ALMA 200 pc Resolution Imaging of Smooth Cold Dusty Disks in Typical z ∼ 3 Star-forming Galaxies

Abstract

We present high-fidelity, 30 milliarcsecond (200-pc) resolution ALMA rest-frame 240 µm observations of cold dust emission in three typical main-sequence star-forming galaxies (SFGs) at z ∼ 3 in the Hubble Ultra-Deep Field (HUDF). The cold dust is distributed within the smooth disk-like central regions of star formation 1 − 3 kpc in diameter, despite their complex and disturbed rest-frame UV and optical morphologies. No dust substructures or clumps are seen down to 1 − 3 M yr −1 (1σ) per 200-pc beam. No dust emission is observed at the locations of UV-emitting clumps, which lie 2 − 10 kpc from the bulk of star formation. Clumpy substructures can contribute no more than 1 − 7% of the total star formation in these galaxies (3σ upper limits). The lack of star-forming substructures in our HUDF galaxies is to be contrasted with the multiple substructures characteristic of submillimeter-selected galaxies (SMGs) at the same cosmic epoch, particularly the far-IR-bright SMGs with similarly high-fidelity ALMA observations of Hodge et al. (2019). Individual star-forming substructures in these SMGs contain ∼ 10 − 30% of their total star formation. A substructure in these SMGs is often comparably bright in the far-infrared as (or in some cases brighter than) our typical SFGs, suggesting that these SMGs originate from a class of disruptive event involving multiple objects at the scale of our HUDF galaxies. The scale of the disruptive event found in our main-sequence SFGs, characterized by the lack of star-forming substructures at our resolution and sensitivity, could be less violent, e.g., gas-rich disk instability or minor mergers.

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

“…Noises could be potentially amplified by deconvolution if the clean is not perfect. We make the same analysis of a dirty cube to check if the results do not depend on the clean (Rujopakarn et al 2019). Figure 8 shows residual channel maps of the dirty cube.…”

Growth of Supermassive Black Hole Seeds in ETG Star-forming Progenitors: Multiple Merging of Stellar Compact Remnants via Gaseous Dynamical Friction and Gravitational-wave Emission

“…Actually, the structure of high-z ETG starforming progenitors is more complex. On kpc scales, both observations (e.g., Genzel et al 2011;Tadaki et al 2017aTadaki et al ,b, 2018Hodge et al 2019;Lang et al 2019;Rujopakarn et al 2019) and simulations (e.g., Bournaud et al 2014;Mandelker et al 2014Mandelker et al , 2017Oklopcic et al 2017) indicate the presence of clumps with masses 10 7 − 10 8 M and sizes of 100 − 200 kpc; note that even more massive and extended clumps can be present but are rarer, and could be real outcomes from collisions of smaller ones (e.g., Tamburello et al 2015) or apparent structures due to blending from observations with limited resolution (e.g., Tamburello et al 2017;Behrend et al 2016;Faure et al 2019, in preparation). The survival of the clumps is still a debated issue, with different simulations favoring short-lived clumps because of feedback and/or collisions (e.g., Hopkins et al 2012;Oklopcic et al 2017), or long-lived clumps that may eventually sink toward the center via gravitational torque and bar instabilities and contribute to the growth of a central bulge (e.g., Ceverino et al 2012;Bournaud et al 2014).…”

“…On a scale of a few kpc, the velocity structure is dominated by rotational motions with v/σ a few, in the way of a clumpy unstable disk (see Genzel et al 2011;Tadaki et al 2017aTadaki et al ,b, 2018Hodge et al 2019). However, on sub-kpc scales both observations (e.g., Barro et al 2016;Rujopakarn et al 2019) and simulations (e.g., Danovich et al 2015;Zolotov et al 2015;Zavala et al 2016) indicate that dynamical friction, gravitational torques, and violent relaxation will operate toward converting such rotational into random motions, setting up a bulge-like structure with v/σ 1 (see also Lapi et al 2018). Provided that the majority of the compact remnants contributing to the growth of the central seed BH come from a scale of 300 pc, our assumption of a dispersion-dominated velocity structure should hold to a good approximation.…”

“…Indubitably, the presence of such a clumpiness in the gaseous medium may in principle affect the dynamical evolution of the rem-nants. However, high-resolution observations with ALMA (see Hodge et al 2019;Rujopakarn et al 2019) have revealed that such clumps contribute less than 10% of the overall star-formation; the latter mainly occurs in a rather smooth gaseous and dust-enshrouded medium within the central kpc scale. Provided that in our treatment most of the compact remnants effectively contributing to the growth of the central BH seed come from initial radii of 300 pc, the assumption of a smooth distribution for the inner star-forming gas should hold to a good approximation.…”

“…• Dust component. The central kpc regions of a star-forming ETG progenitors are very dusty (e.g., Tadaki et al 2017aTadaki et al ,b, 2018Hodge et al 2019;Rujopakarn et al 2019). In principle, dust can cooperate with the gas component in making the dynamical friction of the compact remnants more efficient, and speed up the building up of the central BH seed.…”