1,341 research outputs found
The primordial environment of super massive black holes: large scale galaxy overdensities around QSOs with LBT
We investigated the presence of galaxy overdensities around four
QSOs, namely SDSS J1030+0524 (z = 6.28), SDSS J1148+5251 (z = 6.41), SDSS
J1048+4637 (z = 6.20) and SDSS J1411+1217 (z = 5.95), through deep -, -
and - band imaging obtained with the wide-field () Large
Binocular Camera (LBC) at the Large Binocular Telescope (LBT). We adopted
color-color selections within the vs plane to identify samples of
-band dropouts at the QSO redshift and measure their relative abundance and
spatial distribution in the four LBC fields, each covering
physical Mpc at . The same selection criteria were then applied to
-band selected sources in the 1 deg Subaru-XMM Newton Deep Survey
to derive the expected number of dropouts over a blank LBC-sized field
(0.14 deg). The four observed QSO fields host a number of candidates
larger than what is expected in a blank field. By defining as -band dropouts
objects with and undetected in the -band, we found
16, 10, 9, 12 dropouts in SDSS J1030+0524, SDSS J1148+5251, SDSS J1048+4637,
and SDSS J1411+1217, respectively, whereas only 4.3 such objects are expected
over a 0.14 deg blank field. This corresponds to overdensity significances
of 3.3, 1.9, 1.7, 2.5, respectively. By considering the total number of
dropouts in the four LBC fields and comparing it with what is expected in four
blank fields of 0.14 deg each, we find that high-z QSOs reside in overdense
environments at the level. This is the first direct and unambiguous
measurement of the large scale structures around QSOs. [shortened]Comment: 12 pages, 8 figures. Accepted for publication in A&
Probing black hole accretion in quasar pairs at high redshift
Models and observations suggest that luminous quasar activity is triggered by
mergers, so it should preferentially occur in the most massive primordial dark
matter haloes, where the frequency of mergers is expected to be the highest.
Since the importance of galaxy mergers increases with redshift, we identify the
high-redshift Universe as the ideal laboratory for studying dual AGN. Here we
present the X-ray properties of two systems of dual quasars at z=3.0-3.3
selected from the SDSS-DR6 at separations of 6-8 arcsec (43-65kpc) and observed
by Chandra for 65ks each. Both members of each pair are detected with good
photon statistics to allow us to constrain the column density, spectral slope
and intrinsic X-ray luminosity. We also include a recently discovered dual
quasar at z=5 (separation of 21 arcsec, 136kpc) for which XMM-Newton archival
data allow us to detect the two components separately. Using optical spectra we
derived bolometric luminosities, BH masses and Eddington ratios that were
compared to those of luminous SDSS quasars in the same redshift ranges. We find
that the brighter component of both pairs at z=3.0-3.3 has high luminosities
compared to the distribution of SDSS quasars at similar redshift, with J1622A
having an order magnitude higher luminosity than the median. This source lies
at the luminous end of the z~3.3 quasar luminosity function. While we cannot
conclusively state that the unusually high luminosities of our sources are
related to their having a close companion, for J1622A there is only a 3%
probability that it is by chance.Comment: MNRAS, in pres
The primordial environment of supermassive black holes (II): deep Y and J band images around the z=6.3 quasar SDSS J1030+0524
Many cosmological studies predict that early supermassive black holes (SMBHs)
can only form in the most massive dark matter halos embedded within large scale
structures marked by galaxy over-densities that may extend up to 10 physical
Mpc. This scenario, however, has not been confirmed observationally, as the
search for galaxy over-densities around high-z quasars has returned conflicting
results. The field around the z=6.28 quasar SDSSJ1030+0524 (J1030) is unique
for multi-band coverage and represents an excellent data legacy for studying
the environment around a primordial SMBH. In this paper we present wide-area
(25x25 arcmin), Y- and J-band imaging of the J1030 field obtained with the near
infrared camera WIRCam at the Canada-France-Hawaii Telescope (CFHT). We built
source catalogues in the Y- and J-band, and matched those with our photometric
catalogue in the r, z, i bands presented in Morselli et al. (2014). We used
these new infrared data together with H and K and Spitzer/IRAC data to refine
our selection of Lyman Break Galaxies (LBGs), extending our selection criteria
to galaxies in the range 25.2<zAB<25.7. We selected 21 robust high-z candidates
in the J1030 field with photometric redshift around 6 and colors i-z>=1.3. We
found a significant asymmetry in the distribution of the high-z galaxies in
J1030, supporting the existence of a coherent large-scale structure around the
quasar. We compared our results with those of Bowler et al. (2015), who adopted
similar LBGs selection criteria, and estimated an over-density of galaxies in
the field of delta = 2.4, which is significant at >4 sigma. The over-density
value and its significance are higher than those found in Morselli et al.
(2014), and we interpret this as evidence of an improved LBG selection.Comment: 13 pages, 8 figures, accepted for publication in A&
The G-dwarf distribution in star-forming galaxies: a tug-of-war between infall and outflow
In the past, the cumulative metallicity distribution function (CMDF) turned
out as a useful tool to constrain the accretion history of various components
of the Milky Way. In this Letter, by means of analytical, leaky-box chemical
evolution models (i.e. including both infall and galactic outflows) we study
the CMDF of local star-forming galaxies that follow two fundamental empirical
scaling relations, namely the mass-metallicity and main sequence relations. At
variance with any previous, historical knowledge of this quantity, our analysis
shows that galactic winds, which are dominant mostly in low-mass systems, play
a fundamental role in shaping this function and, in particular, in determining
its steepness and curvature. We show that the CMDF of low-mass
(M/M) and high-mass
(M/M>10) galaxies deviate substantially from the
results of a 'closed-box' model, as the evolution of the former (latter)
systems is mostly dominated by outflows (infall). In the context of galactic
downsizing, we show that downward-concave CMDFs (associated with systems with
extremely small infall timescales and with very strong winds) are more frequent
in low-mass galaxies, which include larger fractions of young systems and
present more substantial deviations from equilibrium between gas accretion and
reprocessing (either via star formation or winds).Comment: 6 pages, 4 figure
X-ray observations of highly obscured τ_(9.7 μm) > 1 sources: an efficient method for selecting Compton-thick AGN?
Observations with the IRS spectrograph onboard Spitzer have found many sources with very deep Si features at 9.7 μm, that have optical depths of τ > 1. Since it is believed that a few of these systems in the local Universe are associated with Compton-thick active galactic nuclei (hereafter AGN), we set out to investigate whether the presence of a strong Si absorption feature is a good indicator of a heavily obscured AGN. We compile X-ray spectroscopic observations available in the literature on the optically-thick (τ_(9.7 μm) > 1) sources from the 12 μm IRAS Seyfert sample. We find that the majority of the high-τ optically confirmed Seyferts (six out of nine)
in the 12 μm sample are probably Compton-thick. Thus, we provide direct evidence of a connection between mid-IR optically-thick galaxies and Compton-thick AGN, with the success rate being close to 70% in the local Universe. This is at least comparable to, if not better than, other rates obtained with photometric information in the mid to far-IR, or even mid-IR to X-rays. However, this technique cannot provide complete Compton-thick AGN samples, i.e., there are many Compton-thick AGN that do not display significant Si
absorption, with the most notable example being NGC1068. After assessing the validity of the high 9.7 μm optical-depth technique in the local Universe, we attempt to construct a sample of candidate Compton-thick AGN at higher redshifts. We compile a sample of seven high-τ Spitzer sources in the Great Observatories Origins Deep Survey (GOODS) and five in the Spitzer First-Look Survey.
All these have been selected to have no PAH features (EW_(6.2 μm) 10^(42) erg s^(−1)) of the detected GOODS sources corroborates that these are AGN. For FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources. SED fitting can support the presence of an AGN in the vast majority of sources. Owing to the limited photon statistics, we cannot derive useful constraints from X-ray spectroscopy on whether these sources are Compton-thick. However, the low L_(X)/L_(6 μm) luminosity ratios, suggest that at least four out of the six detected sources in GOODS may be associated with Compton-thick AGN
NuSTAR reveals that the heavily obscured nucleus of NGC 2785 was the contaminant of IRAS 09104+4109 in the BeppoSAX/PDS hard X-rays
The search for heavily obscured active galactic nuclei (AGNs) has been
revitalized in the last five years by NuSTAR, which has provided a good census
and spectral characterization of a population of such objects, mostly at low
redshift, thanks to its enhanced sensitivity above 10 keV compared to previous
X-ray facilities, and its hard X-ray imaging capabilities. We aim at
demonstrating how NGC2785, a local (z=0.009) star-forming galaxy, is
responsible, in virtue of its heavily obscured active nucleus, for significant
contamination in the non-imaging BeppoSAX/PDS data of the relatively nearby
(~17 arcmin) quasar IRAS 09104+4109 (z=0.44), which was originally
mis-classified as Compton thick. We analyzed ~71 ks NuSTAR data of NGC2785
using the MYTorus model and provided a physical description of the X-ray
properties of the source for the first time. We found that NGC2785 hosts a
heavily obscured (NH~3*10^{24} cm^{-2}) nucleus. The intrinsic X-ray luminosity
of the source, once corrected for the measured obscuration (L(2-10 keV)~10^{42}
erg/s), is consistent within a factor of a few with predictions based on the
source mid-infrared flux using widely adopted correlations from the literature.
Based on NuSTAR data and previous indications from the Neil Gehrels Swift
Observatory (BAT instrument), we confirm that NGC2785, because of its hard
X-ray emission and spectral shape, was responsible for at least one third of
the 20-100 keV emission observed using the PDS instrument onboard BeppoSAX,
originally completely associated with IRAS 09104+4109. Such emission led to the
erroneous classification of this source as a Compton-thick quasar, while it is
now recognized as Compton thin.Comment: Six pages, 3 figures, A&A, in pres
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