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Richard D'souza Unravelling the Andromeda galaxy’s most massive merger Richard D’Souza With Eric Bell !1 Motivation How do mergers affect the properties of L* galaxies? Which properties of galaxies are caused by mergers? Can we derive a merger history from the stellar halo of a galaxy? M31 - PAndDAS Or at least the most massive accreted progenitor !2 M31 in comparison with other galaxies • Require new set of models encoding a diversity of accretion histories • Deason et al. 2016;Cooper et al. 2013; Illustris/Eagle suite of simulations GHOSTS Survey Harmsen et al. 2017 !3 Building Intuition from Models MW-like Large stellar halos are dominated by a single massive accreted progenitor M31-like = Massdom/Massacc Bell et al. 2018 Steepness of the Stellar-Halo mass relationship below L* !5 Constrain the total accreted stellar component of M31 from observations of the outer stellar halo (>27 kpc) If the accreted component of models are robust, then we can estimate: • M31’s total accreted stellar 27 kpc mass log Macc* > 10.3 (PAndAS) • M31’s accreted metallicity [Fe/H]acc > -0.3 (SPLASH) modified from Cooper et al. 2013 M31’s large accreted stellar mass constrains its possible massive progenitors D’Souza & Bell 2018 M31 accreted a satellite of mass log MDom ~ 10.3 in the last 5 Gyrs. The high mass and metallicity of M31’s stellar halo suggests it accreted a single large progenitor (log M* ~ 10.3) in the last 5 Gyrs. Can we narrow down the range of the time of accretion using the debris of the progenitor? !8 Building intuition on the debris field of the progenitor Radial Distribution of the Debris D’Souza & Bell 2018 !10 Identify possible metal-rich debris of the massive progenitor 1. M32 [Fe/H] ~ -0.1 Monachesi et al. 2012 2. Inner Spheroid [Fe/H] ~ -0.5 Bernard et al. 2015 1. 3. Giant Stream [Fe/H] ~ -0.3 2. Bernard et al. 2015 3. M31 accreted a large gas-rich progenitor (log M* ~ 10.3) ~2 Gyrs ago, called M32p. M32 is likely the core of M32p, which had a small bulge. The inner stellar halo contains the debris of the rest of the accreted progenitor. The giant stellar stream was likely caused by the accretion of M32p. !12 The family portrait of the Local Group M32p was the third largest member of the Local Group! Nearby galaxies with the mass of M32p and the central surface brightness of M32 M32p analogues in the local Universe S4G survey out to 24 Mpc Can explain the low number density of M32-like objects! What sets the low number density of M32-like objects in the Universe? Number density of cEs: Number density of M32p’s Major-merger rate since z~2 Local Universe (D<24 Mpc) Local Universe (D<24 Mpc) -4.2 +/- 0.7 log N/Mpc3 -3.5 +/- 0.3 log N/Mpc3 for log M*> 10.0 ~0.1 Chilingarian et al 2015 ~ x Rodriguez-Gomes et al. (2015) (z=0.03-0.05): -4.6 +/- 0.1 log N/Mpc3 The low number density of M32p analogues sets the number density of M32-like objects!!!! !15 What did the merger of M32p do to M31? • M31’s disk predates the merger, and survived a 0.1 to 0.3 merger. • Responsible for the galaxy disk wide star formation ~2 Gyrs ago (Williams et al. 2015) in which 1/5 th of its stars were formed (Williams et al. 2017). • Responsible for disk thickening ( disk scale length ~ 1 kpc, Dalcanton et al. 2015) and the large age - velocity dispersion in the RGB stars ( Vdisp ~ 90 km/s; see also Hammer et al. 2018) • M31’s bulge is substantially older (Olsen et al. 2006) than the merger. !16 What do mergers do to galaxies? HST Images of galaxies in the Toomre sequence (1977) From J. Hubbard, NRAO !17 What do mergers do to galaxies? Visually through examples of identified M32p analogues M81 + M82 M51a + M51b M31+M32 !18 3 Key Takeaways Merger and turmoil in M31’s past Remnants of the merger: Effect of the merger: Massive merger of M31 M32, Global burst ~2 Gyrs ago: much of the inner stellar halo of star formation & Third largest member & the Giant Stellar stream Disk thickening of the Local Group !19 Extra Slides !20 The metallicity of the accreted stellar halo reveals the mass of the dominant progenitor. D’Souza & Bell 2018.
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