galaxies 626 - institute for astronomycowie/ast626_dir/gal_lec23.pdfdark matter in the big spiral...

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

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The Local Group is a fairly typical weak group so very characteristic of the

environments in which many galaxies live

Local Group Substructure

•Sparse group with zerovelocity radius of 1 Mpc

•M31 approaching at 120 km/s

•Reliable orbits unknown

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• about 3540 member galaxies• Milky Way and Andromeda subgroups

The Local Group of Galaxies

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The least luminous galaxies known are in the local group

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Local Group LSB galaxy in nearIR.

HI map has 20 pc resolution

NGC 6822

Milky Way & M31 SatellitesDist. (kpc)TypeGalaxy

27dSphSagDEG89dSphDraco74dSphUrsa Minor230dSphLeo II98dSphSextans276dE3Leo I767Ibm VLeo A7.7dIrrCanis Major110dSphCarina55SB(s)mLMC153dSphFornax490dIrrPhoenix92dSphSculptor64SB(s)mSMC

Dist. (kpc)TypeGalaxy

858dSphAnd VI797dSphAnd VII889dSphAnd II889dSphAnd V889dSphAnd I889E2M32828dSphAnd VIII889E5M110705dE3NGC 185889dSphAnd III735dE5NGC 147

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 well as the Local GroupGrebel 2005

Do these trends result from morphological transformations due to the influence of the massive primary galaxy? (i.e. tidal or ram pressure stripping)

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Dark Matter in M31 and the Milky Way

The first thing we would like to do is see if there is moredark matter in the big spiral galaxies than we are inferringfrom the rotation curves.

That is: do the dark halos extend beyond the maximumradius at which we can measure the rotation curve?

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How large and massive are the dark halos of large spirals like the Milky Way ?

Flat rotation curves => M(r) ~ r, like the isothermalsphere : ρ ~ r2

This cannot go on for ever the halo mass would be infinite.Halos must have a finite extent and mass, and their densitydistribution must truncate or be steeper than ρ ~ r3 at very large radius

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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 ?

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118 km s 1

M31

Milky Way

The stellar mass is about 6 x 1010 M_sun, so the ratio of dark to stellar mass is ~ 20

M31 and the Milky Way are nowapproaching at 118 km s 1. Theirseparation 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 dark halo extends out to ~ 150 kpc, far beyond the disk's radius of ~ 20 kpc

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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 10 11 solarmasses.

A similar argument using the Leo I dwarf at a distance of about230 kpc gives (12 2) x 10 11 solar masses.

Timing argument

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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

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This radius is much larger than the extent of any directly measured rotation curves, so this

“timing argument” gives a realistic lower limit onthe 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

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Satellites of disk galaxies can also be used to estimatethe total mass and extent of the dark halos of otherbright spirals

Individual galaxies have only a few observable satellites each, but we can make a supersystem by combining observations ofmany satellite systems and so get a measure of the mass of atypical dark halo.

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Velocities |∆V| of 3000 satellites relative to their

parent galaxy

error bars show the velocity dispersion decreasing withradius out to ~ 300 kpc !

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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 steeperthan the isothermal: ρ(r) ~ r 3, like most cosmologicalmodels including NFW

The total M/L ratios are typically 100150, compared with the lower limit from the timing argument of 60 forour Galaxy. (The Prada galaxies are bright systems, comparable to the Galaxy)

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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 Groupcould be in the two large spirals

M31 has a similar rotation amplitude so its total massmay be similar to the total mass of the Galaxy.

Evans & Wilkinson (2000) used satellites and GCs in M31 to derive a lower mass of 1.2+3.6

1.7 x 1012 M for M31 similar to the Galaxy, within the uncertainties

So the total mass of MW + M31 ~ 3 x 1012 M

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Conclusion

The total mass of the Milky Way is ~ 1.5 x 1012 MThe MW is one of the few galaxies for which we have anestimate of the total mass, rather than just the mass outto 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

Compare this with the universal Ωbaryon / Ωmatter = 15%

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The satellite galaxies in the Local group can also be used to look at the issue of the dark

matter halo substructure

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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 lotsof faint satellites?

Dark Halo Substructure

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The 21 known satellites of the MW some discovered recently

Planar distribution!

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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 satellitesin the triaxial potential.

Similar planar distribution for the earlytype satellites around M31

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Abundance predicted for CDM subhalos vs. observed for Milky Way dwarfs

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…


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


Very low surface brightness Accretion events High foreground extinction (like ZOA) Unknown orbits for distant Local Group



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 & AccretionOngoing 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


Willman et al 2004

Increased extinction and stellar foreground near the galactic plane limit our ability to detect dwarf galaxies there.

Implies an expected total of 18±4 galaxies with properties similar to the known galaxies (~33% incompleteness).

Increased extinction and stellar foreground near the galactic plane limit our ability to detect dwarf galaxies there.

Implies an expected total of 18±4 galaxies with properties similar to the known galaxies (~33% incompleteness).

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Ultrafaint galaxies are now found indata bases like Sloan....

These are so faint that their properties begin to look intermediate between globular

clusters and dwarf galaxies

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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 subhaloes 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.

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What can we tell from the galaxies themselves?

We can study these in great detail

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

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Dwarf spheroidal galaxies

Faint satellites of our GalaxyMV down to 8Very low surface brightnessTotal 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

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

Even isolated dSphs like Cetus & Tucana sustain gas loss

Cetus dSph

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Velocity dispersion of the Fornax dSph galaxy approximatelyconstant with radius. Fornax is the brightest ofthe galactic dSph galaxies: its M/LV ≈ 10 (expect M/LV = 2from its stellar content alone)

expected for constant M/L

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M/L ratios for dSph galaxies. Some have M/L > 100. The curve is for a luminous component with M/L = 5 plus a halo with M = 2.5 x 107 M .

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The lack of tidal extensions in Dra, Sex, Scl and UMifound by many authors supports the view that thedSph galaxies are immersed in large extended darkhalos with masses ~ 109 M .

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Stellar surface density profile for Draco Steep gradient where velocity dispersion falls

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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 largerat z > 2, but suffer tidal stripping in the hierarchical mergingprocess. The MW dSph formed in such objectswith M > 109 M , so were able to build up some stellar mass and survive reionization despite their present shallowpotential wells ....

This would make some concrete predictions about the age of thestars..

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

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End

galaxies 626 - institute for astronomycowie/ast626_dir/gal_lec23.pdfdark matter in the big spiral...

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