The relationship between molecular gas and star formation in low-mass E/S0 galaxies

Abstract

We consider the relationship between molecular gas and star formation surface densities in 19 morphologically defined E/S0s with stellar mass ≲4 × 10^(10) M_⊙, paying particular attention to those found on the blue sequence in color versus stellar mass parameter space, where spiral galaxies typically reside. While some blue-sequence E/S0s must be young major-merger remnants, many low-mass blue-sequence E/S0s appear much less disturbed and may be experiencing the milder starbursts associated with inner-disk building as spirals (re)grow. For a sample of eight E/S0s (four blue, two mid, and two red sequence) whose CARMA CO(1–0), Spitzer MIPS 24μm, and GALEX FUV emission distributions are spatially resolved on a 750 pc scale, we find roughly linear relationships between molecular gas and star formation surface densities within all galaxies, with power-law indices N = 0.6–1.9 (median 1.2). Adding 11 more blue-sequence E/S0s whose CO(1–0) emission is not as well resolved, we find that most of our E/S0s have global 1–8 kpc aperture-averaged molecular gas surface densities overlapping the range spanned by the disks and centers of spiral galaxies. While many of our E/S0s fall on the same Schmidt–Kennicutt relation as local spirals, ~80% (predominantly on the blue sequence) are offset toward apparently higher molecular gas star formation efficiency (i.e., shorter molecular gas depletion time). Possible interpretations of the elevated efficiencies include bursty star formation similar to that in local dwarf galaxies, H_2 depletion in advanced starbursts, or simply a failure of the CO(1–0) emission to trace all of the molecular gas.

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