Magellanic Cloud Star Formation Bridging the Gap between Milky Way and Distant Galaxies

Star Formation in Different Galaxy Types

Eva K. Grebel Astronomisches Rechen-Institut

Zentrum für Astronomie der Universität Heidelberg Grebel: Star Formation in Different Galaxy Types 0

22.02.2013

Integrating over stellar mass range of 108 < M"/M! < 1013.

Growth of Global Stellar Mass Density

50% 25%

! ~ 45% of the present-day stellar mass was produced in ~ 3.6 Gyr from 1 < z < 3.

Marchesini et al. 2009, ApJ, 701, 1765 22.02.2013

5%

Global stellar mass density

! Remaining 50% formed in the last 7.5 Gyr from 1 < z < 0.

10%

Redshift

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Metallicity Evolution Mass-fraction-weighted metallicity of > 300,000 SDSS galaxies: ! Mass-metallicity relation (metallicity ! w. mass) ! Average metallicity of L" galaxy: solar. ! Below L": decreases by ~ 0.5 dex per dex in mass. ! M" > 1011 M!: flattening.

Panter et al. 2008, MNRAS, 391, 1117

! Small metallicity spread at high masses (0.15 dex); higher at low masses ( ~ 0.5 dex). Caveat: SDSS fibre size # metallicity bias 22.02.2013

Metallicity Evolution

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Ages younger than 0.5 Gyr

Considering only age bins < 0.5 Gyr:

Panter et al. 2008, MNRAS, 391, 1117

! No more massmetallicity relation (flat); not yet clear why. ! Only galaxies with stellar masses M" < 1010 M! contribute significantly to metallicity of young populations (! downsizing).

! Mass range spanned now only 1.5 dex; very few galaxies contribute. Downsizing: Stars in more massive galaxies tend to have formed earlier and over a shorter time period. 22.02.2013

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M87, Virgo

NGC 1316, Fornax

E1

E pec, cD, S0, Sa pec...

Ellipticals: ! Bulk of the star formation at early times. ! Considerable and rapid enrichment. ! Little to no cold gas. Highly ionized gas. ! Generally considered quiescent at present time. 22.02.2013

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The Extremes: High-z QSOs (z > 6; tUniverse ! 900 Myr) Bulk of star formation in ellipticals: how early? $ Metal lines in z > 6 QSOs; sometimes super-solar.

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1500

Mg II

Ly "

1000

2000

2500

3000

Rest frame wavelength [Å]

Grebel: Star Formation in Different Galaxy Types

Fe II

$ Extremely rapid early en! richment within only a few ! 100 Myr (in contrast to spirals).

O I / Si II N V S i IV / O IV C IV

$ $

F# [10–17 erg s–1 cm–2 Å–1])]

! ! ! ! !

! Dust detected $ SNe II-origin, not AGB (time scales). ! Other QSOs dust-free $ onset of massive SF varies. Mg (Mg II): "-element produced in SNe II. SNe II form “as soon” as massive stars form. Fe (Fe II): SNe Ia (minimum Composite spectrum delay time 300 Myr) of QSOs at z > 6 (Kurk et al. 2007, Fe II/Mg II ratio: ~ constant ApJ, 669, 32) as f(z) since z ~ 6 Formation of SN Ia progenitors at z > 10.

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The Extremes: High-z Galaxies

Galaxy A1689-zD1: z ~ 7.6. $ Only detectable in the infrared. " 13 billion lightyears away from us. $ Only 700 Myr after Big Bang! " " "

Strong star formation activity (! 7.6 M!/yr). Star ages of about 45 – 320 Myr. Star-forming knots with < 300 pc diameter.

" " "

Mass in stars: 1.6 – 3.9 " 109 M!.ƞ (Milky Way: 20 – 40 x more.) Size: at least 2 kpc. (Milky Way: > 30 kpc.) Bradley et al. 2008, ApJ, 678, 647

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The Extremes: High-z Galaxies (z = 7 to 8) ! SEDs from NIR imaging data of galaxies at z of 7 and 8 ! (lookback times of 770 – 640 Myr) show median stellar ! population ages of ~ 200 Myr. ! Typical stellar masses of galaxies at z ~ 7: 109 M!; ! at z ~ 8 possibly as low as 107 M!. ! Some 106 M! of dust from SNe II (from 12 – 35 M! stars). ! Time scales: ~ 20 Myr. ! Increase in dust extinction from very low amounts at z ~ 7 ! to AV ~ 0.5 at z = 4. Time scale for this increase consistent ! with low-mass AGB stars forming bulk of the dust. ! Most galaxies at z ~ 7 with metallicities of 0.005 Z!; ! some with 0.02 Z!. ! Colors resemble local, metal-poor star-bursting dwarf galaxies Finkelstein et al. 2010, ApJ, 719, 1250; 2012, ApJ, 756, 164 22.02.2013

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Specific Star Formation Rate as Function of tlookback for EarlyType Gal.

Thomas et al. 2010, MNRAS, 404, 1775

Intermediateand low-mass galaxies get rejuvenated via minor star formation events below redhift z ~ 0.2. Fraction of young, rejuvenated galaxies increases both with decreasing galaxy mass and decreasing environmental density to up to 45%. $ Impact of environment increases with decreasing galaxy mass. Specific star formation rate sSFR = SFR / M# 22.02.2013

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Elliptical Galaxies: SFH Analysis of > 14,000 early-type galaxies in SDSS (volume-limited): % Age, metallicity, "-enhancement increase with galaxy mass (!). % Field early-types younger by ~ 2 Gyr than cluster counterparts. counterparts. % Negative radial metallicity gradients $ masses and environments. environments. % Positive radial age gradients for early-types with ! > 180 km/s. & Low-mass halos with gas & stars accreted mainly at z ≤ 3.5. & Earlier accretion in dense environments than in field. & Fossil populations mainly at large radii (dissipationless stellar accretion)

Clemens et al. 2009, MNRAS, 392, L35

% ~ 30% of the massive early-types shows some recent (< 1 Gyr) Gyr) SF! % Ellipticals: Ellipticals: ~ 29%, lenticulars: lenticulars: ~ 39% % Fraction of UV-bright early-types 25% higher in low-density environments. environments. Schawinski et al. 2007, ApJS, 173, 512 22.02.2013

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E+A Galaxies % Spectra of ellipticals (Mg, Fe, Ca absorption; K") + strong Balmer lines (A") ' SF within last Gyr but no [OII] ' no recent SF. SF. & Post-starburst galaxies (radio: not dusty starbursts obscuring [OII]) % In clusters and in field. % About 30% show disturbed morphologies % or tidal tails. Goto et al. 2005, MNRAS, 357, 937 % Young E+A galaxies have more companion galaxies within 100 kpc. % E+A galaxies have 54% higher probability of having companions than normal galaxies (~ 5%). & Likely merger/interaction origin. 22.02.2013

Yamauchi et al. 2008, MNRAS, 390, 383

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Global (present) Star Formation Rates gas content

~ 0 M!/yr ~ 20 M!/yr

starburst galaxies: up to ~ 100 M!/yr ULIRGs: up to 1000 M!/yr 22.02.2013

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Specific Star Formation Rate vs. Galaxy Mass Mass dependence of the sSFR via dust-corrected FUV measurements of SDSS galaxies: Clear separation between red and blue sequences. Dispersion of sSFRs within blue sequence very small $ Self-regulation mechanism. Sharp increase of inactive galaxies above a few 1010 M!. Blue sequence: sSFR ! with $ galaxy mass such that lower-mass galaxies are forming relatively higher fraction of their stellar mass today.

Schiminovich et al. (2007) Kennicutt & Evans (2012)

Dominant SF galaxy population “migrated” from massive to less massive galaxies over cosmic time. 22.02.2013

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[Fe/H]

Spirals: The Milky Way Age-metallicity evolution of the different Milky Way components

Buser 2000, Science, 287, 69 Freeman & Bland-Hawthorn 2002, ARA&A, 40, 487 Kennicutt & Evans 2012, ARA&A, 50, 531 22.02.2013

Radial distribution of surface densities of atomic gas, molecular gas and SFR for the Milky Way.

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Disk Galaxy Evolution Stellar halos of disk galaxies:

Milky Way stellar halo today (simulation, Cooper et al.)

! Probably formed from inside out. ! ! ! ! ! !

In galaxies with few recent mergers ~ 20 to 50% of the stars formed in situ (according to %CDM simulations). (Zolotov et al. 2009).

Halos dominated by early accretion: higher ["/Fe] expected. If mainly accretion of high-luminosity satellites: higher [Fe/H]. (Johnston et al. 2008)

! MW inner halo: Accretion of a few moderately metal-rich, 108 – ! 1010 M! Magellanic-sized Halo substructure traced by SDSS main-sequence main-sequence stars. ! satellites > 9 Gyr ago. ! (De Lucia & Helmi 2008) ! MW outer halo: Mainly low! mass, low-metallicity Bell ! satellites.

et al. 2008

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Disk Galaxy Evolution Bulges of disk galaxies: ! ! ! ! ! ! ! ! $ ! ! ! ! ! !

Classical bulges: S0 – Sbc. Pseudobulges: later than ~ Sbc. All bulges show some amount of ongoing SF, regardless of type (Fisher et al. 2009).

Small bulges formed 10 – 30% of their mass in past 1 – 2 Gyr (Thomas & Davies 2006).

Massive clumps forming at early times in galactic disks move towards galactic center due to dynamical friction, merge, and form bulge (Noguchi 1999). Trend of ! bulge-to-disk ratios with ! galactic masses. Most galaxies at z ~ 1: disk-like morphologies, but most galaxies at z > 2 look clumpy/chaotic (van den Bergh 2002). Clump masses: 107 – 108 M!. Clump coalescence resembles major merger in terms of orbital mixing, but no increased DM content. (Elmegreen et al. 2008, 2009). Other bulge formation scenarios: Mergers, secular evolution.

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6 Gyr ago

Disk Galaxy Evolution Disks of disk galaxies: ! ! ! ! ! ! !

Even at z < 0.6 – 0.8: 46% of the spirals are still chaotic. Only ~ 5% of Sa/Sab galaxies are peculiar at z ~ 0.7, but almost 75% of Sbc and Sc types are still peculiar.

! ! ! !

SF activity in clumpy disks may be caused by local gravitational collapse w/o external trigger (Elmegreen et al. 2007).

(van den Bergh 2002)

Delgado-Serrano et al. 2010

! Environment: Fraction of ! spirals declines with density ! (e.g., Poggianti et al. 2008). 22.02.2013

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Sites of Star Formation Typical present-day sites of star formation in galaxies: % Extended disks of spirals and irregulars % Dense gas disks in galaxy centers (circumnuclear SF) % Enhanced SF in interacting galaxies and starbursts & Significant contributors to SF in the (local) Universe Occasionally: % Intragalactic SF in tidal tails

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Spirals and Irregulars: Star Formation “Demographics” Area-averaged SFR vs. absolute SFR:

ii rad F S nt a t ns co f o es Lin

SFRs have range of more than 7 orders of magnitude. Largely due to nonequilibrium systems (starbursts). Normal galaxies: Tight range of SFRs per unit area. Quiescent SF galaxies form < 20 M! / year.

Kennicutt & Evans 2012

Starburst galaxies, Absolute SFR LIRGS and ULIRGS: Most of the galaxies in the upper 2 – 3 decades of absolute SFRs and area-averaged SFRs. 22.02.2013

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Spirals and Irregulars: Star Formation “Demographics” Three different star formation regimes: 11HUGS lower specific SFRs

SFR & M(H2) MB ~ –19

~cont. SFRs

SFR & M(HI)

bulgedomin. galaxies Vmax ~ 120 km/s Vmax ~ 50 km/s

MB ~ –15

irregulars

large scatter in spec. SFRs

spiral structure key regulatory factor

internal processes dominate (esp. feedback)

Lee et al. 2007, ApJ, 671, L113 22.02.2013

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Bothwell et al. 2009, MNRAS, 400, 154

Stellar Mass versus Specific Star Formation Rate

22.02.2013

High-mass regime:

Late types

Secularly evolving intermediate-mass populations

Low-mass regime: larger scatter; Some galaxies with anomalously low SFR.

Some tendency for quenched SF

Early types

Grebel: Star Formation in Different Galaxy Types

HI Consumption Time Scale (M(HI)/SFR) ! With increasing luminosity, HI content drops off faster than SFR $ Shorter HI consumpt. time scales.

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Late types Hubble time

Bothwell et al. 2009, MNRAS, 400,154

Early types

! All gas consumption times: > 100 Myr $ & 1 dynamical time scale in a typical galaxy $ lower SFR limit. ! Gas mass / minimum gas assembly time: & free-fall collapse time. $ SFR upper limit. ! Low-mass galaxies: Very low SFR and very extended HI disks (very little of existing gas is available for star formation). 22.02.2013

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Hunter, Elmegreen

Irregulars: Random Gas Motions Dominate % Star formation is occurring in clouds even where the average gas column density is < Toomre 'c. & Difference between dIm & spirals is context in which clouds form. % 60–90% of HI is in cool HI filaments (both spirals and Im) % Dwarfs may contain relatively more warm HI than spirals. % Cool gas is more important in determining star formation. % No correlation between cooler HI component and integrated star formation rate observed in dwarfs. & The immediate reservoir of gas for cloud formation may not be as extensive as integrated HI mass would indicate. % HII regions in dIms are overpressured relative to ambient disk pressures by factor of 10 compared to spirals. & Greater role possible for pressurized triggering and shell formation. % ISM in Im galaxies: structured into clouds of all sizes (fractal) whose distributions resemble those of compressible turbulence. Grebel: Star Formation in Different Galaxy Types

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Grebel & Brandner 1998, inThe Magellanic Clouds and Other Dwarf Galaxies, eds. Richtler & Braun (Shaker Verlag), p. 151

22.02.2013

Spatial Variations: Recent Star Formation History of the LMC Long-lived regions of active star formation in irregular galaxies. Life times a few 10 – 100 Myr. Irregular appearance dominated by young HII regions. Old populations fairly smooth and homogeneous. 22.02.2013

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Magellanic Clouds: Old Populations Old stars in the Magellanic Clouds: Sparse. Hard to find. Traced best by using RR Lyrae and other HB stars. $ Overall no gradients in metallicity, but large spread. (Haschke et al. 2012, AJ, 143, 48)

Most metal-poor LMC star found so far: [Fe/H] = –2.67. Haschke et al. 2012, AJ, 144, 88

(["/Fe])old = 0.36. Individual abundances and trends resemble dSphs and MW ((inner) halo accretion). 22.02.2013

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The SMC Cluster Age-Metallicity Relation

Kayser et al. 2008, Kayser PhD Thesis, Glatt et al. 2008

VLT spectroscopy & ACS photometry of SMC clusters: Metallicity spread at a given age; SMC not well mixed!

[Fe/H]CG97

Parisi et al. 08 NGC 330

Age [Gyr] 22.02.2013

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Dwarf Distance from Primary vs. HI Mass

`Galactic halo’

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Metallicity-Luminosity. relation for the same (old) populations dSphs

– 0.5

dIrrs; but also: evolution as independent entities!)

dSphs: too metal-rich for their luminosity; even allowing for evolution

([Fe/H])

–1

–1.5

–2 Here: “error bars” Indicate true range of – 2.5 metallicities, not uncertainty. 22.02.2013

dIrrs dIrr/dSphs dSphs dEs 6

7

log Lbary [L!]

8

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Shetrone et al. 2001

Koch et al. 2010

Lower ["/Fe] @ [Fe/H] in dSphs than in Galactic halo: ! Low SFRs (little contribution from massive SNe II (")), or ! Loss of metals and SN ejecta by galactic winds, or ! Larger contribution from SNe Ia (Fe enhanced over ") ! Inefficient enrichment.

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Abundance Inhomogeneities in Dwarfs ! Considerable abundance spreads observed in field stars: ! Up to > 1 dex even in dwarfs dominated by old popul. (e.g., Shetrone et al. 2001, ApJ, 548, 592; Norris et al. 2008; ApJ, 689, L113)

! At a given age: scatter in abundances

Carina

e.g., SMC (Glatt et al. 2008, AJ, 136, 1703), Sex B (Kniazev et al. 2005, AJ, 130, 1558).

! At a given metallicity: scatter in " abundance ratios (e.g., Koch et al. 2008, AJ, 135, 1580)

( Slow, stochastic SF, low SFE 22.02.2013

[Fe/H] Koch et al. 2008, AJ, 135, 1580

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Wide Range of Star Formation Histories Observed Downsizing

High-mass regime: % Very rapid, efficient early SF with strong enrichment. % Mostly inactive today. Intermediate-mass regime (MB < –19; M" > ~1010 M!) % Early types: More activity in low-density environments. % Late types: Continuous SF over a Hubble time, well correlated with H2. Low-mass regime (MB < –15; M" < ~108 M!) % Wide variety of properties from bursting to quiescent. % High amount of gas may not imply high SFR. % Generally, stochasticity dominates; upper IMF sparsely sampled. Often: low SFE, low SFR (exception: e.g., BCDs). % Susceptibility to disturbances; environmental dependence. 22.02.2013

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