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 o Ultra-faint dwarfs – the smallest known galaxies o A hypothetical object – ultra low-mass galaxy: o Can these galaxies explain ultra-faint dwarfs? o Kinematics o Chemical signatures o Conclusions “Smallest DM halo that can retain baryons (cold gas) in the event of a single SN explosion” ➙ talk by JBH

The ultra-faint dwarfs (UFDs) – what are they? o Very dim systems (10 3 < L/L sol < 10 5 ) o Very dark matter dominated (M 300 ∼ 10 7 M sol ) o Velocity dispersions σ v ∼ 3 – 8 km/s o Very few number of stars (N ✶ ∼ 1000 – ) o Very old systems (stars among the oldest known) o Very metal-poor (-2.5 < 〈 [Fe/H] 〉 < -2) o Relatively close to the Milky Way (most < 200 kpc) Belokurov et al. (2006)/SDSS collaboration Boötes I

ULMGs – ultra low-mass galaxies Working definition: The smallest type of object that can retain cold gas in the event of a single supernova explosion Gas can be retained in DM haloes of M < 10 7 M sol after SN explosions, if the medium is clumpy (JBH, this morning) Low-mass stars should be able to form in the enriched gas Should be found further out, far away from the Milky Way t = 1 Myr t = 20 Myr (M vir = 3×10 6 M sol ) Bland-Hawthorn, Sutherland & Karlsson (2010)

Kinematics – a poor discriminant? Ultra low-mass galaxy: M vir = 3×10 6 M sol M 300 = 9×10 5 M sol (Einasto DM profile) Motions in the gas generated by the SN explosion produce a stellar velocity dispersion of σ v ∼ 4 km/s! ➙ From a kinematical point of view, the ULMGs appear similar to the ultra-faint dwarfs Strigari et al. (2008) Karlsson, Bland-Hawhtorn & Sutherland (2010) σ v = 3 – 8 km/s

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

Chemical enrichment in ultra-faint dwarfs Real spread in metallicity (Kirby et al. 2008) Evidence of a spread in stellar age (multiple star bursts) for few UFDs (e.g., de Jong et al. 2008) Inhomogeneous chemical evolution modelling: ∼ 30 SNe to reach [Fe/H] ∼ -2.5 – -2 Metal-rich tail! Data bars: Observed metallicity dis- tribution function in UFDs (Kirby et al. 2008) Curve: Model (cf. Karlsson 2005)

Star formation in ultra low-mass galaxies The smallest galaxies have been enriched by ∼ 1-10 SNe In ULMGs, low-number statistics drives the chemical evoultion!

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

Conclusions o ULMGs: “Smallest DM haloes that can retain cold gas in the event of a single SN” ➙ This is possible in DM haloes < 10 7 M sol assuming a clumpy medium o The ULMGs may have kinematics and luminosities similar to the ultra-faint dwarfs (cf. Segue 1) o However: Chemical signatures – various abundance ratios, e.g., [Mg/Ca], and probably a less pronounced [Fe/H]-rich tail – will reveal the true nature of these galaxies o With the advent of the next gen. telescopes and surveys, e.g., the ELTs, LSST, and Gaia, We should start searching for the ULMGs in the outskirts of the Milky Way!