136 research outputs found
Modelling the dust content of spiral galaxies: More dust mass vs. enhanced dust grain emissivity
We present detailed modelling of the spectral energy distribution (SED) of
the spiral galaxies NGC 891, NGC 4013, and NGC 5907 in the far-infrared (FIR)
and sub-millimeter (submm) wavelengths. The model takes into account the
emission of the diffuse dust component, which is heated by the UV and optical
radiation fields produced by the stars, as well as the emission produced
locally in star forming HII complexes. The radiative transfer simulations of
Xilouris et al. (1999) in the optical bands are used to constrain the stellar
and dust geometrical parameters, as well as the total amount of dust. We find
that the submm emission predicted by our model can not account for the observed
fluxes at these wavelengths. We examine two cases, one having more dust
embedded in a second thin disk and another allowing for an enhanced
submillimeter emissivity of the dust grains. We argue that both cases can
equally well reproduce the observed SED. The case of having more dust embedded
in a second disk though, is not supported by the near-infrared observations and
thus more realistic distributions of the dust (i.e., in spiral arms and clumps)
have to be examined in order to better fit the surface brightness of each
galaxy.Comment: To appear in the proceedings of: "The Spectral Energy Distribution of
Gas-Rich Galaxies: Confronting Models with Data", Heidelberg, 4-8 Oct. 2004,
eds. C.C. Popescu and R.J. Tuffs, AIP Conf. Ser., in pres
Is the Galactic submillimeter dust emissivity underestimated?
We present detailed modeling of the spectral energy distribution (SED) of the
spiral galaxies NGC 891, NGC 4013, and NGC 5907 in the far-infrared and submm
wavelengths. The model takes into account the emission produced by the diffuse
dust and the star forming HII complexes. The dust mass is constrained by
radiative transfer simulations in the optical (Xilouris et al. 1999). We find
that the submm emission predicted by our model cannot account for the observed
fluxes. Two scenarios may account for the "missing" submm flux. In the first
scenario (Popescu et al. 2000), additional dust (to that derived from the
optical, and associated with young stars) is embedded in the galaxy in the form
of a thin disk and gives rise to additional submm emission. The other scenario
investigates whether the average submm emissivity of the dust grains is higher
than the values widely used in Galactic environments. In this case, the dust
mass is equal to that derived from the optical observations, and the submm
emissivity is treated as a free parameter calculated by fitting our model to
the observed SED. We find the submm emissivity value to be ~3 times that often
used for our Galaxy. While both scenarios reproduce the observed 850 micron
surface brightness, the extra embedded dust model is not supported by the near
infrared observations. We, thus, find that the enhanced dust submm emissivity
scenario is the most plausible. [abridged]Comment: 12 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
Turbulent and fast motions of H2 gas in active galactic nuclei
Querying the Spitzer archive for optically-selected active galactic nuclei
(AGN) observed in high-resolution mode spectroscopy, we identified radio and/or
interacting galaxies with highly turbulent motions of the H2 gas at a
temperature of a few hundred Kelvin. Unlike all other AGN that have unresolved
H2 line profiles at a spectral resolution of ~600, 3C236, 3C293,
IRAS09039+0503, MCG-2-58-22, and Mrk463E have intrinsic velocity dispersions
exceeding 200 km/s for at least two of the rotational S0, S1, S2, and S3 lines.
In a sixth source, 4C12.50, a blue wing was detected in the S1 and S2 line
profiles, indicating the presence of a warm molecular gas component moving at
-640 km/s with respect to the bulk of the gas at systemic velocity. Its mass is
5.2*10^7 M_sun, accounting for more than one fourth of the H2 gas at 374K, but
less than 1% of the cold H2 gas computed from CO observations. Because no
diffuse gas component of 4C12.50 has been observed to date to be moving at more
than 250 km/s from systemic velocity, the H2 line wings are unlikely to be
tracing gas in shock regions along the tidal tails of this merging system. They
can instead be tracing gas driven by a jet or entrained by a nuclear outflow,
which is known to emerge from the west nucleus of 4C12.50. It is improbable
that such an outflow, with an estimated mass loss rate of 130 M_sun/yr,
entirely quenches the star formation around this nucleus.Comment: A&A letters, revised to match the final print versio
A radio jet drives a molecular and atomic gas outflow in multiple regions within one square kiloparsec of the nucleus of the nearby galaxy IC5063
We analyzed near-infrared data of the nearby galaxy IC5063 taken with the
Very Large Telescope SINFONI instrument. IC5063 is an elliptical galaxy that
has a radio jet nearly aligned with the major axis of a gas disk in its center.
The data reveal multiple signatures of molecular and atomic gas that has been
kinematically distorted by the passage of the jet plasma or cocoon within an
area of ~1 kpc^2. Concrete evidence that the interaction of the jet with the
gas causes the gas to accelerate comes from the detection of outflows in four
different regions along the jet trail: near the two radio lobes, between the
radio emission tip and the optical narrow-line-region cone, and at a region
with diffuse 17.8 GHz emission midway between the nucleus and the north radio
lobe. The outflow in the latter region is biconical, centered 240 pc away from
the nucleus, and oriented perpendicularly to the jet trail. The diffuse
emission that is observed as a result of the gas entrainment or scattering
unfolds around the trail and away from the nucleus with increasing velocity. It
overall extends for >700 pc parallel and perpendicular to the trail. Near the
outflow starting points, the gas has a velocity excess of 600 km/s to 1200 km/s
with respect to ordered motions, as seen in [FeII], Pa alpha, or H2 lines. High
H2 (1-0) S(3)/S(1) flux ratios indicate non-thermal excitation of gas in the
diffuse outflow.Comment: Accepted for publication in Ap
Far-infrared line spectra of active galaxies from the Herschel/PACS Spectrometer: the complete database
We present a coherent database of spectroscopic observations of far-IR
fine-structure lines from the Herschel/PACS archive for a sample of 170 local
AGN, plus a comparison sample of 20 starburst galaxies and 43 dwarf galaxies.
Published Spitzer/IRS and Herschel/SPIRE line fluxes are included to extend our
database to the full 10-600 spectral range. The observations are
compared to a set of CLOUDY photoionisation models to estimate the above
physical quantities through different diagnostic diagrams. We confirm the
presence of a stratification of gas density in the emission regions of the
galaxies, which increases with the ionisation potential of the emission lines.
The new [OIV]25.9/[OIII]88 vs [NeIII]15.6/[NeII]12.8 diagram is proposed as the best diagnostic to separate: AGN activity
from any kind of star formation; and low-metallicity dwarf galaxies from
starburst galaxies. Current stellar atmosphere models fail to reproduce the
observed [OIV]25.9/[OIII]88 ratios, which are much higher when
compared to the predicted values. Finally, the ([NeIII]15.6 +
[NeII]12.8)/([SIV]10.5 + [SIII]18.7) ratio is proposed as
a promising metallicity tracer to be used in obscured objects, where optical
lines fail to accurately measure the metallicity. The diagnostic power of mid-
to far-infrared spectroscopy shown here for local galaxies will be of crucial
importance to study galaxy evolution during the dust-obscured phase at the peak
of the star formation and black-hole accretion activity (). This
study will be addressed by future deep spectroscopic surveys with present and
forthcoming facilities such as JWST, ALMA, and SPICA.Comment: Accepted for publication in the ApJ
Heating of the molecular gas in the massive outflow of the local ultraluminous-infrared and radio-loud galaxy 4C12.50
We present a comparison of the molecular gas properties in the outflow vs. in
the ambient medium of the local prototype radio-loud and ultraluminous-infrared
galaxy 4C12.50 (IRAS13451+1232), using new data from the IRAM Plateau de Bure
interferometer and 30m telescope, and the Herschel space telescope. Previous
H_2 (0-0) S(1) and S(2) observations with the Spitzer space telescope had
indicated that the warm (~400K) molecular gas in 4C12.50 is made up of a
1.4(+-0.2)x10^8 M_sun ambient reservoir and a 5.2(+-1.7)x10^7 M_sun outflow.
The new CO(1-0) data cube indicates that the corresponding cold (25K) H_2 gas
mass is 1.0(+-0.1)x10^10 M_sun for the ambient medium and <1.3x10^8 M_sun for
the outflow, when using a CO-intensity-to-H_2-mass conversion factor alpha of
0.8 M_sun /(K km/s pc^2). The combined mass outflow rate is high, 230-800
M_sun/yr, but the amount of gas that could escape the galaxy is low. A
potential inflow of gas from a 3.3(+-0.3)x10^8 M_sun tidal tail could moderate
any mass loss. The mass ratio of warm-to-cold molecular gas is >= 30 times
higher in the outflow than in the ambient medium, indicating that a
non-negligible fraction of the accelerated gas is heated to temperatures at
which star formation is inefficient. This conclusion is robust against the use
of different alpha factor values, and/or different warm gas tracers (H_2 vs.
H_2 plus CO): with the CO-probed gas mass being at least 40 times lower at 400K
than at 25K, the total warm-to-cold mass ratio is always lower in the ambient
gas than in the entrained gas. Heating of the molecular gas could facilitate
the detection of new outflows in distant galaxies by enhancing their emission
in intermediate rotational number CO lines.Comment: A&A, in pres
X-ray observations of highly obscured τ_(9.7 μm) > 1 sources: an efficient method for selecting Compton-thick AGN?
Observations with the IRS spectrograph onboard Spitzer have found many sources with very deep Si features at 9.7 μm, that have optical depths of τ > 1. Since it is believed that a few of these systems in the local Universe are associated with Compton-thick active galactic nuclei (hereafter AGN), we set out to investigate whether the presence of a strong Si absorption feature is a good indicator of a heavily obscured AGN. We compile X-ray spectroscopic observations available in the literature on the optically-thick (τ_(9.7 μm) > 1) sources from the 12 μm IRAS Seyfert sample. We find that the majority of the high-τ optically confirmed Seyferts (six out of nine)
in the 12 μm sample are probably Compton-thick. Thus, we provide direct evidence of a connection between mid-IR optically-thick galaxies and Compton-thick AGN, with the success rate being close to 70% in the local Universe. This is at least comparable to, if not better than, other rates obtained with photometric information in the mid to far-IR, or even mid-IR to X-rays. However, this technique cannot provide complete Compton-thick AGN samples, i.e., there are many Compton-thick AGN that do not display significant Si
absorption, with the most notable example being NGC1068. After assessing the validity of the high 9.7 μm optical-depth technique in the local Universe, we attempt to construct a sample of candidate Compton-thick AGN at higher redshifts. We compile a sample of seven high-τ Spitzer sources in the Great Observatories Origins Deep Survey (GOODS) and five in the Spitzer First-Look Survey.
All these have been selected to have no PAH features (EW_(6.2 μm) 10^(42) erg s^(−1)) of the detected GOODS sources corroborates that these are AGN. For FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources. SED fitting can support the presence of an AGN in the vast majority of sources. Owing to the limited photon statistics, we cannot derive useful constraints from X-ray spectroscopy on whether these sources are Compton-thick. However, the low L_(X)/L_(6 μm) luminosity ratios, suggest that at least four out of the six detected sources in GOODS may be associated with Compton-thick AGN
A view of the narrow-line region in the infrared: active galactic nuclei with resolved fine-structure lines in the Spitzer archive
We queried the Spitzer archive for high-resolution observations with the
Infrared Spectrograph of optically selected active galactic nuclei (AGN) for
the purpose of identifying sources with resolved fine-structure lines that
would enable studies of the narrow-line region (NLR) at mid-infrared
wavelengths. By combining 298 Spitzer spectra with 6 Infrared Space Observatory
spectra, we present kinematic information of the NLR for 81 z<=0.3 AGN. We used
the [NeV], [OIV], [NeIII], and [SIV] lines, whose fluxes correlate well with
each other, to probe gas photoionized by the AGN. We found that the widths of
the lines are, on average, increasing with the ionization potential of the
species that emit them. No correlation of the line width with the critical
density of the corresponding transition was found. The velocity dispersion of
the gas, sigma, is systematically higher than that of the stars, sigma_*, in
the AGN host galaxy, and it scales with the mass of the central black hole,
M_BH. Further correlations between the line widths and luminosities L, and
between L and M_BH, are suggestive of a three dimensional plane connecting
log(M_BH) to a linear combination of log(sigma) and log(L). Such a plane can be
understood within the context of gas motions that are driven by AGN feedback
mechanisms, or virialized gas motions with a power-law dependence of the NLR
radius on the AGN luminosity. The M_BH estimates obtained for 35 type 2 AGN
from this plane are consistent with those obtained from the M_BH-sigma_*
relation.Comment: ApJ, revised to match the print versio
High-ionization mid-infrared lines as black hole mass and bolometric luminosity indicators in active galactic nuclei
We present relations of the black hole mass and the optical luminosity with
the velocity dispersion and the luminosity of the [Ne V] and the [O IV]
high-ionization lines in the mid-infrared (MIR) for 28 reverberation-mapped
active galactic nuclei. We used high-resolution Spitzer Infrared Spectrograph
and Infrared Space Observatory Short Wavelength Spectrometer data to fit the
profiles of these MIR emission lines that originate from the narrow-line region
of the nucleus. We find that the lines are often resolved and that the velocity
dispersion of [Ne V] and [O IV] follows a relation similar to that between the
black hole mass and the bulge stellar velocity dispersion found for local
galaxies. The luminosity of the [Ne V] and the [O IV] lines in these sources is
correlated with that of the optical 5100A continuum and with the black hole
mass. Our results provide a means to derive black hole properties in various
types of active galactic nuclei, including highly obscured systems.Comment: accepted for publication in ApJ
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