A Census of Early-phase High-mass Star Formation in the Central Molecular Zone

Abstract

We present new observations of C-band continuum emission and masers to assess high-mass (>8 M ) star formation at early evolutionary phases in the inner 200 pc of the Central Molecular Zone (CMZ) of the Galaxy. The continuum observation is complete to free-free emission from stars above 10-11 M in 91% of the covered area. We identify 104 compact sources in the continuum emission, among which five are confirmed ultracompact H II regions, 12 are candidates of ultra-compact H II regions, and the remaining 87 sources are mostly massive stars in clusters, field stars, evolved stars, pulsars, extragalactic sources, or of unknown nature that is to be investigated. We detect class II CH 3 OH masers at 23 positions, among which six are new detections. We confirm six known H 2 CO masers in two high-mass star forming regions, and detect two new H 2 CO masers toward the Sgr C cloud, making it the ninth region in the Galaxy that contains masers of this type. In spite of these detections, we find that current high-mass star formation in the inner CMZ is only taking place in seven isolated clouds. The results suggest that star formation at early evolutionary phases in the CMZ is about 10 times less efficient than expected by the dense gas star formation relation, which is in line with previous studies that focus on more evolved phases of star formation. This means that if there will be any impending, next burst of star formation in the CMZ, it has not yet begun.

CMZoom: Survey Overview and First Data Release

“…In some parts of the CMZ, free-free or synchrotron emission may be significant at these wavelengths. We estimate the contamination from free-free or synchrotron emission using VLA C-band (5.56 GHz) continuum data that cover a total of 26 clouds between SgrB2ext and SgrC in Table 4 (Lu et al 2019a). For each cloud, we extrapolate the 5.56 GHz flux to that at 230 GHz with an assumed spectral index, and use the ratio between it and the SMA flux reported in Table 4 to estimated the fraction of potential contamination.…”

The centres of M83 and the Milky Way: opposite extremes of a common star formation cycle

“…Recent and ongoing Galactic plane surveys across the electromagnetic spectrum (Aguirre et al 2011;H. E. S. S. Collaboration et al 2018), large scale surveys of the Galactic Centre such as HOPS (Walsh et al 2011), SWAG (Krieger et al 2017), CMZoom (Battersby et al 2020;Hatchfield et al 2020), CHIMPS2 (Eden et al 2020) and SOFIA/FORCAST (Hankins et al 2020) as well as more targeted observations (Longmore et al 2012;Henshaw et al 2016c;Walker et al 2018;Ginsburg & Kruijssen 2018;Henshaw et al 2019;Lu et al 2019Lu et al , 2020 continue to elucidate these processes in more detail.…”

A global view on star formation: The GLOSTAR Galactic plane survey

“…Systematic uncertainties in various methods used for determining the SFR as the cause for the much lower value were ruled out by Barnes et al (2017) as they obtained similar average SFRs by comparison of the above YSO counting and free-free emission measurements. Low SFRs have also been found in specific high-density clouds in the CMZ (e.g., Kauffmann et al 2017;Lu et al 2019b). While not applicable to clouds that already show traces of star formation at later stages, as in Sgr B2, some theoretical models suggest that the lower SFR is due to these clouds being in an early evolutionary stage where active star formation is not yet observable (see also, e.g., Kruijssen et al 2014;Krumholz & Kruijssen 2015;Krumholz et al 2017).…”