The Stellar Initial Mass Function of Ultra-faint Dwarf Galaxies: Evidence for Imf Variations With Galactic Environment
Year: 2013
Type: article
Abstract: We present constraints on the stellar initial mass function (IMF) in two ultra-faint dwarf (UFD) galaxies, Hercules and Leo IV, based on deep Hubble Space Telescope Advanced Camera for Surveys imaging. The Hercules and Leo IV galaxies are extremely low luminosity ( M V = −6.2, −5.5), metal-poor (〈[Fe/H]〉 = −2.4, −2.5) systems that have old stellar populations (>11 Gyr). Because they have long relaxation times, we can directly measure the low-mass stellar IMF by counting stars below the main-sequence turnoff without correcting for dynamical evolution. Over the stellar mass range probed by our data, 0.52–0.77 M ☉ , the IMF is best fit by a power-law slope of for Hercules and α = 1.3 ± 0.8 for Leo IV. For Hercules, the IMF slope is more shallow than a Salpeter (α = 2.35) IMF at the 5.8σ level, and a Kroupa (α = 2.3 above 0.5 M ☉ ) IMF slope at 5.4σ level. We simultaneously fit for the binary fraction, f binary , finding for Hercules, and for Leo IV. The UFD binary fractions are consistent with that inferred for Milky Way stars in the same mass range, despite very different metallicities. In contrast, the IMF slopes in the UFDs are shallower than other galactic environments. In the mass range 0.5–0.8 M ☉ , we see a trend across the handful of galaxies with directly measured IMFs such that the power-law slopes become shallower (more bottom-light) with decreasing galactic velocity dispersion and metallicity. This trend is qualitatively consistent with results in elliptical galaxies inferred via indirect methods and is direct evidence for IMF variations with galactic environment.
Source: The Astrophysical Journal
Authors: Marla Geha, Thomas M. Brown, Jason Tumlinson, Jason S. Kalirai, Joshua D. Simon, Evan N. Kirby, Don A. VandenBerg, Ricardo R. Muñoz, Roberto J. Avila, Puragra Guhathakurta, Henry C. Ferguson
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
Awards: G249051994, G433322167, G4518818167, Collaborative Research: M31 Satellites Past and Present, G58316358, G6273510985, G767601387, Collaborative Research: The genomic basis of dramatic, rapid, convergent evolution in the killifish Fundulus heteroclitus, COLLABORATIVE RESEARCH: Searching for Ultra-Faint Galaxies Around the Milky Way
OpenAlex ID: W2013910693