A Study of Cold Gas and Star Formation in Low-Mass Blue-Sequence E/S0s
Abstract
We present a study of cold gas and star formation in low-mass blue-sequence E/S0 galaxies — a population that is morphologically early-type, but resides on the blue sequence in color vs. stellar mass space alongside spirals. A subset of these blue-sequence E/S0s may provide an evolutionary link between traditional red and dead early-type galaxies and star-forming spirals via disk (re)growth. In this dissertation, we use data from the Green Bank Telescope (GBT), the Combined Array for Research in Millimeter-wave Astronomy (CARMA), the <italic>Spitzer Space Telescope</italic>, and the <italic>Galaxy Evolution Explorer</italic> (<italic>GALEX</italic>) to examine the potential for morphological transformation of low-mass blue-sequence E/S0s.
In considering the HI content of these galaxies, we find that, normalized to stellar mass, the atomic gas masses for 12 of the 14 blue-sequence E/S0s range from 0.1 to ≥1.0. These gas-to-stellar mass ratios are comparable to those of spiral and irregular galaxies, and have a similar dependence on stellar mass. Assuming that the HI is accessible for star formation, we find that 9 of 14 blue-sequence E/S0s can increase in stellar mass by 10–60% in 3 Gyr with current HI reservoirs alone. We present evidence that star formation in these galaxies is bursty and likely involves externally triggered gas inflows.
For a sub-sample of eight E/S0s (four blue-, two mid-, and two red-sequence) whose CARMA CO(1–0), <italic>Spitzer</italic> MIPS 24μm, and <italic>GALEX</italic> FUV emission distributions are spatially resolved on a 750pc scale, we find roughly linear relationships between molecular-gas and star-formation surface densities within all galaxies, with power law indices <italic>N</italic> = 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 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% are offset towards apparently higher molecular-gas star formation efficiency. We discuss possible interpretations of the apparently elevated efficiencies.
We examine star formation in blue- and red-sequence E/S0s as traced by the 8μm PAH emission. We find the 8μm PAH/3.6μm emission ratios for most of our blue-sequence E/S0s to be similar to those of local spirals. Ratio images of the two tracers reveal ring-like and non-axisymmetric structures in some of our E/S0s, suggestive of internally and/or externally triggered gas inflow and centrally concentrated star formation. Comparison between the CO(1–0) and 8μm PAH emission distributions shows good agreement globally, although the 8μm PAH/3.6μm emission ratio appears to better trace non-axisymmetric structures observed in CO. Similar to CO observations of spiral galaxies, we find detectable CO emission in our E/S0s to be centrally concentrated, ranging from 0.1&ndash0.6<italic>r<sub>25</sub></italic> (median 0.3<italic>r<sub>25</sub</italic>). We also find that the aperture-averaged 8μm PAH to 3.6μm stellar emission ratio correlates with the atomic and molecular gas mass fractions.