Star Formation in the Universe

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Star Formation in the Universe

• Robert Kennicutt
• Institute of Astronomy
• University of Cambridge

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Lectures

• 1. Diagnostics of Star Formation Rates
• 2. Demographics of Star-Forming Galaxies and
  Starbursts (Mon 1pm)
• 3. Nearby Galaxies as Revealed by the Spitzer
  Space Telescope (colloquium Tues 415pm)
• 4. The Star Formation Law (next Thurs 1pm)

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Motivations

• Observations of external galaxies reveal global
  and local star formation events ranging over
  gt107x in absolute scale--- over a far wider range
  of physical environments than can be found in the
  Milky Way
• Star formation is a primary component of galaxy
  evolution and cosmic evolution
• Despite its central role, galactic-scale SF as a
  physical process is barely understood

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Contributions to the Global Star Formation Budget
IR-luminous 5-8 circumnuclear
3-4 BCGs, ELGs 5-8
Total fraction 10-20
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Hopkins 2004, ApJ, 615, 209
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An Information Explosion

• advent of the mega-survey
• SDSS, 2DF --gt imaging, spectra for gtgt106
  galaxies to z0.5
• GALEX
• SFRs for 107 galaxies to zgt1
• 10000 galaxies within 70 Mpc
• Spitzer
• 3 Legacy surveys MIPS/IRS GTO starburst survey
• large Ha surveys
• SFR maps for gt4000 galaxies
• ISM surveys
• e.g., WHISP, THINGS, BIMA SONG --gt ALMA, Herschel

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Multi-Wavelength SFR Diagnostics
calorimetric IR
? (?m) 1
10 100
1000
24 ?m
70 ?m
160 ?m
8 ?m
P?
OII
H?
UV
Dale et al. 2007, ApJ, 655, 863
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0.1 1 10 100 mag (AV)
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GALEX FUV NUV (1500/2500 A)
Ha R
IRAC 8.0 mm
MIPS 24 mm
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Spitzer Infrared Nearby Galaxies Survey (SINGS)

• complete IRAC, MIPS imaging of 75 nearby galaxies
  (3.5 160 mm)
• IRS, MIPS radial strip maps (10 100 mm)
• IRS maps of centers, 75 extranuclear sources
  (537 mm)
• ancillary imaging campaign covering UV to radio

Kennicutt et al. 2003, PASP, 115, 928
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• UV Continuum Emission

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• Ultraviolet stellar continuum key advantages
• direct photospheric measure of young massive
  stars
• primary groundbased SFR tracer for galaxies at
  zgt2
• However
• heavily attenuated by dust. Dust correction
  methods have limits (age-dust degeneracy).
• dependent on the stellar population mix, usually
  measures timescales of 100 Myr.

Dale et al. 2007, ApJ, 655, 863
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• GALEX Mission
• - all-sky survey
• - 5 arcsec resolution
• - 1500 A, 2500 A to AB 20-21
• - 10,000 galaxies to z0.02
• - deep surveys to AB 25.5, 26.5
• - launched April 2003

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Steidel et al. 1996, ApJ, 462, L17
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Building an Evolutionary Synthesis Model

Kurucz 1979, ApJS, 40, 1
single star SED evolution model
Maeder, Meynet 1988, AAS, 76, 411
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single burst models
continuous star formation models (single age
star clusters)
Leitherer et al. 1999, ApJS, 123, 3
Starburst99
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apply evolutionary synthesis maodels to
constrain IMF
Kennicutt, Tamblyn, Congdon 1994, ApJ, 435, 22
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UV, Dust, and Age
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Starbursts
A dusty stellar population may have similar UV
characteristics of an old population
(Calzetti et al. 1994,1995,1996,1997,2000,
Meurer et al. 1999, Goldader et al. 2002)
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Blue starbursts Red normal SF
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M51 Calzetti et al. 2005, ApJ, 633, 871
FUV, Ha, 24mm
3.6, 4.5, 5.8, 8.0 mm
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Photoionization Methods Emission Lines

• for ionization-bounded region observed
  recombination line flux scales with ionization
  rate
• ionization dominated by massive stars (M gt 10
  Mo), so nebular emission traces SFR in last 3-5
  Myr
• ionizing UV reprocessed through few nebular
  lines, detectable to large distances
• only traces massive SFR, total rates sensitive
  to IMF extrapolation
• SFRs subject to systematic errors from
  extinction, escape of ionizing radiation from
  galaxy

SINGG survey, G. Meurer et al. (NOAO)
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Kennicutt 1992, ApJS, 79, 255
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Local Ha Surveys

• Survey Ngal Selection
  PI
• GOLDMine 277 magnitude
  Coma/Virgo G. Gavazzi
• MOSAIC 1000 Ha
  Abell clusters R. Kennicutt
• HaGS 450 mag/volume field
  (lt40 Mpc) P. James
• SINGG/SUNGG 468 HIPASS field (lt40
  Mpc) G. Meurer
• STARFORM 150 volume field
  (lt25 Mpc) S. Hameed
• 11HUGS 470 volume
  field (lt11 Mpc) R. Kennicutt
• AMIGA 270 magnitude
  isolated field L. Montenegro
• SINGS 75 multi-param lt30
  Mpc R. Kennicutt
• SMUDGES 1000 mag field
  dwarfs L. van Zee
• UCM 376 obj prism
  field J. Gallego
• KISS 2200 obj prism
  field J. Salzer

paired GALEX survey
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Photoionization Methods Emission Lines

• for ionization-bounded region observed
  recombination line flux scales with ionization
  rate
• ionization dominated by massive stars (M gt 10
  Mo), so nebular emission traces SFR in last 3-5
  Myr
• ionizing UV reprocessed through few nebular
  lines, detectable to large distances
• only traces massive SFR, total rates sensitive
  to IMF extrapolation
• SFRs subject to systematic errors from
  extinction, escape of ionizing radiation from
  galaxy

SINGG survey, G. Meurer et al. (NOAO)
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Leakage of Ionizing Flux at z 3
Shapley et al. 2006, ApJ, 651, 688
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composite spectrum
Shapley et al. 2006, ApJ, 651, 688
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galaxies (integrated fluxes)
HII regions
Calzetti et al., ApJ, submitted Kennicutt
Moustakas, in prep
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Other Emission Lines - Hb (0.48
mm) - Paschen-a (1.9 mm) - Brackett-g (2.2
mm) - OII (0.37 mm) - Lyman-a
(0.12 mm)
Scoville et al. 2000, AJ, 122, 3017
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Wavelength

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Moustakas, Kennicutt, Tremonti 2006, ApJ, 642, 775
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Moustakas et al. 2006, ApJ, 642, 775
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11 Mpc Ha/Ultraviolet Survey (11HUGS)
SINGG Survey for Ionization in Neutral-Gas
Galaxies
SINGG Survey for Ionization in Neutral-Gas
Galaxies
M83 NGC 5236 (Sc)
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Lecture 1 Ended Here Extra Slides Follow
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Dust Emission

• Interstellar dust absorbs 50 of starlight in
  galaxies, re-radiates in thermal infrared (31000
  mm)
• Provides near-bolometric measure of SFR in dusty
  starbursts, where absorbed fraction 100
• Largest systematic errors from non-absorbed star
  formation and dust heated by older stars
• Different components of IR trace distinct dust
  species and stellar sub-populations

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FIR observations probe the most luminous
star-forming galaxies, with SFR gtgt SFR (gtgt10
Mo/yr at present epoch).
Martin et al. 2005, ApJ, 619, L59
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NGC 628 (M74)
C. Tremonti
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NGC 7331 Regan et al. 2004, ApJS, 154, 204
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IRAC 8.0 mm
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MIPS 24 mm
Gordon et al. 2004, ApJS, 154, 215
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FIR to SFR?
Dale et al. 2007
calorimetric IR
? (?m) 1
10 100
1000
24 ?m
70 ?m
160 ?m
8 ?m
FIR - sensitive to heating from old stellar
populations 8 ?m - mostly single photon heating
(PAH emission) 24 ?m - both thermal and single
photon heating 70 ?m and 160 ?m - mostly thermal,
also from old stars
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SFR (FIR)

• Idea around since IRAS times (e.g., Lonsdale
  Helou 1987) SFRs from bolometric IR emission
• Depending on luminosity, bolometric IR may be
  measuring star formation or old stars heating
• FIR SEDs depend on dust temperature (stellar
  field intensity Helou 1986) problematic if
  wavelength coverage is not complete.

Higher SFR (stellar radiation field intensity)
higher dust temperature
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Moustakas et al. 2006, ApJ, 642, 775
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SFR(8 ?m, 24 ?m)

• ISO provided ground for investigating
  monochromatic IR emission as SFR tracers, esp.
  UIBAFE(?)PAH (e.g., Madden 2000, Roussel et
  al. 2001, Boselli et al. 2004, Forster-Schreiber
  et al. 2004, Peeters et al. 2004, Tacconi-Garman
  et al. 2005).
• Spitzer has opened a more sensitive window to
  the distant Universe
• A number of studies with Spitzer has already
  looked at the viability of monochromatic IR
  emission (mainly 8 and 24 ?m) as SFR indicator
  (Wu et al, 2005, Chary et al., Alonso-Herrero et
  al. 2006, etc.)
• Appeal of PAH emission (restframe 7.7 ?m
  emission for z2) for investigating star
  formation in high-z galaxy populations (e.g.,
  First Look, GOODS, MIPS GTO, etc. Daddi et al.
  2005)
• Monochromatic 24 ?m (restframe) emission also
  potentially useful for measuring high-z SFRs (see
  Dickinsons Spitzer Cy3 Legacy)

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M81
Ha R
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Calzetti et al. 2007, ApJ, submitted
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Scale 100-600 pc
NGC925

• Use starbursts or SF regions in galaxies
  (SINGS).
• Use P? as ground truth measure of
  instantaneous SFR (Boeker et al. 1999 Quillen
  Yukita 2001)
• Measure 8 ?m, 24 ?m, H?, and P?.

M51
33 normal galaxies (220 regions) 34 starbursts
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• Composite SFR Indices
• Basic Idea
• calibrate 24mm emission (vs Pa, radio, etc) as
  tracer of dust-reprocessed SFR component
• use observed Ha emission to trace unprocessed
  SFR component
• total SFR derived from weighted sum of 24mm
  Ha, calibrated empirically
• applied to UVFIR SFRs, flux ratio method
  (Gordon et al. 2000)

Calzetti et al. 2007, ApJ, submitted
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galaxies (integrated fluxes)
HII regions
Calzetti et al. 2007 Kennicutt Moustakas 2007
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GALEX FUV NUV (1500/2500 A)
Ha R
IRAC 8.0 mm
MIPS 24 mm
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- 8 mm emission is less reliable as a local SFR
tracer, at least for young (HII) regions
Calzetti et al. 2007, ApJ, submitted
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• Rest 8 mm emission traces total IR luminosity
  at factor 3-5 level in metal-rich galaxies, but
  is systematically weak in low-mass galaxies

Dale et al. 2005, ApJ, 633, 857
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PAH Emission vs Metallicity
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M101 Gordon et al., in prep
B UIT FUV G IRAC 8 mm R MIPS 24 mm
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M81 Ho IX 8 mm
GALEX UV
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Spectral Variations in SINGS Galaxies (centers)
Smith et al. 2007, ApJ, 656, 770
- significant variations in absolute and relative
PAH band strengths
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- variations driven by metallicity and ambient
radiation field
HII region dominated AGN dominated
Smith et al. 2007
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Radio Continuum Emission

• exploits tight observed relation between 1.4 GHz
  radio continuum (synchrotron) and FIR luminosity
• correlation may reflect CR particle
  injection/acceleration by supernova remnants, and
  thus scale with SFR
• no ab initio SFR calibration, bootstrapped from
  FIR calibration
• valuable method when no other tracer is
  available

Bell 2003, ApJ, 586, 794
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Cookbook
Extinction-Free Limit (Salpeter IMF,
ZZSun) SFR (Mo yr-1) 1.4 x 10-28 L n (1500)
ergs/s/Hz SFR (Mo yr-1) 7.9 x 10-42 L (Ha)
(ergs/s) Extinction-Dominat
ed Limit SF Dominated SFR (Mo yr-1) 4.5 x
10-44 L (FIR) (ergs/s) SFR (Mo yr-1) 5.5 x
10-29 L (1.4 GHz) (ergs/Hz) Composite SF
Dominated Limit SFR (Mo yr-1) 7.9 x 10-42 L
H?, obs a L24?m (erg s-1) a 0.15
0.31 SFR (Mo yr-1) 4.5 x 10-44 L(UV) L
(FIR) (ergs/s)
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General Points and Cautions

• Different emission components trace distinct
  stellar populations and ages
• nebular emission lines and resolved 24 mm dust
  sources trace ionizing stellar population, with
  ages lt5-10 Myr
• UV starlight mainly traces intermediate age
  population, ages 10-200 Myr
• diffuse dust emission and PAH emission trace same
  intermediate age and older stars 10 Myr to 10
  Gyr(!)
• Consequence it is important to match the SFR
  tracer to the application of interest
• emission lines Schmidt law, early SF phases
• UV time-averaged SFR and SFR in low surface
  brightness systems
• dust emission high optical depth regions
• Multiple tracers can constrain SF history,
  properties of starbursts, IMF, etc.

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GALEX FUV NUV (1500/2500 A)
Ha R
IRAC 8.0 mm
MIPS 24 mm
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