Benne Holwerda - The Origin of The Stellar Streams and Extra-Planar Gas around NGC 891

Speaker: Benne Holwerda, University of Louisville, Department of Physics & Astronomy

Abstract: We report the recovery of parts of the streams in Hubble Space Telescope Advanced Camera for Surveys and Wide-Field Camera 3 (HST ACS & WFC3) observations, using Red Giant Branch stars surrounding NGC 891, at ~600" and ~300" (27 and 13 kpc) from the nucleus on the major and minor axes respectively. The stellar stream does not contain young stars (<1Gyr), and the RGB population is consistent with a metallicity of [Fe/H] ~ -0.8dex, similar to earlier estimates for the combined streams surrounding NGC 891. We argue that the spatial overlap between the major axis stellar stream in the HST observations and a known counter-rotating HI (atomic hydrogen) complex is likely a coincidence or a projection effect. The color-magnitude diagram of stars within this HI complex reveals no recent (massive) star formation.

Here, we argue that the stellar streams surrounding NGC 891 and the extra-planar gas all originate from a single encounter. The similar metallicity at both major and minor intersections implies the streams have a common origin. Both the metallicity and the stream geometry imply a stellar mass of M_* ~ 10⁹ M_☉ satellite. The implied elapsed accretion time from the geometry is 1.5 Gyr. Assuming a typical gas-to-stellar ratios of unity for a late-type galaxies of this stellar mass, the implied HI accretion rate is around 1 M_☉/yr where the current several 10⁷ M_☉ cold extra-planar gas could represent the last few percent of accretion with most of the accreted gas already assimilated by NGC 891 or its halo. Both timescale and proximity argue against the extra-planar HI originating from interaction with UGC 1807. This finding has implications for how Milky Way like galaxies get their fuel; not through cold streams from pristine gas outside but the slow ingestion of a LMC-like galaxy.