6,254 research outputs found
X-ray - Infrared relation of AGNs and search for highly obscured accretion in the AKARI NEP Field
The infrared Astronomical Satellite AKARI conducted deep ( 0.4 deg)
and wide ( deg) surveys around the North Ecliptic Pole (NEP) with
its InfraRed Camera (IRC) with nine filters continuously covering the 2-25
m range. These photometric bands include three filters that fill the
`Spitzer gap' between the wavelength coverages of IRAC and MIPS. This unique
feature has enabled us to make sensitive mid-infrared detection of AGN
candidates at z 1-2, based on the Spectral Energy Distribution (SED)
fitting including hot dust emission in the AGN torus. This enables us to
compare X-rays and the AGN torus component of the infrared emission to help us
identify highly absorbed AGNs, including Compton-thick ones. We report our
results of the Chandra observation of the AKARI NEP Deep Field and discuss the
prospects for upcoming Spectrum-RG (eROSITA+ART-XC) on the AKARI Wide field.Comment: 4 pages, 2 figures. Submitted for publication in the proceedings of
the IAU Symp. 341 "PanModel2018 : Challenges in Panchromatic Galaxy Modelling
with Next Generation Facilities
High-redshift galaxies and black holes in the eyes of JWST: a population synthesis model from infrared to X-rays
The first billion years of the Universe is a pivotal time: stars, black holes
(BHs) and galaxies form and assemble, sowing the seeds of galaxies as we know
them today. Detecting, identifying and understand- ing the first galaxies and
BHs is one of the current observational and theoretical challenges in galaxy
formation. In this paper we present a population synthesis model aimed at
galaxies, BHs and Active Galactic Nuclei (AGNs) at high redshift. The model
builds a population based on empirical relations. Galaxies are characterized by
a spectral energy distribution determined by age and metallicity, and AGNs by a
spectral energy distribution determined by BH mass and accretion rate. We
validate the model against observational constraints, and then predict
properties of galaxies and AGN in other wavelength and/or luminosity ranges,
estimating the contamination of stellar populations (normal stars and high-mass
X-ray binaries) for AGN searches from the infrared to X-rays, and vice-versa
for galaxy searches. For high-redshift galaxies, with stellar ages < 1 Gyr, we
find that disentangling stellar and AGN emission is challenging at restframe
UV/optical wavelengths, while high-mass X-ray binaries become more important
sources of confusion in X-rays. We propose a color-color selection in JWST
bands to separate AGN vs star-dominated galaxies in photometric observations.
We also esti- mate the AGN contribution, with respect to massive, hot,
metal-poor stars, at driving high ionization lines, such as C IV and He II.
Finally, we test the influence of the minimum BH mass and occupa- tion fraction
of BHs in low mass galaxies on the restframe UV/near-IR and X-ray AGN
luminosity function.Comment: Accepted for publication in the Astrophysical Journa
Sub-terahertz, microwaves and high energy emissions during the December 6, 2006 flare, at 18:40 UT
The presence of a solar burst spectral component with flux density increasing
with frequency in the sub-terahertz range, spectrally separated from the
well-known microwave spectral component, bring new possibilities to explore the
flaring physical processes, both observational and theoretical. The solar event
of 6 December 2006, starting at about 18:30 UT, exhibited a particularly
well-defined double spectral structure, with the sub-THz spectral component
detected at 212 and 405 GHz by SST and microwaves (1-18 GHz) observed by the
Owens Valley Solar Array (OVSA). Emissions obtained by instruments in
satellites are discussed with emphasis to ultra-violet (UV) obtained by the
Transition Region And Coronal Explorer (TRACE), soft X-rays from the
Geostationary Operational Environmental Satellites (GOES) and X- and gamma-rays
from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The sub-THz
impulsive component had its closer temporal counterpart only in the higher
energy X- and gamma-rays ranges. The spatial positions of the centers of
emission at 212 GHz for the first flux enhancement were clearly displaced by
more than one arc-minute from positions at the following phases. The observed
sub-THz fluxes and burst source plasma parameters were found difficult to be
reconciled to a purely thermal emission component. We discuss possible
mechanisms to explain the double spectral components at microwaves and in the
THz ranges.Comment: Accepted version for publication in Solar Physic
New insight into the relation between star formation activity and dust content in galaxies
(Abridged) We assemble a sample of 3258 low-redshift galaxies from the SDSS
DR6 with complementary photometric observations by GALEX, 2MASS and IRAS at
far-ultraviolet and infrared wavelengths. We use a recent, simple but
physically motivated model to interpret the observed spectral energy
distributions of the galaxies in this sample in terms of statistical
constraints on physical parameters describing the star formation history and
dust content. The focus on a subsample of 1658 galaxies with highest S/N
observations enables us to investigate most clearly several strong correlations
between various derived physical properties of galaxies. We find that the
typical dust mass of a star-forming correlates remarkably well with the star
formation rate (SFR). We also find that the dust-to-stellar mass ratio, the
ratio of dust mass to star formation rate and the fraction of dust luminosity
contributed by the diffuse interstellar medium all correlate strongly with
specific SFR. A comparison with recent models of chemical and dust evolution of
galaxies suggests that these correlations could arise, at least in part, from
an evolutionary sequence. As galaxies form stars, their ISM becomes enriched in
dust, while the drop in gas supply makes the specific SFR decrease.
Interestingly, as a result, a young, actively star-forming galaxy with low
dust-to-gas ratio may still be highly dusty because it contains large amounts
of interstellar gas. This may be important for the interpretation of the
infrared emission from young, gas-rich star-forming galaxies at high redshift.
Our study provides a useful local reference for future statistical studies of
the star formation and dust properties of galaxies at high redshifts.Comment: 15 pages, 10 figures, accepted for publication in MNRAS.
Full-resolution figures available from
http://users.physics.uoc.gr/~dacunha/paper_dacunha.pd
Analysis of Spitzer-IRS spectra of hyperluminous infrared galaxies
Hyperluminous infrared galaxies (HLIRG) are the most luminous persistent
objects in the Universe. They exhibit extremely high star formation rates, and
most of them seem to harbour an AGN. They are unique laboratories to
investigate the most extreme star formation, and its connection to
super-massive black hole growth. The AGN and SB relative contributions to the
total output in these objects is still debated. Our aim is to disentangle the
AGN and SB emission of a sample of thirteen HLIRG. We have studied the MIR low
resolution spectra of a sample of thirteen HLIRG obtained with the IRS on board
Spitzer. The 5-8 {\mu}m range is an optimal window to detect AGN activity even
in a heavily obscured environment. We performed a SB/AGN decomposition of the
continuum using templates, successfully applied for ULIRG in previous works.
The MIR spectra of all sources is largely dominated by AGN emission. Converting
the 6 {\mu}m luminosity into IR luminosity, we found that ~80% of the sample
shows an IR output dominated by the AGN emission. However, the SB activity is
significant in all sources (mean SB contribution ~30%), showing star formation
rates ~300-3000 solar masses per year. Using X-ray and MIR data we estimated
the dust covering factor (CF) of these HLIRG, finding that a significant
fraction presents a CF consistent with unity. Along with the high X-ray
absorption shown by these sources, this suggests that large amounts of dust and
gas enshroud the nucleus of these HLIRG, as also observed in ULIRG. Our results
are in agreement with previous studies of the IR SED of HLIRG using radiative
transfer models, and we find strong evidence that all HLIRG harbour an AGN.
This work provides further support to the idea that AGN and SB are both crucial
to understand the properties of HLIRG. Our study of the CF supports the
hypothesis that HLIRG can be divided in two different populations.Comment: 17 pages, 9 figures, 4 tables. Accepted for publication in A&
Noncommutative electrodynamics and ultra high energy gamma rays
Plane waves in noncommutative classical electrodynamics (NCED) have a
peculiar dispersion relation. We investigate the kinematical conditions on this
deformed "mass shell" which come from ultra high energy gamma rays and discuss
noncommutative dynamical effects on the gamma absorption by the infrared
background and on the intrinsic spectrum. Finally we note that in NCED there is
a strong correlation between the modified dispersion relation and the presence
of dynamical effects in electromagnetic phenomena such as in the case of the
synchrotron radiation. From this point of view, the limits on the typical
energy scale of the violation of Lorentz invariance obtained by deformed
dispersion relations and by assuming undeformed dynamical effects should be
taken with some caution.Comment: Latex file, 7 pages, to be published in Europhysics Letter
Fundamental Physics With Cosmic High-Energy Gamma Rays
High-energy photons (above the MeV) are a powerful probe for astrophysics and
for fundamental physics under extreme conditions. During the recent years, our
knowledge of the high-energy gamma-ray sky has impressively progressed thanks
to the advent of new detectors for cosmic gamma rays, at ground (H.E.S.S.,
MAGIC, VERITAS, HAWC) and in space (AGILE, Fermi). This presentation reviews
the present status of the studies of fundamental physics problems with
high-energy gamma rays, and discusses the expected experimental developments.Comment: Invited Talk at the 6th International Symposium on High-Energy
Gamma-Ray Astronomy (Gamma2016), Heidelberg, July 201
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