Galactic Outflows

Profile of the CO(1-0) transition of the quasar Mrk231 revealing molegular gas expelled at high velocities (Feruglio, Maiolino, et al. 2010)

First detections of quasar-driven Massive outflows

Galactic outflows, powered either by accreting supermassive black holes (quasars) or by supernovae, are key mechanisms responsible for regulating, or even suppressing, star formation in galaxies by removing and heating gas (i.e. the fuel for star formation).

Observational evidence for quasar-driven massive outflows, in the process of "cleaning" massive galaxies of their gas content, was achieved through the detection of molecular high velocity gas in nearby quasar hosts, extending on large galactic scales, traced thanks to interferometric observations of the CO transitions at millimeter wavelengths (Feruglio, Maiolino, et al. 2010). In parallel, at the same time, molecular outflows were identified through the detection of blueshifted absorption of molecular transitions in the far-IR (Sturm et al. 2011).

Molecular outflow rate as a function of star formation rate, where galaxies are color-coded by fraction of bolometric luminosity contributed by an Active Galactic Nucleus, i.e. an accreting supermassive black hole (Fluetsch, Maiolino et al. 2019).

Relative role of quasars and star formation

Extensions of the same technique to a larger sample of local galaxies have revealed that massive outflows are commonly found in galaxies with vigorous star formation, but the presence a quasar/AGN can boost the outflow rate by a significant factor (Cicone, Maiolino et al. 2014, Fluetsch, Maiolino et al. 2019) supporting models' expectations. The same technique is now routinely being used by several groups to identify outflows of cold gas in several galaxies.

Relative content of molecular, neutral and ionized gas in a sample of nearby active galaxies (Fluesch, Maiolino et al. 2020).

Multi-phase outflows

Multi-band observations have enabled us to investigate the multi-phase and physical properties of outflows, finding that the molecular phase largery dominates the mass and energy budget (Fluesch, Maiolino et al. 2020), with atomic neutral gas less important and the ionized phase generally negligible. Furthermore, we ha found that the molecular gas in outflows is very dense and clumpy (Cicone, Maiolino et al. 2012, Williams, Maiolino et al. 2017, Cicone, Maiolino et al. 2020; Jones, Maiolino et al. 2019).

Extended distribution of cold, atomic (mostly high-velocity) gas around a primeval quasar (Cicone, Maiolino et al. 2015)

Massive outflows in the early Universe

I have also been leading the detection of cold outflows in the early universe. Specifically, by exploiting far-IR fine structure lines we could identify for the first time massive quasar-driven outflows at high redshift, illustrating that this feedback mechanism was already in place in the early univese, soon after the formation of the first galaxies (Maiolino et al. 2012; Cicone, Maiolino et al. 2015, Bischetti, Maiolino et al. 2020).

We have also successfully extended the search of cold, massive molecular outflows by detecting blueshifted molecular transition in absorption in extreme starburst galaxies in the early Universe (Jones et al. 2019).

Distribution of star formation (color) in a distant quasar host galaxy compared with the location of the quasar-driven wind (contours) (Carniani, Marconi, Maiolino et al. 2016)

Star formation quenching in quasar hosts

By also mapping the distribution of star formation in the host galaxies of distant quasars, we could find the first direct evidence of suppression of star formation by quasar-driven outflows (Cano-Diaz, Maiolino et al. 2012). Similar results have subsequently been confirmed by the analysis of other quasar host galaxies at high redshift (Carniani, Marconi, Maiolino et al. 2015, Cresci et al. 2015, Carniani, Marconi, Maiolino et al. 2016). These observations however also reveal that the quenching by quasar-driven outflows is a local process, which does not affect the entire galaxy.

Artist’s impression of stars born in winds from supermassive black holes, from the ESO press release associated with the result presented in Maiolino et al. 2017.

Star formation inside galactic outflows

Several recent models expect that massive galactic outflows may ignite star formation in the outflow itself. This would be a newmode of star formation, in which stars form with high velocities on radial orbits, and would have several major implications.

By using extensive data on a nearby galaxy with a prominent outflow, we have obtained the first unambiguous evidences for stars forming inside galactic outflow (Maiolino et al. 2017, Gallagher et al. 2019). Our results have shown that this new mode of star formation is not uncommon among galactic outflows and may play an important role in the formation and growth of the spheroidal component of galaxies (bulge, halo, elliptical galaxies).

An extensive review of cool galactic outflows is given in Veilleux, Maiolino, Bolatto & Aalto (2020)