[copyrighted background image removed] 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 1 Dwarf Galaxy Types (≤ 1/100 L★; MV ≥ –18) The Galaxy Content of the Local Group ❏ Dwarf elliptical galaxies Certain or probable members: Early-type dwarfs. ≥ 104 galaxies within R ~ 1 Mpc. dEs ❏ Dwarf spheroidal galaxies Gas-deficient and now largely quiescent. 0 dIrrs § 3 spiral galaxies (~ 95% mass). dIrrs spirals ❏ Ultra-compact dwarf galaxies } High-density regions preferred. /dSphs § ≥ 101 dwarf and satellite galaxies ❏ Dwarf spirals / dwarf lenticulars Late-type dwarfs. (typically, MV ≥ –18). ❏ Dwarf irregular galaxies Gas-rich and usually star-forming. § Some satellites have own satellites... ❏ Blue compact dwarf galaxies Low-density regions preferred. } dE dSph dIrr dSphs ❏ Ultra-diffuse galaxies ❏ Tidal dwarf galaxies Pictures not on same scale dE dSph UCD dS0, dS dIrr BCD Gas-deficient, late-type dwarf galaxies: dwarf elliptical (dEs: 3; 1 cE) & dwarf spheroidal galaxies (dSphs: ≥ 83) Gas-rich, early-type dwarf galaxies: dwarf irregular galaxies (dIrrs: 9), transition types (dIrrs/dSphs: 5) 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 2 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 3 New Satellites of Size – Luminosity Relation Morphology- the Milky Way density ❑ New discoveries mainly have mainly very low surface brightnesses. and M31 by Year relation ❑ Note overlap between GC −2 ❑ and dSph locus in L (M ) After Muñoz et al. 2018 of Publication V Blue points: GCs. −2 −2 Mainly thanks to large = 18 mag arcsec 26 mag V imaging surveys in the μ arcsec northern hemisphere 22 mag arcsec Grebel 2017 (esp. SDSS, PAndAS, PS1). −2 Increasingly also southern 30 mag hemisphere (e.g., DES, arcsec VST-ATLAS, Subaru). Muñoz et al. 2018 Total satellite population of the Milky Way estimated (NB: In these two diagrams +53 Walker 2012 142−34 down to MV = 0 in some of the red crosses simulations (Newton et al. 2017). stand for GCs.) 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 4 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 5 At Low Masses: Distinguishing Galaxies & Star Clusters No general definition exists but conventionally the following criteria are used: Galaxies: Star clusters: q Gravitationally bound q Gravitationally bound q Contain dark matter q No dark matter Laevens 1 / Crater I q Considerable metallicity spread q Negligible metallicity spread McConnachie Willman & h 2012 Strader 2012 r within L / dyn M Globular clusters Dark matter Galaxies Sawala et al. 2016 MV 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 6 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 7 Black dots, black circles: Satellites not in the plane. Gaia Collaboration et al. 2018 Green circles, colored dots: Satellites in plane. Orbit backward integration Satellite Planes Satellite Planes for 2.5 Gyr. Thin planes of satellites ❑ Long-term survival of planes depends around MW and M31 ❑ on orientation of dwarfs’ orbit. (e.g., Kunkel & Demers 1976; ➙ Thin plane survives only if aligned with one of the semi-major or semi-minor axes Lynden-Bell 1976; Koch & Grebel 2006; Pawlowski et q of a triaxial halo, or in the polar or equatorial planes of a spherical halo. al. 2012; Ibata et al. 2013). (Bowden et al. 2013; Fernando et al. 2016). q Satellite planes at least partially fortuitous. ΛCDM simulations: q Planes may contain co-rotating pairs of satellites, but planes need not co-rotate. q Planes form through q Planes not kinematically coherent structures as a whole; transitory features. q accretion along large Buck et al. 2015 E.g., Cautun et al. 2015; Buck et al. 2015; Gillet et al. 2015; Bowden et al. 2013; Fernando et al. 2016; q filaments of DM around galaxies at high redshift. Lipnicky & Chakrabarti 2017. q Dwarf galaxy accretion is highly anisotropic, takes place preferentially in the plane ❑ HST & Gaia proper motions: MW q determined by the major and intermediate axes of the DM host halo shape, and, ❑ dwarfs not on single narrow plane. q within this plane, is clustered along the shape major axis. ❑ Orbits typically ^ to MW disk, but Gaia Colla- q High-concentration massive halos tend to have thinner and richer planes. ❑ span broad range of orientations Different boration colors: et al. ❑ (of 39, 11 co-orbit, 6 counter-orbit). different 2018 q Most satellites were accreted along the richest filaments. Galactic ➙ Single major event excluded, but potentials q Group accretion (multiple satellites) is more common for fainter satellites. ❑ multiple infall along cosmic web . q Degree of anisotropic accretion higher for most massive satellites. ❑ filament aligned with Z-axis possible. (Gaia Collaboration et al. 2018; Fritz et al. 2018; Simon E.g., Libeskind et al. 2015; Buck et al. 2015, Shao et al. 2018. ❑ 2018; Casetti-Dinescu et al. 2018, Sohn et al. 2017; Massari & Helmi 2018; Kallivayalil et al. 2018). 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 8 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 9 Infall of Dwarf Groups ❑ Proper ❑ motions ❑ (Gaia DR2) ❑ and radial ❑ velocities ❑ of ultra- ❑ faint dSphs ❑ near MCs: § 4 (Hor 1, § Car 2, 3, § Hyd 1) § came in with Conn et al. 2018 § MCs. § Possibly [copyrighted background image removed] § also Hyd 2, Dra 2. 3 (Ret 2, Tuc 2, Gru 1) uncertain. § 4 are unlikely (Tuc 3, Cra 2, Tri 2, Aqu 2). § Remaining ones: no proper motions yet. (Kallivayalil et al. 2018) § 4 – 6 LMC satellites: Consistent with expectations from ΛCDM. 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 10 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 11 Kirby et al. 2013 De Lucia & Helmi 2008; Cooper et al. 2010 accreted stars (ex situ) in-situ stars Metallicity Scaling Relations Stellar Halo Origins ❑ (Dwarf) galaxies: clear metallicity – luminosity relation q Stellar halos composed in part of ❑ Signature of galaxies’ ability to retain metals in their grav. ❑ potential wells or of correlation between SF efficiency and stellar mass. q accreted stars and in part of stars ❑ GCs: No such relation. Also, don’t extend to as low [Fe/H] as ultrafaint dSphs. q formed in situ. Rodriguez- ❑ Even ultrafaint, very metal-poor dSphs show metallicity spread and extended SFHs. q Halos grow from “from inside out”. Gomez et al. 2016 GCs q Wide variety of satellite accretion histories from smooth growth to discrete events. Dwarf galaxies 8 9 q ≤ 5 luminous satellites (10 – 10 M¤) are the main contributors to stellar halos. q Merged > 9 Gyr ago (inner halo). Satellite accretion mainly between 1 < z < 3. Pillepich et al. 2015 ω Cen M54 Possibly flattening present-day ex-situ stars present-day at metal-poor end? (incl. those still in in-situ stars self-bound satellites) Simon et al. 2017 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 12 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 13 [Fe/H] vs. [α/Fe] in Dwarfs Trends in Individual Element Abundance Ratios Position of turnover (“α knee”) shows how far r-process: n-rich nuclei Beniamini et al. 2016 enrichment could proceed until onset of SNe Ia. formeD rapidly in massive Measure of SFE and retention of enriched ejecta. stars via n-capture (~104s) Sgr dSph: Position of “knee” shows: early accretion and b decay, e.g., Eu: (before knee formed) of Sgr-like galaxies could have q Eu mass in ultra- contributed metal-rich parts of inner MW halo. q faint dSphs and SN II Beniamini et al. 2016 α α Fe SN Ia q large scatter in Fe α q abundances of Stochasticity begins q r - process elements to dominate q (anD Derivatives, mass q number A ≥ 90) Hendricks et al. 2014 in metal-poor dSphs (anD in metal-poor stars in MW): ProDuceD in rare events! Possibly in neutron star mergers. (Beniamini et al. 2016) As with α elements, we see contributions from inDiviDual events. Position of “α knee” correlates r-process retention when events not too energetic. Low r-process frequency (models: Sgr α knee: −1.3 dex. with dSph luminosity (or stellar ~ 0.07 of u.f. dSphs) µ with σ([Eu/Fe]) in MW metal-poor stars. (Beniamini et al. 2018) de Boer et al. 2014 MW α knee: −1.0 dex. mass). MoDels suggest that in an initial, metal-poor ISM stochastic effects dominate. Inhomogeneous pollution, few SNe (Marcolini et al. 2008). 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 14 27.08.2018 Grebel: Dwarf Galaxies in the Local Group 15 Trends in Individual Element Abundance Ratios For –4 ≤ [Fe/H] ≤ –2 (small sample sizes still): q a elements in classical dSphs and MW halo q very similar. Plateau at [a/Fe] ≈ 0.3. q Enrichment by massive stars with “normal” IMF. q Fe peak, Al, Na in dSphs follow MW halo trends ➙ Produced in same nucleosynthetic processes [copyrighted background image removed] (C burning) independent of host galaxy mass. q Ultra-faint dSphs (Boo I, Leo IV) show evidence q for pollution by SNe Ia (SF must have lasted q at least about 1 Gyr). ➙ “Long-lasting” SF. q Ultra-faint dSph UMa II: No SN Ia enrichment! n-capture elements Sr, Ba: Mashonkina et al. 2017 q Classical dSphs and MW halo show large q dispersion in [Sr/Fe] below [Fe/H] ~ –3. (see also Tafelmeyer et al. 2010; Koch et al. 2013; q Same [Sr/Ba] dichotomy in classical dSphs and MW halo: Frebel & Norris 2015; ➙ two nucleosynthesis channels for Sr. Battaglia et al. 2017) q Ultra-faint dSphs: mainly distinctly low [Sr/Ba] values. Lack 2nd channel of Sr q production. Possibly due to undersampling of IMF at high-mass end.