Download Lecture 24, The local group

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
•Sparse group with zero­
velocity radius of 1 Mpc
Local Group Substructure
•M31 approaching at 120 km/s
•Reliable orbits unknown
The Local Group of Galaxies
• about 35­40 member galaxies
• Milky Way and Andromeda subgroups
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The least luminous galaxies known are in the local group
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NGC 6822
Local Group LSB galaxy in near­IR.
HI map has 20 pc
resolution
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Milky Way & M31 Satellites
Galaxy
Type
Dist. (kpc)
SMC
SB(s)m
64
Sculptor
dSph
92
Phoenix
dIrr
490
Fornax
dSph
153
LMC
SB(s)m
55
Carina
dSph
110
Canis Major
dIrr
7.7
Leo A
Ibm V
767
Leo I
dE3
276
Sextans
dSph
98
Leo II
dSph
230
Ursa Minor
dSph
74
Draco
dSph
89
SagDEG
dSph
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Galaxy
Type
Dist. (kpc)
NGC 147
dE5
735
And III
dSph
889
NGC 185
dE3
705
M110
E5
889
And VIII
dSph
828
M32
E2
889
And I
dSph
889
And V
dSph
889
And II
dSph
889
And VII
dSph
797
And VI
dSph
858
Morphological Segregation
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Grebel 2005
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 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?
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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 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
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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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M31
M31 and the Milky Way are now
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 of dark to stellar mass is ~ 20
118 km s ­1
Milky Way
The dark halo extends out to ~ 150 kpc, far beyond the disk's radius of ~ 20 kpc
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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.
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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 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
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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.
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Velocities |∆V| of 3000 satellites relative to their
parent galaxy
error bars show the velocity dispersion decreasing with
radius 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 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)
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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 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
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 Group
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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
Compare this with the universal Ωbaryon / Ωmatter = 15% 21
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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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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
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of faint satellites?
The 21 known satellites of the MW ­ some discovered recently
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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
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Abundance predicted for CDM sub­halos vs. observed for Milky Way dwarfs
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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…
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If the satellites are there, why haven’t we found them?
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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…
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If the satellites are there, why haven’t we found them?
Canis Major Dwarf (?)
Martin et al 2004
Evidence for Mergers & Accretion
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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…
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If the satellites are there, why haven’t we found them?
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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). 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…
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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
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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
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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
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Is there gas in the known dwarves?
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Cetus dSph
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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
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)
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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 UMi
found by many authors supports the view that the
dSph galaxies are immersed in large extended dark
halos with masses ~ 109 M .
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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..
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SF Histories
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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
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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
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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
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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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