Faint progenitors of luminous quasars: where are they?

We studied the visibility of the supermassive black holes’ ancestors in the X- ray band. Our goal was to assess if their detection is at reach of the current observational capabilities. We used the results of a semi-numerical simulation to evaluate the X-ray emis- sion of the progenitors, taking into account the attenuation processes that occurs as the radiation travels through the interstellar medium of the host galaxy. Then we compared our results with the sensitivity of the most powerful observatory available, the Chandra Space Telescope, finding that the expected emission is strong enough to be revealed, although these objects are too rare to be detected with the past surveys. Accounting for the technical difficulties that arise in performing a survey, we planned the best observational strategy to maximize the detection probability. We found that is more advantageous for a survey to explore an area as large as possible to the detriment of the sensitivity. These results are encouraging because none of the past surveys implemented the optimal strategy and therefore there is ample room for improvement: even if they have not been detected so far, we expect the detection to be possible in the near future

X-ray spectroscopy of the z=6.4 quasar J1148+5251

We present the 78-ks Chandra observations of the $z=6.4$ quasar SDSS J1148+5251. The source is clearly detected in the energy range 0.3-7 keV with 42 counts (with a significance $\gtrsim9\sigma$). The X-ray spectrum is best-fitted by a power-law with photon index $\Gamma=1.9$ absorbed by a gas column density of $\rm N_{\rm H}=2.0^{+2.0}_{-1.5}\times10^{23}\,\rm cm^{-2}$. We measure an intrinsic luminosity at 2-10 keV and 10-40 keV equal to $\sim 1.5\times 10^{45}~\rm erg~s^{-1}$, comparable with luminous local and intermediate-redshift quasar properties. Moreover, the X-ray to optical power-law slope value ($\alpha_{\rm OX}=-1.76\pm 0.14$) of J1148 is consistent with the one found in quasars with similar rest-frame 2500 \AA ~luminosity ($L_{\rm 2500}\sim 10^{32}~\rm erg~s^{-1}$\AA$^{-1}$). Then we use Chandra data to test a physically motivated model that computes the intrinsic X-ray flux emitted by a quasar starting from the properties of the powering black hole and assuming that X-ray emission is attenuated by intervening, metal-rich ($Z\geq \rm Z_{\odot}$) molecular clouds distributed on $\sim$kpc scales in the host galaxy. Our analysis favors a black hole mass $M_{\rm BH} \sim 3\times 10^9 \rm M_\odot$ and a molecular hydrogen mass $M_{\rm H_2}\sim 2\times 10^{10} \rm M_\odot$, in good agreement with estimates obtained from previous studies. We finally discuss strengths and limits of our analysis.Comment: 9 pages, 3 figures, 1 table, MNRAS in pres

Black Hole Accretion in Primordial Galaxies

Supermassive black holes (SMBHs) with masses up to 1010M 99 are thought to power the emission from quasars and Active Galactic Nuclei. Surprisingly, these extremely massive, compact objects are already in place within the first billion years from the Big Bang, or redshift z 65 6. In addition, a tight relation between the stellar and central black hole mass in galaxies has been established locally. The SMBH origin, evolution and relation to the host galaxy are key open problems in modern cosmology and astrophysics. They are the main subject of this Thesis. As far as the origin is concerned, the most natural idea considers SMBHs as the end-product of accretion and merging on black hole remnants of massive stars. We have examined this possibility showing that this hypothesis entails several theoretical difficulties. To this aim, we have followed the accretion history of a 100M 99 stellar BH hosted by a typical z = 10 galaxy down to z = 6. We analysed the growth under different conditions linked to the galaxy geometry and BH orbital parameters. We conclude that in all cases, the BH mass can increase at most by 30%, thus making stellar seeds unsuitable to explain the observed masses of SMBH. As a by-product of the study, we have estimated the cumulative X- ray emission from an early BH population and the total energy released in the intergalactic medium. Given the above low accretion rates, we conclude that the X-ray emission from accreting BHs is negligible with respect to that provided by X-ray binaries in the same galaxy. Although sub-dominant, the X-ray preheating of the intergalactic medium by early BHs might have left a specific signature, potentially detectable with SKA, on the HI 21cm line power spectrum. The above results forced us to look for alternative SMBH formation processes. Thus, we explored the scenario in which SMBHs grow by merging of more mas- sive seeds, directly formed through non-stellar channels. These theoretically pre- dicted, massive (104 12 106M 99) \u201cdirect collapse\u201d seeds provide a head-start of the SMBH build-up and overcome the inefficient stellar BH accretion. Lacking an observational confirmation of the existence of these putative SMBH progeni- tors, we first assessed whether these ancestors can be detected by the Chandra X-ray Observatory, and used the upper limits provided by current observations to constrain theoretical model parameters. For this purpose, we exploit a semi- analytical model to predict the number density of progenitors of a z = 6.4 SMBH, their accretion and the amount of obscuring material in their host galaxies. For each ancestor we computed its X-ray spectrum accounting for interstellar absorp- tion and compared it with current observations. Faint progenitors are found to be luminous enough to be detected in the X-ray band of current surveys. Even accounting for a maximum obscuration effect, the number of detectable BHs is reduced at most by a factor of 2. In our simulated sample, observations of faint quasars are mainly limited by their very low active fraction (about 1%), which is the result of short, super-critical growth episodes. We suggest that to detect high-z SMBHs progenitors, large area surveys with shallower sensitivities, such as COSMOS Legacy and XMM-LSS+XXL, are to be preferred with respect to deep surveys probing smaller fields, such as Chandra Deep Field South. The models discussed so far imply that massive black holes ( 48 108M 99) must be present also in galaxies routinely observed in the Epoch of Reionization, such as the so-called Lyman Break Galaxies (LBG) at z > 6. This is an important point because the presence of a (faint) AGN in these systems might substantially alter their physical and observable properties. We addressed this question by combining our semi-analytical model with tight constraints from the 7 Ms Chan- dra survey, and the known high-z SMBH population. Depending on the fraction of early halos planted with a BH direct collapse seed, the model suggests two possible scenarios: (a) if a maximal seeding occurs, massive BH in LBGs mostly grow by merging and must accrete at a low Eddington ratio not to exceed the ex- perimental X-ray luminosity upper bound; (b) if the seeding is inefficient, direct accretion dominates and massive BH emission in LBGs must be heavily obscured. Scenario (a) poses extremely challenging, and possibly unphysical, requirements on seeds formation. Scenario (b) entails testable implications on the physical properties of LBGs involving far-infrared luminosity, emission lines, and presence of outflows that we discuss in detail

Importance of Baseline Prognostic Factors With Increasing Time Since Initiation of Highly Active Antiretroviral Therapy: Collaborative Analysis of Cohorts of HIV-1-Infected Patients

Background: The extent to which the prognosis for AIDS and death of patients initiating highly active antiretroviral therapy (HAART) continues to be affected by their characteristics at the time of initiation (baseline) is unclear. Methods: We analyzed data on 20,379 treatment-naive HIV-1- infected adults who started HAART in 1 of 12 cohort studies in Europe and North America (61,798 person-years of follow-up, 1844 AIDS events, and 1005 deaths). Results: Although baseline CD4 cell count became less prognostic with time, individuals with a baseline CD4 count 350 cells/μL (hazard ratio for AIDS = 2.3, 95% confidence interval [CI]: 1.0 to 2.3; mortality hazard ratio = 2.5, 95% CI: 1.2 to 5.5, 4 to 6 years after starting HAART). Rates of AIDS were persistently higher in individuals who had experienced an AIDS event before starting HAART. Individuals with presumed transmission by means of injection drug use experienced substantially higher rates of AIDS and death than other individuals throughout follow-up (AIDS hazard ratio = 1.6, 95% CI: 0.8 to 3.0; mortality hazard ratio = 3.5, 95% CI: 2.2 to 5.5, 4 to 6 years after starting HAART). Conclusions: Compared with other patient groups, injection drug users and patients with advanced immunodeficiency at baseline experience substantially increased rates of AIDS and death up to 6 years after starting HAART