163 research outputs found
Probing the Circumnuclear Stellar Populations of Starburst Galaxies in the Near-infrared
We employ the NASA Infrared Telescope Facility's near-infrared spectrograph
SpeX at 0.8-2.4m to investigate the spatial distribution of the stellar
populations (SPs) in four well known Starburst galaxies: NGC34, NGC1614,
NGC3310 and NGC7714. We use the STARLIGHT code updated with the synthetic
simple stellar populations models computed by Maraston (2005, M05). Our main
results are that the NIR light in the nuclear surroundings of the galaxies is
dominated by young/intermediate age SPs (yr), summing from
40\% up to 100\% of the light contribution. In the nuclear aperture of
two sources (NGC1614 and NGC3310) we detected a predominant old SP component
(yr), while for NGC34 and NGC7714 the younger component
prevails. Furthermore, we found evidence of a circumnuclear star formation
ring-like structure and a secondary nucleus in NGC1614, in agreement with
previous studies. We also suggest that the merger/interaction experienced by
three of the galaxies studied, NGC1614, NGC3310 and NGC7714 can explain the
lower metallicity values derived for the young SP component of these sources.
In this scenario the fresh unprocessed metal poorer gas from the
destroyed/interacting companion galaxy is driven to the centre of the galaxies
and mixed with the central region gas, before star formation takes place. In
order to deepen our analysis, we performed the same procedure of SP synthesis
using Maraston (2011, M11) EPS models. Our results show that the newer and
higher resolution M11 models tend to enhance the old/intermediate age SP
contribution over the younger ages
Near-IR dust and line emission from the central region of Mrk1066: Constraints from Gemini NIFS
We present integral field spectroscopy of the inner 350 pc of the Mrk1066
obtained with Gemini NIFS at a spatial resolution of 35 pc. This high spatial
resolution allowed us to observe, for the first time in this galaxy, an
unresolved dust concentration with mass 0.014 M_Sun, which may be part of the
dusty torus. The emission-line fluxes are elongated in PA=135/315deg in
agreement with the [OIII] and radio images and, except for the H lines, are
brighter to the north-west than to the south-east. The H emission is stronger
to the south-east, where we find a large region of star-formation. The strong
correlation between the radio emission and the highest emission-line fluxes
indicates that the radio jet plays a fundamental role at these intensity
levels. The H2 flux is more uniformly distributed and has an excitation
temperature of 2100 K. Its origin appears to be circumnuclear gas heated by
X-rays from the AGN. The [FeII] emission also is consistent with X-ray heating,
but with additional emission due to excitation by shocks in the radio jet. The
coronal-line emission of [CaVIII] and [SIX] are unresolved by our observations
indicating a distribution within 18pc from the nucleus. The reddening ranges
from E(B-V) ~ 0 to E(B-V) ~ 1.7 with the highest values defining a S-shaped
structure along PA ~ 135/315deg. The emission-line ratios are Seyfert-like
within the ionization cone indicating that the line emission is powered by the
central active nucleus in these locations. Low ionization regions are observed
away from the ionization cone, and may be powered by the diffuse radiation
field which filters through the ionization cone walls. Two regions at 0.5
arcsec south-east and at 1 arcsec north-west of the nucleus show starburst-like
line ratios, attributed to additional emission from star forming regions.Comment: 14 pages, 10 figures, accepted for publication in MNRA
Molecular Hydrogen and [FeII] in Active Galactic Nuclei
(Abridge) Near-infrared spectroscopy is used to study the kinematics and
excitation mechanisms of the H2 and [FeII] gas in a sample of AGN. The H2 lines
are unresolved in all objects in which they were detected while the [FeII]
lines have widths implying gas velocities of up to 650 km/s. This suggests
that, very likely, the H2 and [FeII] emission does not originate from the same
parcel of gas. Molecular H2 were detected in 90% of the sample, including PG
objects, indicating detectavel amounts of molecular material even in objects
with low levels of circumnuclear starburst activity. The data favors thermal
excitation for the H2 lines. Indeed, in NGC3227, Mrk766, NGC4051 and NGC4151,
the molecular emission is found to be purely thermal. This result is also
confirmed by the rather similar vibrational and rotational temperatures in the
objects for which they were derived. [FeII] lines are detected in all of the
AGN. The [FeII] 1.254mu/Pa-beta ratio is compatible with excitation of the
[FeII] by the active nucleus, but in Mrk 766 it implies a stellar origin. A
correlation between H2/Br-gamma and [FeII]/Pa-beta is found. We confirm that it
is a useful diagnostic tool in the NIR to separate emitting line objects by
their level of nuclear activity. X-ray excitation models are able to explain
the observed H2 and part of the [FeII] emission. Most likely, a combination of
X-ray heating, shocks driven by the radio jet, and circumnuclear star formation
contributes, in different proportions, to the H2 and [FeII] emission. In most
of our spectra, the [FeII] 1.257mu/1.644mu ratio is found to be 30% lower than
the intrinsic value based on current atomic data. This implies either than the
extinction towards the [FeII] emitting clouds is very similar in most objects
or there are possible inaccuracies in the A-values in the [FeII] transitions.Comment: 18 pages, 6 figures, Accepted for publication in Astronomy &
Astrophysic
The first detection of near-infrared CN bands in active galactic nuclei: signature of star formation
We present the first detection of the near-infrared CN absorption band in the
nuclear spectra of active galactic nuclei (AGN). This feature is a recent star
formation tracer, being particularly strong in carbon stars. The equivalent
width of the CN line correlates with that of the CO at 2.3 microns, as expected
in stellar populations (SP) with ages between ~ 0.2 and ~ 2 Gyr. The presence
of the 1.1 microns CN band in the spectra of the sources is taken as an
unambiguous evidence of the presence of young/intermediate SP close to the
central source of the AGN. Near-infrared bands can be powerful age indicators
for star formation connected to AGN, the understanding of which is crucial in
the context of galaxy formation and AGN feedback.Comment: Accepted for publication in The Astrophysical Journal Letters. 4
pages, 3 figure
The continuum and narrow line region of the NLS1 galaxy Mrk 766
We present the first spectroscopic observations in the interval 0.8-4.0
microns, complemented with HST/UV and optical spectroscopy, of the narrow line
Seyfert 1 galaxy Mrk766. The NIR spectrum is characterized by permitted lines
of HI, HeI, HeII and FeII, and forbidden lines of [SII], [SIII] and [FeII]
among others. High ionized species such as [SiIX], [SiX], [SIX] and [MgVII] are
also observed. The continuum has a complex shape, with contribution of the
central engine, circumnuclear stellar population and dust. This last component
is evidenced by the presence of an excess of emission peaking at 2.25 microns,
fitted by blackbody function with T_bb=1200K. That temperature is close to the
evaporation temperature of graphite grains. As such, it provides strong
evidence of hot dust, probably very close to the nucleus. Consistent modeling
of the line and broad band continuum spectrum by composite models, which
account for the photoionizing flux of the central engine and shocks, shows that
the shock velocities are between 100 and 500 km/s, the preshock densities
between 100 and 1000 cm^-3 and the radiation fluxes from the active centre
between 10^9 and 5x10^12 photons cm^-2 s^-1 eV^-1 at 1 Ryd with spectral
indices =-1.5 and =-0.4. Adopting silicon grains,
dust-to-gas ratios are between 10^-{6} and 4x10 by mass. The emitting
clouds are at an average distance of 160 pc from the centre, with high velocity
clouds closer and low velocity clouds farther from the centre. The N/H relative
abundance could be twice solar. In constrast, Fe is depleted from the gaseous
phase by a factor >2. Ratios of calculated to observed line ratios to Hbeta
indicate an average contribution of the broad line region to the observed Hbeta
of about 40%.Comment: 15 pages, 6 figures. Accepted to MNRA
Multi-wavelength properties of the high-energy bright Seyfert 1 galaxy IGR J18027-1455
A new sample of hard X-ray sources in the Galactic Plane is being revealed by
the regular observations performed by the INTEGRAL satellite. The full
characterization of these sources is mandatory to understand the hard X-ray
sky. Here we report new multifrequency radio, infrared and optical observations
of the source IGR J18027-1455, as well as a multi-wavelength study from radio
to hard X-rays. The radio counterpart of IGR J18027-1455 is not resolved at any
observing frequency. The radio flux density is well fitted by a simple power
law with a spectral index alpha=-0.75+/-0.02. This value is typical of
optically thin non-thermal synchrotron emission originated in a jet. The NIR
and optical spectra show redshifted emission lines with z=0.034, and a broad
Halpha line profile with FWHM ~3400 km/s. This suggests an Active Galactic
Nucleus (AGN) of type 1 as the optical counterpart of IGR J18027-1455. We
confirm the Seyfert 1 nature of the source, which is intrinsically bright at
high energies both in absolute terms and when scaled to a normalized 6 cm
luminosity. Finally, comparing its X-ray luminosity with isotropic indicators,
we find that the source is Compton thin and AGN dominated. This indicates that
INTEGRAL might have just seen the tip of the iceberg, and several tens of such
sources should be unveiled during the course of its lifetime.Comment: 6 pages, 6 figures. Accepted for publication in A&A. Minor changes
according to referee repor
Optical/NIR stellar absorption and emission-line indices from luminous infrared galaxies
We analyze a set of optical-to-near-infrared long-slit nuclear spectra of 16
infrared-luminous spiral galaxies. All of the studied sources present H
emission, which reflects the star-forming nature of our sample, and they
clearly display H I emission lines in the optical. Their continua contain many
strong stellar absorption lines, with the most common features due to Ca I, Ca
II, Fe I, Na I, Mg I, in addition to prominent absorption bands of TiO, VO,
ZrO, CN and CO. We report a homogeneous set of equivalent width (EW)
measurements for 45 indices, from optical to NIR species for the 16
star-forming galaxies as well as for 19 early type galaxies where we collected
the data from the literature. This selected set of emission and
absorption-feature measurements can be used to test predictions of the
forthcoming generations of stellar population models. We find correlations
among the different absorption features and propose here correlations between
optical and NIR indices, as well as among different NIR indices, and compare
them with model predictions. While for the optical absorption features the
models consistently agree with the observations,the NIR indices are much harder
to interpret. For early-type spirals the measurements agree roughly with the
models, while for star-forming objects they fail to predict the strengths of
these indices.Comment: accepted for publication in MNRA
Galaxies behind the Galactic plane: First results and perspectives from the VVV Survey
Vista Variables in The Via Lactea (VVV) is an ESO variability survey that is
performing observations in near infrared bands (ZYJHKs) towards the Galactic
bulge and part of the disk with the completeness limits at least 3 mag deeper
than 2MASS. In the present work, we searched in the VVV survey data for
background galaxies near the Galactic plane using ZYJHKs photometry that covers
1.636 square degrees. We identified 204 new galaxy candidates by analyzing
colors, sizes, and visual inspection of multi-band (ZYJHKs) images. The galaxy
candidates colors were also compared with the predicted ones by star counts
models considering a more realistic extinction model at the same completeness
limits observed by VVV. A comparison of the galaxy candidates with the expected
one by Milennium simulations is also presented. Our results increase the number
density of known galaxies behind the Milky Way by more than one order of
magnitude. A catalog with galaxy properties including ellipticity, Petrosian
radii and ZYJHKs magnitudes is provided, as well as comparisons of the results
with other surveys of galaxies towards Galactic plane.Comment: 27 pages, 10 figures, 2 tables; in press at The Astronomical Journa
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