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Register a new userStochastic modeling of star-formation histories I: the scatter of the star-forming main sequence
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We present a framework for modelling the star-formation histories of galaxies as a stochastic process. We define this stochastic process through a power spectrum density with a functional form of a broken power-law. Star-formation histories are correlated on short timescales, the strength of this correlation described by a power-law slope, $alpha$, and they decorrelate to resemble white noise over a timescale that is proportional to the timescale of the break in the power spectrum density, $tau_{rm break}$. We use this framework to explore the properties of the stochastic process that, we assume, gives rise to the log-normal scatter about the relationship between star-formation rate and stellar mass, the so-called galaxy star-forming main sequence. Specifically, we show how the measurements of the normalisation and width ($sigma_{rm MS}$) of the main sequence, measured in several passbands that probe different timescales, give a constraint on the parameters of the underlying power spectrum density. We first derive these results analytically for a simplified case where we model observations by averaging over the recent star-formation history. We then run numerical simulations to find results for more realistic observational cases. As a proof of concept, we use observational estimates of the main sequence scatter at $zsim0$ and $M_{star}approx10^{10}~M_{odot}$ measured in H$alpha$, UV+IR and the u-band, and show that combination of these point to $tau_{rm break}=178^{+104}_{-66}$ Myr, when assuming $alpha=2$. This implies that star-formation histories of galaxies lose memory of their previous activity on a timescale of $sim200$ Myr, highlighting the importance of baryonic effects that act over the dynamical timescales of galaxies.
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The analytic equilibrium model for galaxy evolution using a mass balance equation is able to reproduce mean observed galaxy scaling relations between stellar mass, halo mass, star formation rate (SFR) and metallicity across the majority of cosmic time with a small number of parameters related to feedback. Here we aim to test this data-constrained model to quantify deviations from the mean relation between stellar mass and SFR, i.e. the star-forming galaxy main sequence (MS). We implement fluctuation in halo accretion rates parameterised from merger-based simulations, and quantify the intrinsic scatter introduced into the MS under the assumption that fluctuations in star formation follow baryonic inflow fluctuations. We predict the 1-sigma MS scatter to be ~ 0.2 - 0.25 dex over the stellar mass range 10^8 Mo to 10^11 Mo and a redshift range 0.5 < z < 3 for SFRs averaged over 100 Myr. The scatter increases modestly at z > 3, as well as by averaging over shorter timescales. The contribution from merger-induced star formation is generally small, around 5% today and 10 - 15% during the peak epoch of cosmic star formation. These results are generally consistent with available observations, suggesting that deviations from the MS primarily reflect stochasticity in the inflow rate owing to halo mergers.
The scatter of the spatially resolved star formation main sequence (SFMS) is investigated in order to reveal signatures about the processes of galaxy formation and evolution. We have assembled a sample of 355 nearby galaxies with spatially resolved H{alpha} and mid-infrared fluxes from the Survey for Ionized Neutral Gas in Galaxies and the Wide-field Infrared Survey Explorer, respectively. We examine the impact of various star formation rate (SFR) and stellar mass transformations on the SFMS. Ranging from 10^6 to 10^11.5 M_sun and derived from color to mass-to-light ratio methods for mid-infrared bands, the stellar masses are internally consistent within their range of applicability and inherent systematic errors; a constant mass-to-light ratio also yields representative stellar masses. The various SFR estimates show intrinsic differences and produce noticeable vertical shifts in the SFMS, depending on the timescales and physics encompassed by the corresponding tracer. SFR estimates appear to break down on physical scales below 500 pc. We also examine the various sources of scatter in the spatially resolved SFMS and find morphology does not play a significant role. We identify three unique tracks across the SFMS by individual galaxies, delineated by a critical stellar mass density of log (Sigma_M*)~7.5. Below this scale, the SFMS shows no clear trend and is likely driven by local, stochastic internal processes. Above this scale, all spatially resolved galaxies have comparable SFMS slopes but exhibit two different behaviors, resulting likely from the rate of mass accretion at the center of the galaxy.
Using a sample of 11,478 spaxels in 34 galaxies with molecular gas, star formation and stellar maps taken from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we investigate the parameters that correlate with variations in star formation rates on kpc scales. We use a combination of correlation statistics and an artificial neural network to quantify the parameters that drive both the absolute star formation rate surface density (Sigma_SFR), as well as its scatter around the resolved star forming main sequence (Delta Sigma_SFR). We find that Sigma_SFR is primarily regulated by molecular gas surface density (Sigma_H2) with a secondary dependence on stellar mass surface density (Sigma_*), as expected from an `extended Kennicutt-Schmidt relation. However, Delta Sigma_SFR is driven primarily by changes in star formation efficiency (SFE), with variations in gas fraction playing a secondary role. Taken together, our results demonstrate that whilst the absolute rate of star formation is primarily set by the amount of molecular gas, the variation of star formation rate above and below the resolved star forming main sequence (on kpc scales) is primarily due to changes in SFE.
By using a set of different SFR indicators, including WISE mid-infrared and Halpha emission, we study the slope of the Main Sequence (MS) of local star forming galaxies at stellar masses larger than 10^{10} M_{odot}. The slope of the relation strongly depends on the SFR indicator used. In all cases, the local MS shows a bending at high stellar masses with respect to the slope obtained in the low mass regime. While the distribution of galaxies in the upper envelope of the MS is consistent with a log-normal distribution, the lower envelope shows an excess of galaxies, which increases as a function of the stellar mass but varies as a function of the SFR indicator used. The scatter of the best log-normal distribution increases with stellar mass from ~0.3 dex at 10^{10} M_{odot} to ~0.45 at 10^{11} M_{odot}. The MS high-mass end is dominated by central galaxies of group sized halos with a red bulge and a disk redder than the lower mass counterparts. We argue that the MS bending in this region is due to two processes: i) the formation of a bulge component as a consequence of the increased merger activity in groups, and ii) the cold gas starvation induced by the hot halo environment, which cuts off the gas inflow onto the disk. Similarly, the increase of the MS scatter at high stellar masses would be explained by the larger spread of star formation histories of central group and cluster galaxies with respect to lower mass systems.
We argue that the interplay between cosmic rays, the initial mass function, and star formation plays a crucial role in regulating the star-forming main sequence. To explore these phenomena we develop a toy model for galaxy evolution in which star formation is regulated by a combination of a temperature-dependent initial mass function and heating due to starlight, cosmic rays and, at very high redshift, the cosmic microwave background. This produces an attractor, near-equilibrium solution which is consistent with observations of the star-forming main sequence over a broad redshift range. Additional solutions to the same equations may correspond to other observed phases of galaxy evolution including quiescent galaxies. This model makes several falsifiable predictions, including higher metallicities and dust masses than anticipated at high redshift and isotopic abundances in the Milky Way. It also predicts that stellar mass-to-light ratios are lower than produced using a Milky Way-derived IMF, so that inferences of stellar masses and star formation rates for high redshift galaxies are overestimated. In some cases, this may also transform inferred dark matter profiles from core-like to cusp-like.
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