Forming Super Star Clusters in the Central Starburst of NGC 253

Abstract

NGC 253 hosts the nearest nuclear starburst. Previous observations show a region rich in molecular gas, with dense clouds associated with recent star formation. We used ALMA to image the 350 GHz dust continuum and molecular line emission from this region at 2 pc resolution. Our observations reveal ∼ 14 bright, compact (∼ 2−3 pc FWHM) knots of dust emission. Most of these sources are likely to be forming super star clusters (SSCs) based on their inferred dynamical and gas masses, association with 36 GHz radio continuum emission, and coincidence with line emission tracing dense, excited gas. One source coincides with a known SSC, but the rest remain invisible in Hubble near-infrared (IR) imaging. Our observations imply that gas still constitutes a large fraction of the overall mass in these sources. Their high brightness temperature at 350 GHz also implies a large optical depth near the peak of the IR spectral energy distribution. As a result, these sources may have large IR photospheres and the IR radiation force likely exceeds L/c. Still, their moderate observed velocity dispersions suggest that feedback from radiation, winds, and supernovae are not yet disrupting most sources. This mode of star formation appears to produce a large fraction of stars in the burst. We argue for a scenario in which this phase lasts ∼ 1 Myr, after which the clusters shed their natal cocoons but continue to produce ionizing photons. The strong feedback that drives the observed cold gas and X-ray outflows likely occurs after the clusters emerge from this early phase.

Unveiling kinematic structure in the starburst heart of NGC 253

“…Source S1 is at the IR core (IRC), a region hosting the starburst's brightest NIR source, a ∼6 Myr old, M ∼ 10 6 M SSC (e.g., Watson et al 1996;Kornei & McCrady 2009;Fernández-Ontiveros et al 2009;Davidge 2016;Günthardt et al 2019). Several other candidate SSCs are identified near the bright SSC as fainter thermal IR-radio continuum knots (e.g., Fernández-Ontiveros et al 2009;Günthardt et al 2015;Leroy et al 2018). Broad recombination lines suggest an ionized outflow driven by the SSC formation near the IRC (Günthardt et al 2019).…”

“…5 beam. (7) Coordinates of IR peak (IRC) from Leroy et al (2018), their source #5. Coincident with thermal source TH7 (Turner & Ho 1985).…”

“…Coincident with thermal source TH7 (Turner & Ho 1985). (8) Virial mass estimate from Leroy et al (2018) using the resolved source size of 2.1 pc. (9) Size of the NIRSPEC mapping region, with 31 slit positions.…”

“…The starburst region hosts two regions that could host the forming galactic center: the radio core with the brightest radio source in the galaxy ("TH2"; e.g., Turner & Ho 1985;Ulvestad & Antonucci 1997), and the IR core ("IRC"; e.g., Watson et al 1996;Kornei & McCrady 2009). There are a number of candidate large star clusters detected as radio or sub-mm knots and associated molecular gas clumps, near the radio and IR core (Turner & Ho 1983Ulvestad & Antonucci 1997;Leroy et al 2018;Mangum et al 2019).…”

The MALATANG survey: dense gas and star formation from high-transition HCN and HCO+ maps of NGC 253

“…Note that this approach relies on the assumption that NGC 253 conforms to Milky Way values in terms of gas and dust properties, but there is a large uncertainty in this assumed similarity. Please see Leroy et al (2018) and Knudsen et al (2007) for comparisons of starforming properties between NGC 253 and the Milky Way.…”