Constraints on the star formation histories of galaxies in the Local Cosmological Volume

Abstract

The majority of galaxies with current star-formation rates (SFRs), SF R o 10 −3 M /yr, in the Local Cosmological Volume where observations should be reliable, have the property that their observed SF R o is larger than their average star formation rate. This is in tension with the evolution of galaxies described by delayed-τ models, according to which the opposite would be expected. The tension is apparent in that local galaxies imply the star formation timescale τ ≈ 6.7 Gyr, much longer than the 3.5 − 4.5 Gyr obtained using an empirically determined main sequence at several redshifts. Using models where the SFR is a power law in time of the form ∝ (t − t 1 ) η for t 1 = 1.8 Gyr (with no stars forming prior to t 1 ) implies that η = 0.18 ± 0.03. This suggested near-constancy of a galaxy's SFR over time raises non-trivial problems for the evolution and formation time of galaxies, but is broadly consistent with the observed decreasing main sequence with increasing age of the Universe.

Anomalous Stellar Populations in LSB Galaxies

“…As can be seen in Figure 1 of Schombert, McGaugh & Lelli (2019), the collection of low-mass HSB and LSB galaxies form a well-defined sequence just slightly to the right of the line of constant star formation. Thus, the current SFR in low-mass galaxies is fairly similar to the average SFR over a Hubble time and excludes a scenario where the past SFR is much higher (or lower) as this would dramatically over or under produce the final stellar mass of the galaxy (see also Kroupa et al 2020). One could imagine a much higher past SFR with a later epoch of initial star formation, but this goes against the evidence of old (τ > 10 Gyrs) stars in observed CMD's (Schombert & McGaugh 2014).…”

Very high redshift quasars and the rapid emergence of super-massive black holes

“…While the nuclear regions of spheroids form within the downsizing time scale, late-type galaxies continue to grow in mass (e.g. Speagle et al 2014;Kroupa et al 2020) such that their nuclear star cluster is likely to grow through insitu star formation and mergers with freshly formed massive clusters beyond the mass of that assembled during the initial formation of its spheroidal component. Additional slow growth of the SMBH through the disruption of stars continues until the present, most efficiently though for low-mass SMBHs (Brockamp et al 2011).…”