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Register a new userThe Variability of the Star Formation Rate in Galaxies: I. Star Formation Histories Traced by EW(H$alpha$) and EW(H$delta_A$)
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To investigate the variability of the star formation rate (SFR) of galaxies, we define a star formation change parameter, SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ which is the ratio of the SFR averaged within the last 5 Myr to the SFR averaged within the last 800 Myr. We show that this parameter can be determined from a combination of H$alpha$ emission and H$delta$ absorption, plus the 4000 A break, with an uncertainty of $sim$0.07 dex for star-forming galaxies. We then apply this estimator to MaNGA galaxies, both globally within Re and within radial annuli. We find that galaxies with higher global SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ appear to have higher SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ at all galactic radii, i.e. that galaxies with a recent temporal enhancement in overall SFR have enhanced star formation at all galactic radii. The dispersion of the SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ at a given relative galactic radius and a given stellar mass decreases with the (indirectly inferred) gas depletion time: locations with short gas depletion time appear to undergo bigger variations in their star-formation rates on Gyr or less timescales. In Wang et al. (2019) we showed that the dispersion in star-formation rate surface densities $Sigma_{rm SFR}$ in the galaxy population appears to be inversely correlated with the inferred gas depletion timescale and interpreted this in terms of the dynamical response of a gas-regulator system to changes in the gas inflow rate. In this paper, we can now prove directly with SFR$_{rm 5Myr}$/SFR$_{rm 800Myr}$ that these effects are indeed due to genuine temporal variations in the SFR of individual galaxies on timescales between $10^7$ and $10^9$ years rather than possibly reflecting intrinsic, non-temporal, differences between different galaxies.
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Because the timescale of H$alpha$ emission (several tens of Myr) following star formation is significantly shorter than that of UV radiation (a few hundred Myr), the H$alpha$/UV flux ratio provides insight on the star formation histories (SFHs) of galaxies on timescales shorter than $sim100$ Myr. We present H$alpha$/UV ratios for galaxies at $z=$ 0.02--0.1 on the familiar star-forming main sequence based on the AKARI-GALEX-SDSS archive dataset. The data provide us with robust measurements of dust-corrected SFRs in both H$alpha$ and UV for 1,050 galaxies. The results show a correlation between the H$alpha$/UV ratio and the deviation from the main sequence in the sense that galaxies above/below the main sequence tend to have higher/lower H$alpha$/UV ratios. This trend increases the dispersion of the main sequence by 0.04 dex (a small fraction of the total scatter of 0.36 dex), suggesting that diversity of recent SFHs of galaxies has a direct impact on the observed main sequence scatter. We caution that the results suffer from incompleteness and a selection bias which may lead us to miss many sources with high H$alpha$/UV ratios, this could further increase the scatter from SFHs in the star-forming main sequence.
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 H$beta$ and FUV (1500 AA) (H$beta$--to--FUV ratio). Our sample contains 164 galaxies down to stellar mass (M*) of $10^{8.5} M_odot$ in the CANDELS GOODS-N region, where Team Keck Redshift Survey DEIMOS spectroscopy and HST/WFC3 F275W images from CANDELS and Hubble Deep UV Legacy Survey are available. When the {it ratio} of H$beta$- and FUV-derived SFRs is measured, dust extinction correction is negligible (except for very dusty galaxies) with the Calzetti attenuation curve. The H$beta$--to--FUV ratio of our sample increases with M* and SFR. The median ratio is $sim$0.7 at M*$sim10^{8.5} M_odot$ (or SFR$sim 0.5 M_odot/yr$) and increases to $sim$1 at M*$sim10^{10} M_odot$ (or SFR $sim 10 M_odot/yr$). At M*$<10^{9.5} M_odot$, our median H$beta$--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 of our results, and the importance of the burstiness increases as M* decreases. Due to sample selection effects, our H$beta$--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.
We calculate H$alpha$-based star formation rates and determine the star formation rate-stellar mass relation for members of three SpARCS clusters at $z sim 1.6$ and serendipitously identified field galaxies at similar redshifts to the clusters. We find similar star formation rates in cluster and field galaxies throughout our range of stellar masses. The results are comparable to those seen in other clusters at similar redshifts, and consistent with our previous photometric evidence for little quenching activity in clusters. One possible explanation for our results is that galaxies in our $z sim 1.6$ clusters have been accreted too recently to show signs of environmental quenching. It is also possible that the clusters are not yet dynamically mature enough to produce important environmental quenching effects shown to be important at low redshift, such as ram pressure stripping or harassment.
We present the first measurements of the Lyman-continuum photon production efficiency $xi_{textrm{ion,0}}$ at $zsim4$-5 for galaxies fainter than 0.2 $L^*$ ($-$19 mag). $xi_{textrm{ion,0}}$ quantifies the production rate of ionizing photons with respect to the UV luminosity density assuming a fiducial escape fraction of zero. Extending previous measurements of $xi_{textrm{ion,0}}$ to the faint population is important, as ultra-faint galaxies are expected to contribute the bulk of the ionizing emissivity. We probe $xi_{textrm{ion,0}}$ to such faint magnitudes by taking advantage of 200-hour depth Spitzer/IRAC observations from the GREATS program and $approx$300 3<$z$<6 galaxies with spectroscopic redshifts from the MUSE GTO Deep + Wide programs. Stacked IRAC [3.6]$-$[4.5] colors are derived and used to infer the H$alpha$ rest-frame equivalent widths, which range from 403r{A} to 2818r{A}. The derived $xi_{textrm{ion,0}}$ is $log_{10}(xi_{textrm{ion,0}} / textrm{Hz erg}^{-1}) = 25.36 pm 0.08$ over $-$20.5 < M$_{textrm{UV}}$ < $-$17.5, similar to those derived for brighter galaxy samples at the same redshift and therefore suggesting that $xi_{textrm{ion}}$ shows no strong dependence on $M_{UV}$. The $xi_{textrm{ion,0}}$ values found in our sample imply that the Lyman-continuum escape fraction for $M_{textrm{UV}} approx -19$ star-forming galaxies cannot exceed $approx$8-20% in the reionization era.
We present half-light sizes measured from H${alpha}$ emission tracing star-formation in 281 star-forming galaxies from the KMOS3D survey at 0.7 < z < 2.7. Sizes are derived by fitting 2D exponential disk models, with bootstrap errors averaging 20%. H${alpha}$ sizes are a median (mean) of 1.19 (1.26) times larger than those of the stellar continuum, which due to radial dust gradients places an upper limit on the growth in stellar size via star formation, with just 43% intrinsic scatter. At fixed continuum size the H${alpha}$ size shows no residual trend with stellar mass, star formation rate, redshift or morphology. The only significant residual trend is with the excess obscuration of H${alpha}$ by dust, at fixed continuum obscuration. The scatter in continuum size at fixed stellar mass is likely driven by the scatter in halo spin parameters. The stability of the ratio of H${alpha}$ size to continuum size demonstrates a high degree of stability in halo spin and in the transfer of angular momentum to the disk over a wide range of physical conditions and cosmic time. This may require local regulation by feedback processes. The implication of our results, as we demonstrate using a toy model, is that our upper limit on star-formation driven growth is sufficient only to evolve star-forming galaxies approximately along the observed size-mass relation, consistent with the size growth of galaxies at constant cumulative co-moving number density. To explain the observed evolution of the size-mass relation of star-forming disk galaxies other processes, such as the preferential quenching of compact galaxies or galaxy mergers, may be required.
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