1 / 25

Why care about dust and gas in the ISM ?

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

jerom
Download Presentation

Why care about dust and gas in the ISM ?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Dust/Gas Correlation in the Large Magellanic Cloud:New Insights from the HERITAGE and MAGMA surveysJulia Roman-DuvalJuly 14, 2010HotScI

  2. 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

  3. 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)

  4. Wolfire et al. (2010) dust = gas/GDR Dust, H, C+ Dust, H2, C, C+ Interstellar Radiation Field (ISRF) attenuated by DUST Dust, CO, H2 Unlike CO, H2 self-shielded due to absorption lines in the UV

  5. Metallicity Effects CO fraction determined by AV (photo-dissociation) H2 fraction determined by nZ (formation timescale) Glover et al. (2010)

  6. Metallicity Effects H0/Z = thickness of the C0 region H0/Z = thickness of the C0 region AV Z for a given gas mass Bolatto et al. (1999) Expect a significant fraction of molecular gas to be invisible to CO observations in low metallicity galaxies

  7. FIR Excess from SPITZER Observations of the LMC Excess of FIR emission compared to the measured gas column (from HI 21 cm and CO emission) at high columns => CO dark molecular gas ? 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 Bernard et al. (2008)

  8. FIR excess in the LMC FIR excess correlated with high density regions Nx = GDR Ndust - N(HI) - N(H2CO)

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

  10. DATA 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

  11. New data sets 12CO • MAGMA (MAGellanic MOPRA Assessment (12CO follow up on molecular clouds detected by NANTEN) • 40” resolution, 0.5 K km/s sensitivity 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. Meixner et al. (2010) Gordon et al. (2010) Herschel SDP: Dust surface density

  13. IRAS 100 m resolution (4.3’) dust (IRAS+MIPS) (HI) (ATCA+Parkes HI 21 cm) (H2CO) NANTEN 12CO 1’ resolution dust (MIPS + HERSCHEL) (HI) (ATCA+Parkes HI 21 cm) (H2CO) NANTEN 12CO(2.3’ resolution) NT80 NT80 at 4.3’ resolution dust (IRAS+MIPS) (HI) (ATCA+Parkes HI 21 cm) (H2CO) NANTEN 12CO NT80 at 1’ resolution dust (MIPS + HERSCHEL) (HI) (ATCA+Parkes HI 21 cm) (H2CO) MAGMA 12CO

  14. IRAS 100 m resolution (4.3’) dust (IRAS+MIPS) (HI) (ATCA+Parkes HI 21 cm) (H2CO) NANTEN 12CO 1’ resolution dust (MIPS + HERSCHEL) (HI) (ATCA+Parkes HI 21 cm) (H2CO) NANTEN 12CO(2.3’ resolution) NT71 NT80 at 4.3’ resolution dust (IRAS+MIPS) (HI) (ATCA+Parkes HI 21 cm) (H2CO) NANTEN 12CO NT80 at 1’ resolution dust (MIPS + HERSCHEL) (HI) (ATCA+Parkes HI 21 cm) (H2CO) MAGMA 12CO

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

  16. Dust/gas spatial correlation NT80 dust (1st panel) GDRdust/((HI) + (H2CO)) --- (HI) --- (H2CO) Regions of FIR excess Roman-Duval et al. (2010)

  17. Dust/gas spatial correlation NT71 dust (1st panel) GDRdust/((HI) + (H2CO)) --- (HI) --- (H2CO) Regions of FIR excess Roman-Duval et al. (2010) Excess of FIR emission compared to the observed gas surface density near the envelopes of MCs consistent with CO dark molecular gas

  18. Variations of XCO with Av Transition between CO core and CO-free H2 envelope Glover et al. (2010)

  19. Variations of XCO with Av Roman-Duval et al. (2010) Preliminary results

  20. NT80 NT71 Assumptions Emissivity GDR Other possible causes for observed deviations in the dust/gas correlation • 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

  21. Dust temperature in ISM phases NT80 SFR = 0.018 Mo/kpc2/yr NT71 SFR = 0.042 Mo/kpc2/yr

  22. 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 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

  23. 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

  24. HappyBastilleDay !

More Related