49 research outputs found
Compton Thick AGN in the XMM-COSMOS survey
Heavily obscured, Compton Thick (CT, NH>10^24 cm^-2) AGN may represent an
important phase in AGN/galaxy co-evolution and are expected to provide a
significant contribution to the cosmic X-ray background (CXB). Through direct
X-ray spectra analysis, we selected 39 heavily obscured AGN (NH>3x10^23 cm^-2)
in the 2 deg^2 XMM-COSMOS survey. After selecting CT AGN based on the fit of a
simple absorbed two power law model to the XMM data, the presence of CT AGN was
confirmed in 80% of the sources using deeper Chandra data and more complex
models. The final sample of CT AGN comprises 10 sources spanning a large range
of redshift and luminosity. We collected the multi-wavelength information
available for all these sources, in order to study the distribution of SMBH and
host properties, such as BH mass (M_BH), Eddington ratio (\lambda_Edd), stellar
mass (M*), specific star formation rate (sSFR) in comparison with a sample of
unobscured AGN. We find that highly obscured sources tend to have significantly
smaller M_BH and higher \lambda_edd with respect to unobscured ones, while a
weaker evolution in M* is observed. The sSFR of highly obscured sources is
consistent with the one observed in the main sequence of star forming galaxies,
at all redshift. We also present optical spectra, spectral energy distribution
(SED) and morphology for the sample of 10 CT AGN: all the available optical
spectra are dominated by the stellar component of the host galaxy, and a highly
obscured torus component is needed in the SED of the CT sources. Exploiting the
high resolution Hubble-ACS images available, we conclude that these highly
obscured sources have a significantly larger merger fraction with respect to
other X-ray selected samples of AGN. Finally we discuss implications in the
context of AGN/galaxy co-evolutionary models, and compare our results with the
predictions of CXB synthesis models.Comment: Revised version after referee comments. Accepted for publication in
Astronomy & Astrophysics on 25 November 2014. 23 pages, 2 tables, 16 figure
Hamiltonian form and solitary waves of the spatial Dysthe equations
The spatial Dysthe equations describe the envelope evolution of the
free-surface and potential of gravity waves in deep waters. Their Hamiltonian
structure and new invariants are unveiled by means of a gauge transformation to
a new canonical form of the evolution equations. An accurate Fourier-type
spectral scheme is used to solve for the wave dynamics and validate the new
conservation laws, which are satisfied up to machine precision. Traveling waves
are numerically constructed using the Petviashvili method. It is shown that
their collision appears inelastic, suggesting the non-integrability of the
Dysthe equations.Comment: 6 pages, 9 figures. Other author's papers can be downloaded at
http://www.lama.univ-savoie.fr/~dutykh
Hard X-ray Variability of AGN
Aims: Active Galactic Nuclei are known to be variable throughout the
electromagnetic spectrum. An energy domain poorly studied in this respect is
the hard X-ray range above 20 keV.
Methods: The first 9 months of the Swift/BAT all-sky survey are used to study
the 14 - 195 keV variability of the 44 brightest AGN. The sources have been
selected due to their detection significance of >10 sigma. We tested the
variability using a maximum likelihood estimator and by analysing the structure
function.
Results: Probing different time scales, it appears that the absorbed AGN are
more variable than the unabsorbed ones. The same applies for the comparison of
Seyfert 2 and Seyfert 1 objects. As expected the blazars show stronger
variability. 15% of the non-blazar AGN show variability of >20% compared to the
average flux on time scales of 20 days, and 30% show at least 10% flux
variation. All the non-blazar AGN which show strong variability are
low-luminosity objects with L(14-195 keV) < 1E44 erg/sec.
Conclusions: Concerning the variability pattern, there is a tendency of
unabsorbed or type 1 galaxies being less variable than the absorbed or type 2
objects at hardest X-rays. A more solid anti-correlation is found between
variability and luminosity, which has been previously observed in soft X-rays,
in the UV, and in the optical domain.Comment: 9 pages, 7 figures, accepted for publication in A&
X-ray spectral properties of Seyfert galaxies and the unification scheme
Aims: The unification scheme of Seyfert galaxies predicts that the observed
differences between type 1 and type 2 Seyfert galaxies are solely due to the
differing orientations of the toroidal-shaped obscuring material around AGN.
The observed X-ray spectra of Seyfert type 2s compared to type 1s are expected
to be affected by higher absorbing column density due to the edge-on view of
the obscuring torus. We study the 0.5 - 10 keV X-ray spectral properties of
Seyfert type 1s and type 2s with the aim to test the predictions of Seyfert
unification scheme in the X-ray regime. Methods: We use an optically selected
Seyfert sample in which type 1s and type 2s have matched distributions in the
orientation independent parameters of AGN and host galaxy. Results: The 0.5 -
10 keV XMM-Newton pn X-ray spectra of Seyfert galaxies are in general best
fitted with a model consists of an absorbed power-law, a narrow Gaussian fitted
to the Fe K{\alpha} emission line and an often seen soft excess component
characterized by either a thermal plasma model with temperature kT \sim 0.1 -
1.0 keV and/or a steep power-law. The 2.0 - 10 keV hard X-ray continuum
emission in several Seyfert type 2s is reflection dominated and suggests the
Compton-thick obscuration. Results on the statistical comparison of the
distributions of the observed X-ray luminosities in the soft (0.5 - 2.0 keV)
and hard (2.0 - 10.0 keV) bands, the X-ray absorbing column densities, the
equivalent widths of Fe K{\alpha} line and the flux ratios of hard X-ray to
[OIII] {\lambda}5007{\AA} for the two Seyfert subtypes are consistent with the
obscuration and orientation based unification scheme.Comment: 15 pages, 3 figures, 6 tables, accepted for publication in A&
The Activity of the Neighbours of Seyfert Galaxies
We present a follow-up study on a series of papers concerning the role of
close interactions as a possible triggering mechanism of AGN activity. We have
already studied the close (<100kpc/h) and the large scale (<1 Mpc/h)
environment of a local sample of Sy1, Sy2 and bright IRAS galaxies (BIRG) and
their respective control samples. The results led us to the conclusion that a
close encounter appears capable of activating a sequence where an absorption
line galaxy (ALG) galaxy becomes first a starburst, then a Sy2 and finally a
Sy1. Here we investigate the activity of neighboring galaxies of different
types of AGN, since both galaxies of an interacting pair should be affected. To
this end we present the optical spectroscopy and X-ray imaging of 30
neighbouring galaxies around two local (z<0.034) samples of 10 Sy1 and 13 Sy2
galaxies. Based on the optical spectroscopy we find that more than 70% of all
neighbouring galaxies exhibit star forming and/or nuclear activity (namely
recent star formation and/or AGN), while an additional X-ray analysis showed
that this percentage might be significantly higher. Furthermore, we find a
statistically significant correlation, at a 99.9% level, between the value of
the neighbour's [OIII]/H\beta ratio and the activity type of the central active
galaxy, i.e. the neighbours of Sy2 galaxies are systematically more ionized
than the neighbours of Sy1s. This result, in combination with trends found
using the Equivalent Width of the H\alpha emission line and the stellar
population synthesis code STARLIGHT, indicate differences in the stellar mass,
metallicity and star formation history between the samples. Our results point
towards a link between close galaxy interactions and activity and also provide
more clues regarding the possible evolutionary sequence inferred by our
previous studies.Comment: Accepted for publication in A&A. 17 pages, 4 figures. arXiv admin
note: substantial text overlap with arXiv:0910.1355v1 (withdrawn
The XMM large scale structure survey: optical vs. X-ray classifications of active galactic nuclei and the unified scheme
Our goal is to characterize AGN populations by comparing their X-ray and
optical classifications. We present a sample of 99 spectroscopically identified
X-ray point sources in the XMM-LSS survey which are significantly detected in
the [2-10] keV band, and with more than 80 counts. We performed an X-ray
spectral analysis for all of these 99 X-ray sources. Introducing the fourfold
point correlation coefficient, we find only a mild correlation between the
X-ray and the optical classifications, as up to 30% of the sources have
differing X-ray and optical classifications: on one hand, 10% of the type 1
sources present broad emission lines in their optical spectra and strong
absorption in the X-rays. These objects are highly luminous AGN lying at high
redshift and thus dilution effects are totally ruled out, their discrepant
nature being an intrinsic property. Their X-ray luminosities and redshifts
distributions are consistent with those of the unabsorbed X-ray sources with
broad emission lines. On the other hand, 25/32 are moderate luminosity AGN,
which are both unabsorbed in the X-rays and only present narrow emission lines
in their optical spectra. The majority of them have an optical spectrum which
is representative of the host galaxy. We finally infer that dilution of the AGN
by the host galaxy seems to account for their nature. 5/25 have been defined as
Seyfert 2. In conclusion, most of these 32 discrepant cases can be accounted
for by the standard AGN unified scheme, as its predictions are not met for only
12% of the 99 X-ray sources. ABRIDGEDComment: 25 pages, 19 figures, Accepted for publication in A&
Tracing the cosmic growth of supermassive black holes to z~3 with Herschel
We study a sample of Herschel selected galaxies within the Great Observatories Origins Deep Survey-South and the Cosmic Evolution Survey fields in the framework of the Photodetector Array Camera and Spectrometer (PACS) Evolutionary Probe project. Starting from the rich multiwavelength photometric data sets available in both fields, we perform a broad-band spectral energy distribution decomposition to disentangle the possible active galactic nucleus (AGN) contribution from that related to the host galaxy. We find that 37 per cent of the Herschel-selected sample shows signatures of nuclear activity at the 99 per cent confidence level. The probability of revealing AGN activity increases for bright (L 1â1000 > 10 11 L ? ) star-forming galaxies at z > 0.3, becoming about 80 per cent for the brightest (L 1â1000 > 10 12 L ? )
Infrared (IR) galaxies at zâ„1. Finally, we reconstruct the AGN bolometric luminosity function and the supermassive black hole growth rate across cosmic time up to z ⌠3 from a far-IR perspective. This work shows general agreement with most of the panchromatic estimates from the literature, with the global black hole growth peaking at z ⌠2 and reproducing the observed local black hole mass density with consistent values of the radiative efficiency Erad (âŒ0.07)
Observing Supermassive Black Holes across cosmic time: from phenomenology to physics
In the last decade, a combination of high sensitivity, high spatial
resolution observations and of coordinated multi-wavelength surveys has
revolutionized our view of extra-galactic black hole (BH) astrophysics. We now
know that supermassive black holes reside in the nuclei of almost every galaxy,
grow over cosmological times by accreting matter, interact and merge with each
other, and in the process liberate enormous amounts of energy that influence
dramatically the evolution of the surrounding gas and stars, providing a
powerful self-regulatory mechanism for galaxy formation. The different
energetic phenomena associated to growing black holes and Active Galactic
Nuclei (AGN), their cosmological evolution and the observational techniques
used to unveil them, are the subject of this chapter. In particular, I will
focus my attention on the connection between the theory of high-energy
astrophysical processes giving rise to the observed emission in AGN, the
observable imprints they leave at different wavelengths, and the methods used
to uncover them in a statistically robust way. I will show how such a combined
effort of theorists and observers have led us to unveil most of the SMBH growth
over a large fraction of the age of the Universe, but that nagging
uncertainties remain, preventing us from fully understating the exact role of
black holes in the complex process of galaxy and large-scale structure
formation, assembly and evolution.Comment: 46 pages, 21 figures. This review article appears as a chapter in the
book: "Astrophysical Black Holes", Haardt, F., Gorini, V., Moschella, U and
Treves A. (Eds), 2015, Springer International Publishing AG, Cha