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Register a new userA Closer look at Bursty Star Formation with $L_{Halpha}$ and $L_{UV}$ Distributions
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We investigate the bursty star formation histories (SFHs) of dwarf galaxies using the distribution of log($L_{Halpha}/L_{UV}$) of 185 local galaxies. We expand on the work of Weisz et al. 2012 to consider a wider range of SFHs and stellar metallicities, and show that there are large degeneracies in a periodic, top-hat burst model. We argue that all galaxies of a given mass have similar SFHs and we can therefore include the $L_{Halpha}$ distributions (subtracting the median trend with stellar mass, referred to as $Delta text{log}(L_{Halpha})$) in our analyses. $Delta text{log}(L_{Halpha})$ traces the amplitude of the bursts, and log($L_{Halpha}/L_{UV}$) is a function of timescale, amplitude, and shape of the bursts. We examine the 2-dimensional distribution of these two indicators constrain the SFHs. We use exponentially rising/falling bursts to determine timescales ($e$-folding time, $tau$). We find that galaxies below $10^{7.5}$ M$_{odot}$ undergo large (amplitudes of $sim 100$) and rapid ($tau < 30$ Myr) bursts, while galaxies above $10^{8.5}$ M$_{odot}$ experience smaller (maximum amplitudes $sim 10$), slower ($tau gtrsim 300$ Myr) bursts. We compare to the FIRE-2 hydrodynamical simulations and find that the burst amplitudes agree with observations, but they are too rapid in more massive galaxies ($M_* > 10^8$ M$_{odot}$). Finally, we confirm that stochastic sampling of the stellar mass function can not reproduce the observed distributions unless the standard assumptions of cluster and stellar mass functions are changed. With the next generation of telescopes, measurements of $L_{UV}$ and $L_{Halpha}$ will become available for dwarf galaxies at high-redshift, enabling similar analyses of galaxies in the early universe.
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The star formation histories (SFHs) of dwarf galaxies are thought to be emph{bursty}, with large -- order of magnitude -- changes in the star formation rate on timescales similar to O-star lifetimes. As a result, the standard interpretations of many galaxy observables (which assume a slowly varying SFH) are often incorrect. Here, we use the SFHs from hydro-dynamical simulations to investigate the effects of bursty SFHs on sample selection and interpretation of observables and make predictions to confirm such SFHs in future surveys. First, because dwarf galaxies star formation rates change rapidly, the mass-to-light ratio is also changing rapidly in both the ionizing continuum and, to a lesser extent, the non-ionizing UV continuum. Therefore, flux limited surveys are highly biased toward selecting galaxies in the emph{burst} phase and very deep observations are required to detect all dwarf galaxies at a given stellar mass. Second, we show that a $log_{10}[ u L_{ u}(1500{rm AA})/L_{{rm H}alpha}]>2.5$ implies a very recent quenching of star formation and can be used as evidence of stellar feedback regulating star formation. Third, we show that the ionizing continuum can be significantly higher than when assuming a constant SFH, which can affect the interpretation of nebular emission line equivalent widths and direct ionizing continuum detections. Finally, we show that a star formation rate estimate based on continuum measurements only (and not on nebular tracers such as the hydrogen Balmer lines) will not trace the rapid changes in star formation and will give the false impression of a star-forming main sequence with low dispersion.
We present an analysis of the $Rlesssim 1.5$ kpc core regions of seven simulated Milky Way mass galaxies, from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite, for a finely sampled period ($Delta t = 2.2$ Myr) of 22 Myr at $z approx 0$, and compare them with star formation rate (SFR) and gas surface density observations of the Milky Ways Central Molecular Zone (CMZ). Despite not being tuned to reproduce the detailed structure of the CMZ, we find that four of these galaxies are consistent with CMZ observations at some point during this 22 Myr period. The galaxies presented here are not homogeneous in their central structures, roughly dividing into two morphological classes; (a) several of the galaxies have very asymmetric gas and SFR distributions, with intense (compact) starbursts occurring over a period of roughly 10 Myr, and structures on highly eccentric orbits through the CMZ, whereas (b) others have smoother gas and SFR distributions, with only slowly varying SFRs over the period analyzed. In class (a) centers, the orbital motion of gas and star-forming complexes across small apertures ($R lesssim 150$pc, analogously $|l|<1^circ$ in the CMZ observations) contributes as much to tracers of star formation/dense gas appearing in those apertures, as the internal evolution of those structures does. These asymmetric/bursty galactic centers can simultaneously match CMZ gas and SFR observations, demonstrating that time-varying star formation can explain the CMZs low star formation efficiency.
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