A contribution of star-forming clumps and accreting satellites to the mass assembly of z ∼ 2 galaxies

Abstract

We investigate the contribution of clumps and satellites to the galaxy mass assembly. We analysed spatially resolved HubbleSpace Telescope observations (imaging and slitless spectroscopy) of 53 star-forming galaxies at z ∼ 1–3. We created continuum and emission line maps and pinpointed residual ‘blobs’ detected after subtracting the galaxy disc. Those were separated into compact (unresolved) and extended (resolved) components. Extended components have sizes ∼2 kpc and comparable stellar mass and age as the galaxy discs, whereas the compact components are 1.5 dex less massive and 0.4 dex younger than the discs. Furthermore, the extended blobs are typically found at larger distances from the galaxy barycentre than the compact ones. Prompted by these observations and by the comparison with simulations, we suggest that compact blobs are in situ formed clumps, whereas the extended ones are accreting satellites. Clumps and satellites enclose, respectively, ∼20 per cent and ≲80 per cent of the galaxy stellar mass, ∼30 per cent and ∼20 per cent of its star formation rate. Considering the compact blobs, we statistically estimated that massive clumps (M⋆ ≳ 109 M⊙) have lifetimes of ∼650 Myr, and the less massive ones (108 < M⋆ < 109 M⊙) of ∼145 Myr. This supports simulations predicting long-lived clumps (lifetime ≳ 100 Myr). Finally, ≲30 per cent (13 per cent) of our sample galaxies are undergoing single (multiple) merger(s), they have a projected separation ≲10 kpc, and the typical mass ratio of our satellites is 1:5 (but ranges between 1:10 and 1:1), in agreement with literature results for close pair galaxies.

CO emission in distant galaxies on and above the main sequence

“…Moreover, the high gas fractions inducing clumping in turbulent high-redshift massive disks further enhance this effect (Bournaud et al 2015), increasing the level of CO excitation of distant main-sequence galaxies with respect to local Milky Way-like objects. In other words, the same mechanisms that we consider here as acting on global galaxy scales might well be in action in subgalactic massive clumps, effectively mimicking starburst environments (Zanella et al 2015(Zanella et al , 2019. This is also consistent with the results from LVG modeling, where the density of the collisionally excited gas is the term driving the high-J emission, given the fast deexcitation rates.…”

“…This trend could be explained by the availability of copious molecular gas at high redshift (Daddi et al 2010a;Tacconi et al 2010Tacconi et al , 2018Scoville et al 2017a;Riechers et al 2019;Decarli et al 2019;Liu et al 2019a), ultimately regulated by the larger accretion rates from the cosmic web (Kereš et al 2005;Dekel et al 2009a). Moreover, higher SFRs could be induced by an increased efficiency of star formation due to the enhanced fragmentation in gas-rich, turbulent, and gravitationally unstable high-redshift disks (Bournaud et al 2007(Bournaud et al , 2010Dekel et al 2009b;Ceverino et al 2010;Dekel & Burkert 2014), reflected on their clumpy morphologies (Elmegreen et al 2007;Förster Schreiber et al 2011;Genzel et al 2011;Guo et al 2012Guo et al , 2015Zanella et al 2019). IR-bright galaxies with prodigious SFRs well above the level of the MS are observed also in the distant Universe, but their main physical driver is a matter of debate.…”

The diversity and variability of star formation histories in models of galaxy evolution

“…Constraints on the timescales of physical processes: In addition to the two approaches described above, observations can also directly measure timescales for gas depletion (Kennicutt Jr 1998;Wong & Blitz 2002;Bigiel et al 2008), stellar winds (Sharp & Bland-Hawthorn 2010;Ho et al 2016), disk formation (Kobayashi et al 2007), bulge growth (Lang et al 2014;Tacchella et al 2015), black hole growth (Hopkins et al 2005) and more, albeit for limited samples of galaxies. On short timescales, a large body of work also exists studying GMC lifetimes (∼ 10 − 30 Myr) (Zanella et al 2019;Kruijssen et al 2019;Chevance et al 2020), measuring the extent to which this depends on environment, and the extent to which it is decoupled from galactic dynamics. Krumholz et al (2017) also find episodic starbursts lasting ∼ 5−10 Myr with intervals of ∼ 20−40 Myr in a ring around the Milky-Way's central molecular zone.…”

Origin of giant stellar clumps in high-redshift galaxies

“…Some observed clumps appear to be old: e.g., Guo et al (2012) found clump ages in the range 10 8 − 10 9 yr, Soto et al (2017) in the range 10 6 − 10 10 yr, Zanella et al (2019) in the range 10 6 −10 9 yr. The oldest end of these intervals is used to argue the longevity of the clumps.…”