Probing Star Formation at Low Metallicity: The Radio Emission of Super Star Clusters in SBS 0335–052

Abstract

We present high-resolution radio-continuum observations of the nascent starburst in the metal-poor galaxy SBS 0335−052. These radio data were taken with the Very Large Array and include observations at 0.7 cm, 1.3 cm, 2 cm, 3.6 cm, and 6 cm. These observations enable us to probe the thermal radio nebulae associated with the extremely young star-forming regions in this galaxy. Two discrete and luminous star-forming regions are detected in the south of the galaxy that appear to be associated with massive star clusters previously identified at optical wavelengths. However, the remaining optically identified massive star clusters are not clearly associated with radio emission (either thermal or nonthermal) down to the sensitivity limits of these radio data. The spectral energy distributions (SEDs) of the two radio-detected clusters are consistent with being purely thermal, and the entire region has an inferred ionizing flux of ∼1.2 × 10 53 s −1 , which is equivalent to ∼12,000 "typical" O-type stars (type O7.5 V). The observations presented here have resolved out a significant contribution from diffuse nonthermal emission detected previously, implying a previous episode of significant star formation. The current star formation rate (SFR) for this southern region alone is ∼1.3 M yr −1 , or ∼23 M yr −1 kpc −2 , which is nearing the maximum starburst intensity limit. This SFR derived from thermal radio emission also suggests that previous optical recombination line studies do not detect a significant fraction of the current star formation in SBS 0335−052. From model fits to the radio SED, we infer a global mean density in the two youngest clusters of n e 10 3 -10 4 cm −3 . In addition, a comparison between the compact and diffuse radio emission indicates that up to ∼50% of the ionizing flux could be leaking out of the compact H ii regions; this in is agreement with previous work, which suggests that the interstellar medium surrounding the natal clusters in SBS 0335−052 is porous and clumpy.

ALMA observations of cool dust in a low-metallicity starburst, SBS 0335−052

“…Free-free emission at 870 μm in SBS 0335−052 870 μm emission can contain contributions from different physical processes: cool dust, thermal free-free emission (or bremsstrahlung) from ionized gas, and synchrotron emission from supernova remnants and their diffused non-thermal emission after longer timescales. In SBS 0335−052, synchrotron emission contributes a significant fraction of the total at long cm wavelengths and low spatial resolution (Hunt et al 2004), but not at the highest radio frequencies observed (43 GHz, Johnson et al 2009). Extrapolating from the 1.3 cm flux of ∼0.52 mJy (Hunt et al 2004;Johnson et al 2009), we estimate a free-free flux at 870 μm of 0.40 ± 0.07 mJy.…”

“…2, we present the data we have acquired and compiled in order to better constrain the dust mass and gas content in SBS 0335−052. We then estimate the free-free contribution to Izotov et al (1999); (3) Reines et al (2008); (4) Johnson et al (2009);(5) in the entire SBS 0335−052 E region (Ekta et al 2009); (6) Thuan & Izotov (1997a).…”

The size-density relation of extragalactic H II regions

“…Thus, the authors were able to estimate the size D of the emitting region by fitting a Gaussian, and from the observed flux, determine the EM, and infer the rms electron density n e . In other cases (NGC 4214, NGC 1741, Mrk 8, Mrk 33, VII Zw 19, Pox 4, Tol 35, Mrk 1236NGC 6946, NGC 253, M 33: Johnson et al 2001;SBS 0335-052: Hunt et al 2004;Johnson et al 2009), the spatial resolution is insufficient to resolve the regions. Then the radio spectrum can be fit by models of homogeneous, isothermal, dust-free, ionization bounded regions of ionized gas, to obtain the turnover frequency ν t , the EM, n e , and infer the size D of the region (e.g., Deeg et al 1993;Johnson et al 2001;Hunt et al 2004).…”

“…In fact, ultra-dense H ii regions or radio "supernebulae" have been proposed as SSC progenitors, observed in their very young embedded state (Kobulnicky & Johnson 1999;Johnson et al 2003;Johnson et al 2009). This would be consistent with our finding that extreme conditions are necessary to explain the radio sample.…”

Radio Continuum Observations of Local Star-Forming Galaxies Using the Caltech Continuum Backend on the Green Bank Telescope

“…T72 denotes galaxies that were observed, but not detected, by Tovmassian (1972) at 9.5 mm (31.6 GHz). T94 (Turner & Ho 1994), B00 (Beck et al 2000), B02 (Beck et al 2002), B03 (Beswick et al 2003), C04 (Chyży & Beck 2004), J04 (Johnson et al 2004), L04 (Lisenfeld et al 2004), R05 (Rosa-González 2005), T06 (Tsai et al 2006), B07 (Beck et al 2007), R08 (Reines et al 2008), J09 (Johnson et al 2009), D11 (Dumas et al 2011), K11 (Kodilkar et al 2011), and A11 (Aversa et al 2011) denote galaxies that were observed at lower frequencies. The galaxies are described in more detail in these papers.…”