formation of satellites and rings in local group and ... -...

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

01/04/2011 School of Physics & Astronomy, St Andrews 3

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


Tidal dwarf galaxies


Duc et al.

Dwarf galaxies on tidal rings


Duc et al.

Satellites have high dynamical mass large M/L


No dwarfs form by DM recollapse


Lumps in Gas-rich mergers


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.


Satellite galaxies have young-ish stars


Tolstoy et al. 2009 ARAA

Grebel 0005296

7

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.


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

01/04/2011 School of Physics & Astronomy, St Andrews 26 Klypin,Zhao,Somerville 2002 ApJ 573,597

Encounter 7 or 10 Gyrs


Equation of motion Under Force F12 ~ (200km/s)2/r12


Explore the ranges of parameters


Fly-by 10 Gyrs ago


External field effect (Xufen Wu)


Dynamical Friction small in Modified Kepler-law


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

formation of satellites and rings in local group and ... -...

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