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I. Satellite Galaxies
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1. DGs around the Milky Way
The Magellanic Clouds: LMC and SMC 2
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1.1. The Magellanic System
On the southern sky 2 large diffuse and faint patches are optically visible: 3 The Magellanic Clouds
Small Magellanic Cloud (SMC); dIrr; dist.: ~ 58 kpc 4
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Large Magellanic Cloud (LMC), dIrr; dist.: ~ 58 kpc
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optical : stars + lumin. gas 1.2. The many faces of the LMC
Star-forming Regions: H
6 HI withl21cm
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(J. van Loon)
4/29/2018 Cosmic Matter Circuit 8
(J. van Loon)
4/29/2018 Cosmic Matter Circuit 9
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(J. van Loon)
4/29/2018 Cosmic Matter Circuit 10
1.3. The LMC, an gas-rich Dwarf Irregular Gakaxy (dIrr)
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(J. van Loon)
4/29/2018 Cosmic Matter Circuit 12
Panchromatic picture (J. van Loon)
4/29/2018 Cosmic Matter Circuit 13
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The Magellanic Clouds
Computer model of a small satellite galaxy orbiting a larger (edge-on) disk galaxy. As the satellite orbits, stars are stripped from the satellite and orbit in the halo of the larger galaxy. (Kathryn Johnston, Wesleyan): see the bending and tumbling of the satellite‘s figure axis!
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Further evidence for ram pressure: at its front-side LMC gas is compressed leading to molecular cloud formation triggered star formation
Star-forming regions: H
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(J. van Loon)
4/29/2018 Cosmic Matter Circuit 19
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Small Magellanic Cloud (SMC); dIrr; dist.: ~ 58 kpc 20
1.4. More distant dIrrs
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Carina II dIrr
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IC 10, dIrr, Dist.: 4200 kly, Local Group Cloud 24
Leo A in the solar vicinity
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1.5. Gas-free Dwarf Galaxies: dSphs
Fornax dSph D= 138 kpc
Mv = -13.5 27
Faint dSphs pure stellar systems, no gas,
metal-poor: Z< Z, faint end of dwarf Es, m extremely faint: Mv>-8 , very small: ~ few kpc, close to the MWG
Leo I with Regulus = Leo 28
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Ursa Minor dSph
Leo I: D = 250 kpc
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1.6. Intrinsic properties of dSphs
van den Bergh (2008)
dSphs are less
Mv = 16.2 – 14.26 log Rh concentrated and flatter than dSphs
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1.7. Spatial Distribution of MW satellites
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The MW Satellites
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Marla Geha 38
39 Grebel, 1998
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NGC 147 2MASS 2. The M31 system
NGC 205
NGC 185 BVR
NGC 221
Star-formation regions in NGC 185 and NGC 205 of similar size as in dIrrs
M32
NGC 205
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And VII And VI
3. Stellar populations
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The chart above demonstrates the previous conclusions by showing the abundances of alpha elements in dSphs versus solar metallicity. The symbols are as follows: blue triangles, Carina blue triangles plus circles, Leo I red triangles, Sculptor red triangles plus circles, Fornax green triangles, Draco, Ursa Minor, and Sextans from SCS01 black crosses, Glactic disk stars open squares, halo data from McWilliam et al. 1995 light blue stars, UVES data from a study of LMC star clusters of different ages 60 light blue crosses, Galactic globular cluster measurements
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4.1. SF timescale from element
Assumptions: abundances closed box,
constant Yields yi O+Fe from SNeII of massive stars, Fe by SN Ia from WD-WD or WD-RG slow evolution fast gas consumption Effects: The ratio of element abundances from particular precursor stars allow the age dating of their lifetimes and the derivation of the
gas consumption. 63
SNeII of massive stars produce a constant ratio [O/Fe]0.5, while Fe increases continuously. After the typical formation timescale of SN Ia Fe is further enhanced independently of the O enrichment. Thus, O/Fe decreases. From the age of the disk, the SN Ia timescale must be of the same order.
Tolstoy & Venn, 2003
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4.2. Radial gradients in [Fe/H]
EK et al., ApJ, submitted
67/42
Walker et al.
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, 2004
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The metallicity distributions of dwarf galaxies evolve with luminosity.
EK et al., ApJ
Kirby et al. 2008, ApJ,685
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Kirby et al. 2008, ApJ,685
4.3. The Population Box
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with courtesy by Eva Grebel SF continues also through the re-ion.epoch
dIrrs of the MW show stronger and more continuous star formation with an increase of Z. Phoenix is in a stage of morphological transition.
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(2009) A&A 500
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4.4. Gas in dSph‘s: almost gas free, but gas infall!
Carignan et al. (1988) HI gas outside Sculptor dSph, Welsh et al. (1998) flocculent HVCs gas infall in NGC 205 enhances SF (see also Bouchard et al. 2003, 2006)
Satellites with gas
Sculptur (Carignan 1996) Leo A Dwarf Galaxies (low masses) can easily expel all(?) their gas into their intergalactic environment. Gas is stripped off by tidal and dynamical drag, by this, transforming dIrrs into dEs and dSphs(?). 80
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HI Environment?
Carignan et al. (1998)
Bouchard et al. (2003) AJ, 126
Gas clouds around Sculptor dSphs from expulsion?
Grcevich & Putman, 09, ApJ, 696
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Gas-poor Dwarf Galaxies in the Local Group
Antlia83 DG
5. Detection of the Sagittarius Satellite Galaxy
If no bright star-forming regions exist, the stellar component of satellite
The Sagittarius Dwarf Galaxy galaxies is hardly detectible84 at D 24 kpc due to their low brightness.
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Southwards of the MWG center a sample of stars was detected at a dist. of 24 kpc due to their collective kinematics:
SagDIG Dwarf Galaxy 85
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Belokurov et al. (2007) ApJ, 658
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Belokurov et al. (2007) ApJ, 658
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Search for tidal streamers.
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6. Satellite Accretion
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Computer model of a small satellite galaxy orbiting a larger (edge-on) disk galaxy. As the satellite orbits, stars are stripped from the satellite and orbit in the halo of the larger galaxy. (Kathryn Johnston): see the bending and tumbling of the satellite‘s figure axis!
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Satellites on elliptical orbits experience
1) stretching along the trajectory on their approach to perigalacticum and crushing along the orbit towards apogalacticum because of velocity differences between leading anf trailing part, 2) radial stretching due to the tidal force of the mature galaxy,
3) by this a revolving gravitational potential along the orbit (due to tidal torque),
4) an oscillating equipotential (:= tidal radius) 95 that facilitates tidal stripping.
6.1. Tidal Force
Bodies that are extended over d and located at distance D in the central gravitational field of any mass M experience a Tidal Force
d Ftide GM 3 D This results in a mode-2 deformation in radial direction towards and away from the center of mass.
The detection of the leading arm confirms the tidal stripping effect. The stripped gas approachs the MW disk. 96
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7. The Magellanic Stream Gas bridges between the Magellanic Clouds are formed (Magellanic Stream), showing that this complex is tidally disrupted by the Milky Way. Stripped-off gas drops down to the Galactic disk and feeds the MWG.
[Dwarf satellite galaxies are swallowed by larger parent galaxies (see next 97 Chapt.)]
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The tidal stream of Sgr I is detected from enhanced star density. 99
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The tidal stream of Sgr DG is detected from enhanced star density.
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8. The Canis Major Satellite Galaxy Canis Major DG discovered recently: 17/11/03, close to the galactic plane at 7.5 kpc distance
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The Canis Major tidal Stream
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8.2. Tidal Streams Recent sensitive observations of galaxy halos have revealed tidal streamers of satellite galaxies under disruption in a few of them. In M31 HVCs accumulate along the tidal path.
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Tidal streams around NGC 5907
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Search for tidal streamers.
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The Aquarius stellar stream
CDM models of Galaxy Stellar Halos
Cosmological models based on CDM predict many accretion events through lifetime of a big galaxy. Infalling satellites are torn apart by tidal forces.
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8.3. Are halo stars witnesses of accretion?
The accretion of satellite galaxies by host galaxies like the Milky Way are observed by 1. Satellite infall 2. Tidally disrupted stellar streams
Most halo stars do not agree with Z of the present-day dSphs, but some could be the remnants of accretion. 110
Tolstoy & Venn, 2004 Koch (2009) Rev. Mod. Astron.
Conclusion: dSph stars do not match the MW halo stars!!
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Tolstoy et al. (2009) ARAA
At low Z dSphs coincide with halo star abundance ratios
Tolstoy & Venn, 2004
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8.4. Gas stripping
Ram-pressure stripping by the ram-pressure stripping 2 motion through hot halo gas if Pram = IGM·v rel > P0(r)
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Gas displacement by tidal and ram-pressure effects
Carignan 1999
HI gas displaced of Phoenix Welsh et al. (1998) Gas infall in NGC 205 enhances117 SF
8.5. Cosmological implications
Satellite galaxies move in the tidal field and the halo gas of mature galaxies.
Cosmological models predict numerous satellite galaxies around Hubble types.
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The Disk of Satellites see e.g. : Metz et al. (2009) ApJ, 697
Proposed solution: Group infall but too improbable (Knebe et al. 2008, MN, 388)
Kroupa et al. (2010) A&A, 523
The evolution for 2 Gyrs
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Petrov & Hensler (2011) in prep.
Interactions of Sat.s important; e.g. Satellites merge; Gas is removed from the Satellites And contributes to the Galactic halo gas: Small subhalos survive, but without gas! Large Satellites are tidally stretched and partly disrupted dSphs merge: Mass spectrum?
The MW dSphs‘ distribution Stadel et al. (2008)
Jerjen (2008)
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8.5.1. The Disk of Satellites see e.g. : Metz et al. (2009) ApJ, 697
Proposed solution: Group infall but too improbable (Knebe et al. 2008, MN, 388)
Kroupa et al. (2010) A&A, 523
8.5.2. Dark Matter content of dSphs
Assumptions: virial equil. + spherical symmetry + isotropic veloc. disp.
Ekin = - Etot
2 = G M/R log (M/L) = 2.5 + 107/(L/L ) tot L = R2 I M 1 2 L G R I
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8.5.3. BUT!!
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The elongated shape of the Her dSph
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The case of Draco
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UMi
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Grebel & Gallagher (2004) ApJ, 610
Most of the star formation in dSphs continued through the era of re- ionization: evidence for ionization inhomogeneity in the Universe!133
8.5.4. Conclusions The DM content is doubtful, because of Elongated shapes S shapes of tidal stripping Substructures Probable anisotropic velocity dispersions low star-formation rates but gas loss by galactic wind + stripping challenging CDM cosmology, because of low DM orbits in a single plane Solution: formation scenario ??
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