radiation.Comment: 32 pages, to appear in Publications of the Astronomical Society of
Japa
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