Outflows in star-forming galaxies: Stacking analyses of resolved winds and the relation to their hosts’ properties

Abstract

Outflows form an integral component in regulating the gas cycling in and out of galaxies, although their impact on the galaxy hosts is still poorly understood. Here we present an analysis of 405 high mass (log M * /M 10), star-forming galaxies (excluding AGN) with low inclinations at z ∼0, using stacking techniques of the Na D λλ5889,5895 Å neutral gas tracer in IFU observations from the MaNGA DR15 survey. We detect outflows in the central regions of 78/405 galaxies and determine their extent and power through the construction of stacked annuli. We find outflows are most powerful in central regions and extend out to ∼1R e , with declining mass outflow rates and loading factors as a function of radius. The stacking of spaxels over key galaxy quantities reveals outflow detections in regions of high Σ SFR ( 0.01 M yr −1 kpc −2 ) and Σ M * ( 10 7 M kpc −2 ) along the resolved main sequence. Clear correlations with Σ SFR suggest it is the main regulator of outflows, with a critical threshold of ∼0.01 M yr −1 kpc −2 needed to escape the weight of the disk and launch them. Furthermore, measurements of the Hδ and D n 4000 indices reveal virtually identical star formation histories between galaxies with outflows and those without. Finally, through stacking of H I 21 cm observations for a subset of our sample, we find outflow galaxies show reduced H I gas fractions at central velocities compared to their non-detection control counterparts, suggestive of some removal of H I gas, likely in the central regions of the galaxies, but not enough to completely quench the host.

Centrally concentrated molecular gas driving galactic-scale ionized gas outflows in star-forming galaxies

“…After falling back to the disk, a separate mechanism may then act to drive the gas inwards to the central regions of the outflow-type object (see Sections 4.2.1 & 4.2.2). In contrast to our findings concerning the molecular gas fraction, Roberts-Borsani et al (2020) find evidence that the neutral gas fraction is depleted in the central regions of their outflow-type sample (where H i is measured through stacking of H i 21 cm observations). The combination of these results suggest that while molecular H2 gas is driven towards the centre of outflow-type galaxies, H i is simultaneously being propelled towards their outer regions, perhaps tracing the ionised wind.…”

Cool outflows in galaxies and their implications

“…( 39)) are ubiquitous in powerful dusty starburst and active galaxies, where dust in the hosts and within the outflowing gas shields Na I (5.1 eV) against the 2432 Å radiation emitted by the starbursts and AGN (e.g., nearby U/LIRGs, Heckman et al 2000;Rupke et al 2002Rupke et al , 2005bMartin 2005Martin , 2006Cazzoli et al 2016). The detection rate (and velocities) of Na I outflows drops precipitously with decreasing dust content A V , SFR, SFR per unit area, AGN power, and stellar mass, averaging a value 1% among the general population of optically selected star-forming and active galaxies in SDSS (e.g., Sarzi et al 2016;Bae et al 2017;Concas et al 2019;Nedelchev et al 2019;Roberts-Borsani and Saintonge 2019), in MaNGA-selected star-forming galaxies (Roberts-Borsani et al 2020), as well as in IRfaint quasars (Krug et al 2010). Na I outflows in non-ULIRGs show a dependence on galaxy disk inclination that indicates a preference for Na I outflows to align along the minor axis of the host galaxy disk (Fig.…”

“…The sizes of the outflows studied in the above HST data (r 1 kpc) are derived indirectly from detailed modeling of the photoionization conditions in the outflowing material, carefully taking into account object-by-object variations in the metallicity and ionization corrections (e.g., Chisholm et al 2016a, b). In contrast, direct size measurements exist for a significant fraction of the cool neutral-atomic outflows based on the Na I absorption line; they typically extend to a few kpc, with a range from 1 kpc to ∼ 15 kpc (Rupke et al 2005c;Martin 2006;Rupke et al 2017;Roberts-Borsani et al 2020). These measurements should be considered lower limits since they are limited by the sensitivity of optical spectrographs to detect this absorption line against the faint galaxy continuum at large r .…”

Frequency and nature of central molecular outflows in nearby star-forming disk galaxies

“…Generation of the spectra. For the 80 galaxies that we consider, we extracted integrated spectra from circular regions (within the plane of the sky) centered on the galaxy's center following previous works (e.g., Fluetsch et al 2019;Roberts-Borsani et al 2020;Roberts-Borsani 2020). The adopted center positions and systemic velocities are taken from Lang et al (2020) as described in Leroy et al (2021b) and listed in Table C.1.…”

“…Observationally, thanks to advances in recent instrumentation, galactic outflow studies are now reaching statistically significant sample sizes of several tens or even several hundreds of galaxies (e.g., Veilleux et al 2013;Concas et al 2019;Förster Schreiber et al 2019;Roberts-Borsani et al 2020). These statistical works now provide typical outflow properties at given galaxy properties and redshift, although many of these lack spatial resolution sufficient to separate the non-circular motions that signpost outflowing gas from the rotation-dominated kinematics of the gas settled in the disk, except for very massive outflows with large velocities of several 100 km s −1 .…”