The gravitationally unstable gas disk of a starburst galaxy 12 billion years ago

Abstract

Galaxies in the early Universe that are bright at submillimetre wavelengths (submillimetre-bright galaxies) are forming stars at a rate roughly 1,000 times higher than the Milky Way. A large fraction of the new stars form in the central kiloparsec of the galaxy, a region that is comparable in size to the massive, quiescent galaxies found at the peak of cosmic star-formation history and the cores of present-day giant elliptical galaxies. The physical and kinematic properties inside these compact starburst cores are poorly understood because probing them at relevant spatial scales requires extremely high angular resolution. Here we report observations with a linear resolution of 550 parsecs of gas and dust in an unlensed, submillimetre-bright galaxy at a redshift of z = 4.3, when the Universe was less than two billion years old. We resolve the spatial and kinematic structure of the molecular gas inside the heavily dust-obscured core and show that the underlying gas disk is clumpy and rotationally supported (that is, its rotation velocity is larger than the velocity dispersion). Our analysis of the molecular gas mass per unit area suggests that the starburst disk is gravitationally unstable, which implies that the self-gravity of the gas is stronger than the differential rotation of the disk and the internal pressure due to stellar-radiation feedback. As a result of the gravitational instability in the disk, the molecular gas would be consumed by star formation on a timescale of 100 million years, which is comparable to gas depletion times in merging starburst galaxies.

The Redshift and Star Formation Mode of AzTEC2: A Pair of Massive Galaxies at z = 4.63

“…Then, we derive v rot /σ 5, indicating that AzTEC2-A is rather an unperturbed, rotation-dominated disk that resembles the v rot /σ ratio of more evolved disk galaxies at z ∼ 1 (e.g., Di Teodoro et al 2016, and references therein). This finding, therefore, provides more evidence of kinematically mature disks that can be found at even z ∼ 4.5, such as GN20 (Hodge et al 2012), ALESS 73.1 (De Breuck et al 2014), AzTEC/C159, J1000+0234, Jiménez-Andrade et al 2018) and AzTEC1 (Tadaki et al 2018).…”

“…The gas depletion time-scale (τ gas ) of AzTEC2-A, in particular, exhibit a mild excess with respect to the average for SMGs at similar redshifts (τ gas ∼ 45 Myr; Aravena et al 2016), but resembles the one of the massive, star-forming disk galaxies GN20, AzTEC/C159 and AzTEC1 at z The diamonds represent the parameter space covered by the SMGs reported in . Large symbols correspond to the SMGs studied here and the massive, rotating disk galaxies at z ∼ 4.5: GN20, AzTEC1, and AzTEC/C159 Hodge et al 2012;Jones et al 2017;Jiménez-Andrade et al 2018;Tadaki et al 2018). In the case of AzTEC2-A and AzTEC2-B, we use lensing-corrected luminosities.…”

“…The evidence of rotation-dominated, gas-rich, starforming disks at z > 4 (e.g., Carilli et al 2010;Hodge et al 2012;De Breuck et al 2014;Jones et al 2017;Tadaki et al 2018) indicates that high-redshift SMGs are not only merging, strongly perturbed systems (Tacconi et al 2008;Younger et al 2010;Engel et al 2010;Riechers et al 2011;Menéndez-Delmestre et al 2014;Chen et al 2015;Jones et al 2017;Chang et al 2018). High-resolution (0.07 arcsec) dust continuum imaging with ALMA has even shown evidence for spiral arms, bars, and rings in z 2 SMGs ).…”

“…This is often due to major/minor mergers providing the energetic and baryonic input to abruptly enhance the star formation rate (SFR) of galaxies (e.g., Narayanan et al 2010;L´Huillier et al 2012;Ellison et al 2013). Whereas both regimes of star formation have been widely explored out to intermediate redshifts (z ∼ 2; e.g., Genzel et al 2010;Wuyts et al 2011;Magnelli et al 2012Magnelli et al , 2014Daddi et al 2015;Elbaz et al 2018;Jiménez-Andrade et al 2019), the relative role of the cold gas accretion and merger mode in driving the intense production of stars in galaxies at higher redshifts (z 3) remains an open issue (e.g., Carilli et al 2010;Hayward et al 2012;Hodge et al 2012;Hayward et al 2018;Jiménez-Andrade et al 2018;Tadaki et al 2018).…”

A Noncorotating Gas Component in an Extreme Starburst at z = 4.3

“…These findings suggest an evolutionary link between bright SMGs and compact quiescent galaxies at z ∼ 2 (Toft et al 2014) although it is not necessarily the case in faint SMGs with a flux density of <3.5 mJy at 850 µm (Valentino et al 2019). The star formation rate surface density in the central 1-2 kpc region exceeds 100 M ⊙ yr −1 kpc −2 (e.g., Younger et al 2008;Casey et al 2014;Tadaki et al 2018). Understanding the physical mechanism triggering such an extreme starburst in early Universe is a main topic in this paper.…”