10 research outputs found
Evidence for a Physically Compact Narrow-Line Region in the Seyfert 1 Galaxy NGC 5548
We have combined HST/FOS and ground-based spectra of the Seyfert 1 galaxy NGC
5548 to study the narrow emission lines over the 1200 -- 10,000 angstrom
region. All of the spectra were obtained when the broad emission line and
continuum fluxes were at an historic low level, allowing us to accurately
determine the contribution of the narrow-line region (NLR) to the emission
lines. We have generated multicomponent photoionization models to investigate
the relative strength of the high ionization lines compared to those in Seyfert
2 galaxies, and the weakness of the narrow Mg II 2800 line. We present evidence
for a high ionization component of NLR gas that is very close to the nucleus
(~1 pc). This component must be optically thin to ionizing radiation at the
Lyman edge (tau = 2.5) to avoid producing [O I] and Mg II in a partially
ionized zone. The very high ionization lines (N V, [Ne V], [Fe VII], [Fe X])
are stronger than the predictions of our standard model, and we show that this
may be due to supersolar abundances and/or a ``blue bump'' in the extreme
ultraviolet (although recent observations do not support the latter). An outer
component of NLR gas (at only ~70 pc from the continuum source) is needed to
produce the low ionization lines. We show that the outer component may contain
dust, which further reduces the Mg II flux by depletion and by absorption of
the resonance photons after multiple scatterings. We show that the majority of
the emission in the NLR of NGC 5548 must arise within about ~70 pc from the
nucleus. Thus, the NLR in this Seyfert 1 galaxy is very physically compact,
compared to the typical NLR in Seyfert 2 galaxies.Comment: 38 pages, Latex, includes 2 figures (postscript), to appear in Ap
The Narrow Line Region of Ark 564
The continuum and emission-line spectrum of the narrow-line Seyfert 1 galaxy
Ark 564 is used to investigate, for the first time, the physical conditions and
structure of its narrow line region (NLR). For this purpose, composite models,
accounting for the coupled effect of photoionization and shocks, are employed.
The emission-line spectrum of Ark 564, which ranges from the ultraviolet to the
near-infrared, shows a rich forbidden line spectrum. Strong emphasis is given
to the study of the coronal line region. The diversity of physical conditions
deduced from the observations requires multi-cloud models to reproduce the
observed lines and continuum. We find that a combination of high velocity (Vs =
1500 km/s) shock-dominated clouds as well as low velocity (Vs = 150 km/s)
radiation-dominated clouds explains the coronal lines, while the optical
low-ionization lines are mainly explained by shock-dominated clouds. The
results for Ark 564 are compared with those obtained for other Seyfert galaxies
previously analyzed such as NGC 5252, Circinus, NGC 4051 and NGC 4151. The
model results for the ultraviolet and optical permitted lines suggest that the
broad line region may contribute up to 80%, depending on the emission-line,
being of about 30% for Hbeta. The consistency of the multi-cloud model is
checked by comparing the predicted and observed continuum, from radio to X-ray,
and indicate that the dust-to-gas ratio in the clouds varies from 10^{-15} to
10^{-12}.Comment: 18 pages, 1 figure. Accepted in A&
Extended High-Ionization Nuclear Emission-Line Region in the Seyfert Galaxy NGC 4051
We present an optical spectroscopic analysis of the well-known Seyfert galaxy
NGC 4051. The high-ionization nuclear emission-line region (HINER) traced by
[Fe X]6374 is found to be spatially extended to a radius of 3a rcseconds (150
pc) west and southwest from the nucleus; NGC 4051 is the third example which
has an extended HINER.
The nuclear spectrum shows that the flux of [Fe X]6374 is stronger than that
of [Fe VII] 6087 in our observation. This property cannot be interpreted in
terms of a simple one-zone photoionization model.
In order to understand what happens in the nuclear region in NGC 4051, we
investigate the physical condition of the nuclear emission-line region in
detail using new photoionization models in which the following three
emission-line components are taken into account; (1) optically thick,
ionization-bounded clouds; (2) optically thin, matter-bounded clouds; and (3) a
contamination component which emits H and H lines. Here the
observed extended HINER is considered to be associated with the low-density,
matter-bounded clouds. Candidates of the contamination component are either the
broad-line region (BLR) or nuclear star forming regions or both. The complexity
of the excitation condition found in NGC 4051 can be consistently understood if
we take account of these contamination components.Comment: 16 pages, including figures. To Appear in the Astronomical Journal
February 2000 Issu
The peculiar galaxy Mkn 298 revisited through integral field spectroscopy
Spectroscopic and imaging data of the peculiar galaxy Mkn 298 are presented
in this paper. Narrow-band Halpha and broad-band R images are used to study the
star formation rate in the galaxy and its morphology, which is typical of a
merging system. Long-slit and integral field spectra are used to assess the
kinematics of gas and stars, and the nature of the ionizing source at different
distances from the nucleus. In particular, the nucleus of Mkn 298 is
characterized by peculiar line ratios: [NII] 6583/Halpha is typical of HII-like
regions, while [OI] 6300/Halpha could indicate the presence of an active
galactic nucleus. We show that models where a shock component is added to
photoionization from a starburst allow to reproduce the observed line ratios.
Mkn 298 is thus most likely a star forming galaxy, rather than a galaxy hosting
an active nucleus.Comment: 9 figures, 12 pages, accepted for publication on Astronomy &
Astrophysic