Star formation inside a galactic outflow

Abstract

Recent observations have revealed massive galactic molecular outflows 1-3 that may have physical conditions (high gas densities 4-6 ) required to form stars. Indeed, several recent models predict that such massive galactic outflows may ignite star formation within the outflow itself 7-11 . This star-formation mode, in which stars form with high radial velocities, could contribute to the morphological evolution of galaxies 12 , to the evolution in size and velocity dispersion of the spheroidal component of galaxies 11,13 , and would contribute to the population of highvelocity stars, which could even escape the galaxy 13 . Such star formation could provide in-situ chemical enrichment of the circumgalactic and intergalactic medium (through supernova explosions of young stars on large orbits), and some models also predict that it may contribute substantially to the global star formation rate observed in distant galaxies 9 . Although there exists observational evidence for star formation triggered by outflows or jets into their host galaxy, as a consequence of gas compression, evidence for star formation occurring within galactic outflows is still missing. Here we report new spectroscopic observations that unambiguously reveal star formation occurring in a galactic outflow at a redshift of 0.0448. The inferred star formation rate in the outflow is larger than 15 M ¤ /yr. Star formation may also be occurring in other galactic outflows, but may have been missed by previous observations owing to the lack of adequate diagnostics 14,15 .IRAS F23128-5919 is a merging system (Fig.1a), in which the southern nucleus hosts an obscured active nucleus (AGN), detected in the X-rays 16 . Past observations had already revealed a prominent outflow developing from the southern nucleus 14,15,[17][18][19] , driven by the nuclear starburst, or by the AGN, or both. We analyzed archival Very Large Telescope (VLT) spectroscopic observations, obtained with the MUSE instrument, of the optical nebular lines to better characterize the outflow. The nebular emission line profiles can be clearly separated into a narrow component, associated with the interstellar medium in the two galactic disks, and a very broad (Full Width Half Maximum, FWHM~600-1,000 km/s), predominantly blueshifted component tracing the outflow. The velocity field, velocity dispersion, surface brightness maps of the two components are shown in Fig. 1a. The narrow component (bottom row in Fig. 1a) is probably tracing the bulk of the dynamics of the two merging disks. The outflow traced by the broad blueshifted component (top row in Fig. 1a) of the nebular lines extends towards the East of the southern nucleus for about 7-9 kpc (8"-10"), beyond the optical galactic disk. A receding counter-outflow is also observed in the opposite direction, i.e. towards the West (although weaker, owing to extinction by the galactic disk).We have observed the central and eastern outflows with the X-shooter spectrograph at the VLT, which enabled the detection of spectral diagnostics over the en...

Active galactic nucleus outflows in galaxy discs

“…Radio observations reveal large molecular outflows with velocities of 100 km s −1 up to a few times 1000 km s −1 (Aalto et al 2012;Cicone et al 2014). Recently, Maiolino et al (2017) report star formation inside a galactic outflow, another strong indication for the presence of an outflow of cold gas. In Mrk231, the closest and best-studied quasar, we observe velocities of ∼ 1000 km s −1 and mass outflow rates of the order ∼ 1000 M yr −1 (Feruglio et al 2010;Rupke & Veilleux 2011;Sturm et al 2011).…”

Ultrafast outflows disappear in high-radiation fields

“…Alternatively, AGN outflows may trigger star formation by compressing the gas within the process known as positive feedback (see e.g. Maiolino et al 2017). Ultrafast outflows (UFOs, e.g.…”

Negative and Positive Outflow-Feedback in Nearby (U)LIRGs

“…In all the five diagnostic diagrams (Figure 2 in Maiolino et al, 2017) we have found that the line ratios for the broad outflowing component are typical of HII star-forming regions. Furthermore, on the one hand coronal lines associated with AGN excitation (e.g., S IXλ1.252 and Al IXλ2.045µm) are absent in the near IR spectra.…”

“…The southern galaxy is characterized by a prominent outflow studied by various authors through emission and absorption lines (Piqueras López et al, 2012;Bellocchi et al, 2013;Arribas et al, 2014;Cazzoli et al, 2016). In order to investigate if the multiphase outflow is able to undergo vigorous SF as expected in case of positive feedback (section 1) we have combined archival optical MUSE/VLT data with new X-shooter/VLT observations (Maiolino et al, 2017).…”