The radio continuum of the extremely metal-poor blue compact dwarf galaxy I Zw 18

Abstract

Abstract. We present 1.4, 4.8 and 8.4 GHz Very Large Array observations of the lowest metallicity blue compact dwarf (BCD) galaxy known, I Zw 18, with a heavy element abundance of ∼2% that of the Sun. The 1.4 and 4.8 GHz images reveal a halo of mostly non-thermal extended emission, with asymmetric lobes extending laterally in the east-west direction. We interpret the radio halo as produced by a superbubble carved out in the interstellar medium (ISM) by supernovae, with a bipolar outflow oriented along the galaxy's rotation axis. The overall spectral index is −0.39 from 1.4 Ghz to 4.8 Ghz and −0.13 from 4.8 to 8.4 GHz. The radio luminosity of I Zw 18 has a thermal to total emission fraction of 0.30 at 1.4 GHz. This fraction increases to 0.41 at 4.8 GHz and to 0.47 at 8.4 GHz. The thermal radio luminosity gives a total of 1200 O7 V stars and a star formation rate of 0.1 M yr −1 . Unlike the BCD SBS 0335−052 which has a similar metallicity and forms stars with a high rate in regions which are dense and compact, I Zw 18 makes stars at a smaller rate in complexes which are diffuse and extended. Star formation in BCDs thus appears to occur in a bimodal fashion, independently of the metallicity of the interstellar medium.Key words. galaxies: dwarf -galaxies: starburst -galaxies: star clusters -radio continuum: galaxiesradio continuum: ISM -ISM: supernova remnants IntroductionHow and when galaxies formed is a fundamental problem of modern astrophysics. To date however the quest for truly young galaxies in the process of forming at redshifts >5-6 is still unsuccessful. The high-redshift objects discovered so far are typically enriched in heavy elements, implying that the starformation events which formed them took place in an earlier yet unobserved epoch. Instead of looking for young systems at high redshifts, an alternative approach is to study star formation in environments which are similar to primordial ones, in that they are unevolved chemically. Chemically unenriched interstellar media in the local universe, such as those in metalpoor blue compact dwarf galaxies (BCDs), approximate the best those which hosted primordial starbursts; but because they are nearby, they are easier to study than the high-redshift Lyα absorption systems. Therefore, low-metallicity BCDs may be useful "templates" for studying how the first stars and galaxies formed.Because of its extremely low metal abundance, I Zw 18 is arguably one of the best nearby BCDs with which to study primordial-like starbursts. First discovered by Zwicky (1966), I Zw 18 was originally described as a double system of compact galaxies, which are instead two massive star clusters, the brighter northwest (NW) and fainter southeast (SE) components, separated by ∼300 pc. The galaxy, together with a starforming region ∼1.3 kpc to the NW (component C), is embedded within a massive H cloud (van Zee et al. 1998).I Zw 18 has the lowest metal abundance (∼1/50 Z Izotov et al. 2001, and references therein) of known star-forming systems in the local universe. Very metal-deficie...

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

“…These clusters are more extended than those in SBS 0335−052 1 (∼150−200 pc in diameter), similar to concentrations of normal OB associations in the Milky Way and the Large Magellanic Cloud (Hunter & Thronson 1995) 2 . The mass of stars in I Zw 18, ∼1.3 × 10 6 M (Fumagalli et al 2010), is about 4 times lower than in SBS 0335−052, and I Zw 18's SFR, estimated from radio free-free emission (Hunt et al 2005b) is 0.17 M yr −1 , about 5 times lower than in SBS 0335−052. The SFR surface density Σ SFR is more than 30 times lower than in SBS 0335−052.…”

“…The SFR surface density Σ SFR is more than 30 times lower than in SBS 0335−052. Unlike SBS 0335−052, there is no evidence for self-absorption in the radio spectrum of I Zw 18 (Hunt et al 2005b;Cannon et al 2005), implying the absence of dense gas.…”

“…Based on I Zw 18's SFR = 0.17 M yr −1 (Hunt et al 2005b, scaled to D = 18.2 Mpc), this turns out to be an important correction: ∼50% of the 3.6 μm and 4.5 μm emission is from nebular continuum + Brα. I Zw 18 is extended at short IRS wavelengths (see Wu et al 2007), and judging from the IRAC global photometry, the 3.…”

The applicability of far-infrared fine-structure lines as star formation rate tracers over wide ranges of metallicities and galaxy types

“…Total infrared luminosities L TIR are taken from Madden et al (2013), as determined from Spitzer bands using the prescriptions in Dale & Helou (2002). Radio continuum measurements are retrieved from the NRAO VLA Sky Survey (NVSS) catalog (Condon et al 1998), Cannon & Skillman (2004), Thuan et al (2004), andHunt et al (2005). The emission from PAHs usually dominates the IRAC 8 µm band in metal-rich galaxies.…”

The size-density relation of extragalactic H II regions

“…There is one galaxy in our data set, I Zw 18, in which multifrequency observations show no sign of a rising spectrum (Hunt et al 2005). Here also the data have been fit to a model of an homogeneous, isothermal, dust-free ionization bounded region of ionized gas, as described above.…”