dust/gas correlation in the large magellanic cloud: new insights from the heritage and magma surveys...
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Dust/Gas Correlation in the Large Dust/Gas Correlation in the Large Magellanic Cloud:Magellanic Cloud:
New Insights from the HERITAGE New Insights from the HERITAGE and MAGMA surveysand MAGMA surveys
Julia Roman-DuvalJulia Roman-DuvalJuly 14, 2010July 14, 2010
HotScIHotScI
Why care about dust and gas in the ISM ?Why care about dust and gas in the ISM ?
• Constrain galactic evolution models
SFR g1.4-1.5 (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)
• H2 does not have a dipole moment = no emission at gas temperature
• Use CO as a tracer of H2, problem in low metallicity galaxies
OBJECTIVEOBJECTIVE1. 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, H2
Dust, H2, C, C+
Dust, H, C+
Wolfire et al. (2010)
Unlike CO, HUnlike CO, H22 self-shielded due to self-shielded due to
absorption lines in the UVabsorption lines in the UV
Interstellar Radiation Field (ISRF) attenuated by DUST
dust = gas/GDR
Metallicity EffectsMetallicity Effects
CO fraction determined by AV (photo-dissociation)
Glover et al. (2010)
H2 fraction determined by nZ (formation timescale)
Metallicity EffectsMetallicity Effects
Bolatto et al. (1999)
Expect a significant fraction of molecular gas to be invisible to CO observations in low metallicity galaxies
H0/Z = thickness of the C0 regionH0/Z = thickness of the C0 region
AV Z for a given gas mass
FIR Excess from SPITZER FIR Excess from SPITZER Observations of the LMCObservations of the LMC
Bernard et al. (2008)
Correlation between FIR emission from dust and NH 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 ?
Nx = GDR Ndust - N(HI) - N(H2
CO)
FIR excess in the LMCFIR excess in the LMC
FIR excess correlated with high density regions
FIR excess and CO-dark FIR excess and CO-dark molecular gasmolecular gas
1. Is the FIR excess due to CO dark molecular gas ?
2. Can dust/FIR measurements trace this CO-dark molecular gas ?
DATA setsDATA sets
• LMC (Z = 0.5 Zo), SMC (Z = 0.2 Zo) closest low-Z galaxies
• Pre-Herschel data sets from the Surveying the Agents of Galactic Evolution (SAGE) project (Meixner et al. 2006):– SPITZER/MIPS (24-160 m, 40” resolution)– SPITZER/IRAC (3-8 m)– IRAS (12-100 m, 4.3’ resolution)– NANTEN 12CO (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 (AV = 0.7)– L = AVN0/(<n>Z) = 0.8 pc @ n= 1000 cm-3
– L = 8pc @ n = 100 cm-3
• Need higher resolution to better resolve the structure of molecular clouds envelopes
New data setsNew 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.
12CO
• MAGMA (MAGellanic MOPRA Assessment (12CO follow up on molecular clouds detected by NANTEN)
• 40” resolution, 0.5 K km/s sensitivity
HerschelHerschel SDP: Dust surface density SDP: Dust surface density
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are needed to see this picture.QuickTime™ and a decompressor
are needed to see this picture.
Gordon et al. (2010)
Meixner et al. (2010)
IRAS 100 m resolution (4.3’)dust (IRAS+MIPS) (HI) (ATCA+Parkes HI 21 cm)(H2
CO) NANTEN 12CO
1’ resolution dust (MIPS + HERSCHEL) (HI) (ATCA+Parkes HI 21 cm)(H2
CO) NANTEN 12CO (2.3’ resolution)
NT80 at 1’ resolution dust (MIPS + HERSCHEL) (HI) (ATCA+Parkes HI 21 cm)(H2
CO) MAGMA 12CO
NT80 at 4.3’ resolutiondust (IRAS+MIPS) (HI) (ATCA+Parkes HI 21 cm)(H2
CO) NANTEN 12CO
NT80
IRAS 100 m resolution (4.3’)dust (IRAS+MIPS) (HI) (ATCA+Parkes HI 21 cm)(H2
CO) NANTEN 12CO
1’ resolution dust (MIPS + HERSCHEL) (HI) (ATCA+Parkes HI 21 cm)(H2
CO) NANTEN 12CO (2.3’ resolution)
NT80 at 4.3’ resolutiondust (IRAS+MIPS) (HI) (ATCA+Parkes HI 21 cm)(H2
CO) NANTEN 12CO
NT80 at 1’ resolution dust (MIPS + HERSCHEL) (HI) (ATCA+Parkes HI 21 cm)(H2
CO) MAGMA 12CO
NT71
Dust/Gas correlationDust/Gas correlation
Roman-Duval et al. (2010)
Dust/gas spatial correlationDust/gas spatial correlation
Roman-Duval et al. (2010)
NT80dust (1st panel)GDRdust/((HI) + (H2
CO))--- (HI)--- (H2
CO) Regions of FIR excess
Dust/gas spatial correlationDust/gas spatial correlation
Roman-Duval et al. (2010)
NT71dust (1st panel)GDRdust/((HI) + (H2
CO))--- (HI)--- (H2
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 XVariations of XCO CO with Awith Avv
Glover et al. (2010)
€
XCO =N(H2)
WCO
Transition between CO core and CO-free H2 envelope
Variations of XVariations of XCO CO with Awith Avv
Roman-Duval et al. (2010)
Preliminary results
€
XCO =GDR Σdust − Σ(HI)
WCO
• 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 Other possible causes for observed deviations in the dust/gas correlationdeviations in the dust/gas correlation
Emissivity
GDR
NT80 NT71
Assumptions
Dust temperature Dust temperature in ISM phasesin ISM phases
NT80SFR = 0.018 Mo/kpc2/yr
NT71SFR = 0.042 Mo/kpc2/yr
Ongoing/future workOngoing/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 H2 radiative transfer, cooling and heating, and basic H2 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 H2 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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