Emanuele Paolo Farina Research Interest

Multiple quasar systems

Soon after the discovery of the first quasar (3C 273, Schmidt 1963), it became clear that its enormous luminosity and short-timescale variability require intense gas accretion onto a supermassive black hole (SMBH) at the center of a galaxy (e.g., Salpeter 1964, Zel'dovich et al. 1964, Lynden-Bell 1969). This scenario is now widely accepted, but the mechanism responsible for the gas inflow is still a matter of debate. Some theoretical models suggest that gas-rich galaxy mergers provide an efficient way to channel gas down to the circumnuclear regions, possibly triggering both star formation and quasar activity in the host galaxies (e.g., Hernquist 1989, Kauffmann et al. 2000, Di Matteo et al. 2005, Hopkins et al. 2008). From an observational point of view, this scenario is supported by the detection of features associated with recent or ongoing mergers in quasar host galaxies (e.g., Canalizo et al. 2007, Bennert et al. 2008), and by evidence that quasars preferentially reside in richer-than-average galactic environments (e.g., Wold et al. 2001, Söchting et al. 2002, 2004, Serber et al. 2006, Hutchings et al. 2009). Moreover, the observed excess of small-separation quasar pairs (<100 kpc) can be explained if mergers trigger quasar activity (e.g., Djorgovski et al. 1991, Hennawi et al. 2006, Myers et al. 2007, 2008, Shen et al. 2010, Kayo et al. 2012).

One of my main research topics is the study of physical systems of quasars (i.e., two or more gravitationally bound quasars), which are an expected outcome of the current hierarchical framework of galaxy formation and evolution. In this framework, we know that:

galaxies regularly interact and merge (e.g., Toomre & Toomre 1972);
SMBHs reside at the centers of almost all massive galaxies (e.g., Kormendy et al. 1995, Richstone et al. 1998);
major mergers can trigger inflows of gas down to the circumnuclear regions, which may result in a burst of quasar activity (e.g., Di Matteo et al. 2005).

These facts suggest that binary SMBHs should be common, and so should quasar pairs (if both SMBHs are accreting at the same time).

For a long time, the search for physical groupings of quasars focused only on binary systems. However, thanks to the enormous amount of data made available by the SDSS (Richards et al. 2009, Schneider et al. 2010), it is now possible to carry out a systematic search for systems of more than two quasars, which are even more elusive (for instance, the probability of finding a chance superposition of two quasars within 500 kpc of a third one is Pr(pd<500kpc)≈10^-8). The follow-up of candidate quasar triplets with the Magellan telescopes, the ESO NTT, and the 3.5 m telescope at Calar Alto Observatory led to the discovery of QQQ J1519+0627, the second quasar triplet known to date (after QQQ J1432−0106, Djorgovski et al. 2007). More recently, another quasar triplet has been reported (Assef et al. 2018). In addition, Hennawi et al. (2015) discovered the first quasar quartet, dramatically challenging our understanding of structure formation.

The circum-galactic medium of quasars

The circum-galactic medium (CGM) is the interface between the interstellar medium (ISM) and the large-scale intergalactic medium (IGM). In this crucial region surrounding a galaxy (empirically, within a radius of about 300 kpc), all the mechanisms responsible for galaxy formation and evolution take place. Since the pioneering works of Bahcall & Spitzer (1969) and Boksenberg & Sargent (1978), the traditional way to probe the CGM has been through the detection of narrow metal absorption lines imprinted on the spectra of bright background sources.

Projected quasar pairs (i.e., two quasars that are closely aligned on the sky but lie at different redshifts) are a powerful tool for linking the nuclear activity of quasars with their immediate environment. In the standard scenario, major mergers between galaxies are believed to be responsible for intense starbursts and for channeling large amounts of gas down to the circumnuclear regions, eventually triggering the activity of the central supermassive black hole (e.g., Hernquist 1989; Kauffmann & Haehnelt 2000; Canalizo & Stockton 2001; Di Matteo, Springel & Hernquist 2005). In this picture, the CGM of quasars is expected to harbor a large amount of enriched gas. This was confirmed by Hennawi et al. (2006), who studied the distribution of neutral hydrogen around quasars: using a sample of 149 projected quasar pairs (projected separation <2.5 Mpc), they found that the probability of finding an absorber with N_HI>10¹⁹cm^-2 within 200 kpc of a quasar is around 50%, and that the absorbers are highly anisotropically distributed (Hennawi & Prochaska 2007, Prochaska et al. 2013a,b). In Farina et al. (2013, 2014), we confirmed these results using MgII as a tracer of the cool gas in the CGM of quasars. A great advantage of MgII is that it allows a direct comparison with the vast literature on MgII absorption systems associated with “normal” galaxies. The CGM of quasars appears to contain more cool gas than that of any other type of galaxy (see also Prochaska et al. 2014), and one of my current research interests is to understand the physical reasons behind this overabundance.

The hunt for high-z quasars

Quasars are among the brightest non-transient sources in the sky. Thanks to their high luminosity, they can be observed even at early cosmic times, when the young Universe was emerging from the dark ages (z>6.5; Mortlock et al. 2011, Venemans et al. 2013, 2015, Bañados et al. 2018). High-redshift quasars are discovered in large multi-band surveys through the detection of a sharp drop in the reddest optical filters, caused by the so-called Gunn–Peterson trough, i.e., the suppression of the flux blueward of the Lyα emission line by neutral hydrogen in the intergalactic medium. In particular, together with the quasar team led by Fabian Walter, we are successfully mining the Pan-STARRS survey to find new z>5.7 quasars (Bañados et al. 2014, 2015) and to push beyond the SDSS redshift and magnitude limits (Venemans et al. 2015, Mazzucchelli et al. 2017).

Surprisingly, these first quasars already appear to be evolved systems, with black holes as massive as 10⁸–10⁹M_☉ (e.g., Jiang et al. 2007, Mortlock et al. 2011, De Rosa et al. 2014, and the spectacular discovery by Wu et al. 2015) hosted by massive (Walter et al. 2004, Wang et al. 2010, Willott et al. 2015) and vigorously star-forming (Walter et al. 2009) galaxies. In addition, studies of the metallicity of the broad-line regions show that high-redshift quasars are already metal-enriched, and that there is no evolution in metallicity from z≈4 to z>6 (Jiang et al. 2007, De Rosa et al. 2011, 2014). To explain such fast growth and evolution (at z=6 the Universe was only 0.9 Gyr old), Volonteri & Rees (2006) proposed that the first quasars must reside in particularly rich environments, where huge gas reservoirs favor the rapid accretion of material onto the seeds of supermassive black holes. At low redshift, the most effective way to probe the gas surrounding quasars is through the detection of the so-called Lyα fuzz, which appears when the cool gas reservoir is fluorescently illuminated by the emission of the bright quasar and by UV photons generated by intense star formation. The success obtained at z≈2 (Hennawi & Prochaska 2013, Martin et al. 2014, Cantalupo et al. 2014, Hennawi et al. 2015) motivated us to play this game at the highest accessible redshifts. Redshift dimming makes this research particularly hard at z>6 (the surface brightness scales with redshift as SB(Lyα)∝(1+z)^-4), and current evidence of extended Lyα emission associated with the first quasars is scarce (Willott et al. 2011, Decarli et al. 2012, Roche et al. 2014). Together with the other members of the quasar team, I am trying to improve our knowledge of this feature using 8m-class telescopes and the exquisite spatial resolution of HST. More details are available on the websites of our REQUIEM and Aether surveys.