Inefficient star formation in extremely metal poor galaxies

Abstract

The first galaxies contain stars born out of gas with little or no metals. The lack of metals is expected to inhibit efficient gas cooling and star formation 1, 2 but this effect has yet to be observed in galaxies with oxygen abundance relative to hydrogen below a tenth of that of the Sun 2-4 . Extremely metal poor nearby galaxies may be our best local laboratories for studying in detail the conditions that prevailed in low metallicity galaxies at early epochs. Carbon Monoxide (CO) emission is unreliable as tracers of gas at low metallicities 5-7 , and while dust has been used to trace gas in lowmetallicity galaxies 5, 8-10 , low-spatial resolution in the far-infrared has typically led to large uncertainties 9, 10 . Here we report spatiallyresolved infrared observations of two galaxies with oxygen abundances below 10 per cent solar, and show that stars form very inefficiently in seven star-forming clumps of these galaxies. The star formation efficiencies are more than ten times lower than found in normal, metal rich galaxies today, suggesting that star formation may have been very inefficient in the early Universe.The two galaxies that are the focus of this study are Sextans A, a dwarf irregular at 1.4 Mpc with oxygen abundance of 7% Solar 11,12 , and ESO 146-G14, a low-surface-brightness galaxy at 22.5 Mpc with 9% solar oxygen abundance 11,13 . Their metallicities may be similar to that of gas out of which Population II stars form in the early Universe around redshift from 7 to 12 14 . An effective way to estimate the total gas content in extremely metal poor galaxies is to employ spatially resolved maps of the far-infrared (IR) emitting dust as tracers of the atomic and molecular gas by multiplying with an appropriate gas-todust ratio (GDR) that is inferred from regions with little or no active star formation, a methodology that has been extensively demonstrated in relatively metal rich galaxies 7, 15 .The infrared observations described in this paper were carried out at 70, 160, 250, 350 and 500 µm with the Photodetector Array Camera and Spectrometer (PACS) 16 and Spectral and Photometric Imaging REceiver (SPIRE) 17 onboard the Herschel Space Observatory. We complement our far-IR data with mid-IR images from the Spitzer Space Telescope to construct the full IR spectral energy distributions (SED). Far-ultraviolet (UV) images from the GALEX Space Telescope archive are used to trace un-obscured star formation. Maps of atomic gas are available in the literature for Sextans A . Figure 1 shows the multi-wavelength images of our sample galaxies. Based on the far-UV image we defined the star-forming disk as an ellipse to closely follow the 10-σ (∼ 26 AB-mag/arcsec 2 ) contour of the far-UV emission as shown in the Fig. 1 gions with elevated (> 3σ) emission relative to local disk backgrounds in both far-UV and 160 µm bands after smoothing images to 28 arcsec resolutions. The diffuse emission is measured by subtracting the total emission of all star-forming clumps in the disk from the integrated disk emission. For Se...

OB stars at the lowest Local Group metallicity

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