Revisiting the Integrated Star Formation Law. I. Non-starbursting Galaxies

We use new and updated gas- and dust-corrected star formation rate (SFR) surface densities to revisit the integrated star formation law for local "quiescent" spiral, dwarf, and low surface brightness galaxies. Using UV-based SFRs with individual IR-based dust corrections, we find that "normal" spiral galaxies alone define a tight Σ_(H I + H2)–Σ_(SFR) relation described by an n = 1.41^(+0.07)_(-0.07) power law with a dispersion of 0.28^(+0.02)_(-0.02) (errors reflect fitting and statistical uncertainties). The SFR surface densities are only weakly correlated with H I surface densities alone, exhibiting a stronger and roughly linear correlation with H2 surface densities, similar to what is seen in spatially resolved measurements of disks. However, many dwarf galaxies lie below the star formation law defined by spirals, suggesting a low-density threshold in the integrated star formation law. We consider alternative scaling laws that better describe both spirals and dwarfs. Our improved measurement precision also allows us to determine that much of the scatter in the star formation law is intrinsic, and we search for correlations between this intrinsic scatter and secondary physical parameters. We find that dwarf galaxies exhibit second-order correlations with the total gas fraction, stellar mass surface density, and dynamical time, which may explain much of the scatter in the star formation law. Finally, we discuss various systematic uncertainties that should be kept in mind when interpreting any study of the star formation law, particularly the X(CO) conversion factor and the diameter chosen to define the star-forming disk in a galaxy

Revisiting the Integrated Star Formation Law. Paper I: Non-Starbursting Galaxies

This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research was supported in part by the STFC through a consolidated grant to the Institute of Astronomy, University of Cambridge. M. A. de los Reyes also acknowledges the financial support of the Winston Churchill Foundation and the NSF Graduate Research Fellowship Program. The authors would like to thank the anonymous referee for their thoughtful and constructive comments, as well as M. Irwin, A. Saintonge, L. Hunt, and J. Wang for their useful comments and advice. Finally, we would like to express our deep gratitude to the staff at academic and telescope facilities, particularly those whose communities are excluded from the academic system, but whose labor maintains spaces for scientific inquiry. Software: Matplotlib (Hunter 2007), Linmix (Meyers 2015), Astropy (Astropy Collaboration et al. 2013)

Revisiting the Integrated Star Formation Law. I. Non-starbursting Galaxies

We use new and updated gas- and dust-corrected star formation rate (SFR) surface densities to revisit the integrated star formation law for local "quiescent" spiral, dwarf, and low surface brightness galaxies. Using UV-based SFRs with individual IR-based dust corrections, we find that "normal" spiral galaxies alone define a tight Σ_(H I + H2)–Σ_(SFR) relation described by an n = 1.41^(+0.07)_(-0.07) power law with a dispersion of 0.28^(+0.02)_(-0.02) (errors reflect fitting and statistical uncertainties). The SFR surface densities are only weakly correlated with H I surface densities alone, exhibiting a stronger and roughly linear correlation with H2 surface densities, similar to what is seen in spatially resolved measurements of disks. However, many dwarf galaxies lie below the star formation law defined by spirals, suggesting a low-density threshold in the integrated star formation law. We consider alternative scaling laws that better describe both spirals and dwarfs. Our improved measurement precision also allows us to determine that much of the scatter in the star formation law is intrinsic, and we search for correlations between this intrinsic scatter and secondary physical parameters. We find that dwarf galaxies exhibit second-order correlations with the total gas fraction, stellar mass surface density, and dynamical time, which may explain much of the scatter in the star formation law. Finally, we discuss various systematic uncertainties that should be kept in mind when interpreting any study of the star formation law, particularly the X(CO) conversion factor and the diameter chosen to define the star-forming disk in a galaxy

The Stellar Kinematics of Void Dwarf Galaxies Using KCWI

Dwarf galaxies located in extremely under-dense cosmic voids are excellent test-beds for disentangling the effects of large-scale environment on galaxy formation and evolution. We present integral field spectroscopy for low-mass galaxies ($M_{\star}=10^{7}-10^{9}~M_{\odot}$) located inside (N=21) and outside (N=9) cosmic voids using the Keck Cosmic Web Imager (KCWI). Using measurements of stellar line-of-sight rotational velocity $v_{\mathrm{rot}}$ and velocity dispersion $\sigma_{\star}$, we test the tidal stirring hypothesis, which posits that dwarf spheroidal galaxies are formed through tidal interactions with more massive host galaxies. We measure low values of $v_{\mathrm{rot}}/\sigma_{\star}\lesssim2$ for our sample of isolated dwarf galaxies, and we find no trend between $v_{\mathrm{rot}}/\sigma_{\star}$ and distance from a massive galaxy $d_{L^{\star}}$ out to $d_{L^{\star}}\sim10$ Mpc. These suggest that dwarf galaxies can become dispersion-supported "puffy" systems even in the absence of environmental effects like tidal interactions. We also find indications of an upward trend between $v_{\mathrm{rot}}/\sigma_{\star}$ and galaxy stellar mass, perhaps implying that stellar disk formation depends on mass rather than environment. Although some of our conclusions may be slightly modified by systematic effects, our main result still holds: that isolated low-mass galaxies may form and remain as puffy systems rather than the dynamically cold disks predicted by classical galaxy formation theory.Comment: 19 pages including references; submitted to ApJ. Code used for analysis and figures can be found here: https://github.com/mdlreyes/void-dwarf-analysi