GALAXIES 626 The Local Group: Why is the local group interesting? Can study the dynamics of the group and how it relates to the individual galaxies Can find the faintest galaxies and study their dark matter properties, stellar populations and star formation histories Can look in detail at the disruption and merging processes in galaxies GALAXIES 626 The Local Group is a fairly typical weak group so very characteristic of the environments in which many galaxies live ·Sparse group with zero- velocity radius of Local Group Substruct1u Mrpce ·M31 approaching at 120 km/s ·Reliable orbits unknown The Local Group of Galaxies · about 35-40 member galaxies · Milky Way and Andromeda subgroups 5 The least luminous galaxies known are in the local group 6 NGC 6822 Local Group LSB galaxy in near-IR. HI map has 20 pc resolution 7 Milky Way & M31 Satellites Galaxy Type Dist. (kpc) Galaxy Type Dist. (kpc) SMC SB(s)m 64 NGC 147 dE5 735 Sculptor dSph 92 And III dSph 889 Phoenix dIrr 490 NGC 185 dE3 705 Fornax dSph 153 M110 E5 889 LMC SB(s)m 55 And VIII dSph 828 Carina dSph 110 M32 E2 889 Canis Major dIrr 7.7 And I dSph 889 Leo A Ibm V 767 And V dSph 889 Leo I dE3 276 And II dSph 889 Sextans dSph 98 And VII dSph 797 Leo II dSph 230 And VI dSph 858 Ursa Minor dSph 74 Draco dSph 89 SagDEG dSph 27 Morphological Segregation Gas-poor, low-mass dwarfs (dSphs) tend to cluster around massive galaxies (Cetus & Tucana are exceptions) Gas-rich, high-mass dwarfs (dIs) are found to be widely distributed Observed in nearby groups and clusters as Grebel 2005 well as the Local Group Do these trends result from morphological transformations due to the influence of the massive primary galaxy? (i.e. tidal or ram pressure stripping) Dark Matter in M31 and the Milky Way The first thing we would like to do is see if there is more dark matter in the big spiral galaxies than we are inferring from the rotation curves. That is: do the dark halos extend beyond the maximum radius at which we can measure the rotation curve? 10 How large and massive are the dark halos of large spirals like the Milky Way ? Flat rotation curves => M(r) ~ r, like the isothermal sphere : ρ ~ r-2 This cannot go on for ever - the halo mass would be infinite. Halos must have a finite extent and mass, and their density distribution must truncate or be steeper than ρ ~ r-3 at very large radius 11 Tracers of dark matter in the Galaxy (rotation curve to ~ 20 kpc, kinematics of metal poor stars, globular clusters and satellites out to ~ 50 kpc) indicate that the halo mass M(r) = r(kpc) x 1010 solar masses. Again, this is what we expect if ρ ~ r-2 ie the rotation curve stays approximately flat at 220 km/s out to 50 kpc. How large are dark halos - how far in radius do they extend ? 12 M31 and the Milky Way are now M31 approaching at 118 km s -1. Their separation is about 750 kpc To acquire this velocity of approach in the life of the universe means that the total mass of the Milky Way is at least 13 x 10 11 M_sun. The stellar mass is about 6 x 1010 M_sun, so the ratio 118 km s -1 of dark to stellar mass is ~ 20 Milky Way The dark halo extends out to ~ 150 kpc, far beyond the disk©s radius of ~ 20 kpc 13 Timing argument M31 (Andromeda) is now approaching the Galaxy at 118 km s-1. Its distance is about 750 kpc. Assuming their initial separation was small and the age of the universe is say 18 Gyr, we can estimate a lower limit on the total mass of the Andromeda + Galaxy system. The Galaxy's share of this mass is (13 ± 2) x 1011 solar masses. A similar argument using the Leo I dwarf at a distance of about 230 kpc gives (12 ± 2) x 1011 solar masses. 14 The relation for the mass of the galactic halo M(r) = r (kpc) x 1010 solar masses out to r ~ 50 kpc then indicates that the dark halo extends out beyond r = 120 kpc if the rotation curve remains flat ie if ρ(r) ~ r -2 and possibly much further than 120 kpc if the density distribution declines more rapidly at large radius 15 This radius is much larger than the extent of any directly measured rotation curves, so this ªtiming argumentº gives a realistic lower limit on the total mass of the dark halo. For our Galaxy, the luminous mass (disk + bulge) is about 6 x 1010 solar masses The luminosity is about 2 x 1010 solar luminosities The ratio of total dark mass to stellar mass is then at least 120/6 = 20 and the total M/L ratio is at least 60 16 Satellites of disk galaxies can also be used to estimate the total mass and extent of the dark halos of other bright spirals Individual galaxies have only a few observable satellites each, but we can make a super-system by combining observations of many satellite systems and so get a measure of the mass of a typical dark halo. 17 Velocities |∆V| of 3000 satellites relative to their parent galaxy error bars show the velocity dispersion decreasing with radius out to ~ 300 kpc ! 18 With a careful treatment of interlopers, they find that the velocity dispersion of the super-satellite-system decreases slowly with radius The halos typically extend out to about 300 kpc but the density distribution at large radius is steeper than the isothermal: ρ(r) ~ r -3, like most cosmological models including NFW The total M/L ratios are typically 100-150, compared with the lower limit from the timing argument of 60 for our Galaxy. (The Prada galaxies are bright systems, comparable to the Galaxy) 19 M31 has a similar rotation amplitude so its total mass may be similar to the total mass of the Galaxy. Evans & Wilkinson (2000) used satellites and GCs +3.6 12 in M31 to derive a lower mass of 1.2 -1.7 x 10 M for M31 - similar to the Galaxy, within the uncertainties So the total mass of MW + M31 ~ 3 x 1012 M For comparison, from least action arguments, the likely mass of the local group is 4-8 x 1012 M Within the uncertainties, most of the mass in the Local Grou20 p could be in the two large spirals Conclusion The total mass of the Milky Way is ~ 1.5 x 1012 M The MW is one of the few galaxies for which we have an estimate of the total mass, rather than just the mass out to the end of a rotation curve. The stellar mass is about 6 x 1010 M So the stellar baryons are only about 4% of the total mass Ω Ω Compare this with the universal baryon / matter = 15% 21 GALAXIES 626 The satellite galaxies in the Local group can also be used to look at the issue of the dark matter halo substructure 23 Dark Halo Substructure In simulations of galaxy formation, the virialized halos are quite lumpy, with a lot of substructure - a lot more satellites and dwarf galaxies than observed. From simulations, we would expect a galaxy like the Milky Way to have ~ 500 satellites with bound masses > 108 M . These are not seen optically or in HI. What is wrong ? Could be a large number of baryon-depleted dark satellites, or some problem with details of CDM or could we missing lots of faint satellites? 24 The 21 known satellites of the MW - some discovered recently 25 Planar distribution! In dissipationless simulation, satellites are preferentially aligned along the major axes of the host©s triaxial mass distribution. Consistent with planarity of the MW satellite system, if major axis of the MW mass distribution is ~ perpendicular to the disk. The anisotropy is associated partly with accretion of satellites along filaments and partly due to evolution of satellites in the triaxial potential. Similar planar distribution for the early-type satellites around M31 26 Abundance predicted for CDM sub-halos vs. observed for Milky Way dwarfs 27 If the satellites are there, why haven't we found them? Very low surface brightness Accretion events High foreground extinction Unknown orbits for distant Local Group candidates Scarcity of stars¼ If the satellites are there, why haven't we found them? Found via data mining techniques mv=14.4, Mv=-10.1 DMW=775±50 kpc, DLG=615±40 kpc Fe/H = -1.9±0.2 If the satellites are there, why haven't we found them? Very low surface brightness Accretion events High foreground extinction (like ZOA) Unknown orbits for distant Local Group candidates Scarcity of stars¼ If the satellites are there, why haven't we found them? Canis Major Dwarf (?) Martin et al 2004 Evidence for Mergers & Accretion If primary mechanism for growth of large galaxies is accretion of low-mass dwarfs, we should find evidence of present-day merger events in the Local Group. Evidence for Mergers & Accretion On-going Accretion: · Sagittarius dSph galaxy · Metal-rich giants in M31 halo Stellar Overdensities: · Monoceros feature (possible tail connected to Canis Major overdensity) · Triangulum-Andromeda (possible tail of more distant dwarf) Twists & Distortions: · Ursa Minor shows distorted, S-shaped surf density profile If the satellites are there, why haven't we found them? Very low surface brightness Accretion events High foreground extinction (like ZOA) Unknown orbits for distant Local Group candidates Scarcity of stars¼ If the satellites are there, why haven't we found them? ● IInnccreased extinctnction and stellar foregroround near the galactic planeane limit our ability to dedetect dwarf galaxies there.