Inner stellar halos of spiral galaxies: accretion or in-situ formation?
Abstract
The stellar halos surrounding spiral galaxies like the Milky Way are thought to be old and metal-poor. Indeed, we see little evidence for recent star formation in the form of main sequence stars or He burning stars in the color-magnitude diagrams (CMDs) of the stellar halos of the 16 nearby disk galaxies in our GHOSTS HST survey. It is therefore surprising, that we do see bright Asymptotic Giant Branch (AGB) stars in our CMDs, stars that are typically between 0.5 and 2.5 Gyr old. Recent simulations show two viable models for the origin of these intermediate age halo stars: 1) they have been brought in by gas-rich, star-forming, recently-disrupted satellites or 2) they have been dislodged from the main galaxy disk by recent interactions. To constrain their origin, we propose to use WFC3 F673N narrowband imaging on two stellar halos to determine the fraction of carbon (C) stars in the detected AGB population. Classical carbon stars only form effectively at lower metallicities ([Fe/H]<-0.3) and hence will be low in number compared to normal (M-type) AGB stars if this mysterious AGB population originates from the main disk, but the C star fraction will be high if they originate from accreted metal-poor dwarf galaxies. Such measurements are hard to do in the Milky Way and M31, and NGC253 is the only disk galaxy where this halo C star fraction has been mapped before. We will triple this sample by measuring M-to-C fractions at distances 15-30kpc above the midplane of NGC891 and NGC4565. The relative distributions of C stars, M-giants and RGB stars will be compared to our suite of halo formation models to determine the relative importance of accretion vs. in situ formation.
- Publication:
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HST Proposal
- Pub Date:
- August 2017
- Bibcode:
- 2017hst..prop15230D