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Unraveling the Star Formation
Scaling Laws in Galaxies
(review + 2 new results)
Robert Kennicutt
Institute of Astronomy
University of Cambridge
M51: FUV, Ha, 24mm
Basic Observations: circa 1998
• Galaxies exhibit an immense diversity in star formation
properties, varying by >107 in absolute SFR, SFR/mass
and SFR/area.
• Over this range the SFR/area is correlated with gas
surface density, following a truncated Schmidt power
law with index N = 1.4 +-0.1
– the correlation of with dense gas (e.g., HCN) is roughly linear
• The Schmidt law shows a turnover below a threshold
surface density that varies between galaxies.
– in gas-rich, actively star-forming galaxies this transition is
seen as a radial transition in the SFR/area
– some gas-poor disks reside in the threshold regime at all radii
IR-luminous starbursts
circumnuclear starbursts
BCGs, ELGs
starbursts
normal galaxies
Kennicutt 1998, ApJ, 498, 541
Gao, Solomon 2004, ApJ, 606, 271
NGC 1291
Blue: Carnegie Atlas
Sandage & Bedke 1994
Ha + R: SINGG survey
Meurer et al. 2006
Questions: Schmidt Law
• Is the correlation really this good (or this bad)?
– Why all the discrepant results yesterday (and today)?
– Do all galaxies follow the same Schmidt law?
– Is the scatter driven by a second parameter?
• Is the Schmidt law the result of a more fundamental
underlying SF scaling law?
– over what range of physical scales is the law valid?
• Is the SFR correlated more strongly with the total
(atomic + molecular) surface density or with the
molecular surface density alone?
• What is the physical origin of the relation?
“Schmidt law”:
SFR vs gas density power law
“Silk law”:
SFR vs gas density/dynamical time
“pressure law”
Blitz & Rosolowsky 2006, ApJ, 650, 933
Questions: Schmidt Law
• Is the correlation really this good (or this bad)?
– Why all the discrepant results?
– Do all galaxies follow the same Schmidt law?
– Is the scatter driven by a second parameter?
• Is the Schmidt law the result of a more fundamental
underlying SF scaling law?
– over what range of physical scales is the law valid?
• Is the SFR correlated more strongly with the total
(atomic + molecular) surface density or with the
molecular surface density alone?
• What is the physical origin of the relation?
Questions: Thresholds
• Do the observed Ha edges of galaxies trace
proportional changes in the SFR/area?
• Does the SFR in the sub-threshold regime follow a
(modified) Schmidt law? Or is it triggered entirely
by local compression events?
• What is the physical nature of the threshold?
Key Caveats, Considerations (aka Rant #1)
• The “star formation law” and “star formation rate”
mean different things on different linear scales
– disk-averaged scale (1-20 kpc, >100 Myr): useful empirical
recipes, but physical significance difficult to infer
– radial averages (~1 kpc, 20-300 Myr): breaks some parametric
degeneracies, but still smooths nonlinear phenomena over 10100’s of cloud scales
– cloud scale or below (50-500 pc, 3-10 Myr): probes “star
formation efficiency”, but with large observational scatter.
“SFRs” really are measures of cluster luminosities. SF law on
larger scales may have very different form.
Rant #2: Beware of the local coincidence
between ISM phase vs gravitational instabilities
Log P/k
(pressure)
Rant #3
• It is important to match the SF and gas tracers to the
application of interest
– color-magnitude diagrams: nirvana: range of ages, stellar masses
– Ha, Pa, 24mm knots: massive stars, last 3-10 Myr
– FUV continuum: massive stars, last 0-200 Myr
– diffuse dust continuum emission, 20-200mm + PAH emission:
massive and intermediate mass stars, last 0-2 Gyr
– CO, HI clumps: probably bound clouds, <10 Myr
– diffuse CO, HI: anybody’s guess, probably ~0.1-1 Gyr
Draine et al. 2007, astro-ph/0703213
GALEX FUV + NUV (1500/2500 A)
IRAC 8.0 mm
Ha + R
MIPS 24 mm
M81
Ha + R
Calzetti et al. 2007, ApJ, 666, 870
M 81
24µm
70µm
160µm
PHOENIGS: from the SINGS `ashes’
Project for Herschel On an Extragalactic Normal
Infrared Galaxies Survey
R. Kennicutt (IoA, Cambridge), L. Armus (SSC), A. Bollato (UMd), B. Brandl (Leiden),
D. Calzetti (UMass), D. Dale (UWyo), B. Draine (Princeton), C. Engelbracht (UofA, USA),
K. Gordon (UoA, USA), B. Groves (Leiden), L. Hunt (Oss Arcetri, Italy), J. Koda (Caltech),
O. Krause, A. Leroy, H.W. Rix (MPIA), H. Roussel (IAP), M. Sauvage (CEA), E. Schinnerer
(MPIA), J.D. Smith (Toledo), L. Vigroux (IAP), F. Walter (MPIA), M. Wolfire (UMd) + TBD
Broad Science Objectives:
• Trace and characterize the flow of energy through the ISM in galaxies;
• Link heaters-emitters: use Herschel spatial resolution to enable definitive modeling of
radiative transfer of dust and gas cooling in galaxies;
• Probe the nature/origin of extended cold dust envelopes; link warm-cold dust emission;
• Improve dust and spectral diagnostics of star formation and ISM properties.
Approach:
• An objectively selected sample of nearby galaxies (SINGS-inspired), optimized to cover a
broad and representative range of properties, and broad range of local physical environments;
• Exploit angular resolution for resolving infrared components and dust heating populations.
• Leverage existing and new ancillary data: from UV to radio
• Data and high-level data products would be delivered quickly to the broad community.
Observational Wishlist
• Spatially resolved measurements, vs disk-averaged
or azimuthally-averaged data
• Extinction corrections (Ha, UV), and corrections for
unextincted star formation (infrared, radio)
• Accurate molecular mass measurements (how
reliable is CO?)
• IMF
NGC 628
(M74)
C. Tremonti
SINGS Galaxies
The Global Schmidt Law Revisited
• analyze galaxies with spatiallymapped star formation (Ha, Pa,
FIR), HI, and CO
• enlarged, diversified samples
– normal galaxy sample 3x larger
– larger ranges in gas and SFR
densities
– large subsamples of circumnuclear
starbursts, low-metallicity galaxies
incorporated
• densities averaged within active
SF regions
• explicit corrections for [NII],
extinction
• point-by-point analysis of
SINGS + BIMA SONG galaxies
Kennicutt (1998) sample
expanded sample
constant extinction
expanded sample
constant extinction
Ha + 24mm extinction corrections
total FIR
Pa + Ha
Ha + 24mm
Ha + Hb
HI + CO (const X)
HI + H2
HI
H2
metal-poor dwarf galaxies
The Spatially Resolved Star Formation Law in M51
Kennicutt et al. 2007, ApJ, in press (astro-ph/0708.0922)
FUV, Ha, 24mm
- Use spatially-resolved measures
of CO, HI, and SFR to characterize
SFR vs gas surface density relation
on a point-by-point basis
- Use combinations of Ha + Pa and
Ha + 24 mm emission to correct for
extinction in SFR measurements
- Probe scales from 300 - 1850 pc
(IR/HII regions to unbiased
sampling of the disk)
Calzetti et al. 2005, ApJ, 633, 871
M51: BIMA SONG Survey
Helfer et al. 2003
NGC 6946– THINGS VLA HI Survey
F. Walter et al.
Local Schmidt Law in M51
Kennicutt et al. 2007
Tentative Conclusions
• The disk-averaged Schmidt law in galaxies is rooted in
a local relationship that persists to scales of <500 pc
• In M51 the SF density is tightly coupled to the local
H2 surface density, and not with HI density
• A kinematic star formation law does not seem to
extend as well to local scales
• The disk-averaged SF law is confirmed with
more/better observations. Some metal-poor galaxies
lie systematically above the mean relation.
Tentative Conclusions
• The combination of Ha and 24 mm imaging provides a
reliable method for obtaining extinction-corrected
ionizing fluxes of HII regions. The combination of Ha
and FIR luminosities can provide reliable extinction-
corrected SFRs of galaxies as a whole.
• As SFR estimators become more reliable, empirical
characterization of the SF law will become increasingly
limited by the accuracy and depth of cold gas tracers,
especially for molecular gas.
Primary Datasets
• Spitzer Infrared Nearby Galaxies Survey (SINGS)
– resolved UV  radio mapping of 75 galaxies
– selection: maximize diversity in type, mass, IR/optical
• 11 Mpc Ha/Ultraviolet Survey (11HUGS)
– resolved Ha, UV imaging, integrated/resolved IR of 400 galaxies
– selection: volume-complete within 11 Mpc (S-Irr)
• Survey for Ionization in Neutral-Gas Galaxies (SINGG)
– resolved Ha, UV imaging, integrated/resolved IR of 500 galaxies
– selection: HI-complete in 3 redshift slices
• Integrated Measurements
– Ha flux catalogue (+IR, UV) for >3000 galaxies within 150 Mpc
- integrated spectra (+IR, UV) for ~600 galaxies in same volume
(Moustakas & Kennicutt 2006, 2007)
Physical Origins of SF Law
• Beware the treacheries of correlation vs
causation!
– For a Q ~ 1 disk: Sgas ~ kc/pG
k a W so Sgas a W
– Likewise:
Sgas a Stot , so P a S2
– Also, for local Galactic ISM pressures, Scrit for
self-gravitating clouds is approximately the same as
Scrit for self-shielding of molecular clouds
– And--- in SF regions much of HI may be a
photodissociation product of UV radiation on H2
Thanks to:
S. Akiyama, J. Lee, C. Tremonti, J. Moustakas, C. Tremonti
(Arizona), J. Funes (Vatican), S. Sakai (UCLA), L. van Zee (Indiana)
+
The SINGS Team: RCK, D. Calzetti, L. Armus, G. Bendo, C. Bot,
J. Cannon, D. Dale, B. Draine, C. Engelbracht, K. Gordon, G. Helou,
D. Hollenbach, T. Jarrett, S. Kendall, L. Kewley, C. Leitherer, A. Li,
S. Malhotra, M. Meyer, E. Murphy, M. Regan, G. Rieke, M. Rieke,
H. Roussel, K. Sheth, JD Smith, M. Thornley, F. Walter
SFR surface density
starburst galaxies
normal galaxies
HI+H2 mass surface density
HII regions
Calzetti et al., ApJ, submitted
Kennicutt & Moustakas, in prep
galaxies (integrated fluxes)
Kennicutt 1998, ApJ, 498, 541
Wang & Heckman 1996, ApJ, 547, 965
Martin et al. 2005, ApJ, 619, L59
Ha + R
Ha + R