Dust/Gas Correlation in the Large Magellanic Cloud: New Insights from the HERITAGE and MAGMA surveys Julia Roman-Duval July 14, 2010 HotScI

Why care about dust and gas in the ISM ? Constrain galactic evolution models  SFR   g (Schmidt 1959, Kennicutt 1998),  SFR   H2 (Bigiel et al. 2008, Leroy et al. 2008) Dust shields molecular clouds from interstellar radiation field, allows molecules to form and gas to cool, and SF to proceed We can observe dust easily (emission of absorbed stellar light in FIR) H 2 does not have a dipole moment = no emission at gas temperature Use CO as a tracer of H 2, problem in low metallicity galaxies

OBJECTIVE 1. Provide a brief summary of the effects of metallicity on dust/gas correlation and the structure of MCs 2. CO-dark molecular gas problem 3. Present current and new data sets that will shed more light on the issue 4. Present preliminary results for the LMC derived from Herschel data from the Science Demonstration Phase (SDP)

Dust, CO, H 2 Dust, H 2, C, C + Dust, H, C + Wolfire et al. (2010) Unlike CO, H 2 self-shielded due to absorption lines in the UV Interstellar Radiation Field (ISRF) attenuated by DUST  dust =  gas /GDR

Metallicity Effects CO fraction determined by A V (photo-dissociation) Glover et al. (2010) H 2 fraction determined by nZ (formation timescale)

Metallicity Effects Bolatto et al. (1999) Expect a significant fraction of molecular gas to be invisible to CO observations in low metallicity galaxies H 0 /Z = thickness of the C 0 region  H 0 /Z = thickness of the C 0 region A V  Z for a given gas mass

FIR Excess from SPITZER Observations of the LMC Bernard et al. (2008) Correlation between FIR emission from dust and N H expected from a constant GDR measured in diffuse regions where no molecular gas is expected to exist Excess of FIR emission compared to the measured gas column (from HI 21 cm and CO emission) at high columns => CO dark molecular gas ?

N x = GDR N dust - N(HI) - N(H 2 CO ) FIR excess in the LMC FIR excess correlated with high density regions

FIR excess and CO-dark molecular gas 1.Is the FIR excess due to CO dark molecular gas ? 2.Can dust/FIR measurements trace this CO- dark molecular gas ?

DATA sets LMC (Z = 0.5 Z o ), SMC (Z = 0.2 Z o ) closest low-Z galaxies Pre-Herschel data sets from the Surveying the Agents of Galactic Evolution (SAGE) project (Meixner et al. 2006): –SPITZER/MIPS (  m, 40” resolution) –SPITZER/IRAC (3-8  m) –IRAS (  m, 4.3’ resolution) –NANTEN 12 CO (J = 1-0) (2.6’ resolution) –ATCA+Parkes HI 21 cm (1’ resolution) Limited by IRAS resolution (63 pc at the distance of the LMC) to estimate dust SED and dust/gas surface density Width of the CO dark molecular gas region (  A V = 0.7) – L =  A V N 0 /( Z) = 0.8 n= 1000 cm -3 – L = n = 100 cm -3 Need higher resolution to better resolve the structure of molecular clouds envelopes

New data sets FIR Herschel HERITAGE (HERschel Inventory of The Agents of Galactic Evolution) Resolution limit: SPIRE 500 (40”) Images of the entire LMC and SMC at 100, 160 (PACS), 250, 350, 500 (SPIRE)  m. 12 CO MAGMA (MAGellanic MOPRA Assessment ( 12 CO follow up on molecular clouds detected by NANTEN) 40” resolution, 0.5 K km/s sensitivity

Herschel SDP: Dust surface density Gordon et al. (2010) Meixner et al. (2010)

IRAS 100  m resolution (4.3’)  dust (IRAS+MIPS)  (HI) (ATCA+Parkes HI 21 cm)  (H 2 CO ) NANTEN 12 CO 1’ resolution  dust (MIPS + HERSCHEL)  (HI) (ATCA+Parkes HI 21 cm)  (H 2 CO ) NANTEN 12 CO (2.3’ resolution) NT80 at 1’ resolution  dust (MIPS + HERSCHEL)  (HI) (ATCA+Parkes HI 21 cm)  (H 2 CO ) MAGMA 12 CO NT80 at 4.3’ resolution  dust (IRAS+MIPS)  (HI) (ATCA+Parkes HI 21 cm)  (H 2 CO ) NANTEN 12 CO NT80

IRAS 100  m resolution (4.3’)  dust (IRAS+MIPS)  (HI) (ATCA+Parkes HI 21 cm)  (H 2 CO ) NANTEN 12 CO 1’ resolution  dust (MIPS + HERSCHEL)  (HI) (ATCA+Parkes HI 21 cm)  (H 2 CO ) NANTEN 12 CO (2.3’ resolution) NT80 at 4.3’ resolution  dust (IRAS+MIPS)  (HI) (ATCA+Parkes HI 21 cm)  (H 2 CO ) NANTEN 12 CO NT80 at 1’ resolution  dust (MIPS + HERSCHEL)  (HI) (ATCA+Parkes HI 21 cm)  (H 2 CO ) MAGMA 12 CO NT71

Dust/Gas correlation Roman-Duval et al. (2010)

Dust/gas spatial correlation Roman-Duval et al. (2010) NT80  dust (1 st panel) GDR  dust /(  (HI) +  (H 2 CO )) ---  (HI) ---  (H 2 CO ) Regions of FIR excess

Dust/gas spatial correlation Roman-Duval et al. (2010) NT71  dust (1 st panel) GDR  dust /(  (HI) +  (H 2 CO )) ---  (HI) ---  (H 2 CO ) Regions of FIR excess Excess of FIR emission compared to the observed gas surface density near the envelopes of MCs consistent with CO dark molecular gas

Variations of X CO with A v Glover et al. (2010) Transition between CO core and CO-free H 2 envelope

Variations of X CO with A v Roman-Duval et al. (2010) Preliminary results

Emissivity variations –Grain coagulation in dense, molecular regions (Paradis et al. 2009) Gas-to-dust ratio variations –Dust destruction in the diffuse ISM by shocks (Jones et al. 1996) –Grain growth in the dense molecular phase Other possible causes for observed deviations in the dust/gas correlation Emissivity GDR NT80NT71 Assumptions

Dust temperature in ISM phases NT80 SFR = M o /kpc 2 /yr NT71 SFR = M o /kpc 2 /yr

Ongoing/future work Coming soon: full HERITAGE Mosaics of the LMC and SMC in PACS100, PACS160, SPIRE250, SPIRE350, SPIRE500  m Epoch 1 has been reduced, scientific analysis under way Include H 2 radiative transfer, cooling and heating, and basic H 2 and CO chemistry in DIRTY radiative transfer code Extend the SDP analysis to larger sample of MCs in the SMC, where metallicity effects are more important

Conclusion Molecular cloud envelopes in low metallicity galaxies (e.g., LMC, SMC) galaxies probably hide large amounts of molecular gas not traced by CO Deviations in the dust/gas surface density correlation (FIR excess) are not likely to be caused by gas-to-dust ratio or emissivity variations between the diffuse and dense phases Envelopes of H 2 not traced by CO are more likely Dust emission in the FIR is potentially a good tracer of this CO-dark molecular gas (see also, Isarel et al. 1997, Leroy et al. 2007, 2009) The dust temperature is lower by a few degrees In the molecular phase compared to the diffuse phase

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