| Description |
A surprising number (virgul50%) of field early-type galaxies (E and S0s) contain a cool interstellar medium phase detected through the 21cm hyperfine transition of neutral hydrogen. In some cases, this gas is distributed in extremely extensive outer distributions of up to a few 10^9 solar masses of HI. The origin and relation of such outer gas reservoirs to their host early-type galaxies is currently unknown. The gas may be directly accreted from the IGM, stripped during a tidal interaction, or acquired during a gas-rich merger. Alternatively, such gas may have been long associated with the early-type galaxy, not being exhausted due to a low star formation efficiency. In each of these cases the gas is expected to have a different enrichment history and thus a different present-day metallicity and dust-to-gas ratio. With Herschel PACS and SPIRE photometry, we propose to measure dust masses in the outer HI distributions of 15 nearby early-type galaxies. With interferometric HI data already in hand, we can then calculate dust-to-gas ratios and constrain the origin of such gas. The sample size and variety of HI masses, radial extents and morphologies will let us test if the dust-to-gas ratios vary among the sample in a way described best by a single accretion scenario. Additionally, the temperature derived for the dust in the outer gas will let us constrain the possible heating sources for the dust. In some sample galaxies, coincident UV emission suggests young stars are present. If heating from these young stars is important, we expect a variation of dust temperature in systems with and without UV emission. |
