Ionized outflows in z~2.4 quasar host galaxies

One of the most important astronomical discoveries in the last 20 years is that most local massive galaxies host a supermassive black hole (BH) in their centre with a mass between 10⁷ and 10¹⁰ solar masses, typically 1000 times lighter than their host galaxy. A physical connection between the BH and its host galaxy is suggested by the observed correlation between the BH mass and the stellar velocity dispersion (a proxy for the mass) of the spherical component of the galaxy (the "bulge"). This observation led astronomers to believe that BHs and bulges co-evolve by regulating each other's growth despite the very different scales of these objects. However, the nature of the co-evolution mechanisms of supermassive BHs and their host galaxies is still elusive.

When a BH is actively accreting gas, i.e. when it is in the Active Galactic Nucleus (AGN) phase, the energy released can have a significant impact on the larger-scale surrounding gas of the host galaxy. During their bright phase, AGN can drive energetic outflows capable of expelling the gas on large scales. These energetic winds would shut off the supply of cold gas, necessary for both star formation and BH growth. This AGN "feedback" on the surrounding gas would thus quench the star formation in the galaxy, leaving the system to evolve passively and to grow in mass only through minor and major mergers with other galaxies. Despite impressive observational advances in the past decade, the main properties of these energetic outflows and their actual impact on the galaxy star formation activity remain largely unknown.

A detailed study of AGN-driven outflows has been recently published in the Astronomy & Astrophysics journal by Stefano Carniani, a PhD student at the University of Florence, in collaboration with Alessandro Marconi (University of Florence), Roberto Maiolino (Cambridge), Barbara Balmaverde and Giovanni Cresci (Arcetri Observatory) and others. The work analyses a sample of bright AGN at redshift 2.4, a cosmic epoch for massive galaxies and BH growth, when the most powerful, fast outflows are expected.

The targets were observed using the Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) at the Very Large Telescope (VLT). The observations were obtained in H band (λ~1.45-1.85μm) where the emission line [OIII] (λ_rest = 5007Å), a good tracer of ionized gas, is detectable. The analysis of the [OIII] velocity field (Figure 1) suggests conical, fast (>300 km/s) ionized outflows extending up to ~2 kpc and associated with regions of high velocity dispersion (>500 km/s).

Figure 1: velocity maps of each target composing the quasar sample. The velocity maps display regions with a conical morphology where the ionized gas is blue-shifted. The contours represent the position of the AGN. Image from Carniani et al (2015).

The properties of the ionized outflows (Figure 2), i.e. mass outflow rate, momentum rate and kinetic power, are all correlated with the AGN luminosity. The increase in outflow rate with increasing AGN luminosity is consistent with the idea that a luminous AGN pushes away the surrounding gas through fast outflows that are driven by radiation pressure, which depends on the emitted luminosity. At the same time, the lower outflow rates of the ionized gas compared to molecular gas (circles versus squares in Fig. 2) can be explained with the fact that ionized gas traces a smaller fraction of the total gas mass. Alternatively, this indicates different acceleration mechanisms for the molecular and the ionized gas.

Figure 2: Mass outflow rates as a function of the AGN bolometric luminosity. The blue circles denote the results from this work; the open circles mark the velocities of the ionized gas, traced by either the [OIII] or the Hβ emission line: the red, orange, purple and black circles are the estimates obtained from Harrison et al (2014), Brusa et al (2015), Cresci et al (2015) and Greene et al (2012), respectively. The green, black and brown squares denote the velocities of the molecular outflows from Cicone et al (2014), Sun et al (2014) and Feruglio et al (2015). The solid line is the best-fit relation to the averages of filled and empty circles. Image from Carniani et al (2015).

This work is one of the first to compare the properties of ionized outflows with those of molecular ones. These results, presented in the article "Ionized outflows in z~2.4 quasar host galaxies" to be published in Astronomy & Astrophysics, open the door to further analysis connecting molecular and ionized outflows to the AGN and host galaxy properties.

Edited by Anna Gallazzi and Stefano Carniani, July 2015