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 35­40 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 near­IR.

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

 Gas­poor, low­mass dwarfs (dSphs) tend to cluster around massive galaxies (Cetus & Tucana are exceptions)  Gas­rich, high­mass 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 ~ r­2

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 ~ r­3 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 ~ r­2 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 s­1. 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 “timing argument” 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 super­system 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 super­satellite­system 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 100­150, 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 4­8 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 baryon­depleted 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 early­type satellites around M31

26 Abundance predicted for CDM sub­halos 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 stars… 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 stars… 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 low­mass dwarfs, we should find evidence of present­day merger events in the Local Group. Evidence for Mergers & Accretion On­going Accretion: • Sagittarius dSph galaxy • Metal­rich giants in M31 halo

Stellar Overdensities: • Monoceros feature (possible tail connected to Canis Major overdensity) • Triangulum­Andromeda (possible tail of more distant dwarf)

Twists & Distortions: • Ursa Minor shows distorted, S­shaped 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 stars… If the satellites are there, why haven’t we found them?

● IInnccreased e extinctnction aand stellar foregroround near the galactic planeane limit o our ability to o dedetect ddwarf galaxies there. ● IImmplies aan expexpected total ofof 18±4 galaxies with prrooppertrties s similarar tto the known galaxiess ((~3333% incompletenesss)). Willman et al 2004 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 stars… GALAXIES 626

Ultra­faint galaxies are now found in data bases like Sloan....

These are so faint that their properties begin to look intermediate between globular clusters and dwarf galaxies 38 GALAXIES 626

None of this brings the numbers up enough so probable really that many of the dark halos don't have much star formation.... Possible Star Formation Quenching in Sub­Halos

 Reionization: The gas in the early universe becomes very hot and difficult for sub­haloes to accrete after reionization occurs. Thus sub­haloes formed after reionization cannot form stars as easily as those sub­ haloes that formed before (if they can form stars at all).  Tidal Disruption & Heating: When sub­haloes get too close to their parent galaxy they can be stretched or even cannibalized and suffer an increase in temperature.  Feedback from Star Formation: Supernovae eject large amounts of gas, and given a small enough sub­halo, this ejection could have a significant effect on its evolution. GALAXIES 626

What can we tell from the galaxies themselves? We can study these in great detail

Most of the local group galaxies are dwarf spheroidals.... Dwarf spheroidal galaxies

Faint satellites of our Galaxy

MV down to ­8 Very low surface brightness Total masses ~ 107 solar masses

Radial velocities of individual stars in several of these dSph galaxies show that their M/L ratios can be very high: the fainter ones have M/L ratios > 100

42 Is there gas in the known dwarves?

 For many dSphs only upper limits for neutral & ionized gas can be determined  Even those limits lie well below amounts expected from gas loss from old red giants in the dSphs  Fornax, only dSph w/ star formation as recent as ~200 Myr ago also devoid of gas Cetus dSph  Even isolated dSphs like Cetus & Tucana sustain gas loss

expected for constant M/L

Velocity dispersion of the Fornax dSph galaxy ­ approximately constant with radius. Fornax is the brightest of ≈ the galactic dSph galaxies: its M/LV 10 (expect M/LV = 2 from its stellar content alone) 44 M/L ratios for dSph galaxies. Some have M/L > 100. The curve is for a luminous component with M/L = 5 45 plus a halo with M = 2.5 x 107 M . The lack of tidal extensions in Dra, Sex, Scl and UMi found by many authors supports the view that the dSph galaxies are immersed in large extended dark halos with masses ~ 109 M .

46 Stellar surface density profile for Draco

Steep gradient where velocity dispersion falls 47 Why do most of the subhalos not have stars but some do?

One possibility is that about 10% of halos that are small now (Vc < 30 km/s) were much larger at z > 2, but suffer tidal stripping in the hierarchical merging process. The MW dSph formed in such objects with M > 109 M , so were able to build up some stellar mass and survive reionization despite their present shallow potential wells ....

This would make some concrete predictions about the age of the stars.. 48 SF Histories

 If dwarfs are building blocks for more massive galaxies, then old stellar populations in both should have similar properties. Also, oldest stars in massive galaxies must be as old as or younger than the oldest stars in dwarf galaxies.  If cosmic reionization squelches star formation due to heating and gas­loss (as many cold dark matter models predict), then we should see a slowing of star­forming activity in the star formation histories of these dwarf galaxies. Determining the Age of Old Stellar Populations

 Most age­sensitive feature is the main sequence turn­off  Need photometry reaching at least 2 magnitudes below the turn­off and enough stars to produce a measurable turn­off (Population II stars only)  Despite drawbacks, internally ages are accurate to within 1 Gyr  Absolute ages are harder because they require isochrones (globular clusters) within galaxies

• Not a single dwarf galaxy lacks an old population although how dominant that population varies. • Evidence for a common episode of star formation (ancient Pop II stars found in Galactic halo and “galactic dSphs” to be same age within 1 Gyr). • Even the least massive dSphs show evidence of some kind of continuous star formation (but with decreasing intensity) over several Gyr ­ no cessation of star formation during or after reionization. • No two histories look the same. Summary

 Nearly all the dwarfs have ongoing star formation continuing well after the reionization epoch  No two dwarfs have the same star formation history despite similar masses.  Suggests star formation history is very much a function of what happens to the individual galaxy GALAXIES 626

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