formation of satellites and rings in local group and ... -...
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Formation of Satellites and Rings in Local Group and Elsewhere
Hongsheng Zhao Univ. of St Andrews, [email protected]
Empirical start of Newtonian/Einstein gravity
ircamera.arizona.as.edu Sirius A and B orbiting each other, from 1900 through 1970 (animated by G. Rieke)
Study of Empirical Keplerian law leads to Newtonian/Einstein gravity
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a3 / P2 ~ GM constant for all planets
q What is the “Kepler-law” in galaxies?
(Modified) Kepler-law seen in galaxy rotation
Baryonic Tully-Fisher relation: Log Mb = 4 log V – log β Zero-point defines an acceleration constant a0 ≈ V4/(GMb) ≈ 10-10 m/s2
Such that β=Ga0
McGaugh (2005, 2011) Famaey & McGaugh (2012)
Polar rings
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Tidal dwarf galaxies
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Duc et al.
Dwarf galaxies on tidal rings
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Duc et al.
Satellites have high dynamical mass large M/L
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No dwarfs form by DM recollapse
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Lumps in Gas-rich mergers
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All Dwarfs have high M/L
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Back to the Milky Way
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Most satellites has 10 Gyrs old populations.
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Satellite galaxies have young-ish stars
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Tolstoy et al. 2009 ARAA
Grebel 0005296
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Knapp et al. 1978), and subsequent VLA synthesis studiescovering much of the optical extent of some of the dSphsdid not reveal H i in emission or in absorption down tocolumn densities of 1017 – 1018 cm−2 (Young 1999; 2000).
The apparent absence of gas in dSphs is an unsolvedpuzzle. Simulations by Mac Low & Ferrara (1999) indicatethat gas loss through starbursts becomes efficient only atmasses < 10−6, an order of magnitude less than the typicalmass of a dSph. The observed upper H i limits are lowerthan even what is expected from evolutionary mass lossfrom red giants in dSphs. The lack of gas is particularlypuzzling in dSphs and dEs with pronounced intermediate-age populations (Carina: 3 Gyr, Hurley-Keller et al. 1998;Leo I: 2 Gyr, Gallart et al. 1999; NGC 147, Han et al.1997), or very recent star formation (Fornax: ∼ 200 Myr,Grebel & Stetson 1999). However, Carignan (1998) andCarignan (1999) found extended gas lobes around the Sculp-tor dSph, whose radial velocities are similar to the stellarradial velocity of this galaxy. A recent re-investigation ofthe Leiden-Dwingeloo survey led to the detection of simi-lar gas concentrations with matching radial velocities out-side the optical boundaries of several other dSphs and tonon-detections for others (Blitz & Robishaw 2000). Theseauthors suggest that tidal effects are the most likely agentfor the displacement of the gas.
8. Star Formation in Local Group Dwarfs
As illustrated in earlier reviews (Grebel 1997, 1998, Ma-teo 1998, Grebel 1999) dwarf galaxies vary widely in theirstar formation histories, age distribution, and enrichmenthistory. No two dwarfs are alike even within the same mor-phological type. In spite of this diversity, galaxy mass andproximity to a massive spiral appear to play a definingrole in dwarf galaxy evolution.
8.1. Modes of star formation
The following modes of star formation are representedamong LG dwarfs:
– Continuous star formation with a constant or varyingstar formation rate over a Hubble time and gradualenrichment (see also Hunter 1997). Examples for thismode include irregulars and dIrrs such as the Mag-ellanic Clouds, which are massive enough to have asufficiently large gas supply and to hold on to gas andmetals.
– Continuous star formation with decreasing star forma-tion rate that ceases eventually. A good example is theFornax dSph (e.g., Grebel & Stetson 1999). This modemay be dominant in low-mass dIrrs and transition-type galaxies as well. External effects such as tidal orram-pressure stripping may have contributed to thegradual loss of star-forming material in these galax-ies. Extreme cases at the low-mass end are two ofthe closest and least massive Milky Way dSph com-
panions, Draco and Ursa Minor, which are dominatedby ancient populations (see also Section 5). However,the abundance spread found in Draco (Shetrone et al.1998) and the presence of a few Carbon stars indicatesthat the early star formation episodes must have beenfairly extended.
– Distinct star formation episodes separated by Gyr-longperiods of quiescence. So far only one example of thismode is known, the Carina dSph (e.g., Hurley-Kelleret al. 1998). It is unclear what caused the gaps and thesubsequent onset of star formation. One possible expla-nation is the episodic scenario proposed by Lin & Mur-ray 1999: star formation in a dwarf heats and dispersesthe gas, which remains bound and eventually cools andcontracts during the apogalactic passages of the dwarfaround the massive parent. According to their simula-tions this leads to star formation separated by gaps ofa few Gyr. — The distinct episodes of star formation inCarina seem to have proceeded without enrichment, apossible indication of star formation through accretionor else metal loss as described by Mac Low & Ferrara(1999).
Our present lack of knowledge of dwarf galaxy orbitsmakes it impossible to evaluate the impact of past closeencounters and interactions on dwarf galaxy star forma-tion histories.
8.2. Evolution from dIrrs to dSphs
in age and metallicitySpatial variations No HI detectionSome evidence for
No HI detection
HI cloud unassociated?
No HI detectionsingle metallicity
HI in surroundings?Spatial variations
radial variations
Basically single age,
possible radial gradientin age and metallicity;Spatial variations
HI in surroundings? HI in surroundings?Radial gradientNo deep photometryAbundance spread
No HI detection
-1-2
-1 -1
-2-1
000
-2-1-1-1
0
0000 0
-2
-1
-1
-2
0
Sextans Sculptor
15 10 5 0 15 10 5 0 15 1510 10 105
Ursa Minor Draco
0 015
88 kpc69 kpc 79 kpc 86 kpcSagittarius
4 GCs
C
MSRR
spread
MSRR
anCepspr
eadRRHB
anCepMS
AGB
?? spr
ead
5
RGB
HB
[Gyr]
C,BSAGB,CanCep
MSdwCep
MS
?
MSRR
HB PNMSAGB AGB,C
anCep
Star Formation Histories of MW dSph Galaxies
Fornax Leo II138 kpc 205 kpc
Leo I270 kpc
00005 5 5 515151515 10 10 10 10
5
Carina
0
25 kpc
94 kpcRCMS
MS
gradie
nt
RRHB
RC
const?
spread
AGB,CMS
AGB,CHBMS
uncertai
n
MS
MS
5 GCs
HBMS
AGB,CRC
Age[Fe/H
]HB S
FR
anCep= anomalous Cepheids, RGB = red giant branch, RR = RR Lyrae, HB= horizontal branchMS= main sequence, C = Carbon stars, AGB = asympt. giants, RC= red clump, PN=planetary nebulae,
Mv = -8.6 magMv = -8.9 mag
Mv = -9.5 mag Mv = -9.8 mag
Mv = -9.4 magMv = -13.1 mag
Mv = -10.1 magMv = -11.9 mag
Figure 3. Star formation histories of Milky Way dwarfspheroidal companions. Each population box gives aschematic representation of star formation rate (SFR)as a function of age and metallicity. The fraction ofintermediate-age populations tends to increase with in-creasing Galactocentric distance and dwarf galaxy mass.On the other hand, the distant, isolated dSph Tucana ap-pears to have predominantly old populations.
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Kroupa, Famaey, et al. (2010)
Milky Way Andromeda
Ibata et al. (2013)
A bold hypothesis
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Galaxies tell us a modified “Kepler-law” n V2 / r = a = G M/r n G = 6.67 x 10-11 (1+ 10-10/a )
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In the DM framework rhodm = rho0r03/[(r + r0)(r2 + r0
2)] a typical DM halo surface density rho0r0 ≈a0 /(2πG) defines an acceleration constant a0
Donato et al. (2009); Gentile, Famaey, Zhao, Salucci (Nature 2009)
Empirically Modified Kepler-law force ~ (250km/s)2 /r
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Encounter 7 or 10 Gyrs
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Equation of motion Under Force F12 ~ (200km/s)2/r12
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Explore the ranges of parameters
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Fly-by 10 Gyrs ago
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External field effect (Xufen Wu)
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Dynamical Friction small in Modified Kepler-law
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Tiret & Combes 2007, A&A 464, 517
Conclusion
n Modified Kepler-law predicts M31-MW frictionless fly-by. q which must produce thick disk and tidal arms
n Satellites/star clusters born in tidal arm q will show large M/L (e.g. LMC, Draco, YHG pal5).
n CDM dynamical friction produces low M/L baryon-only satellites on polar tidal arms of a merged elliptical. 01/04/2011 School of Physics & Astronomy, St Andrews 33