RETURN TO [Log-]NORMALCY: RETHINKING QUENCHING, THE STAR FORMATION MAIN SEQUENCE, AND PERHAPS MUCH MORE

Abstract

Knowledge of galaxy evolution rests on cross-sectional observations of different objects at different times. Understanding of galaxy evolution rests on longitudinal interpretations of how these data relate to individual objects moving through time. The connection between the two is often assumed to be clear, but we use a simple "physics-free" model to show that it is not and that exploring its nuances can yield new insights. Comprising nothing more than 2094 loosely constrained lognormal star formation histories (SFHs), the model faithfully reproduces the following data it was not designed to match: stellar mass functions at z ≤ 8; the slope of the star formation rate/stellar mass relation (the SF "Main Sequence") at z ≤ 6; the mean sSFR(≡ SFR/M * ) of low-mass galaxies at z ≤ 7; "fast-" and "slow-track" quenching; downsizing; and a correlation between formation timescale and sSFR(M * ,t) similar to results from simulations that provides a natural connection to bulge growth. We take these findings-which suggest that quenching is the natural downturn of all SFHs affecting galaxies at rates/times correlated with their densities-to mean that: (1) models in which galaxies are diversified on Hubble timescales by something like initial conditions rival the dominant grow-and-quench framework as good descriptions of the data; or (2) absent spatial information, many metrics of galaxy evolution are too undiscriminating-if not inherently misleading-to confirm a unique explanation. We outline future tests of our model but stress that, even if ultimately incorrect, it illustrates how exploring different paradigms can aid learning and, we hope, more detailed modeling efforts. TOWARDS AN EXPLICITLY QUENCHING-FREE WORLDVIEWOf the papers mentioned in Section 1, Oemler et al. (2013, hereafter O13) and G13 are most germane to the present discussion. These works summarize the motivation and construction of the G13 model, respectively. Motivation: A Diversity of Smooth SFHsO13 reached two conclusions based on analyses of specific star formation rate (sSFR ≡ SFR/M * ) distributions at z 1.The first was that canonical SFH parameterizations-τ and delayed-τ models (Tinsley 1972;Gavazzi et al. 2002)-could not explain the global decline in sSFRs observed over the above interval: they cannot generate both the tail of high sSFRs at z = 1 and the low values seen today. This problemcentral to understanding how G13 bypasses explicit quenching (Section 3.2.1, Appendix A)-is not small: ∼ 25% of z ≈ 0 galaxies with M * 4 × 10 10 M ⊙ -a fair fraction of the Universe's stellar mass-cannot have evolved via the above prescriptions (see also Dressler et al. 2016).The second conclusion was that large discontinuities-e.g., starbursts-cannot be invoked to remedy this discrepancy. Such events (briefly) modulate the sSFRs of individual galaxies, but they cannot drive the evolution of that quantity for whole populations over many Gyr (as is necessary): if some objects are in high states, others are in low ones, largely nulling their global effect (O13 Figu...

The Star Formation Histories of Disk Galaxies: The Live, the Dead, and the Undead

“…Finally, we should comment on the connection between these findings and the log-normal model for star formation histories which we have previously described (G13, Abramson et al 2015Abramson et al , 2016. That model is characterized by star formation rates which peak at intermediate epochs and are characterized by two independent timescales.…”

Structural and Star-forming Relations since z ∼ 3: Connecting Compact Star-forming and Quiescent Galaxies

“…A dichotomy is also present in the sizemass relations, where quiescent galaxies exhibit a much steeper slope than SFGs and a lower normalization, i.e., higher densities (Williams et al 2010;Newman et al 2012;Shibuya et al 2015). Although both the SFR-MS and the size-mass scaling relations evolve with time, the fundamental structural differences in SFGs and quiescent galaxies are always present, suggesting that having concentrated (denser) surface density profiles is a requisite for quenching (Kauffmann et al 2003(Kauffmann et al , 2006Schiminovich et al 2007;Bell 2008;Cheung et al 2012;Fang et al 2013;Lang et al 2014;Abramson et al 2016;Margalef-Bentabol et al 2016;Whitaker et al 2017). In other words, SFGs must grow dense cores before quenching.…”

“…The increased gas mass relative to the SFR makes such systems prone to gravitational collapse on scales of ∼1 kpc, causing substantial core growth resulting from a gasfed central starburst and/or an inward migration of clumps (Dekel et al 2009b;Ceverino et al 2010Ceverino et al , 2015Genel et al 2014;Wellons et al 2015Wellons et al , 2016Zolotov et al 2015;Bournaud 2016;Tacchella et al 2016). At lower redshifts, SFGs with dense cores have clearly recognizable disk structures, but their profiles are dominated by a central bulge (Bruce et al 2012(Bruce et al , 2014aWuyts et al 2012;Mortlock et al 2013;Lang et al 2014;Morishita et al 2015;Abramson et al 2016;Margalef-Bentabol et al 2016;Schreiber et al 2016). Interestingly, quiescent galaxies at low zalso seem to have bulge+disk morphologies (McGrath et al 2008;Bundy et al 2010;van der Wel et al 2011;Chang et al 2013;Dullo & Graham 2013), suggesting that quenching takes place among galaxies with similar morphologies.…”

Spatially-resolved star formation histories of CALIFA galaxies

“…However, alternative views are emerging in which galaxy growth is a more heterogeneous phenomenon, as suggested by the diversity of star formation histories (SFH hereafter) obtained in several surveys Oemler et al 2013;Dressler et al 2016;Abramson et al 2016). These works show that the SFH may be extended or compressed in time (Asari et al 2007;Pacifici et al 2016).…”