The interstellar medium and star formation in nearby galaxies

Abstract

In this overview article we present some of the key projects we pursue in our Emmy Noether group. Our work is focused on nearby galaxies, where we use multi-wavelength, state-of-the-art survey data to probe distribution, abundance and properties of gas and dust in the interstellar medium (ISM) on ∼ kpc scales. We study the average, radial distributions of atomic (H I) and molecular hydrogen (H2) across the disks of spiral galaxies and assess local (on 1 kpc scales) correlations between H I, H2 and star formation rate (SFR) surface densities across the inner, optical disks of our sample of ∼ 30 spiral galaxies. The short H2 depletion times (∼ 2 Gyr) we find raises the question of if and how star formation is refueled in galactic disks. We look for such signatures of radial gas flows in our H I data and find compelling evidence at least in one case. We extend and compare our gas-SFR studies to the outer disks of galaxies, where conditions change significantly in the ISM, e.g., low metallicity and dust abundance. We focus on star formation at low-metallicity further with detailed ISM studies in dwarf galaxies, where we combine spectroscopic observations in the infrared with detailed modelling to learn about composition and detailed physical properties of the ISM. Of particular interest is the question of what drives large scale star formation in galaxies at low metallicity.

The chemical evolution of local star-forming galaxies: radial profiles of ISM metallicity, gas mass, and stellar mass and constraints on galactic accretion and winds

“…Bigiel et al (2014) identify clear inflow signatures form their study of HI velocity fields for NGC 2403, 3198, 2903, 7331, for which we also determine significant rates of mass infall (see also de Blok et al 2014). Bigiel et al (2014) also detect infall for NGC 6946. We note that while our fit for NGC 6946 finds a solution with Λ = 0 as the best value, the shape of the isocontours in Fig.…”

“…Red filled circles refer to the metallicities given in Table 1 Λ as high as 0.4. For NGC 925, Bigiel et al (2014) find no signs of inflows but instead significant outflows. With η = 0.67 and Λ = 0.54, we detect in NGC 925 both outflows and inflows, but the inflow rate within 3σ can still be consistent with zero (Fig.…”

Revisiting the Integrated Star Formation Law. I. Non-starbursting Galaxies

“…More recent studies on local scales have considered another explanation, again suggesting that the low-density threshold may simply result from the multi-phase ISM. In this picture, the low-density turnover is a phase transition below which the ISM becomes predominantly atomic gas, producing a steep, near-vertical star formation law (e.g., Leroy et al 2008Leroy et al , 2013bBigiel et al 2014). Both the gravitational stability and phase transition models may explain our observed low-Σ gas threshold.…”

“…On global scales, K98 found that SFR correlates strongly with total gas surface density, moderately with atomic gas surface density (Σ HI ), and only weakly with molecular gas surface density (Σ H2 ). Most of the spatiallyresolved studies, on the other hand, find that SFR correlates most strongly with Σ H2 and almost not at all with Σ HI (e.g., Kennicutt et al 2007;Bigiel et al 2008;Leroy et al 2008;Bigiel et al 2014).…”

Phylogeny of the Milky Way’s inner disk and bulge populations: Implications for gas accretion, (the lack of) inside-out thick disk formation, and quenching

“…Observations of high redshift (z>1) galaxies suggest that the gas depletion time scales are < ∼ 1 Gyr (Tacconi et al 2013;Genzel et al 2015;Schinnerer et al 2016;Tacconi et al 2017). Locally gas depletion timescales are much longer, about 1-3 Gyr (Leroy et al , 2013Bigiel et al 2014). These gas depletion timescales are defined relative to the H 2 content of the galaxies and not the total gas content as we have done.…”