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

The jet-disc connection in AGN

We present our latest results on the connection between accretion rate and relativistic jet power in AGN, by using a large sample which includes mostly blazars, but contains also some radio--galaxies. The jet power can be traced by $\gamma$--ray luminosity in the case of blazars, and radio luminosity for both classes. The accretion disc luminosity is instead traced by the broad emission lines. Among blazars, we find a correlation between broad line emission and the $\gamma$--ray or radio luminosities, suggesting a direct tight connection between jet power and accretion rate. We confirm that the observational differences between blazar subclasses reflect differences in the accretion regime, but with blazars only we cannot properly access the low--accretion regime. By introducing radio--galaxies, we succeed in observing the fingerprint of the transition between radiatively efficient and inefficient accretion discs in the jetted AGN family. The transition occurs at the standard critical value $L_{\rm d}/L_{\rm Edd}\sim10^{-2}$ and it appears smooth. Below this value, the ionizing luminosity emitted by the accretion structure drops significantly.Comment: 11 pages, 6 figures. Accepted for publication in MNRA

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

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

The role of relativistic jets in the heaviest and most active supermassive black holes at high redshift

In powerful radio-quiet active galactic nuclei (AGN), black holes heavier than one billion solar masses form at a redshift ~1.5-2. Supermassive black holes in jetted radio-loud AGN seems to form earlier, at a redshift close to 4. The ratio of active radio-loud to radio-quiet AGN hosting heavy black holes is therefore a rather a strong function of redshift. We report on some recent evidence supporting this conclusion, gathered from the Burst Alert Telescope (BAT, onboard Swift) and by the Large Area Telescope (LAT, onboard Fermi). We suggest that the more frequent occurrence of relativistic jets in the most massive black holes at high redshifts, compared to later times, could be due to the average black hole spin being greater in the distant past, or else to the jet helping a fast accretion rate (or some combination of the two scenarios). We emphasize that the large total accretion efficiency of rapidly spinning black holes inhibits a fast growth, unless a large fraction of the available gravitational energy of the accreted mass is not converted into radiation, but used to form and maintain a powerful jet.Comment: 6 pages, 3 figures, accepted for publication in MNRAS, main journa

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 $\gamma$-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

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 ($\sim$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