Early formation of massive, compact, spheroidal galaxies with classical profiles by violent disc instability or mergers

Abstract

We address the formation of massive stellar spheroids between redshifts z = 4 and 1 using a suite of AMR hydro-cosmological simulations. The spheroids form as bulges, and the spheroid mass growth is partly driven by violent disc instability (VDI) and partly by mergers. A kinematic decomposition to disc and spheroid yields that the mass fraction in the spheroid is between 50% and 90% and is roughly constant in time, consistent with a cosmological steady state of VDI discs that are continuously fed from the cosmic web. The density profile of the spheroid is typically "classical", with a Sersic index n = 4.5 ± 1, independent of whether it grew by mergers or VDI and independent of the feedback strength. The disc is characterized by n = 1.5±0.5, and the whole galaxy by n = 3±1. The high-redshift spheroids are compact due to the dissipative inflow of gas and the high universal density. The stellar surface density within the effective radius of each galaxy as it evolves remains roughly constant in time after its first growth. For galaxies of a fixed stellar mass, the surface density is higher at higher redshifts.

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

“…Cheung et al 2012;Fang et al 2013; and suggests that the immediate starforming progenitors of quiescent galaxies experience a phase of stronger core growth that increases their concentration with respect to galaxies growing along the M  Srelations. At high redshifts, this phase is consistent with the compaction evolutionary tracks predicted in hydrodynamical simulations (Ceverino et al 2015;Zolotov et al 2015;Tacchella et al 2016), which are typically associated with strongly dissipational (gas-rich) processes such as major mergers and interaction driven gravitational instabilities (Dekel et al 2009a;Ceverino et al 2010;. At lower redshifts, these gas-rich processes probably coexist with minor mergers and secular instabilities (e.g., bars and spiral arms; Kormendy & Kennicutt 2004).…”

“…The blue and red lines show the best-fit e S and 1 S relations at z 1 and z 2 from Figure 2 (the z = 2 lines are higher). The model tracks are in excellent agreement with the observed distributions in e S and 1 S (see also Figure 12 of Ceverino et al 2015) and exhibit good agreement with the main evolutionary phases discussed in Section 4.1 (blue arrows), namely (1) a phase of relatively smooth structural growth that follows approximately the best-fit e S and 1 S relations from the data ( 0.6 a~, 0.9 b~) and (2) a phase of compaction, a period of steeper core growth ( 1.5 a~, 1.3 b~), usually triggered by a strongly dissipational event. The simulations exhibit a downsizing trend, such that the most massive galaxies evolve earlier and experience a stronger compaction event (open star) due to higher gas fraction at high z.…”

“…Some of these processes, like mergers and disk instabilities, are indeed expected to be more frequent at earlier times due to the higher gas-to-total mass ratios in SFGs (Daddi et al 2010;Tacconi et al 2010Tacconi et al , 2013. 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).…”

Characterization of star-forming dwarf galaxies at 0.1 ≲z ≲ 0.9 in VUDS: probing the low-mass end of the mass-metallicity relation

“…The models proposed by Tassis et al (2008) introduce a critical density threshold for the activation of star-formation, without requiring outflows to reduce the star formation efficiency and the metal content. Finally, the observed MZR can be explained assuming accretion of metal-poor gas along filaments from the cosmic web (cold-flows) (e.g., Dalcanton et al 2004;Ceverino et al 2015;Sánchez Almeida et al 2014b), for which also indirect evidences have been found in recent observations (Sánchez Almeida et al 2015).…”

Morphology Dependence of Stellar Age in Quenched Galaxies at Redshift ∼1.2:Massive Compact Galaxies Are Older than More Extended Ones

“…In other words, does some physical process associated with stellar compactness predispose galaxies to quench? Alternatively, are the earliest galaxies to form and complete their evolution simply the densest because the universe was denser at early times (e.g., Lilly & Carollo 2016), or is itbecause of some highly dissipative gaseous process that could take place predominantly at high redshift (e.g., Dekel et al 2009;Johansson et al 2012;Dekel & Burkert 2014;Ceverino et al 2015;Zolotov et al 2015)?…”