343 research outputs found
First CO(17-16) emission line detected in a z > 6 quasar
We report the serendipitous detection of the CO(17-16) emission line toward
the quasar SDSSJ114816.64+525150.3 (J1148) at redshift z = 6.4 obtained with
the Plateau de Bure Interferometer. The CO(17-16) line is possibly contaminated
by OH+ emission, that may account for ~ 35 - 60% of the total flux observed.
Photo-Dissociation and X-ray Dominated Regions (PDRs and XDRs) models show that
PDRs alone cannot reproduce the high luminosity of the CO(17-16) line relative
to low-J CO transitions and that XDRs are required. By adopting a composite
PDR+XDR model we derive molecular cloud and radiation field properties in the
nuclear region of J1148. Our results show that highly excited CO lines
represent a sensitive and possibly unique tool to infer the presence of X-ray
faint or obscured supermassive black hole progenitors in high-z galaxies.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Lette
Gas Metallicity in the Narrow-Line Regions of High-Redshift Active Galactic Nuclei
We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs
at redshift 1.5 < z < 3.7 found in the Chandra Deep Field South and of
narrow-line radio galaxies at redshift 1.2 < z < 3.8 to investigate the gas
metallicity of the narrow-line regions and their evolution in this redshift
range. Such spectra are also compared with UV spectra of local Seyfert 2
galaxies. The observational data are inconsistent with the predictions of shock
models, suggesting that the narrow-line regions are mainly photoionized. The
photoionization models with dust grains predict line flux ratios which are also
in disagreement with most of the observed values, suggesting that the
high-ionization part of the narrow-line regions (which is sampled by the
available spectra) is dust-free. The photoionization dust-free models provide
two possible scenarios which are consistent with the observed data: low-density
gas clouds (n < 10^3 cm^-3) with a sub-solar metallicity (0.2 < Z/Z_sun < 1.0),
or high-density gas clouds (n ~ 10^5 cm^-3) with a wide range of gas
metallicity (0.2 < Z/Z_sun < 5.0). Regardless of the specific interpretation,
the observational data do not show any evidence for a significant evolution of
the gas metallicity in the narrow-line regions within the redshift range 1.2 <
z < 3.8. Instead, we find a trend for more luminous active galactic nuclei to
have more metal-rich gas clouds (luminosity-metallicity relation), which is in
agreement with the same finding in the studies of the broad-line regions. The
lack of evolution for the gas metallicity of the narrow-line regions implies
that the major epoch of star formation in the host galaxies of these active
galactic nuclei is at z > 4.Comment: 16 pages, 12 figures, submitted to Astronomy and Astrophysic
AGN Obscuration and the Unified Model
Unification Models of Active Galactic Nuclei postulate that all the observed
differences between Type 1 and Type 2 objects are due to orientation effects
with respect to the line-of-sight to the observer. The key ingredient of these
models is the obscuring medium, historically envisaged as a toroidal structure
on a parsec scale. However, many results obtained in the last few years are
clearly showing the need for a more complex geometrical distribution of the
absorbing media. In this paper we review the various pieces of evidence for
obscuring media on different scales, from the vicinity of the black hole to the
host galaxy, in order to picture an updated unification scenario explaining the
complex observed phenomenology. We conclude by mentioning some of the open
issues.Comment: 14 pages, 8 figures, review article accepted for publication on the
special issue of Advances in Astronomy "Seeking for the Leading Actor on the
Cosmic Stage: Galaxies versus Supermassive Black Holes
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Hardness of ionizing radiation fields in MaNGA star-forming galaxies
We investigate radiation hardness within a representative sample of 67 nearby (0.02 ≲z≲0.06) star-forming (SF) galaxies using the integral field spectroscopic data from the MaNGA survey. The softness parameter η = O+/O2+S+/S2+ is sensitive to the spectral energy distribution of the ionizing radiation. We study η via the observable quantity η′ (=[OII]/[OIII][SII][SIII]) We analyse the relation between radiation hardness (traced by η and η′) and diagnostics sensitive to gas-phase metallicity, electron temperature, density, ionization parameter, effective temperature and age of ionizing populations. It is evident that low metallicity is accompanied by low log η′, i.e. hard radiation field. No direct relation is found between radiation hardness and other nebular parameters though such relations can not be ruled out. We provide empirical relations between log η and strong emission line ratios N2, O3N2 and Ar3O3 which will allow future studies of radiation hardness in SF galaxies where weak auroral lines are undetected. We compare the variation of [O III]/[O II] and [S III]/[S II] for MaNGA data with SF galaxies and H II regions within spiral galaxies from literature, and find that the similarity and differences between different data set is mainly due to the metallicity. We find that predictions from photoionizaion models considering young and evolved stellar populations as ionizing sources in good agreement with the MaNGA data. This comparison also suggests that hard radiation fields from hot and old low-mass stars within or around SF regions might significantly contribute to the observed η values.STFC
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De re metallica: the cosmic chemical evolution of galaxies
The evolution of content of heavy elements in galaxies, the relative chemical abundances, their spatial distribution, and how these scale with various galactic properties, provide unique information on the galactic evolutionary processes across the cosmic epochs. In recent years major progress has been made in constraining the chemical evolution of galaxies and inferring key information relevant to our understanding of the main mecha- nisms involved in galaxy evolution. In this review we provide an overview of these various areas. After an overview of the methods used to constrain the chemical enrichment in galax- ies and their environment, we discuss the observed scaling relations between metallicity and galaxy properties, the observed relative chemical abundances, how the chemical elements are distributed within galaxies, and how these properties evolve across the cosmic epochs. We discuss how the various observational findings compare with the predictions from theo- retical models and numerical cosmological simulations. Finally, we briefly discuss the open problems the prospects for major progress in this field in the nearby future.STFC
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The Building Up of the Black Hole Mass - Stellar Mass Relation
We derive the growth of SMBHs relative to the stellar content of their host
galaxy predicted under the assumption of BH accretion triggered by galaxy
encounters occurring during their merging histories. We show that, within this
framework, the ratio Gamma=(M_BH/M_*)(z)/(M_BH/M_*)(z=0) between the Black Hole
mass and the galactic stellar mass (normalized to the local value) depends on
both BH mass and redshift. While the average value and the spread of Gamma(z)
increase with z, such an effect is larger for massive BHs, reaching values
Gamma=5 for massive Black Holes (M>10^9 M_{\odot}) at z>4, in agreement with
recent observations of high-redshift QSOs; this is due to the effectiveness of
interactions in triggering BH accretion in high-density environments at high
redshifts. To test such a model against observations, we worked out specific
predictions for sub-samples of the simulated galaxies corresponding to the
different observational samples for which measurements of Gamma have been
obtained. We found that for Broad Line AGNs at 1<z<2 values of Gamma=2 are
expected, with a mild trend toward larger value for increasing BH mass.
Instead, when we select from our Monte Carlo simulations only extremely gas
rich, rapidly star forming galaxies at 2<z<3, we find low values 0.3<Gamma<1.5,
consistent with recent observational findings on samples of sub-mm galaxies; in
the framework of our model, these objects end up at z=0 in low-to-intermediate
mass BHs (M<10^9 M_{\odot}), and they do not represent typical paths leading to
local massive galaxies. The latter have formed preferentially through paths
passing above the local M_*-M_BH relation. We discuss how the global picture
emerging from the model is consistent with a downsizing scenario, where massive
BHs accrete a larger fraction of their final mass at high redshifts z>4.Comment: 13 pages, 9 figures. Accepted to MNRA
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