Abstract
ABSTRACT It remains an open question as to how long ago the morphology that we see in a present-day galaxy was typically imprinted. Studies of galaxy populations at different redshifts reveal that the balance of morphologies has changed over time, but such snapshots cannot uncover the typical time-scales over which individual galaxies undergo morphological transformation, nor which are the progenitors of today’s galaxies of different types. However, these studies also show a strong link between morphology and star formation rate (SFR) over a large range in redshift, which offers an alternative probe of morphological transformation. We therefore derive the evolution in SFR and stellar mass of a sample of 4342 galaxies in the SDSS-IV MaNGA survey through a stellar population ‘fossil record’ approach, and show that the average evolution of the population shows good agreement with known behaviour from previous studies. Although the correlation between a galaxy’s contemporaneous morphology and SFR is strong over a large range of lookback times, we find that a galaxy’s present-day morphology only correlates with its relatively recent ($\sim \! 2\, \textrm {Gyr}$) star formation history. We therefore find strong evidence that morphological transitions to galaxies’ current appearance occurred on time-scales as short as a few billion years.
Highlights
Dividing a galaxy sample by morphology was the earliest classification scheme for these objects (Hubble 1926, 1936), and morphology is still considered to be one of the defining characteristics of a galaxy, which is closely tied to its other properties
These results are in good agreement with with the equivalent findings from studies of galaxy populations at different redshifts (e.g. Bell et al 2012; Muzzin et al 2013; Ilbert et al 2013; Davidzon et al 2017), offering some assurance as to the reliability of star-formation histories derived through this fossil record
Over most of the range in stellar mass of our sample, the population of early-type galaxies here will be dominated by fastrotator galaxies, but we find that the progenitors of the most massive present-day early-type galaxies — likely ‘true’ slow-rotator ellipticals — have always had high stellar mass compared to the galaxy population as a whole, reflecting the minimal change in rank of galaxy masses: massive galaxies have always been massive, as we previously found in spirals (Peterken et al 2020)
Summary
Dividing a galaxy sample by morphology was the earliest classification scheme for these objects (Hubble 1926, 1936), and morphology is still considered to be one of the defining characteristics of a galaxy, which is closely tied to its other properties. By analysing the spatially-resolved stellar populations contained within galaxies, it is possible to start to understand their evolution on a galaxy-bygalaxy basis We previously used such a “fossil record” approach to investigate the morphological evolution of spiral galaxies (Peterken et al 2020; see Ibarra-Medel et al 2016). López Fernández et al (2018) and Sánchez et al (2019) have recently shown that it is possible to derive the distribution of galaxies in the M★–SFR plane at different lookback times using a fossil record approach. Fossil record analysis allows for the evolution of a single galaxy population to be studied, thereby ensuring that low-mass galaxies are included at all lookback times and avoiding redshift-dependent sampling effects such as progenitor bias. We use the weightings calculated by Calette et al (in preparation), which are robust for stellar masses above ∼ 109 M (see Sánchez et al 2019; Rodriguez-Puebla et al 2020 for further details)
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