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Decay of OB asso ciations and evolution of dwarf galaxies
Evgeny Kurbatov
Institute of astronomy, Russian Acad. Sci.

Nearby Dwarf Galaxies SAO 2009


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The exchange of the matter between galaxy and IGM (ICM) can affect the chemical composition of IGM, the chemical composition of galactic gas and stars, and even the morphology of galaxy. Possible mechanisms of exchange are:


galactic wind driven by multiple explosions of SNae ram pressure of intergalactic gas tidal action from nearby galaxies evaporation of gas by hot IGM blow out of dust by star radiation pressure accretion from IGM escape of stars with velo cities large enough achieved in:


scatter of stars on the stars decay of closed binaries decay of OB asso ciations


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Some mechanisms of gas exchange

Galactic wind
The efficiency of wind strongly depends on gas distribution, i.e. whether the gas stratified or not (Co oper et al. 2008ApJ...674..157C), and on the details of the distribution for continuous one (Mac Low et al. 1989ApJ...337..141M). De Young & Gallagher 1990ApJ...356L..15D showed that the galaxies with mass 1.4 в 109 M eject gas of mass fraction 0.6 but observations can't find yet the gas ejection from galaxies with masses 109 M (van Eymeren et al. 2009A&A...493..511V). So the question on quantitative reliability of the theoretical mo dels of galactic wind is still open.


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Some mechanisms of gas exchange

Tidal action
Every galaxy in cluster undergo a collision at least once in a lifetime (Tutukov 2006ARep...50..439T). Two galaxies can merge, lose one or both of their gaseous components, or totally disintegrate as a result of a collision; ultimately, a new galaxy may form from the gas lost by the colliding galaxies.

Ram pressure, evap oration, and dust ejection
Thought to be less effective than noted above


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Star ejection by decaying OB asso ciation
The common opinion is that majority of stars form in stellar asso ciations (but see Elmegreen & Efremov 1996ApJ...466..802E). OB asso ciations has short life time till their decay, about few million years. In decay time stars gain the velo cities with mean values from 2 to 8 km/s (Brown et al. 1997MNRAS.285..479B). The value of virial velo cities in dwarf galaxies may be as low as few km/s (Karachentsev et al. 2004AJ....127.2031K) and the escape velo city may not exceed 10 . . . 20 km/s (Bovill & Ricotti 2009ApJ...693.1859B, Dijkstra et al. 2004ApJ...601..666D). If the galaxy has disk morphology the ordered motion of matter can help to stars to escape. The aim of this work is to estimate this effect and clear its observational consequences.


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The mechanism of star ejection
The condition for star to escape: (v + u)2 2 The probability of escape: (v, -) =
(v+u)2 2

-

(1)

d3 u u2 exp - 2 2 2O (2 OB )3/2
-

(2)
B

I.e. = ( , ) where = v
OB

(3) 2 2 OB

=

r
2 OB

r

=-

(4)


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The mechanism of star ejection

=

V

dV V V dV V

(5)

Plummer sphere
= 2 2 (1 + r 2 /a2 )1/2 = GM 2 OB a (6)

SFR law of Firmani & Tutukov 1992A&A...264...37F: V 2 (7)


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The mechanism of star ejection
dS S S dS S

=

S

(8)

Exp onential disk
1 - (1 + r /a) e- r /a
r /a

=

= 2

2

1 - e- r /a

r /a

(9)

SFR law of Kennicutt 1998ApJ...498..541K: S
3/2

(10)


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The mechanism of star ejection
The mean probability of ejection for plummer sphere (solid line) and exponential disk (dashed line) as a function of dimensionless virial velo city.
1

0.8

0.6 0.4 0.2 0 0 1 2 3 4 5 6


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The mechanism of star ejection
Dimensionless virial velo city of galaxies for limiting values of 2 km/s (triangles) and 8 km/s (circles).
10 9 8 7 6 5 4 3 2 1 0 5 5.5 6 6.5 7 7.5 lg M25 [Msol] 8 8.5 9 5 5.5 6 6.5 7 7.5 lg M25 [Msol] 8 8.5 9 0.2 0.6 0.8 1

OB

:

0.4


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The mo del series
The one-zone mo del of galaxy evolution of Tutukov & Firmani 1992A&A...264...37F (see also Shustov et al. 1997A&A...317..397S and Wiebe et al. 1998AZh...75.....3W) was the basis of the series of mo delling of galaxies with barionic masses from 106.5 M to 108.5 M :


series of closed mo dels with the same M /L and [Z] mo dels with ejection of stars for ... ...
OB OB OB

= 2 km/s

= 8 km/s = 8 km/s + Dark halo
OB = 2 km/s OB = 8 km/s 0.70 1.07 1.62 2.45 3.72 0.92 0.743 0.481 0.21 0.075 OB = 8 km/s +Dark halo 1.38 0.6 2.08 0.32 3.16 0.13 4.78 0.035 7.23 0

lg M /M 6.5 7 7.5 8 8.5

R, p c 79.5 141.4 251.5 447 795

2.73 4.14 6.26 9.48 14.25

0.21 0.09 0.012 0 0


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magenta (filled diamonds) closed mo dels blue (open diamonds) OB = 2 km/s green (filled triangles) OB = 8 km/s + Dark halo red (open triangles) OB = 8 km/s
8 7.5 7 6.5 6 5.5 5 4.5 5.5 6 6.5 7 7.5 8 8.5 9 -1.33 -1.34 -1.35 -1.36 -1.37 -1.38 4.5 5 5.5 6 6.5 7 7.5 8 L - [Z] 0.4 0.38 0.36 0.34 0.32 0.3 0.28 4.5 5 5.5 6 6.5 7 7.5 8 L - [Fe] 0.85 0.8 5.5 6 6.5 7 7.5 8 8.5 9 0.95 0.9 M-L 1.05 1 M - M/L


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M = 106.5 Msol 0.3 [Fe/H] 0.2

OB = 2 km/s 0.3

M = 106.5 Msol

OB = 8 km/s 0.3

M = 107.5 Msol

OB = 8 km/s 0.3

M = 108.5 Msol

OB = 8 km/s

[Fe/H] 0.2 0.2

[Fe/H] 0.2

[Fe/H]

0.1

0.1

0.1

0.1

0 -2.5 0.3 [O/H] 0.2 -2 -1.5 -1 -0.5 0 0.5

0 -2.5 0.3 [O/H] 0.2 -2 -1.5 -1 -0.5 0 0.5

0 -2.5 0.3 [O/H] 0.2 -2 -1.5 -1 -0.5 0 0.5

0 -2.5 0.3 [O/H] 0.2 -2 -1.5 -1 -0.5 0 0.5

0.1

0.1

0.1

0.1

0 -2.5 0.3 [Z] 0.2 -2 -1.5 -1 -0.5 0 0.5

0 -2.5 0.3 [Z] 0.2 -2 -1.5 -1 -0.5 0 0.5

0 -2.5 0.3 [Z] 0.2 -2 -1.5 -1 -0.5 0 0.5

0 -2.5 0.3 [Z] 0.2 -2 -1.5 -1 -0.5 0 0.5

0.1

0.1

0.1

0.1

0 -2.5 -2 -1.5 -1 -0.5 0 0.5

0 -2.5 -2 -1.5 -1 -0.5 0 0.5

0 -2.5 -2 -1.5 -1 -0.5 0 0.5

0 -2.5 -2 -1.5 -1 -0.5 0 0.5


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Conclusions



Stars with life time greater than 107 yrs can leave the galaxies of lowest mass contributing to IGM abundance as SNIa and SNI I (less significant) The "mass­luminosity" rate increases just slightly Metallicity can vary about 0.05 dex Iron abundance can vary about 0.1 dex Disk galaxies of the lowest mass can effectively turn to spheroidal type The galaxies of lowest mass can eject nearly all of stars becoming to o faint to observe