Molecular Gas and Star Formation in Nearby Galaxies

Tony WongTony Wong

Bolton Fellow

Australia Telescope National Facility

Outline1. Observations of molecular gas in galaxies

– CO single-dish

– CO interferometry

– (Sub)millimetre dust emission

– UV absorption

2. Current issues in relating H2 to star formation

– Radial CO distributions, vs. HI and stellar light

– The Schmidt law within galaxies

– Triggered (sequential) star formation

CO as a Tracer of H2

Advantages of the CO molecule:

1. Most abundant trace molecule: 10-5 of H2

2. Rotational lines easily excited: E10/k = 5.5 K

3. Effective critical density quite low, due to high opacity: ncr/ ~ 300 cm-3

Disadvantages:

1. Optically thick in most regions

2. Not as self-shielding as H2

3. Expect low abundance in metal-poor regions

CO Single-Dish Studies

1. 300 galaxies, incl. most bright northern ones

2. CO usually peaked toward galaxy centres (Young et al. 1995)

3. CO linearly related to star formation tracers (Rownd & Young 1996) except in merging or interacting galaxies (Young et al. 1996)

4. Molecular gas not easily stripped by intracluster medium (Kenney & Young 1986, 1989)

The baseline for our understanding of H2 in galaxies

FCRAO Extragalactic CO Survey:

Local Group: LMC

CO (1-0)

4m NANTEN telescope (2.6’ ~ 40 pc)

Fukui et al. 1999, 2001

168 GMCs identified

Local Group: M31

30m IRAM (23” ~ 70 pc)

Neininger et al. 2001

• CO in narrow arms extending into inner disk

• No structure comparable to Milky Way’s Molecular Ring

•CO appears to trace H2 well (no dust extinction w/o CO)

CO InterferometryIndividual case studies (e.g. NGC 4736)

Wong & Blitz 2000, BIMA E. Schinnerer, PdB

Large-Scale Mapping: BIMA SONG

44 nearby spirals

6”-9” resolution

Most maps extend to 100” radius or more

Single-dish data included

Helfer et al. 2003,ApJS 145:259

High Resolution Towards Nuclei

IRAM PdB NUGA

NGC 1068

(Baker 2000)

NGC 4826

(García-Burillo et al. 2003)

OVRO MAIN

Other Probes of H2

(Sub)millimetre dust emission

• Reveals cold dust not seen by IRAS

• Conversion to NH depends on Td (but only linearly),

grain parameters, and gas-to-dust ratio

• Very good correlation with CO (Alton et al. 2002)

UV absorption towards continuum sources

• Extremely sensitive tracer of diffuse H2

• Tumlinson et al. 2002: diffuse H2 fraction in MCs

very low (~1% vs. ~10% in Galaxy)

CO Profiles from BIMA SONGR

eg

an

et a

l. (2001)

CO Profiles from BIMA SONGOf 27 SONG galaxies for which reliable CO profiles could be derived, 19 show evidence of a central CO excess corresponding to the stellar bulge.

12

15

3

6

9

SA SAB/SB

Central excessNo central excess

(5) (6)

(14)

(2)

2

4

6

8

10

Sab/Sb Sbc Sc/Scd

Central excessNo central excess

Thornley, Spohn-Larkins, Regan, & Sheth (2003)

CO excesses are found in galaxies of all Hubble types, and preferentially in galaxies with some bar contribution (SAB-SB).

CO vs. HI Radial ProfilesOverlaid CO (KP 12m) and HI (VLA)

images

Crosthwaite et al. 2001, 2002

CO vs. HI Radial ProfilesIC 342 M83

Crosthwaite et al. 2001, 2002

HI

CO

Atomic to Molecular Gas Ratio

Wong & Blitz (2002) found evidence for a

strong dependence of the HI/H2 ratio on the

hydrostatic midplane pressure.

Consistent with ISM modelling (e.g.

Elmegreen 1993) & observations of star formation “edges.”

The Edge-On Spiral NGC 891

WSRT HI

Sw

ate

rs,

San

cisi

, &

van

der

Hu

lst

(19

97

)

BIMA CO

10

kp

c

Won

g,

How

k, &

van

der

Hu

lst

The Star Formation Law

Various empirical “laws” have been devised to explain correlations between SFR and other quantities, the most popular being the Schmidt law:

SFR (gas)n

Ken

nicu

tt 1998

n=1.4 ± 0.15

Determining the SFR

A difficulty with such studies is estimating SFRs from H fluxes, which are subject to extinction.

Determining the SFR

Kewley et al. (‘02) derive a correction factor of ~3 for H, and conclude that LIR is a

better SFR indicator.

Considering HI and H2 Separately

Within galaxies, the SFR surface density is roughly proportional to (H2) but is poorly correlated with HI.

Wo

ng

& B

litz 2002

Origin of Schmidt Law Index

1. Stars form on dynamical timescale of gas:

5.15.0)( gas

gas

gasSFR G

2. Stars form on a constant timescale from H2 only:

,molSFR mgasmolf )4.0( m

Normalisation of the Schmidt Law

Elmegreen (2002) derives the observed SF timescale from the fraction of gas above a critical density of ~105 cm–3, which in turn is determined by the density PDF resulting from turbulence.

See also Kravtsov (2003).

Sequential Star FormationCan pressures from one generation of stars compress surrounding gas to form a new generation?

Ya

ma

gu

ch

i et a

l. 200

1

Summary1.High-resolution observations of molecular gas in

nearby galaxies, using the CO line as a tracer, are becoming available for large numbers of galaxies.

2.At high resolution, CO radial profile often shows a depression or excess relative to exponential.

3.The CO/HI ratio decreases strongly with radius, mainly due to decreasing interstellar pressure.

4.The SFR (traced by Ha or IR emission) is well-correlated with CO but not necessarily HI.

5.The ‘universality’ of the Schmidt law may be related to the generic nature of turbulence.