DO HOT HALOS AROUND GALAXIES CONTAIN THE MISSING BARYONS?
Galaxies are missing most of their baryons, and many models predict these baryons lie in a hot halo around galaxies. We establish observationally motivated constraints on the mass and radii of these halos using a variety of independent arguments. First, the observed dispersion measure of pulsars in the Large Magellanic Cloud allows us to constrain the hot halo around the Milky Way: if it obeys the standard Navarro, Frenk, and White (NFW) profile, it must contain less than 4%-5% of the missing baryons from the Galaxy. This is similar to other upper limits on the Galactic hot halo, such as the soft X-ray background and the pressure around high-velocity clouds. Second, we note that the X-ray surface brightness of hot halos with NFW profiles around large isolated galaxies is high enough that such emission should be observed, unless their halos contain less than 10%-25% of their missing baryons. Third, we place constraints on the column density of hot halos using nondetections of O VII absorption along active galactic nucleus (AGN) sightlines: in general they must contain less than 70% of the missing baryons or extend to no more than 40 kpc. Flattening the density profile of galactic hot halos weakens the surface brightness constraint so that a typical L{sub *} galaxy may hold half its missing baryons in its halo, but the O VII constraint remains unchanged, and around the Milky Way a flattened profile may only hold 6%-13% of the missing baryons from the Galaxy ((2-4) x 10{sup 10} M{sub sun}). We also show that AGN and supernovae at low to moderate redshift-the theoretical sources of winds responsible for driving out the missing baryons-do not produce the expected correlations with the baryonic Tully-Fisher relationship and, therefore, are insufficient to explain the missing baryons from galaxies. We conclude that most of missing baryons from galaxies do not lie in hot halos around the galaxies, and that the missing baryons never fell into the potential wells of protogalaxies in the first place. They may have been expelled from the galaxies as part of the process of galaxy formation.
- OSTI ID:
- 21448926
- Journal Information:
- Astrophysical Journal, Vol. 714, Issue 1; Other Information: DOI: 10.1088/0004-637X/714/1/320; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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COSMOLOGY AND ASTRONOMY
ABSORPTION
BARYONS
BRIGHTNESS
MAGELLANIC CLOUDS
MILKY WAY
PULSARS
RED SHIFT
SOFT X RADIATION
SUPERNOVAE
BINARY STARS
COSMIC RADIO SOURCES
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
ERUPTIVE VARIABLE STARS
FERMIONS
GALAXIES
HADRONS
IONIZING RADIATIONS
OPTICAL PROPERTIES
PHYSICAL PROPERTIES
RADIATIONS
SORPTION
STARS
VARIABLE STARS
X RADIATION