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Chemical Signatures of the Smallest Galaxies

Chemical Signatures of the Smallest Galaxies. Torgny Karlsson SIfA , School of Physics , The University of Sydney Collaborators: Joss Bland-Hawthorn and Ralph S. Sutherland. Outline. Ultra-faint dwarfs – the smallest known galaxies A hypothetical object – ultra low-mass galaxy :

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Chemical Signatures of the Smallest Galaxies

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  1. Chemical Signatures of the Smallest Galaxies Torgny Karlsson SIfA, School of Physics, The University of Sydney Collaborators: JossBland-Hawthorn and Ralph S. Sutherland

  2. Outline • Ultra-faintdwarfs – the smallestknowngalaxies • A hypotheticalobject – ultra low-massgalaxy: • Can thesegalaxiesexplainultra-faintdwarfs? • Kinematics • Chemical signatures • Conclusions “Smallest DM halo that canretainbaryons (cold gas) in the event of a single SN explosion” ➙ talk by JBH

  3. The ultra-faintdwarfs (UFDs) – what are they? Boötes I • Verydim systems (103 < L/Lsol < 105 ) • Very dark matterdominated (M300 ∼107Msol) • Velocity dispersions σv ∼ 3 – 8 km/s • Veryfewnumber of stars (N✶ ∼ 1000 – 100 000) • Veryold systems (stars among the oldestknown) • Verymetal-poor (-2.5 < 〈[Fe/H]〉 < -2) • Relativelyclose to the Milky Way (most < 200 kpc) Belokurov et al. (2006)/SDSS collaboration

  4. ULMGs – ultra low-massgalaxies • Working definition: The smallesttype of object that canretaincold gas in the • event of a single supernova explosion • Gas can be retained in DM haloes of M < 107Msolafter SN explosions, if the • medium is clumpy (JBH, this morning) • Low-mass stars should be able • to form in the enriched gas • Should be foundfurtherout, • far away from the Milky Way t = 1 Myr t = 20 Myr (Mvir = 3×106Msol) Bland-Hawthorn, Sutherland & Karlsson (2010)

  5. Kinematics – a poordiscriminant? Karlsson, Bland-Hawhtorn & Sutherland (2010) • Ultra low-massgalaxy: • Mvir = 3×106Msol • M300 = 9×105Msol (Einasto DM profile) • Motions in the gas generated by the SN • explosion produce a stellar velocity • dispersion of σv ∼ 4 km/s! • ➙ From a kinematicalpoint of view, the • ULMGsappearsimilar to the ultra-faint • dwarfs σv = 3 – 8 km/s Strigari et al. (2008)

  6. Segue 1 – what is it? Velocity [km/s] No. of stars Radius [arcmin] Velocity [km/s] σv = 4.3 km/s (Geha et al. 2009)

  7. Chemical enrichment in ultra-faintdwarfs Data bars: Observedmetallicity dis- tributionfunction in UFDs (Kirby et al. 2008) Curve:Model (cf. Karlsson 2005) Metal-richtail! • Real spread in metallicity (Kirby et al. 2008) • Evidence of a spread in stellar age (multiple starbursts) for fewUFDs (e.g., de Jong et al. 2008) • Inhomogeneouschemical evolution modelling: ∼30 SNe to reach [Fe/H] ∼ -2.5 – -2

  8. Star formation inultra low-massgalaxies The smallestgalaxieshavebeenenriched by ∼1-10 SNe In ULMGs, low-numberstatistics drives the chemicalevoultion!

  9. Chemical signatures in the ULMGs t = 1 Myr 1st SN [Fe/H] t = 20 Myr 4th SN 2nd SN 5th SN 3rd SN [Fe/H] Feltzing et al. (2009)

  10. Conclusions • ULMGs: “Smallest DM haloes that canretaincold gas in the event of a single SN” ➙ This is possible in DM haloes < 107Msolassuming a clumpy medium • The ULMGsmayhavekinematics and luminositiessimilar to the ultra-faintdwarfs (cf. Segue 1) • However: Chemical signatures – variousabundanceratios, e.g., [Mg/Ca], and probably a less pronounced [Fe/H]-richtail – willreveal the truenature of thesegalaxies • With the advent of the next gen. telescopes and surveys, e.g., the ELTs, LSST, and Gaia, Weshould start searching for the ULMGs in the outskirts of the Milky Way!

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