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Satellite Galaxies
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1. DGs around the Milky Way
The Magellanic Clouds: LMC SS2019and SMC 2
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1.1. The Magellanic System
On the southern sky 2 large diffuse and faint patches are optically visible: SS2019 3 The Magellanic Clouds
Small Magellanic Cloud (SMC);SS2019 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
SS2019 6 HI withl21cm
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(J. van Loon)
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(J. van Loon)
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(J. van Loon)
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1.3. The LMC, a gas-rich Dwarf Irregular Galaxy (dIrr)
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(J. van Loon)
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Panchromatic picture (J. van Loon)
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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
SS2019 14 Hot Supernova Gas: X-ray
(J. van Loon)
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8.8. 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 SS2019 17 Chapt.)]
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1.4. The Magellanic System
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Besla et al. (2016) ApJ, 825
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The Magellanic Stream
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1.5. Modeling the Magellanic System (2012) ApJ, 421
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Besla et al. (2012) ApJ, 421
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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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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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1.6. Gas-free Dwarf Galaxies: dSphs
Fornax dSph D= 138 kpc
Mv = -13.5 SS2019 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 SS2019 28
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dE‘s in the vicinity of the Milky Way populate the low-mass end and are named Dwarf Spheroidals: Their surface brightness is only slightly larger than the background; almost no gas
LeoI
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Ursa Minor dSph
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Leo I: D = 250 kpc
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1.7. 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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Correlations of different galaxy types
Tolstoy et al. (2010) ARAA, 47
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2. The MWG Satellites
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SS2019 38 Grebel, 1998
2.1. The 12 major MWG satellites
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2.2. Their Location
Marla Geha SS2019 42
Spatial Distribution of MW satellites
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Holtzman et al. (2006) ApJS, 166 SS2019 46
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2.3. The Population Box
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with courtesy by SS2019 48 Eva Grebel SF continues also through the re-ion.epoch
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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 ionizationSS2019 inhomogeneity in the Universe!49
2.4. More distant satellites
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Gas-poor Dwarf Galaxies in the Local Group
SS2019 Antlia51 DG
2.5. 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 SS2019 and dSphs(?). 52
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Carina II dIrr
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IC 10, dIrr, Dist.: 1400 SS2019kpc, V = 10.3m 55
Leo A in the solar vicinity
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2.6. 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 SS2019 57 (see also Bouchard et al. 2003, 2006)
Gas displacement by tidal and ram-pressure effects
Carignan 1999
HI gas displaced of Phoenix Welsh et al. (1998) SSGas2019 infall in NGC 205 enhances58 SF
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HI Environment?
Carignan et al. (1998)
Bouchard et al. (2003) AJ, 126
Gas clouds around Sculptor dSphs from expulsion? SS2019 59
The Satellites’ gas content
Grcevich & Putman, 09, ApJ, 696
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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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NGC 147 2MASS 3. 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
4. Detection of the Sagittarius Satellite Galaxy
If no bright star-forming regions exist, the stellar component of satellite
The Sagittarius Dwarf Galaxy SS2019 galaxies is hardly detectible66 at D 24 kpc due to their low brightness.
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4.1.
Southwards of the MWG center a sample of stars was detected at a distance of 24 kpc due to their collective kinematics: Sgr I Dwarf Galaxy SS2019 67
SS2019 68
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4.2. Satellite Accretion
SS2019 70
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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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The tidal stream of Sgr I is detected from enhanced star density. SS2019 72
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The tidal stream of Sgr DG is detected from enhanced star density.
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4.3. 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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4.4. 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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4.5. Search for MW tidal streamers
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Belokurov et al. (2007) ApJ, 658
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Belokurov et al. (2007) ApJ, 658
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The Aquarius stellar stream
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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 (:=SS 2019tidal radius) 85 that facilitates tidal stripping.
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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 approachsSS2019 the MW disk. 86
Metz et al. (2008) ApJ, 680 SS2019 89
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5.2. 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 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?SS2019 93
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5.3. Gas stripping
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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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4 major questions:
1. Is the Milky Way built-up of dSphs?
2. How did the dSphs evolve?
3. How did the dSphs gained their gas?
4. What does the dSphs distribution tell us?
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6. Halo Stars by Satellite Accretion
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 clustersSS2019 of different ages 99 light blue crosses, Galactic globular cluster measurements
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6.2. 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. SS2019 102
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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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. SS2019 105
Tolstoy & Venn, 2004 Koch (2009) Rev. Mod. Astron.
Conclusion: dSph stars doSS not2019 match the MW halo stars106!!
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Tolstoy et al. (2009) ARAA
At low Z dSphs coincide with halo star abundance ratios
SS2019 108
Tolstoy & Venn, 2004
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Kirby et al. 2008, ApJ,685 SS2019 110
The metallicity distributions of dwarf galaxies evolve with luminosity
SS2019 EK et al., ApJ, submitted 112/42
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6.3. Radial gradients in [Fe/H]
EK et al., ApJ, submitted SS2019
113/42
Walker et al.
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SS2019 115
, 2004
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Kirby et al. 2008, ApJ,685
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(2009) A&A 500
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7. 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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7.1. 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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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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