Variation in the stellar initial mass function from the chromospheric activity of M dwarfs in early-type galaxies

Abstract

Mass measurements and absorption line studies indicate that the stellar initial mass function (IMF) is bottom-heavy in the central regions of many early-type galaxies, with an excess of low mass stars compared to the IMF of the Milky Way. Here we test this hypothesis using a method that is independent of previous techniques. Low mass stars have strong chromospheric activity characterized by non-thermal emission at short wavelengths. Approximately half of the UV flux of M dwarfs is contained in the $\lambda 1215.7$ Ly$\alpha$ line, and we show that the total Ly$\alpha$ emission of an early-type galaxy is a sensitive probe of the IMF with a factor of $\sim 2$ flux variation in response to plausible variations in the number of low mass stars. We use the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure the Ly$\alpha$ line in the centers of the massive early-type galaxies NGC1407 and NGC2695. We detect Ly$\alpha$ emission in both galaxies and demonstrate that it originates in stars. We find that the Ly$\alpha$ to i-band flux ratio is a factor of 2.0$\pm$0.4 higher in NGC1407 than in NGC2695, in agreement with the difference in their IMFs as previously determined from gravity-sensitive optical absorption lines. Although a larger sample of galaxies is required for definitive answers, these initial results support the hypothesis that the IMF is not universal but varies with environment.

Accepted for publication in ApJ. The COS spectra are shown in Fig. 8. Figure 13 compares the measured Ly alpha fluxes to model predictions. Figure 14 compares the IMF derived from Ly alpha to that derived from optical absorption lines. Some background on this HST project is provided in footnotes 3 and 6

References in corpus (26)

• The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies
• A systematic variation of the stellar initial mass function in early-type galaxies
• The Subaru/XMM-Newton Deep Survey (SXDS). IV. Evolution of Lya Emitters from z=3.1 to 5.7 in the 1 deg^2 Field: Luminosity Functions and AGN
• The First Direct Measurements of Magnetic Fields on Very Low-Mass Stars
• The Majority of Compact Massive Galaxies at z~2 are Disk Dominated
• Radial variations in the stellar initial mass function of early-type galaxies
• Magnetic activity in the HARPS M-dwarf sample. The rotation-activity relationship for very low-mass stars through R'HK
• HAZMAT I: The Evolution of Far-UV and Near-UV Emission from Early M Stars
• Planetary nebulae as tracers of galaxy stellar populations
• A massive, dead disk galaxy in the early Universe
• The SINFONI Nearby Elliptical Lens Locator Survey: Discovery of two new low-redshift strong lenses and implications for the initial mass function in giant early-type galaxies
• Variations in the initial mass function in early-type galaxies: A critical comparison between dynamical and spectroscopic results
• The High-Energy Radiation Environment Around a 10 Gyr M Dwarf: Habitable at Last?
• IMF shape constraints from stellar populations and dynamics from CALIFA
• Flare Rates, Rotation Periods. and Spectroscopic Activity Indicators of a Volume-Complete Sample of Mid-to-Late M dwarfs within 15 Parsecs
• The Stellar Initial Mass Function in Early-Type Galaxies From Absorption Line Spectroscopy. IV. A Super-Salpeter IMF in the center of NGC 1407 from Non-Parametric Models
• A Very Bright, Very Hot, and Very Long Flaring Event from the M Dwarf Binary System DG CVn
• Calibration of the Halpha Age-Activity relation for M dwarfs
• Spatially resolved variations of the IMF mass normalisation in early-type galaxies as probed by molecular gas kinematics
• The relative emission from chromospheres and coronae: dependence on spectral type and age
• The early-type galaxies NGC 1407 and NGC 1400 - II: star formation and chemical evolutionary history
• The kinematics of massive quiescent galaxies at $1.4<z<2.1$: dark matter fractions, IMF variation, and the relation to local early-type galaxies
• Consistent dynamical and stellar masses with potential light IMF in massive quiescent galaxies at $3<z<4$ using velocity dispersions measurements with MOSFIRE
• The MASSIVE Survey VI: The Spatial Distribution and Kinematics of Warm Ionized Gas in the Most Massive Local Early-type Galaxies
• The high-energy spectrum of the nearby planet-hosting inactive mid-M dwarf LHS 3844
• Hubble Space Telescope Constraints on the Winds and Astrospheres of Red Giant Stars

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