THE STELLAR INITIAL MASS FUNCTION OF ULTRA-FAINT DWARF GALAXIES: EVIDENCE FOR IMF VARIATIONS WITH GALACTIC ENVIRONMENT

By M. Geha, T. Brown, Jason Tumlinson, J. Kalirai, J. Simon, E. Kirby, D. VandenBerg, R. Muñoz, R. Avila, P. Guhathakurta and 1 more. 2013

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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 (MV = −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, fbinary, 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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/2e29663c306e941e397cf2fdecc4e39b8615117a

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UniverseMachine: The correlation between galaxy growth and dark matter halo assembly from z = 0−10 (2018, background): “…disper- MNRAS 000, 000–000 (2018) sions, although the exact amount and redshift dependence is debated (Conroy & van Dokkum 2012; Conroy et al. 2013; Geha et al. 2013; Martín-Navarro et al. 2015; La Barbera et al. 2016; Posacki et al. 2015). van Dokkum et al. (2017) note that correcting for the…”
The birth of a galaxy – III. Propelling reionization with the faintest galaxies (2014, background): “…2560–2579 (2014) by guest on Septem ber 12, 2016 http://m nras.oxfordjournals.org/ D ow nloaded from stars, whereas recent observations of ultrafaint dwarf galaxies have suggested that the IMF slope is more shallow at α = 1.2+0.4−0.5 for Hercules and α = 1.3 ± 0.8 for Leo IV (Geha et al. 2013).”
A BUDGET AND ACCOUNTING OF METALS AT z ∼ 0: RESULTS FROM THE COS-HALOS SURVEY (2013, background): “…that all galaxies have— and have always had—this same IMF, though there is increasing evidence in the literature that the true picture may be more complicated (e.g., Auger et al. 2010; Brewer et al. 2012; Conroy & van Dokkum 2012; Sonnenfeld et al. 2012; Dutton et al. 2013; Geha et al. 2013).”
THE QUENCHING OF THE ULTRA-FAINT DWARF GALAXIES IN THE REIONIZATION ERA (2014, methodology): “For CVn II, Com Ber, Hercules, and Leo IV, the younger component is small ( 25% of the population).”
Observational Clues to the Progenitors of Type Ia Supernovae (2013, unknown): “…massive galaxies could perhaps be related to the recently noted dependence of IMF on galaxy mass (Cappellari et al. 2012, Conroy & van Dokkum 2012, Geha et al. 2013), or to the well know excess of metals in clusters, compared to expectations from SN yields and the present-day stellar population…”
The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, background): “For example, Geha et al. (2013) m asure the IMF in two ultra-faint dwarf satellite galaxies of the Milky Way via direct counting of stars in the mass range 0.52 − 0.77 M , below he main equ nce turnoff mass for se stellar populations.”
The Close Binary Fraction of Solar-type Stars Is Strongly Anticorrelated with Metallicity (2018, background): “Although some observations indicate that the IMF becomes top-heavy toward lower metallicities (Marks et al. 2012; Geha et al. 2013; Kroupa et al. 2013), this trend is not statistically significant until the metallicity falls below at least [Fe/H]−1.5.”