The Resolved Properties of Extragalactic Giant Molecular Clouds Alberto D. Bolatto University of Maryland Adam Leroy (MPIA-H) Erik Rosolowsky (UBC) Fabian Walter (MPIA-H) Leo Blitz (UCB)

Take home punch line CO-bright portions of extragalactic giant molecular clouds are almost* identical to Galactic giant molecular clouds CO-bright portions of extragalactic giant molecular clouds are almost* identical to Galactic giant molecular clouds But CO tells only part of the story * Details are important

Tying gas to star formation Gas  SF is at the core of structure formation models Gas  SF is at the core of structure formation models GMCs mediate the process GMCs mediate the process Leroy et al. (2008), submitted Kennicutt (1989/98)- Schmidt Law SFR  ∑(H I +H 2 ) 1.4

GMCs obey simple scaling relations First recognized by Larson (1979,1981) First recognized by Larson (1979,1981) Attributed to Kolmogorov-like turbulence Attributed to Kolmogorov-like turbulence Today we attribute it to compressible, shock-dominated supersonic (Burger’s) turbulence Today we attribute it to compressible, shock-dominated supersonic (Burger’s) turbulence σ=0.72 R pc 0.5 km/s σ=0.72 R pc 0.5 km/s The coefficient is related to surface density The coefficient is related to surface density M vir ~Rσ 2  Σ=M/R 2 ~const M vir ~Rσ 2  Σ=M/R 2 ~const Σ=170 Mo/pc 2 Σ=170 Mo/pc 2 Solomon et al. (1987)

Mass-Luminosity relation Luminosity-line width and luminosity-size relations yield mass-luminosity relation Luminosity-line width and luminosity-size relations yield mass-luminosity relation M vir =39 L co 0.8 M sun M vir =39 L co 0.8 M sun Quasi-linearity gives rise to X co, the CO-to-H 2 conversion factor Quasi-linearity gives rise to X co, the CO-to-H 2 conversion factor In the MW, virial,  - ray, and dust continuum –derived values of X co are consistent In the MW, virial,  - ray, and dust continuum –derived values of X co are consistent Solomon et al. (1987)

Galaxy sample A combination of BIMA, OVRO, PdBI, and SEST datasets A combination of BIMA, OVRO, PdBI, and SEST datasets Analyzed in a consistent manner ( CPROPS code: Rosolowsky & Leroy 2006) Analyzed in a consistent manner ( CPROPS code: Rosolowsky & Leroy 2006) Spatial resolution pc Spatial resolution pc Metallicity 1/5 to solar Metallicity 1/5 to solar A large range of environments A large range of environments Bolatto et al. (2008), ApJ, in press

Measuring extragalactic GMC properties The tip of the iceberg in the sea of noise… NGC 3077 in CO CPROPS: Rosolowsky & Leroy 2006

The size-line width relation in galaxies Solomon et al. (1987) points corrected for methodology and GC distance Solomon et al. (1987) points corrected for methodology and GC distance

The size-line width relation in galaxies Solomon et al. (1987) points corrected for methodology and GC distance Solomon et al. (1987) points corrected for methodology and GC distance M31/M33 from Rosolowsky et al. (2003,2007) M31/M33 from Rosolowsky et al. (2003,2007)

The size-line width relation in galaxies Solomon et al. (1987) points corrected for methodology and GC distance Solomon et al. (1987) points corrected for methodology and GC distance M31/M33 from Rosolowsky et al. (2003,2007) M31/M33 from Rosolowsky et al. (2003,2007)

The size-line width relation in galaxies Solomon et al. (1987) points corrected for methodology and GC distance Solomon et al. (1987) points corrected for methodology and GC distance M31/M33 from Rosolowsky et al. (2003,2007) M31/M33 from Rosolowsky et al. (2003,2007) Fairly consistent with MW relation! Fairly consistent with MW relation! Suggests similar surface densities Suggests similar surface densities Worst outliers: small SMC/LMC/IC10 clouds Worst outliers: small SMC/LMC/IC10 clouds

The size-line width relation in galaxies Solomon et al. (1987) points corrected for methodology and GC distance Solomon et al. (1987) points corrected for methodology and GC distance M31/M33 from Rosolowsky et al. (2003,2007) M31/M33 from Rosolowsky et al. (2003,2007) Fairly consistent with MW relation! Fairly consistent with MW relation! Suggests similar surface densities Suggests similar surface densities Worst outliers: small SMC/LMC/IC10 clouds Worst outliers: small SMC/LMC/IC10 clouds σ=0.44 R pc 0.6 km/s for all eGMCs σ=0.31 R pc 0.65 km/s for dwarf eGMCs Impossing R 0.5, Σ dwarfs ~85 M o /pc 2

Luminosity-mass relation M vir ~Lco 0.8 in MW M vir ~Lco 0.8 in MW Solomon et al. (1987) justifies the “0.8” in some detail Solomon et al. (1987) justifies the “0.8” in some detail We find a remarkably compatible and “flat” relation We find a remarkably compatible and “flat” relation M vir ~Lco 1.0 for eGMCs M vir ~Lco 1.0 for eGMCs Implications for CO-to-H2 factor Implications for CO-to-H2 factor

Photoionization-regulated star formation? Star forming clouds need similar extinctions at their centers to decouple from magnetic support and collapse (McKee 1989) Theory predicts σ =0.72 (Av/7.5 δ gr ) 0.5 R 0.5 Measurements show no evidence for that trend Caveats: are we reaching the relevant scales? Is the prediction overly simplistic?

The CO-to-H2 conversion factor Ratio of luminous to virial mass Ratio of luminous to virial mass Not surprisingly no strong trend with metallicity Not surprisingly no strong trend with metallicity Note that SMC clouds are completely compatible with MW and the Solomon 0.8 slope: M vir /Lco~Lco -0.2 Note that SMC clouds are completely compatible with MW and the Solomon 0.8 slope: M vir /Lco~Lco -0.2

The Spitzer view of H 2 in the SMC at 70 pc resolution Use 100 and 160 um to model τ dust Use 100 and 160 um to model τ dust Use τ dust ~N(HI)+2N(H 2 ) Use τ dust ~N(HI)+2N(H 2 ) Determine DGR locally Determine DGR locally M H2 ~3x10 7 M sun total molecular mass, compared to M HI ~2x10 8 M sun M H2 ~3x10 7 M sun total molecular mass, compared to M HI ~2x10 8 M sun This means Xco~30-60 times Galactic! This means Xco~30-60 times Galactic! Furthermore, Σ FIR ~180 M o /pc 2, while Σ VIR ~45 M o /pc 2 ! Furthermore, Σ FIR ~180 M o /pc 2, while Σ VIR ~45 M o /pc 2 ! Leroy, Bolatto, et al. (2007)

What happens at higher resolution? We work out a “VSG-corrected” emissivity in the large scales, and assume it in the small scales We work out a “VSG-corrected” emissivity in the large scales, and assume it in the small scales We map the CO and H 2 distribution at 15 pc scales We map the CO and H 2 distribution at 15 pc scales CO emission is seen only at Av>1.6, Σ~350 M o /pc 2 CO emission is seen only at Av>1.6, Σ~350 M o /pc 2 How do we put together this picture with the kinematic studies? How do we put together this picture with the kinematic studies? Leroy, Bolatto, et al. (2008, in prep.)

What is really going on? Metallicity and cloud structure GMCs are just the peaks of the density distribution in the SMC GMCs are just the peaks of the density distribution in the SMC GMC internal kinematics only sample the potential of the CO emitting volume (hence low Σ VIR ) GMC internal kinematics only sample the potential of the CO emitting volume (hence low Σ VIR ) But there are are vast envelopes with most of the mass, just not enough Av, surrounding them (hence large Σ FIR ) But there are are vast envelopes with most of the mass, just not enough Av, surrounding them (hence large Σ FIR ) This picture is consistent with the observer GMC-to-GMC velocity dispersion This picture is consistent with the observer GMC-to-GMC velocity dispersion It requires H 2 self-shielding to be important at Z~1/5 It requires H 2 self-shielding to be important at Z~1/5 Low Av H 2 somehow partakes in star-formation? Low Av H 2 somehow partakes in star-formation? SMC Outer disks? MW Israel et al. (1987); Maloney & Black (1988); Elmegreen (1989); Rubio et al. (1993)

The future

GLAST: launch in June. May allow direct  -ray H 2 estimate in the Magellanic Clouds ALMA: end of The panacea for every mm-wave sensitivity problem HERSCHEL: launch in early Mapping and FIR spectroscopy - KINGFISH

Summary and Conclusions Our study of extragalactic GMCs shows remarkable similarities in the properties of the CO-bright regions: Our study of extragalactic GMCs shows remarkable similarities in the properties of the CO-bright regions: 1. Larson relations are universal 2. Surface densities are similar to MW (Σ~85 Mo/pc 2 ) 3. Mass-Luminosity relation is similar too: Xco is approximately Galactic inside resolved GMCs Nevertheless metallicity plays a role: large molecular envelopes are invisible in CO in the SMC Nevertheless metallicity plays a role: large molecular envelopes are invisible in CO in the SMC Photoionization-regulated star formation? Photoionization-regulated star formation?

Mapping molecular gas in the Blue Sequence

Multiwavelength study of NGC 604 Nearest extragalactic giant star forming region accessible from the north (M33) Nearest extragalactic giant star forming region accessible from the north (M33) Multiwavelength effort to understand the GMCs in relation to the star formation Multiwavelength effort to understand the GMCs in relation to the star formation R: CARMA CO G: HST H  B: HST V

The present The CARMA STING: Survey Toward IR- bright Nearby Galaxies The CARMA STING: Survey Toward IR- bright Nearby Galaxies Purpose: to extend the interferometric CO mapping to the H I /H 2 transition and beyond Purpose: to extend the interferometric CO mapping to the H I /H 2 transition and beyond Bolatto, Wong, Blitz, Ott, Leroy, Walter, Rosolowsky, West, Calzetti, et al. SONG STING

The size-line width relation is universal Individual parts of GMCs also follow the size-line width relation, down to very small scales Individual parts of GMCs also follow the size-line width relation, down to very small scales Heyer & Brunt (2004); Rosolowsky et al. (2008)

Departures from the standard relation The size-lw relation breaks down for small clouds in the outer disk The size-lw relation breaks down for small clouds in the outer disk These are likely not self- gravitating, but pressure confined These are likely not self- gravitating, but pressure confined Oka et al. (2001) find higher Σ near GC Oka et al. (2001) find higher Σ near GC Heyer et al. (2001)

Luminosity-line width relation New: excitation of CO New: excitation of CO In “CO mist” (Dickman et al. 1986) model Lco~M In “CO mist” (Dickman et al. 1986) model Lco~M Lco~σ 5 for MW Lco~σ 5 for MW Lco~ σ 3.4 for eGMCs, driven by SMC points Lco~ σ 3.4 for eGMCs, driven by SMC points

Luminosity-size relation Lco~25 R 2.5 for MW Lco~25 R 2.5 for MW Lco~7 R 2.5 for eGMCs Lco~7 R 2.5 for eGMCs Is the offset methodological? Difficult to say, but displacements look too large for that Is the offset methodological? Difficult to say, but displacements look too large for that

Resolved brightness temperature Product of CO kinetic temperature and filling factor Product of CO kinetic temperature and filling factor Indicator of cloud porosity and temperature Indicator of cloud porosity and temperature “Typical” MW cloud fits most galaxies, a smaller version fits most of the rest “Typical” MW cloud fits most galaxies, a smaller version fits most of the rest Some galaxies need to be hotter, the SMC seems more porous Some galaxies need to be hotter, the SMC seems more porous

Is self-shielding dominant at low Z? (D. Hollenbach) Is self-shielding dominant at low Z? (D. Hollenbach) Assume dust shielding is unimportant. Equate H 2 formation and photodestruction Assume dust shielding is unimportant. Equate H 2 formation and photodestruction R n n(H) = G 0 I 0 f s n(H 2 ) where the H 2 self-shielding and formation coefficients are f s ~  N(H 2 ) -0.5, R ~ R 0 Z Integrate the above equation Integrate the above equation R n n(H) dx = G 0 I 0  N(H 2 ) -0.5 n(H 2 ) dx  R n N(H) = 2 G 0 I 0  N(H 2 ) 0.5 Where is the H-H2 transition? (N(H)=N(H 2 )=½N ½ ) Where is the H-H2 transition? (N(H)=N(H 2 )=½N ½ ) ½ R n N ½ = 2 (½) 0.5 G 0 I 0  N ½ 0.5  N ½  (G 0 I 0  ) 2 (n R ) -2 Therefore Therefore N ½  Z -2 for self-shielding!

Caveats Linearity of the MIPS 160 um detectors Linearity of the MIPS 160 um detectors Checked against DIRBE Checked against DIRBE Systematic differences in dust properties or DGR between “molecular” and “reference” regions Systematic differences in dust properties or DGR between “molecular” and “reference” regions Large spatial scales Large spatial scales Works for the MW (Dame et al. 2001) Works for the MW (Dame et al. 2001) Missing cold dust Missing cold dust