New Insights on the Galactic Bulge Initial Mass Function

Abstract

We have derived the Galactic bulge initial mass function (IMF) of the Sagittarius Window Eclipsing Extrasolar Planet Search field in the mass range 0.15 \lt M/{M}<SUB>☉ </SUB> 1.0, using deep photometry collected with the Advanced Camera for Surveys on the Hubble Space Telescope. Observations at several epochs, spread over 9 years, allowed us to separate the disk and bulge stars down to very faint magnitudes, F814W ≈ 26 mag, with a proper-motion accuracy better than 0.5 mas yr<SUP>-1</SUP> (20 km s<SUP>-1</SUP>). This allowed us to determine the IMF of the pure bulge component uncontaminated by disk stars for this low-reddening field in the Sagittarius window. In deriving the mass function, we took into account the presence of unresolved binaries, errors in photometry, distance modulus and reddening, as well as the metallicity dispersion and the uncertainties caused by adopting different theoretical color-temperature relations. We found that the Galactic bulge IMF can be fitted with two power laws with a break at M̃ 0.56 {M}<SUB>☉ </SUB>, the slope being steeper (α =-2.41+/- 0.50) for the higher masses, and shallower (α =-1.25+/- 0.20) for the lower masses. In the high-mass range, our derived mass function agrees well with the mass function derived for other regions of the bulge. In the low-mass range however, our mass function is slightly shallower, which suggests that separating the disk and bulge components is particularly important in the low-mass range. The slope of the bulge mass function is also similar to the slope of the mass function derived for the disk in the high-mass regime, but the bulge mass function is slightly steeper in the low-mass regime. We used our new mass function to derive stellar mass-to-light values for the Galactic bulge and we obtained 2.1 \lt M/{L}<SUB>F814W</SUB> \lt 2.4 and 3.1 \lt M/{L}<SUB>F606W</SUB> \lt 3.6 according to different assumptions on the slope of the IMF for masses larger than 1{M}<SUB>☉ </SUB>. <P />Based on observations made with the NASA/ESA Hubble Space Telescope, obtained by the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.