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Isolated Dwarf Galaxies As Probes of Dark Matter Isolated dwarf galaxies as probes of dark matter Michelle Collins University of Surrey Isolated dwarf galaxies as probes of dark matter Michelle Collins University of Surrey Low density dark matter halos 4 M. Boylan-Kolchin, J. S. Bullock and M. Kaplinghat ‘Too big to fail’ ‘Cusp-Core’ spherical Jeans equation, Thomas et al. (2011)haveshown that this mass estimator accurately reflects the mass as de- rived from axisymmetric orbit superposition models as well. This result suggests that Eqns. (1)and(2) are also applica- ble in the absence of spherical symmetry, a conclusion that is also supported by an analysis of Via Lactea II subhalos (Rashkov et al. 2012). We focus on the bright MW dSphs – those with LV > 105 L – for several reasons. Primary among them is that these systems have the highest quality kinematic data and the largest samples of spectroscopically confirmed member stars to resolve the dynamics at r1/2.Thecensusofthese bright dwarfs is also likely complete to the virial radius of the Milky Way ( 300 kpc), with the possible exception of ⇠ yet-undiscovered systems in the plane of the Galactic disk; the same can not be said for fainter systems (Koposov et al. 2008; Tollerud et al. 2008). Finally, these systems all have half-light radii that can be accurately resolved with the high- est resolution N-body simulations presently available. The Milky Way contains 10 known dwarf spheroidals 5 satisfying our luminosity cut of LV > 10 L :the9clas- sical (pre-SDSS) dSphs plus Canes Venatici I, which has a V -band luminosity comparable to Draco (though it is sig- Figure 1. Observed V values of the nine bright dSphs nificantly more spatially extended). As in BBK, we remove circ (symbols, with sizes proportional to log LV ), along with ro- the Sagittarius dwarf from our sample, as it is in the pro-Boylan-Kolchintation curves et al. corresponding 2012 to NFW subhalos with Vmax = Bramante et al 2015 1 cess of interacting (strongly) with the Galactic disk and is (12, 18, 24, 40) km s− .Theshadingindicatesthe1σ scatter in likely not an equilibrium system in the same sense as the rmax at fixed Vmax taken from the Aquarius simulations. All of other dSphs. Our final sample therefore contains 9 dwarf the bright dSphs are consistent with subhalos having Vmax 1 1 spheroidals: Fornax, Leo I, Sculptor, Leo II, Sextans, Ca- 24 km s− ,andmostrequireVmax . 18 km s− .OnlyDraco,the rina, Ursa Minor, Canes Venatici I, and Draco. All of these least luminous dSph in our sample, is consistent (within 2σ)with 1 amassiveCDMsubhaloof 40 km s− at z =0. galaxies are known to be dark matter dominated at r1/2 ⇡ (Mateo 1998): Wolf et al. (2010)findthattheirdynamical mass-to-light ratios at r range from 10 300. 1/2 ⇠ − The Large and Small Magellanic Clouds are dwarf ir- regular galaxies that are more than an order of magnitude provides a somewhat better match to the profiles of both brighter than the dwarf spheroidals. The internal dynamics simulated halos (Navarro et al. 2004; Merritt et al. 2006; of these galaxies indicate that they are also much more mas- Gao et al. 2008; Ludlow et al. 2011)andsubhalos(Springel 1 sive than the dwarf spheroidals: Vcirc(SMC) = 50 60 km s− et al. 2008) even when fixing the Einasto shape parameter − (Stanimirovi´c et al. 2004; Harris & Zaritsky 2006)and (thereby comparing models with two free parameters each). 1 Vcirc(LMC) = 87 5kms− (Olsen et al. 2011). Abun- To connect this work to the analysis of BBK, Figure 1 com- ± dance matching indicates that galaxies with luminosities pares the measured values of Vcirc(r1/2) for the nine bright equal to those of the Magellanic Clouds should have Vinfall MW dSphs to a set of dark matter subhalo rotation curves 1 ⇡ 80 100 km s− (BBK); this is strongly supported by the based on NFW fits to the Aquarius subhalos; the shaded − analysis of Tollerud et al. (2011). A conservative estimate bands show the 1 σ scatter from the simulations in rmax at of subhalos that could host Magellanic Cloud-like galaxies fixed Vmax. More detailed modeling of subhalos’ density pro- 1 1 is therefore Vinfall > 60 km s− and Vmax > 40 km s− .Asin files will be presented in subsequent sections. BBK, subhalos obeying these two criteria will be considered It is immediately apparent that all of the bright dSphs 1 Magellanic Cloud analogs for the rest of this work. are consistent with NFW subhalos of Vmax =12 24 km s− , − and only one dwarf (Draco) is consistent with Vmax > 1 24 km s− . Note that the size of the data points is pro- portional to galaxy luminosity, and no obvious trend exists 3COMPARING⇤CDM SUBHALOS TO between L and Vcirc(r1/2)orVmax (see also Strigari et al. MILKY WAY SATELLITES 2008). Two of the three least luminous dwarfs, Draco and Ursa Minor, are consistent with the most massive hosts, 3.1 A preliminary comparison while the three most luminous dwarfs (Fornax, Leo I, and Density and circular velocity profiles of isolated dark mat- Sculptor) are consistent with hosts of intermediate mass 1 ter halos are well-described (on average) by Navarro et al. (Vmax 18 20 km s− ). Each of the Aquarius simulations ⇡ − 1 (1997, hereafter, NFW) profiles, which are specified by two contains between 10 and 24 subhalos with Vmax > 25 km s− , parameters – i.e., virial mass and concentration, or Vmax almost all of which are insufficiently massive to qualify as and rmax. Average dark matter subhalos are also well-fitted Magellanic Cloud analogs, indicating that models populat- by NFW profiles inside of their tidal radii, though recent ing the most massive redshift zero subhalos with the bright- work has shown that the 3-parameter Einasto (1965) profile est MW dwarfs will fail. c 2012 RAS, MNRAS 000, 1–17 A problem for CDM? Or fixed by baryons? • Importance of feedback effects Supernovae feedback can ‘smooth’ density profiles - e.g. Pontzen & Governato 2012, Brooks et al. 2015, Read, Agertz & Collins 2016, Wheeler + 2015 FIRE, LATTE, APOSTLES, CLUES… Vc Milky Way dsphs R A problem for CDM? Or fixed by baryons? • Importance of feedback effects Supernovae feedback can ‘smooth’ density profiles - e.g. Pontzen & Governato 2012, Brooks et al. 2015, Read, Agertz & Collins 2016, Wheeler + 2015 FIRE, LATTE, APOSTLES, CLUES… Vc Milky Way dsphs R Dark matter ‘heating’ - testing with high resolution simulations 8.0 8.0 8.0 8 8 9 2 (a) 10 M 2 (b)7.6 5 10 M 2 (c)7.6 10 M 7.6 ⇥ Isolated dwarf galaxies - idealised 7.2 7.2 7.2 1 1 6.8 1 6.8 6.8 3 3 Leo T 3 Aquarius − − − 6.4 6.4 6.4 kpc kpc kpc ⇥ ⇥ ⇥ kpc simulations kpc 0 kpc 0 6.0 0 6.0 6.0 M M M y/ y/ y/ 10 5.6 10 5.6 10 5.6 log 1 1 5.2 log 1 5.2 log 5.2 − − − Gas density 4.8 4.8 4.8 2 2 4.4 2 4.4 4.4 − ⇢gas − − Resolution: 250 Msun,4pc 4.0 4.0 4.0 2 1 0 1 2 26.0 1 0 1 2 26.0 1 0 1 2 6.0 − − − − − − 2 (d) x/kpc 2 (e)5.6 x/kpc 2 (f)5.6 x/kpc 5.6 5.2 5.2 5.2 1 1 4.8 1 4.8 4.8 4.4 4.4 4.4 Can resolve individual supernova K K K kpc kpc kpc 0 0 4.0 10 0 4.0 10 4.0 10 y/ y/ y/ 3.6 log 3.6 log 3.6 log 1 1 3.2 1 3.2 3.2 − − − Gas temperature 2.8 2.8 2.8 2 2 2.4 2 2.4 2.4 − Tgas − − 2.0 2.0 2.0 2 1 0 1 2 2 1 0 1 2 2 1 0 1 2 − − − − − − x/kpc 2 (g) x/kpc 2 (h)7.2 x/kpc 7.2 6.8 6.8 1 1 6.4 3 6.4 3 − − 6.0 kpc 6.0 kpc ⇥ ⇥ kpc 0 kpc 0 M M y/ y/ 5.6 5.6 10 10 5.2 log 5.2 log 1 1 − − 4.8 4.8 2 2 4.4 4.4 − ⇢ Stellar density − ⇤ 4.0 4.0 2 1 0 1 2 2 1 0 1 2 − − x/kpc − − x/kpc Read, Agertz & Collins (2016) Have lower masses than dark matter only Can reduce central densities of DM halo, even at low stellar masses Cores can be formed Lowers central mass by factor ~4 Can ‘correct’ measured circular velocities New, updated EDGE simulations to test non-idealized case Read, Agertz & Collins (2016) Implies there should be high density/cusped dwarfs • Significant ‘heating’ of dark matter can only occur if star formation is extended (tsf > 4-5 Gyrs) • Expect some dwarf galaxies to retain their cusps • If galaxies found with cores, but little star formation, need other mechanisms Searching for cusps - Tucana Tucana potentially in a ‘massive failure’ halo. But, could be much lower mass. Star formation mostly quenched ~ 10 Gyrs ago (Savino +19) Revisited with FLAMES Fraternali et al. 2009 Savino et al. 2019 Searching for cusps - Tucana With velocities for more member stars, Tucana still resides in a high Gregory et al. 2019 density halo Searching for cusps - Tucana And, mass modelling with GravSphere NFW halos (Read & Steger 2017) implies it is cusped. 1011 Msun Would need more data in the centre of 1010 Msun confirm… 109 Msun Gregory et al.
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