Build-up of structure within galaxies:
spheroids and disks

"Morphology" refers to the structural properties of galaxies. Galaxy morphology is a product of how galaxies formed, how they interacted with their large-scale environment and how they were influenced by internal processes, active black holes, and dark matter. The above classification is the Hubble tuning fork and it provides one way of describing galaxy morphology. However, this classification scheme only considers the most prominent features such as disks, bulges and bars. Furthermore, beside the visual morphology that is based on images, galaxies' morphology can also characterized by the kinematics of the stars or gas. For the latter, more expensive integral-field unit data needs to be available.

We focus on understanding the correlation between galaxy morphology and the galactic star formation history. Galaxies where star formation ceased billion of years ago tend to look very different from those where star formation continues at the present time (like our own Milky Way galaxy). Old, quiescent galaxies (to the left in the above diagram) are more spheroidal. On the other hand, young, star-forming galaxies are disk galaxies with spiral arms and sometimes bars. We investigate the build-up of bulges and disks by studying young galaxies at high redshifts. Specifically, we address the following questions:

  • How and when did bulge and disk components in galaxies form?

  • How do galaxies grow in time? How important is in-situ star formation versus ex-situ accretion of stars ("mergers")?

  • What does the morphology of a galaxy tell us about it past and present star formation?

Evidence for mature bulges 3 billion years after the Big Bang

In Tacchella et al. (2015a) and Tacchella et al. (2018b) we measure the star-formation rate and stellar mass distribution in galaxies at the cosmic noon. At masses below 1011 M, the dust-corrected specific star-formation rate (sSFR) profiles are on average radially constant at a mass-doubling timescale of ∼300 Myr, pointing at a synchronous growth of bulge and disk components (see below). At masses above 1011 M, the specific star-formation rate profiles are typically centrally suppressed by a factor of ∼10 relative to the galaxy outskirts. With total central obscuration disfavored, this indicates that at least a fraction of massive z ∼ 2 SFMS galaxies have started their inside-out star-formation quenching that will move them to the quenched sequence. In combination with other observations, galaxies above and below the ridge of the star-forming main sequence relation have, respectively, centrally enhanced and centrally suppressed specific star-formation rate relative to their outskirts, supporting a picture where bulges are built owing to gas “compaction” that leads to a high central star-formation rate as galaxies move toward the upper envelope of the star-forming main sequence (see also regulation of star formation).

Central density of quiescent galaxies

The massive z~2 star-forming galaxies have central stellar mass densities that are saturated to those of similar mass quiescent spheroids in the local universe (Tacchella et al. 2015). These galaxies will become members the quiescent galaxy population soon. In order to quantify the role of the addition of newly quenched galaxies in driving the apparent evolution of population-averaged quantities of quiescent galaxies (progenitor bias effect), such as the size and the central stellar mass density, we measure in Tacchella et al. (2017) the relation between stellar age and central stellar mass density of today's quiescent galaxies. As with star-forming galaxies, the quiescent galaxies shape a narrow locus in the central density - stellar mass plane, which we refer to as the S1 ridgeline. At fixed stellar mass, old quiescent galaxies on the S1 ridgeline have higher central density than young quiescent galaxies. This shows explicitly that galaxies landing on the S1 ridgeline today arrive with lower central density, which tends to drive the population-averaged zeropoint of the ridgeline down with time. We conclude that the zeropoint evolution of the ridgeline is mainly driven by progenitor bias effects.

Formation of spheroids and disks in IllustrisTNG

As highlighted above, observations deliver views of the galaxy population at different epochs. It is, however, challenging to infer from this how individual galaxies evolve, since at each epoch we observe different galaxies. A way to understand how individual galaxies evolve with time is to use galaxy formation models. In Tacchella et al. (2019), we use the IllustrisTNG simulations to study how galaxies - in the numerical simulation - are building up their bulge and disk component and change their star formation (and color) with cosmic time. These simulation overall reproduce well the observed color-morphology relation (left figure). We find that disk formation efficiency peaks for galaxies roughly around the Milky Way mass, while galaxies formed at early times are typically more spheroidal. In particular, the most massive galaxies assemble about half of the spheroidal mass while star-forming and the other half through mergers while quiescent.