28 research outputs found
Dark bubbles around high-redshift radio-loud AGN
At redshift larger than 3 there is a disagreement between the number of
blazars (whose jet is pointing at us) and the number of expected parents (whose
jet is pointing elsewhere). Now we strengthen this claim because (i) the number
of blazars identified within the SDSS+FIRST survey footprint increased,
demanding a more numerous parent population, and (ii) the detected blazars have
a radio flux large enough to be above the FIRST flux limit even if the jet is
slightly misaligned. The foreseen number of these slightly misaligned jets, in
principle detectable, is much larger than the radio-detected sources in the
FIRST+SDSS survey (at redshift larger than 4). This argument is independent of
the presence of an isotropic radio component, such as the hot spot or the radio
lobe, and does not depend on the bulk Lorentz factor Gamma. We propose a
scenario that ascribes the lack of slightly misaligned sources to an
over-obscuration of the nucleus by a "bubble" of dust, possibly typical of the
first high-redshift quasars.Comment: 5 pages, 2 figures, to appear in Monthly Notices Letter
Jets and accretion in heavy black holes across cosmic time.
Blazars are Active Galactic Nuclei (AGN) characterized by relativistic jets launched in the vicinity of the central engine (i.e. a supermassive black hole; SMBH), that are oriented close to our line of sight.
How jets in AGN form, collimate and accelerate is still an open issue, but a connection with the accretion process is sometimes suggested. We first investigated this issue, finding that in jetted AGN the SMBH can accrete both through a radiatively efficient, optically thick, geometrically thin accretion disc, and through a radiatively inefficient, geometrically thick hot accretion flow. The occurrence of these two accretion regimes depends on the accretion rate: if it is larger than ˙M ~ 0.1˙MEdd, the accretion is radiatively efficient, while if it is less than ~ 0.1˙MEdd the accretion is inefficient.
After this first insight on accretion and jets, we used these components as tools to study the extremely massive black hole population at high redshift (i.e. MBH > 109M, z > 4). A deep knowledge of these objects can provide fundamental clues to the models of formation and growth of the first supermassive black holes (106M_ < MBH < 109M). The peculiar orientation of blazars makes them the most effective tracers of their parent population, namely all the jetted AGN with similar intrinsic properties, but oriented in random directions. For this reason, we set up a systematic search of blazar candidates from a large quasar sample, in order to collect a complete sample of high–redshift blazars. We selected a sample of 19 extremely radio–loud, high–redshift quasars. We characterized their nuclear features (i.e. SMBH mass and accretion rate) by fitting their accretion disc spectra: we find that our criteria are efficient in selecting very massive and fast accreting black holes. We started our classification campaign, through X–ray observations. We successfully classified three quasars from our sample as blazars, along with a serendipitously selected (but analogous) candidate from the same area of sky. This means that our criteria are efficient in selecting good blazar candidates.
Comparing our findings with the known distribution of non–jetted AGN, we find that there are different formation epochs for extremely massive black holes hosted in jetted (z ~ 4) and non–jetted systems (z ~ 2.5). This is not easy to explain, according to the current black hole formation models in the early Universe. The search of very high–redshift jetted sources must be pursued, to learn more about the early stages of heavy black hole formation
Jets and accretion in heavy black holes across cosmic time.
Blazars are Active Galactic Nuclei (AGN) characterized by relativistic jets launched in the vicinity of the central engine (i.e. a supermassive black hole; SMBH), that are oriented close to our line of sight.
How jets in AGN form, collimate and accelerate is still an open issue, but a connection with the accretion process is sometimes suggested. We first investigated this issue, finding that in jetted AGN the SMBH can accrete both through a radiatively efficient, optically thick, geometrically thin accretion disc, and through a radiatively inefficient, geometrically thick hot accretion flow. The occurrence of these two accretion regimes depends on the accretion rate: if it is larger than ˙M ~ 0.1˙MEdd, the accretion is radiatively efficient, while if it is less than ~ 0.1˙MEdd the accretion is inefficient.
After this first insight on accretion and jets, we used these components as tools to study the extremely massive black hole population at high redshift (i.e. MBH > 109M, z > 4). A deep knowledge of these objects can provide fundamental clues to the models of formation and growth of the first supermassive black holes (106M_ < MBH < 109M). The peculiar orientation of blazars makes them the most effective tracers of their parent population, namely all the jetted AGN with similar intrinsic properties, but oriented in random directions. For this reason, we set up a systematic search of blazar candidates from a large quasar sample, in order to collect a complete sample of high–redshift blazars. We selected a sample of 19 extremely radio–loud, high–redshift quasars. We characterized their nuclear features (i.e. SMBH mass and accretion rate) by fitting their accretion disc spectra: we find that our criteria are efficient in selecting very massive and fast accreting black holes. We started our classification campaign, through X–ray observations. We successfully classified three quasars from our sample as blazars, along with a serendipitously selected (but analogous) candidate from the same area of sky. This means that our criteria are efficient in selecting good blazar candidates.
Comparing our findings with the known distribution of non–jetted AGN, we find that there are different formation epochs for extremely massive black holes hosted in jetted (z ~ 4) and non–jetted systems (z ~ 2.5). This is not easy to explain, according to the current black hole formation models in the early Universe. The search of very high–redshift jetted sources must be pursued, to learn more about the early stages of heavy black hole formation
AGN mass estimates in large spectroscopic surveys: the effect of host galaxy light
Virial-based methods for estimating active supermassive black hole masses are
now commonly used on extremely large spectroscopic quasar catalogues. Most
spectral analyses, though, do not pay enough attention to a detailed continuum
decomposition. To understand how this affects virial mass estimate results, we
test the influence of host galaxy light on them, along with Balmer continuum
component. A detailed fit with the new spectroscopic analysis software QSFit
demonstrated that the presence or absence of continuum components do not affect
significantly the virial-based results for our sample. Taking or not in
consideration a host galaxy component, instead, affects the emission line
fitting in a more pronounced way at lower redshifts, where in fact we observe
dimmer quasars and more visible host galaxies.Comment: 5 pages, 3 figures, accepted for publication on A&
An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes?
High energy observations of extreme BL Lac objects, such as 1ES 0229+200 or
1ES 0347-121, recently focused interest both for blazar and jet physics and for
the implication on the extragalactic background light and intergalactic
magnetic field estimate. However, the number of these extreme highly peaked BL
Lac objects (EHBL) is still rather small. Aiming at increase their number, we
selected a group of EHBL candidates starting from the BL Lac sample of Plotkin
et al. (2011), considering those undetected (or only barely detected) by the
Large Area Telescope onboard Fermi and characterized by a high X-ray vs. radio
flux ratio. We assembled the multi-wavelength spectral energy distribution of
the resulting 9 sources, profiting of publicly available archival observations
performed by the Swift, Galex and Fermi satellites, confirming their nature.
Through a simple one-zone synchrotron self-Compton model we estimate the
expected VHE flux, finding that in the majority of cases it is within the reach
of present generation of Cherenkov arrays or of the forthcoming Cherenkov
Telescope Array (CTA).Comment: 12 pages, 5 figures, accepted for publication in MNRA
Difficulties in Mid-Infrared selection of AGN in dwarf galaxies
While massive black holes (MBHs) are known to inhabit all massive galaxies,
their ubiquitous presence in dwarf galaxies has not been confirmed yet, with
only a limited number of sources detected so far. Recently, some studies
proposed infrared emission as an alternative way to identify MBHs in dwarfs,
based on a similar approach usually applied to quasars. In this study, by
accurately combining optical and infrared data taking into account resolution
effects and source overlapping, we investigate in detail the possible
limitations of this approach with current ground-based facilities, finding a
quite low (0.4 per cent) fraction of active MBH in dwarfs that are
luminous in mid-infrared, consistent with several previous results. Our results
suggest that the infrared selection is strongly affected by several limitations
that make the identification of MBHs in dwarf galaxies currently prohibitive,
especially because of the very poor resolution compared to optical surveys, and
the likely contamination by nearby sources, although we find a few good
candidates worth further follow-ups. Optical, X-ray and radio observations,
therefore, still represent the most secure way to search for MBH in dwarfs.Comment: 7 pages, 7 figures, 1 table, accepted for publication on MNRA
A new panchromatic classification of unclassified Burst Alert Telescope active galactic nuclei
We collect data at all frequencies for the new sources classified as unknown
active galactic nuclei (AGNs) in the latest Burst Alert Telescope (BAT) all-sky
hard X-ray catalog. Focusing on the 36 sources with measured redshift, we
compute their spectral energy distribution (SED) from radio to -rays
with the aim to classify these objects. We apply emission models that attempt
to reproduce the obtained SEDs, including: i) a standard thin accretion disk
together with an obscuring torus and a X-ray corona; ii) a two temperature
thick advection-dominated flow; iii) an obscured AGN model, accounting for
absorption along the line of sight at kiloelectronvolt energies and in the
optical band; and iv) a phenomenological model to describe the jet emission in
blazar-like objects. We integrate the models with the SWIRE template libraries
to account for the emission of the host galaxy. For every source we found a
good agreement between data and our model. Considering that the sources were
selected in the hard X-ray band, which is rather unaffected by absorption, we
expected and found a large fraction of absorbed radio-quiet AGNs (31 out of 36)
and some additional rare radio-loud sources (5 out of 36), since the jet
emission in hard X-rays is important for aligned jets owing to the boost
produced by the beaming effect. With our work we can confirm the hypothesis
that a number of galaxies, whose optical spectra lack AGN emission features,
host an obscured active nucleus. The approach we used proved to be efficient in
rapidly identifying objects, which commonly used methods were not able to
classify.Comment: 11 pages, LaTeX; Acknowledgments adde
On the orientation of Narrow Line Seyfert I
We study a sample of Narrow-line Seyfert 1 galaxies (NLS1) in their optical
and radio features, in order to understand the differences between their radio
silent, radio-loud and radio-quiet subclasses. We first show that the different
redshift and mass distributions of radio-loud and -quiet NLS1s could be
ascribed to observational biases. We then present a geometrical model according
to which most of the different observational features of radio-loud and
radio-quiet NLS1s are ascribed to the orientation of an intrinsically
structured jet. We estimate the fraction of intrinsically jetted sources among
NLS1s that justifies the observed radio-detected population. Noticeably, under
the assumptions of the geometrical model, we derive a fraction of jetted
sources significantly larger than in standard AGN.Comment: 6 pages, 3 figures, accepted for publication on A&
Black hole mass and spin estimates of the most distant quasars
We investigate the properties of the most distant quasars ULASJ134208.10 + 092838.61 (z = 7.54), ULASJ112001.48 + 064124.3 (z = 7.08) and DELSJ003836.10-152723.6 (z = 7.02) studying their Optical-UV emission that shows clear evidence of the presence of an accretion disk. We model such emission applying the relativistic disk models KERRBB and SLIMBH for which we have derived some analytical approximations to describe the observed emission as a function of the black hole mass, accretion rate, spin and the viewing angle. We found that: 1) our black hole mass estimates are compatible with the ones found using the virial argument but with a smaller uncertainty; 2) assuming that the virial argument is a reliable method to have a black hole mass measurement (with no systematic uncertainties involved), we found an upper limit for the black hole spin of the three sources: very high spin values are ruled out; 3) our Eddington ratio estimates are smaller than those found in previous studies by a factor similar to 2: all sources are found to be sub-Eddington. Using our results, we explore the parameter space (efficiency, accretion rate) to describe the possible evolution of the black hole assuming a similar to 10(2-4) M-circle dot seed: if the black hole in these sources formed at redshift z = 10-20, we found that the accretion has to proceed at the Eddington rate with a radiative efficiency eta similar to 0.1 in order to reach the observed masses in less than similar to 0.7 Gyr