Star Formation: Near and Far

Neal J. Evans IIwith Rob Kennicutt

Far: Whole Galaxy RelationsSolid circles are disk-averaged normal spiralsOpen circles are central regions of normal disksSquares are circumnuclear starbursts

Slope is 1.4±0.15

Kennicutt 1998, ARAA 36, 189

StarburstsSpirals

black: normal galaxiesred: starburstsgreen: circumnuclear starburstsblue open: Low metals (<~1/3 solar), mostly dwarfsBlue line: slope of 1.4, not a fit

RCK, in preparation

SFR/Mass Increases with SFR SFR/Mass of molecular

gas increases with SFR Factor of ~ 100 “Efficiency” increasing But what does this really

mean?

Solomon & Vanden Bout (2005 ARAA)

Sta

r for

mat

ion

effic

ienc

y

Star formation Rate

The Dense Gas SF Relation

LFIR correlates better with L(HCN)

Smaller scatter Higher rate SFR rate linearly

proportional to amount of dense gas

“Efficiency” for dense gas stays the same

Gao & Solomon (2004) ApJ 606, 271

Amount of dense molecular gas

Sta

r for

mat

ion

r ate

Whole Galaxy Prescriptions

Kennicutt (1998) SFR(Msun yr–1 kpc–2) = 2.5x10–4

gas(Msun pc–2) Gao and Solomon (2004)

SFR (Msun/yr) ~ 1.8 x 10–8 M(dense) (Msun) SFR(Msun yr–1 kpc–2) = 1.8x10–2 0

dense(Msun pc–2)

What Does SFR Mean?

SFR is grand average over: Whole galaxy, with huge variations in

SFR, gas, metallicity, … Time

~5 Myr for Ha ~ 30-100 Myr for UV, 5-100 Myr for FIR (short for starbursts)

What Does gas Mean?

“gas” is not the mean surface density of any structure.

At best, the filling factor x mean cloud emission times X(CO)

Higher “gas” really means more clouds in beam

CO: Limited Dynamic Range

Heiderman et al. 2010

CO can be off by large factors in some regions. It clearly fails for AV > 10 mag.

Need AV >0.4 mag for CO, but issues below 3 mag (Pineda et al. 2010)

Not so Bad on Average

12CO underestimates AV at gas > 200 M pc–

2 by 30%

Constant value of 13CO vs gas, underestimating gas by factors of 4-5

Correcting for 12CO, would flatten the slope of the Kennicutt-Schmidt relation (but does not explain big offset)

Intermediate: Resolved Studies

Radial cuts or averages Martin and Kennicutt (2001): threshold Schruba et al. (2011): SF continues even

when HI > H2

Pixel by pixel: e.g., Kennicutt et al. (2007) Bigiel et al. (2008) Blanc et al. (2009)

Sub-kpc scales

Bigiel et al. 2008

Study of 18 nearby galaxies with sub-kpc resolution in HI, CO. SFR from UV+24 micronThreshold around 10 Msunpc–2 in total gas: transition from HI to H2

CO, SF continue into HI region

Schruba et al. 2011

SFR ~ I(CO) even in HI dominated outer parts

Star Formation Prescriptionsfor sub-kpc scales

Kennicutt et al. (2007) M51 SFR(Msun yr–1 kpc–2) = 1.7x10–4 37

mol(Msun pc–2) Bigiel et al. (2008)

SFR(Msun yr–1 kpc–2) = 7.9x10–3 0mol(10 Msun pc–2)

SFR(Msun yr–1 kpc–2) = 7.9x10–4 0mol(Msun pc–2)

Blanc et al. (2009) M51 SFR(Msun yr–1 kpc–2) = 5.1x10–2 0.82

mol(Msun pc–2) Includes 0.43 dex scatter in SFR and includes limits

Issues of tracer, diffuse emission, fitting method

Star Formation PrescriptionsTheory

Schmidt (1959) SFR ~ n, n = 1 or 2 (or n, 2009)

Krumholz et al. (2009) SFR = f(gas, f(H2), Z, clumping) Nearly linear with mol below ~ 100 Msun pc–2

Steepens above 100 Msun pc–2

Other dynamical relations

The Predictions

17

Very Near: Clouds in Solar Neighborhood

Spitzer Programs

c2d + Gould Belt:20 nearby molecular clouds (blue circles)

Cluster Project:35 young stellar clusters(red circles)

90% of known stellar groups and clusters within 1 kpc(complete to ~ 0.1 MSun)

Whole Clouds (2-16 pc)

Heiderman et al. 2010

Almost all clouds within 300 pcTotal SFR from YSO counting /areaTotal mass/area

Clouds within 1 kpc

Adds Orion, Mon R2, S140, Cep OB3, all forming more massive stars, and North America nebula, less active

not complete to 1 kpc, but representative

It’s Worse than that…

Gray is extinction, red dots are YSOs, contours of volume density (blue is 1.0 Msun pc–3; yellow is 25 Msun pc–3)

Really Near: Within Clouds

Heiderman et al. 2010

Less Near: Add Clouds to 1 kpc

Gutermuth et al. subm.

N = 2.67

N = 1.87

Cep OB3

Gutermuth et al. subm.

Still Less Near: Dense Clumps

L(HCN J = 1-0)

L(IR

)

Wu et al. (2005)

Survey of dense clumps across MW.(n ~ 105 to 106 cm–3)Birthsites of large clusters.

Follow linear relation very similar to dense gas relation for starbursts, as long as LFIR > 104.5 Lsun.

Dense Clumps on gas-SFR

Using LFIR to get SFR, likely underestimates.Includes fit from Wu et al.

Combine with Nearby CloudsFit with broken powerlaw with slopes of 4.6 below and 1.1 above a turnover gas = 129+-14 Msun pc–2.(see Lada et al. 2010)

Gutermuth et al. favor continued rise withSFR ~ gas

2 throughout.

All agree: well above all exgal relations except for dense gas relation.

Lessons from Nearby Clouds

SFR >10 times prediction of relations for galaxies

SFR determined on sub-pc scales << exgal resolution

On scales where SF actually happens… Dependence on Smol may be very strong, at least up to

Smol~ 100 Msun pc–2

Speculation The underlying SF law is linear in gas above a noisy

threshold ~ 100 Msunpc–2

10 times exgal relations around threshold. Fraction of gas above threshold (fdense) increases with

<> as 0.5 for <> >100 Msunpc–2

When <> ~ 100 th, fdense ~1 KS prescription and Dense gas prescription agree

What about linear relations in resolved studies of non-starbursts? fdense ~ constant below <> ~ 100 Msunpc–2?

Issues for Resolved Studies

SFR have be restricted to local SF Remove diffuse emission Use tracer with short timescale

Clouds are not resolved, much less clumps “gas” is still not that of any structure Small number statistics cause larger spread

Massive stars can destroy clouds SF tracers and gas may even anti-correlate

30

Observe the Solar Neighborhood from Outside

Size and location of beam/pixel causes huge variations

All centered on Sun100 pc: No SF, no CO300 pc: SF, CO, but no Ha, little 24 mm500 pc: SF, Ha, CO

What would we see?

300 pc, count YSOs,

500 pc, count YSOs

300 pc, using Ha, remove diffuse emission

500 pc, using Ha, remove diffuse emission, assume standard L(Ha) to SFR

Bigiel et al. 2011

The Larger Context of MW

Surveys in mm continuum finding 1000’s of dense clumps Bolocam Galactic Plane Survey (>8000 sources) http://irsa.ipac.caltech.edu/data/BOLOCAM_GPS/ ATLASGAL survey from APEX Future SCUBA2 survey Herschel Galactic Plane Survey (HIGAL)

Infrared Dark Clouds (IRDC) MSX, GLIMPSE, MIPSGAL

New models of Galaxy, VLBA distances, … Provide link to extragalactic star formation

The Improved Milky Way Model

Green and blue dots show VLBA measurements of distance, which align star-forming regions along spiral arms much better than previous distances.

Summary

Star formation highly concentrated to dense regions Steep increase in SFR to at least gas > 120 Msun pc–2

10-20 x more SF than predicted by any prescriptions SFR ~ Mass of gas above a threshold density Non-linear nature of KS relation:

A consequence of fdense ~ <gas>0.5? Resolved studies of galaxies must watch for systematic

issues

Backup Slides

A Popular Explanation for Non-linear Relation

Free-fall time depends on volume density tff ~ r–0.5

Common theoretical approach Krumholz and Thompson Narayanan et al. SFR ~ Mass/tff dr*/dt ~ r/r–0.5 ~ r1.5

Local version of Kennicutt relation

Any evidence for this?

Mean density from virial mass and radius of well-studiedsample of dense clumps. <n> ~ M/r3 (Wu et al. 2010)

~ S

FR

Nor in YSO Counting

Yellow stars are from Class I and Flat SED SFRs in c2d+GBClouds.

Milky Way Estimates

Volume filling factor of molecular gas (as traced by CO) is about 0.005 (Heyer, prelim estimate)

Volume filling factor of clumps (density of few x 103 cm–3) < 10–5 (M. K. Dunham, prelim estimate)