Are the red halos of galaxies made of low-mass stars? Constraints from subdwarf star counts in the Milky Way halo
arXiv:0809.2991 · doi:10.1086/591659
Abstract
Surface photometry detections of red and exceedingly faint halos around galaxies have resurrected the old question of whether some non-negligible fraction of the missing baryons of the Universe could be hiding in the form of faint, hydrogen-burning stars. The optical/near-infrared colours of these red halos have proved very difficult to reconcile with any normal type of stellar population, but can in principle be explained by advocating a bottom-heavy stellar initial mass function. This implies a high stellar mass-to-light ratio and hence a substantial baryonic mass locked up in such halos. Here, we explore the constraints imposed by current observations of ordinary stellar halo subdwarfs on a putative red halo of low-mass stars around the Milky Way. Assuming structural parameters similar to those of the red halo recently detected in stacked images of external disk galaxies, we find that a smooth halo component with a bottom-heavy initial mass function is completely ruled out by current star count data for the Milky Way. All viable smooth red halo models with a density slope even remotely similar to that of the stacked halo moreover contain far too little mass to have any bearing on the missing-baryon problem. However, we note that these constraints can be sidestepped if the red halo stars are locked up in star clusters, and discuss potential observations of other nearby galaxies that may be able to put such scenarios to the test.
11 pages, 5 figures, ApJ, in press
References in corpus (15)
- Limits on the Macho Content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds
- Two Stellar Components in the Halo of the Milky Way
- The accretion origin of the Milky Way's stellar halo
- On the mass-to-light ratio of the local Galactic disc and the optical luminosity of the Galaxy
- The Search for the Missing Baryons at Low Redshift
- M dwarfs: effective temperatures, radii and metallicities
- The baryon fraction of LambdaCDM haloes
- Where are the ``Missing'' Galactic Baryons?
- Microlensing constraints on the Galactic Bulge Initial Mass Function
- MACHOs in M31? Absence of evidence but not evidence of absence
- Point spread function tails and the measurements of diffuse stellar halo light around edge-on disc galaxies
- The M31 microlensing event WeCAPP-GL1/Point-AGAPE-S3: evidence for a MACHO component in the dark halo of M31?
- The Red Halo Phenomenon
- High-redshift microlensing and the spatial distribution of dark matter in the form of MACHOs
- Red Halos of Galaxies - Reservoirs of Baryonic Dark Matter?