Starburst Dwarf Galaxies

Transcription

1 Starburst Dwarf Galaxies 1 Starburst Dwarf Galaxies The star-formation history does in general not show a continuous evolution but preferably an episoidal behaviour. 2 1

2 Definition: Starburst ( t0) ( ) G from stellar population synthesis SFR M HI ( t ) Examples: at former times: 0 G Hubble HI mass Globular Clusters Dwarf Ellipticals giant Ellipticals at present: giant HII regions > 30 Dor SBDGs NGC 1569, NGC 4449, NGC 5253 He 2-10, NGC 1705, III Zw 102 M82, NGC 253 nuclear SBs NGC 1808, NGC 2903, Mkr 297 ULIRGs Merger 4 Dwarf Galaxies and Galactic Winds In NGC Super Star Clusters at the base of the gas stream are the engines of the galactic wind due to their 5 cumulative supernova II explosions. 2

3 Model: bipolar outflow; the S part towards observer is unobscured; disk inclination known. X-ray in colors according to hardness (blue: hard, red: soft) overlaid with HI contours (white) Martin et al. (2002) 6 Abundances in the galactic wind from X-ray spectra Martin 7 et al. (2002) ApJ 574 3

4 The mass loss can be determined from the effective yield y eff of the HI ISM. The loss of metals should be visible in the hot gas outflow. 8 Martin et al. (2002) ApJ 574 Galactic winds MacLow & Ferrara (1999) courtesy Simone Recchi Effective yields of dirrs < solar! Outflow of SNII gas reduces e.g. O, y SS eff but: simple outflow models cannot account for gas mixing + turb. heating 4

5 Galactic winds in Dwarf Galaxies? Continuous mechanical L over 50 Myrs for various galaxy masses M g and gas densities n: at t=50 Myrs No consistent star formation! MacLow & Ferrara (1999) 11 t=100 Myrs MacLow 12 & Ferrara (1999) 5

6 Mass (M ) Galactic winds in DGs and the fate of metals Luminosity (10 38 erg s -1 ) 0, , , e-3 8.4e-3 4.8e e-4 3.4e-4 1.3e-3 0,8 1 1 Mass ejection efficiencies Metal ejection efficiencies MacLow & Ferrara (1999) 14 Testing galactic winds in different gas disks Recchi & Hensler (2013) A&A, 551 Initial conditions: Baryonic mass: M b = 10 7, 10 8, 10 9 M Stellar disk by Myamoto-Nagai potential Isothermal gas disk with flattening b/a = 0.2, 1.0, 5.0 Gas fraction: 60% (L), 90% (H) Spherical DM halo with M DM = 10 M b SFR~

7 Testing galactic winds in different gas disks (2013) A&A, 551 SN ~ 5% Blow-away of total gas almost impossible: Only in lowest-mass DGs and flat, light gas disks, but then all metals are lost. Gas disk, mixing, turbulence, external halo gas, and embedded clouds hamper outflow. 16 Testing galactic winds in different gas disks Oxygen ejection: Only lowest-mass DGs with lower gas fraction lose large O fractions. Thick gas disk and massive DGs retain their supernova elements. Recchi & Hensler (2013) A&A,

8 Testing galactic winds in different gas disks H models ISM abundances: The thicker the gas disk, the higher the metal enrichment. The larger the mass, the larger the metallicity. The larger the gas content, the greater the metal enrichment. Blow-away of total gas almost impossible: Only in lowest-mass DGs + flat, light gas disks, lose all metals. Gas disk, mixing, turbulence, external halo gas, and embedded clouds hamper outflow. 18 Can strong Galactic Winds solve the CDM cusp/core problem? Loss of low-ang.mom. gas! (2010) Nature, 463 (2012) MNRAS, 422 NO solution when metal. mismatch! 20 8

9 Large-scale outflows + gas-phase mixing (2006) A&A, 445 In reality: blowaway almost impossible: gas halo, infalling clouds, turbulence metals retained Timescales of gas return cooling of blow-out hot gas and fall back: ~ Gyrs turbulent mixing: ~ 20 Myrs evaporative mixing: Myrs (Rieschick 21 & G.H. 2002) Refill of Superbubbles Due to cooling and buoyancy the surrounding gas pressure refills the superbubble cave after a few 100 Myrs Recchi et al. (2006) A&A,

10 What triggers the high star-formation rates? Gas Infall? Consider the effects of external gas infall! 23 Gas Infall to DGs What triggers the high star-formation rates? Consider the effects of external 24 gas infall! 10

11 Gas infall triggers Starburst The case od NGC 1569: HI clouds (each ~10 6 M ) fall towards in from a disk 2 huge super star clusters are formed. (2005, AJ, 130) 25 H NGC 1569 Gas Infall confirmed! see Muehle et al. (2005) and in many other SBDGs! HI Stil & Isreal (2002) 26 11

12 NGC 4449 a triggered starburst 27 (Hunter et al. 1995) 28 (van Zee et al. 1997) II Zw 40 12

13 (Östlin & Kunth 2000) I Zw 18 - a perturbed dirr with gas infall? (van Zee et al. 1997) Associated HI Clouds I Zw 18 is associated with a kinematically disjunct HI complex. van Zee et al. (1998) 30 13

14 BCD NGC Dwarf Starburst Galaxies are experiencing an epoch of strong Star Formation: Massive stars illuminate their surrounding gas; Exploding stars release hot, vehemently expanding gas NGC 1705 But: super star cluster not formed in the center X-ray contours overlaid on H (Hensler et al. 1997) 2 kpc H HI 32 14

15 Galactic outflows and infalling clouds Recchi & Hensler (2007) A&A, 477 Although clouds hamper the outflow by evaporated mass-loading, by this, reducing the metal loss, over-running clouds pierce holes into the superbubble shell: nozzle-like outflows are facilitated. 33 He 2-10 A dirr colliding with an intergalactic cloud? UV X Vacca & Conti (1992) Hensler et al. (1997) Papaderos et al. (1998) Kobulnicky et al. (1995) CO V 34 15

16 Metal differences between outflow vs. infall Metal content of the cool ( 10 4 K) circumgalactic medium around 28 HI-selected LLS at z 1 observed in absorption against background QSOs 35 Star formation is self-regulated! SF = g_cons = M g / Hubble or ssfr:= s = /M gal 0.01 Gyr -1 What triggers high star-formation rates? Gas Infall? The effects of external gas infall vs. outflow! Both act simultaneously: e.g. at high z~2-3: Erb (2008) ApJ, 674 at low z 1: Lehner et al. (2013) ApJ, 770 present: starburst DGs 36 16

17 Can Gas Infall explain a chemical rejuvenation of dirrs and their abundance peculiarities? 37 The N/O problem of dirrs/bcds Henry, Edmunds, Köppen, (1999) N/O production: O is produced in massive stars and released by SNeII (hot gas); N is mainly produced in intermediate-mass stars (warm gas); Massive stars live shorter than IMS; N also produced and released by massive stars as primary and secondary element N/O signatures: HII regions in gss along second.-n production track; outer HII regions resemble dirrs scatter; dirrs show low N/O (~ -1.5) at low O! Henry, R.B.C. & Worthey, G. (1999) Stellar evolution tracks pass dirr regime too fast! 38 17

18 Gas Infall: its Effect on Abundances Model assumptions: Yields same as in Henry et al. (2000): van der Hoek & Groenewegen (1997), Maeder (1992) Galaxy models evolve for 13 Gyrs with different y eff of reaching different loc.s in (N/O)-(O/H) diagram Infall of clouds with primordial abund. and masses of M produces loops. Koeppen & G.H. (2005) A&A, Gas Infall and its Effect on Abundances Koeppen & Hensler (2005) A&A, 434 Extension of tracks depends on y eff (N/O) scatter reproducible by age diff s of start models