Highly efficient star formation in NGC 5253 possibly from stream-fed accretion

Abstract

Gas clouds in present-day galaxies are inefficient at forming stars. Low star formation efficiency is a critical parameter in galaxy evolution: it is why stars are still forming nearly fourteen billion years after the Big Bang 1 and why star clusters generally do not survive their births, instead dispersing to form galactic disks, halos, or bulges 2 . Yet the existence of ancient massive star clusters in the Milky Way, globular clusters, suggests that efficiencies were higher when they formed ten billion years ago. A local dwarf galaxy, NGC 5253, has a young star cluster that may provide an example of highly efficient star formation 3 . Here we report the detection of the J= 3-2 rotational transition of CO at the location of the massive cluster. The gas cloud is hot, dense, quiescent, and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by the embedded star cluster. Its star formation efficiency exceeds 50%, ten times higher than clouds in the Milky Way: this cloud is a factory of stars and soot. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy.The Submillimeter Array image of NGC 5253, shown in Figure 1, reveals a bright CO(3-2) source coincident with the giant cluster and its "supernebula" 4 . "Cloud D" 3 is one of only two molecular clouds detected within the galaxy; the second cloud is smaller and located ~5" (90 pc) to the southwest. A "streamer" of gas extending along the minor axis is also detected in CO(3-2). This streamer, previously detected in lower J CO lines, appears to be falling into the galaxy near the supernebula 3,5 . Both the streamer and Cloud D emit 870µm continuum emission, as shown in Figure 2. Also shown is an image of 350µm continuum, in which both Cloud D and the streamer are detected. Figure 1. CO J=3-2 emission in NGC 5253. The SMA CO(3-2) integrated line intensity, in red, is shown atop a λ814nm Hubble Space Telescope image. The SMA beam is 4" x 2" (74 pc x 37 pc). The field covers 40" x 40" (740 pc x740 pc), north up, east left. Image registration is to < 1". The CO streamer coincides with the optical dust lane to the east. The massive star cluster is located at the bright CO peak, Cloud D; it is embedded 8.9 , and not visible here. Cloud F is to the southwest of Cloud D.The molecular gas in Cloud D is hot. This is clear from the increase in brightness from CO(2-1) 3 to CO(3-2). The intensity ratio of the two lines is I 32 /I 21 = 2.6±0.5 (I line = ∫T line dv). This ratio is nonthermal, although the thermal limit of 2.25 (~ν 2 ) is within the uncertainties, and is what we adopt. Non-LTE modeling of this ratio using RADEX 6 indicates a minimum kinetic temperature of T K > 200 K for the 1σ lower limit, and T K > 350 K for the adopted value of I 32 /I 21 = 2.25 (see Methods). The high gas temperature is consistent with a thermal origin for H 2 2.2µm emission in the region 7 . Cloud D appears to be a Photon-Dominated Region (PDR), heated by ultraviol...

The very massive star content of the nuclear star clusters in NGC 5253

“…The young ages of the two nuclear star clusters are in apparent contradiction with the ages of 3-5 Myr inferred from the presence of W-R stars in cluster #5 (e.g. Monreal-Ibero et al 2010;Turner et al 2015). In this paper, we reexamine this aspect by considering whether the W-R features arise from hydrogen-rich very massive stars (VMS; masses > 100 M ).…”

“…3, we presented spectra for nuclear cluster #5 only. Cluster #11 has a projected separation from cluster #5 of 0 .3-0 .4 (5 pc) and is more massive and extreme in its properties (Turner et al 2015). Calzetti et al (2015) find that both clusters are 1±1 Myr old but #11 is heavily reddened and very faint at optical wavelengths.…”

“…Onethird of this flux is confined to the central 5 pc region containing clusters #5 and 11. Turner et al (2015) take the total ionizing flux, an age of 4 Myr, the virial mass limit for this region of 1.8 × 10 6 M , and compare to the predicted ionizing fluxes from Starburst99 (Leitherer et al 1999). They find that to match the observations, a top-heavy initial mass function (IMF) is required with a lower mass cutoff of 3 M .…”

“…The metallicity of NGC 5253 is fairly low with the recent study of Monreal-Ibero et al (2012) giving 12 + log O/H= 8.26 or 35% solar (us-ing the solar oxygen abundance of Asplund et al 2009). Recently, Turner et al (2015) presented Submillimeter Array images of NGC 5253 showing evidence for a CO streamer falling into the center of the galaxy, and possibly fuelling the current star formation. Turner et al (1998) found that radio maps of the central region are dominated by a single unresolved source with the characteristics of a massive ultracompact H ii region.…”

“…However, 50% of the ionizing photon rate is unaccounted for in the region of the supernebula (Calzetti et al 2015). Turner et al (2015) find that they need to invoke a top-heavy initial mass function (IMF) for the two nuclear clusters to provide sufficient Lyman ionizing photons given the age constraint of 4 Myr and an upper limit to the combined cluster mass (derived from the dynamical mass of the gas in the region).…”