THE BURSTY STAR FORMATION HISTORIES OF LOW-MASS GALAXIES AT 0.4 < z < 1 REVEALED BY STAR FORMATION RATES MEASURED FROM Hβ AND FUV

Abstract

We investigate the burstiness of star formation histories (SFHs) of galaxies at 0.4<z<1 by using the ratio of star formation rates (SFRs) measured from Hβ and FUV (1500 Å) (Hβ-to-FUV ratio). Our sample contains 164 galaxies down to stellar mass (M * ) of 10 8.5 M e in the CANDELS GOODS-N region, where Team Keck Redshift Survey Keck/DEIMOS spectroscopy and Hubble Space Telescope/WFC3 F275W images from CANDELS and Hubble Deep UV Legacy Survey are available. When the ratio of Hβ-and FUV-derived SFRs is measured, dust extinction correction is negligible (except for very dusty galaxies) with the Calzetti attenuation curve. The Hβ-to-FUV ratio of our sample increases with M * and SFR. The median ratio is ∼0.7 at M * ∼10 8.5 M e (or SFR∼0.5 M e yr −1 ) and increases to ∼1 at M * ∼10 10 M e (or SFR∼10 M e yr −1 ). At M * <10 9.5 M e , our median Hβ-to-FUV ratio is lower than that of local galaxies at the same M * , implying a redshift evolution. Bursty SFH on a timescale of a few tens of megayears on galactic scales provides a plausible explanation for our results, and the importance of the burstiness increases as M * decreases. Due to sample selection effects, our Hβ-to-FUV ratio may be an upper limit of the true value of a complete sample, which strengthens our conclusions. Other models, e.g., non-universal initial mass function or stochastic star formation on star cluster scales, are unable to plausibly explain our results.

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

“…We thus caution against interpreting such time-averaged coherent star formation as evidence that galaxies maintain their positions relative to the MS owing to, e.g., systematic differences in gas accretion rates and thus gas fractions. A crucial next step is to place observational constraints on the timescale over which galaxies oscillate across the main sequence, perhaps via measurement of SFR tracers that probe different timescales (e.g., Guo et al 2016). Although our analysis does not rule out the possibility that galaxies maintain their positions relative to the MS for long periods of time, it demonstrates that simulations in which this is not the case yield stacked SFR profiles consistent with those observed, including spatially coherent star formation in a time-averaged sense.…”

ZFIRE: using Hα equivalent widths to investigate the in situ initial mass function at z ∼ 2

“…Multiple studies have investigated possibilities for IMF variation in galaxies using Balmer line flux in the context of probing SFHs (Meurer et al 2009;Weisz et al 2012;Zeimann et al 2014;Guo et al 2016;Smit et al 2016). Modelling effects of IMF variation using Hα or Hβ to UV flux ratios have strong dependence on the assumed SFH and dust extinction of the galaxies and is only sensitive to the upper end of the high mass IMF.…”

“…Modelling effects of IMF variation using Hα or Hβ to UV flux ratios have strong dependence on the assumed SFH and dust extinction of the galaxies and is only sensitive to the upper end of the high mass IMF. Apart from IMF variation (Boselli et al 2009;Meurer et al 2009;Pflamm-Altenburg et al 2009), stochasticity in SFH (Boselli et al 2009;Fumagalli et al 2011;Guo et al 2016), non-constant SFHs (Weisz et al 2012), and Lyman leakage (Relaño et al 2012) can provide viable explanations to describe offsets between expected Balmer line to UV flux ratios and observed values. Kauffmann (2014) used SFRs derived via multiple nebular emission line analysis with the 4000Å break and Hδ A absorption to probe the recent SFHs of SDSS galaxies with log 10 (M * /M ) < 10 and infer possibilities for IMF variation.…”

The HDUV Survey: A Revised Assessment of the Relationship between UV Slope and Dust Attenuation for High-redshift Galaxies

“…Quantifying this scatter can elucidate the degree to which the attenuation curve may vary from galaxy to galaxy, or highlight the sensitivity of the UV slope to factors other than dust obscuration. In general, the effects of age, metallicity, and star formation history on the UV slope may become important for ultrafaint galaxies at high redshift, which have been suggested to undergo bursty star formation (e.g., Weisz et al 2012;Hopkins et al 2014;Domínguez et al 2015;Guo et al 2016;Sparre et al 2017;Faucher-Giguere 2018).…”