64 research outputs found
A complete census of silicate features in the mid-infrared spectra of active galaxies
We present a comprehensive study of the silicate features at 9.7 and 18
micron of a sample of almost 800 active galactic nuclei (AGN) with available
spectra from the Spitzer InfraRed Spectrograph (IRS). We measure the strength
of the silicate feature at 9.7 micron, S9.7, before and after subtracting the
host galaxy emission from the IRS spectra. The numbers of type 1 and 2 AGN with
the feature in emission increase by 20 and 50%, respectively, once the host
galaxy is removed, while 35% of objects with the feature originally in
absorption exhibit it in even deeper absorption. The peak of S9.7, lambda_peak,
has a bimodal distribution when the feature is in emission, with about 65% of
the cases showing lambda_peak > 10.2 micron. Silicates can appear in emission
in objects with mid-infrared (MIR) luminosity spanning over six orders of
magnitude. The derived distributions of the strength of the silicate features
at 9.7 and 18 micron provide a solid test bed for modeling the dust
distribution in AGN. Clumpiness is needed in order to produce absorption
features in unobscured AGN and can also cause the silicates to be in absorption
at 9.7 micron and in emission at 18 micron in type 1 sources. We find the
`cosmic' silicates of Ossenkopf et al. to be more consistent with the
observations than Draine's `astronomical' silicates. Finally, we discuss the
possibility of a foreground absorber to explain the deep silicate absorption
features in the MIR spectra of some type 2 AGN.Comment: 10 pages, 11 figures. Accepted for publication in Ap
Multi-band Emission of Active Galactic Nuclei: the Relationship of Stellar and Gravitational-Accretion Activity
One of the remaining open issues in the context of the analysis of active galactic nuclei is the evidence that nuclear gravitational accretion is often accompanied by a concurrent starburst activity. What is, in this picture, the role played by the obscuring dust around the nucleus and what does the state of the art models have to say? Can the infrared data provided by Spitzer and Herschel help us in extensively investigate both phenomena and, if so, how and with what limitations? Does the presence of an active nucleus have an impact in the mid- and far-infrared properties of galaxies? Which are the effects of simultaneous nuclear gravitational accretion and starburst activities in these same galaxies?
This Thesis presents our contribution to the efforts of answering these questions. I report on results coming from a comparative study of various approaches adopted while modelling active galactic nuclei, focusing mostly on the much-debated issue about the morphology of the dust distribution in the toroidal structure surrounding their nuclear centre. We largely illustrate that properties of dust in active galactic nuclei as measured by matching observations (be it broad band infrared photometry or infrared spectra) with models strongly depend on the choice of the dust distribution.
Further, I describe a spectral energy distribution fitting tool appositely developed to derive simultaneously the physical properties of active nuclei and coexisting starbursts. The procedure was developed to make the best use of Spitzer and Herschel mid- and far-infrared observations. Such data play a crucial role in this context, providing much stronger constraints on the models with respect to the previous observing facilities. The tool has been applied to a large sample of extragalactic sources representing the Herschel/Multi-tiered Extragalactic Survey population with mid-infrared spectra from Spitzer and with a plethora of multi-wavelength data (SDSS, Spitzer and Herschel/SPIRE). The goal of such work is to study the impact of a possible presence of an active nucleus on the host galaxy's properties. Finally, I present the main results obtained. In particular, I focus on the analysis of the star formation rate in connection to the presence of an active nucleus and on the
comparison of the properties of the hot, heated by the active nucleus, and cold, starburst heated, dust components
Extreme star formation events in quasar hosts over
We explore the relationship between active galactic nuclei and star formation
in a sample of 513 optically luminous type 1 quasars up to redshifts of 4
hosting extremely high star formation rates (SFRs). The quasars are selected to
be individually detected by the \textit{Herschel} SPIRE instrument at 3 at 250 m, leading to typical SFRs of order of 1000
Myr. We find the average SFRs to increase by almost a factor
10 from to , mirroring the rise in the comoving SFR density
over the same epoch. However, we find that the SFRs remain approximately
constant with increasing accretion luminosity for accretion luminosities above
10 L. We also find that the SFRs do not correlate with black
hole mass. Both of these results are most plausibly explained by the existence
of a self-regulation process by the starburst at high SFRs, which controls SFRs
on time-scales comparable to or shorter than the AGN or starburst duty cycles.
We additionally find that SFRs do not depend on Eddington ratio at any
redshift, consistent with no relation between SFR and black hole growth rate
per unit black hole mass. Finally, we find that high-ionisation broad
absorption line (HiBAL) quasars have indistinguishable far-infrared properties
to those of classical quasars, consistent with HiBAL quasars being normal
quasars observed along a particular line of sight, with the outflows in HiBAL
quasars not having any measurable effect on the star formation in their hosts.Comment: 12 pages, 6 figure
Ultraviolet spectra of extreme nearby star-forming regions --- approaching a local reference sample for JWST
Nearby dwarf galaxies provide a unique laboratory in which to test stellar
population models below . Such tests are particularly important for
interpreting the surprising high-ionization UV line emission detected at
in recent years. We present HST/COS ultraviolet spectra of ten nearby
metal-poor star-forming galaxies selected to show He II emission in SDSS
optical spectra. The targets span nearly a dex in gas-phase oxygen abundance
() and present uniformly large specific star
formation rates (sSFR ). The UV spectra confirm
that metal-poor stellar populations can power extreme nebular emission in
high-ionization UV lines, reaching C III] equivalent widths comparable to those
seen in systems at . Our data reveal a marked transition in UV
spectral properties with decreasing metallicity, with systems below
() presenting minimal
stellar wind features and prominent nebular emission in He II and C IV. This is
consistent with nearly an order of magnitude increase in ionizing photon
production beyond the -ionizing edge relative to H-ionizing flux
as metallicity decreases below a fifth solar, well in excess of standard
stellar population synthesis predictions. Our results suggest that often
neglected sources of energetic radiation such as stripped binary products and
very massive O-stars produce a sharper change in the ionizing spectrum with
decreasing metallicity than expected. Consequently, nebular emission in C IV
and He II powered by these stars may provide useful metallicity constraints in
the reionization era.Comment: 27 pages, 13 figures, 11 tables, accepted for publication in MNRA
Spectroscopic detection of CIV in a galaxy at z=7.045: Implications for the ionizing spectra of reionization-era galaxies
We present Keck/MOSFIRE observations of UV metal lines in four bright
gravitationally-lensed z~6-8 galaxies behind the cluster Abell 1703. The
spectrum of A1703-zd6, a highly-magnified star forming galaxy with a
Lyman-alpha redshift of z=7.045, reveals a confident detection of the nebular
CIV emission line (unresolved with FWHM < 125 km/s). UV metal lines are not
detected in the three other galaxies. At z~2-3, nebular CIV emission is
observed in just 1% of UV-selected galaxies. The presence of strong CIV
emission in one of the small sample of galaxies targeted in this paper may
indicate hard ionizing spectra are more common at z~7. The total estimated
equivalent width of the CIV doublet (38 A) and CIV/Lyman-alpha flux ratio (0.3)
are comparable to measurements of narrow-lined AGNs. Photoionization models
show that the nebular CIV line can also be reproduced by a young stellar
population, with very hot metal poor stars dominating the photon flux
responsible for triply ionizing carbon. Regardless of the origin of the CIV, we
show that the ionizing spectrum of A1703-zd6 is different from that of typical
galaxies at z~2, producing more H ionizing photons per unit 1500A luminosity
and a larger flux density at 30-50 eV. If such extreme radiation fields are
typical in UV-selected systems at z>7, it would indicate that reionization-era
galaxies are more efficient ionizing agents than previously thought.
Alternatively, we suggest that the small sample of Lyman-alpha emitters at z>7
may trace a rare population with intense radiation fields capable of ionizing
their surrounding hydrogen distribution. Additional constraints on high
ionization emission lines in galaxies with and without Lyman-alpha detections
will help clarify whether hard ionizing spectra are common in the reionization
era.Comment: 11 pages, 6 figures, submitted to MNRA
Ultraviolet/Optical Emission Of The Ionised Gas In Agn: Diagnostics Of The Ionizing Source And Gas Properties
Spectroscopic studies of active galactic nuclei (AGN) are powerful means of probing the physical properties of the ionized gas within them. In particular, near future observational facilities, such as the James Webb Space Telescope (JWST), will allow detailed statistical studies of rest-frame ultraviolet and optical spectral features of the very distant AGN with unprecedented accuracy. In this proceedings, we discuss the various ways of exploiting new dedicated photoionization models of the narrow-line emitting regions (NLR) of AGN for the interpretation of forthcoming revolutionary datasets
Exploring HeII1640 emission line properties at
Deep optical spectroscopic surveys of galaxies provide us a unique
opportunity to investigate rest-frame ultra-violet (UV) emission line
properties of galaxies at . Here we combine VLT/MUSE Guaranteed
Time Observations of the Hubble Deep Field South, Ultra Deep Field, COSMOS, and
several quasar fields with other publicly available data from VLT/VIMOS and
VLT/FORS2 to construct a catalogue of HeII1640 emitters at
. The deepest areas of our MUSE pointings reach a line
flux limit of 3.110-19 erg/ s/ cm. After discarding broad line
active galactic nuclei we find 13 HeII1640 detections from MUSE with
a median MUV = and 21 tentative HeII1640 detections from
other public surveys. Excluding Ly, all except two galaxies in our
sample show at least one other rest-UV emission line, with
CIII]1907,1909 being the most prominent. We use
multi-wavelength data available in the Hubble legacy fields to derive basic
galaxy properties of our sample via spectral energy distribution fitting
techniques. Taking advantage of the high quality spectra obtained by MUSE
(h of exposure time per pointing), we use photo-ionisation
models to study the rest-UV emission line diagnostics of the
HeII1640 emitters. Line ratios of our sample can be reproduced by
moderately sub-solar photo-ionisation models, however, we find that including
effects of binary stars lead to degeneracies in most free parameters. Even
after considering extra ionising photons produced by extreme sub-solar
metallicity binary stellar models, photo-ionisation models are unable to
reproduce rest-frame HeII1640 equivalent widths ( 0.2 - 10
A), thus additional mechanisms are necessary in models to match the observed
HeII1640 properties.Comment: Accepted to A&A, 31 pages, 17 figure
Chemical abundances in Seyfert galaxies, VII : direct abundance determination of neon based on optical and infrared emission lines
For the first time, neon abundance has been derived in the narrow line region from a sample of Seyfert 2 nuclei. In view of this, we compiled from the literature fluxes of optical and infrared (IR) narrow emission lines for 35 Seyfert 2 nuclei in the local universe (z ∼ 8.80]) an in- crease in Ne/O with O/H is found, which likely indicates secondary stellar production for the neon.Fil: Armah, Mark. Universidade Do Vale Do Paraíba; BrasilFil: Dors, Oli L.. Universidade Do Vale Do Paraíba; BrasilFil: Aydar, C. P.. Universidade de Sao Paulo; BrasilFil: Cardaci, Monica Viviana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Hägele, Guillermo Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Feltre, Anna. Universidad de Bologna; ItaliaFil: Riffel, R.. Universidade Federal do Rio Grande do Sul; BrasilFil: Riffel, R. A.. Universidade Federal de Santa Maria; BrasilFil: Krabbe, A. C.. Universidade Do Vale Do Paraíba; Brasi
Chemical abundances in Seyfert galaxies : VII. Direct abundance determination of neon based on optical and infrared emission lines
For the first time, neon abundance has been derived in the narrow line region from a sample of Seyfert 2 nuclei. In view of this, we compiled from the literature fluxes of optical and infrared (IR) narrow emission lines for 35 Seyfert 2 nuclei in the local universe (z 0.06). The relative intensities of emission lines were used to derive the ionic and total neon and oxygen abundances through electron temperature estimations (Te-method). For the neon, abundance estimates were obtained by using both Te-method and IR-method. Based on photoionization model results, we found a lower electron temperature [te(Ne iii)] for the gas phase where the Ne2 + is located in comparison with t3 for the O2 + ion. We find that the differences (D) between Ne2 +/H+ ionic abundances calculated from IR-method and Te-method (assuming t3 in the Ne2 +/H+ derivation) are similar to the derivations in star-forming regions (SFs) and they are reduced by a mean factor of ∼3 when te(Ne iii) is considered. We propose a semi-empirical Ionization Correction Factor (ICF) for the neon, based on [Ne II]12.81μm, [Ne III]15.56μm, and oxygen ionic abundance ratios. We find that the average Ne/H abundance for the Seyfert 2s sample is nearly 2 times higher than similar estimate for SFs. Finally, for the very high metallicity regime (i.e. [12 + log(O/H) 8.80]) an increase in Ne/O with O/H is found, which likely indicates secondary stellar production for the neon
- …