(Star)bursts of FIRE: observational signatures of bursty star formation in galaxies

Abstract

Galaxy formation models are now able to reproduce observed relations such as the relation between galaxies' star formation rates (SFRs) and stellar masses (M * ) and the stellar masshalo mass relation. We demonstrate that comparisons of the short-timescale variability in galaxy SFRs with observational data provide an additional useful constraint on the physics of galaxy formation feedback. We apply SFR indicators with different sensitivity timescales to galaxies from the Feedback in Realistic Environments (FIRE) simulations. We find that the SFR-M * relation has a significantly greater scatter when the Hα-derived SFR is considered compared with when the far-ultraviolet (FUV)-based SFR is used. This difference is a direct consequence of bursty star formation because the FIRE galaxies exhibit order-of-magnitude SFR variations over timescales of a few Myr. We show that the difference in the scatter between the simulated Hα-and FUV-derived SFR-M * relations at z = 2 is consistent with observational constraints. We also find that the Hα/FUV ratios predicted by the simulations at z = 0 are similar to those observed for local galaxies except for a population of low-mass (M * 10 9.5 M ) simulated galaxies with lower Hα/FUV ratios than observed. We suggest that future cosmological simulations should compare the Hα/FUV ratios of their galaxies with observations to constrain the feedback models employed.

Stacked Star Formation Rate Profiles of Bursty Galaxies Exhibit “Coherent” Star Formation

“…Moreover, on average, the below-MS galaxies tend to have lower SFRs at all radii than above-MS galaxies (but this is not true of the individual profiles) because these galaxies have, on average, lower gas surface densities (Figure 3) than the above-MS galaxies (owing to stochasticity in gas accretion from both the intergalactic medium and galactic fountains and/or recent strong outflows driven by stellar feedback; Muratov et al 2015;Anglés-Alcázar et al 2017;. However, in the simulations, these differences are stochastic rather than long-lived, as is evident from the bottom panel of Figure 4, and the FIRE galaxies can cross the MS multiple times within 100 Myr (Figure 2; see also Sparre et al 2017). Note, however, that the galaxies considered here are of relatively "low" stellar mass (   M 10 10  M ), and more massive simulated galaxies tend to exhibit less bursty star formation and smoother mass, metallicity, and SFR profiles, especially at low redshift (Ma et al 2017;Sparre et al 2017).…”

“…The observed MS (Whitaker et al 2014) and scatter (Speagle et al 2014) in two redshift bins intersecting this interval are shown. In this redshift and mass range, the individual simulated galaxies experience significant (sometimes an order of magnitude or more), rapid (timescales100 Myr) SFR variations (see Sparre et al 2017 for a detailed study) and clearly do not evolve parallel to the MS.…”

“…In Figure 4, we see that galaxies selected to be above the MS have preferentially recently formed several massive clumps of stars; this is true whether the SFR is averaged over 10 or 100 Myr (i.e., whether Hα-or UV+IR-based SFRs are used). Conversely, galaxies below the MS are unlikely to have formed many massive clumps within the past ∼100 Myr and rather are likely to be in a low-sSFR period, which can last for a few hundreds of Myr in the simulations (Muratov et al 2015;Sparre et al 2017); if they have formed a few clumps, the associated SFRs are not as high as in the above-MS galaxies. Moreover, on average, the below-MS galaxies tend to have lower SFRs at all radii than above-MS galaxies (but this is not true of the individual profiles) because these galaxies have, on average, lower gas surface densities (Figure 3) than the above-MS galaxies (owing to stochasticity in gas accretion from both the intergalactic medium and galactic fountains and/or recent strong outflows driven by stellar feedback; Muratov et al 2015;Anglés-Alcázar et al 2017;.…”

“…a Except for m11h383 (Chan et al 2015), all simulations are from Hopkins et al (2014). 9 In the FIRE simulations, galaxies with   ☉ M M 10 10 exhibit highly bursty star formation at high redshift and transition to steady star formation at  z 1; lower-mass galaxies always exhibit bursty star formation (Sparre et al 2017). 10 For < < z 0.7 1.5, the snapshot spacing of D = z 0.01 corresponds to time spacing of -25 56 Myr.…”

Characterization of star-forming dwarf galaxies at 0.1 ≲z ≲ 0.9 in VUDS: probing the low-mass end of the mass-metallicity relation

“…How SFDGs assemble their stellar mass remains one of the unanswered questions surrounding these galaxies. Differently from high-mass (M * > 10 9 M ) galaxies, which show a continuous rate of star formation (SF), the most common scenario for dwarf galaxies is the cyclic bursty mode, as pointed out by theoretical models (e.g., Hopkins et al 2014;Sparre et al 2017) and observations (Guo et al 2016b). Intense SF episodes produce stellar feedback through strong winds and supernova, which heat and expel the surrounding gas in outflows, eventually resulting in a temporary quenching of SF on timescales of tens of Myr (e.g., Olmo-Garcia et al 2017;Pelupessy et al 2004).…”