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Properties of LMC Molecular Clouds from MAGMA, the ... · MAGMA’s observations in the LMC are...
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III. Properties of the Atomic Gas surrounding LMC Molecular Clouds The interstellar medium of the LMC appears to be dominated by atomic gas (e.g [11]). CO emission from molecular clouds caps the high-density peaks and ridges of the pervasive HI emission, which often shows complex line profiles with multiple velocity components. In some cases where the HI line profiles are relatively simple, however, it is possible to isolate the HI emission that is associated with an individual molecular cloud. These HI+CO complexes are interesting because they provide test cases for theories of molecular cloud formation. Models in which atomic gas contracts under self-gravity, gradually becoming opaque to FUV radiation and allowing H 2 molecules to form on dust grain surfaces suggest that molecular clouds might lie at the core of HI envelopes. Models of turbulence in the interstellar gas, on the other hand, suggest that molecular clouds form rapidly behind shock fronts (e.g. [12]). We have isolated the HI emission associated with seven molecular clouds in the MAGMA survey. To identify this emission, we generate a map of the local HI peak brightness for the velocity range ±ΔV/2 around the radial velocity of the molecular cloud, where ΔV is the FWHM CO linewidth. A 3-D mask is generated from the 2-D map by incorporating all pixels with significant (>5σ) HI emission that are contiguous with regions in the 2-D map that lie above a brightness threshold, T c =40K (Figure 4, see also [13,14]). Example images of the atomic gas surrounding two molecular clouds are shown in Figure 3. Assuming that the HI emission is optically thin, we find that the atomic gas surrounding a molecular cloud is typically ~1.2 to 3 times more massive than the molecular cloud itself. Dynamical mass estimates for these atomic gas structures are considerably higher, suggesting that they are far from self-gravitating (see also [15]). We note that our mass estimates derived from the HI integrated intensity are probably lower limits, since our identification method imposes a brightness threshold on the HI emission, and the atomic gas surrounding a molecular cloud should have significant optical depth. For the HI+CO complexes to be self- gravitating, the atomic gas would require optical depths of τ=5-9, higher than the values obtained by HI absorption experiments in the LMC (τ=0.5-1.5, [16,17]). Properties of LMC Molecular Clouds from MAGMA, the Magellanic Mopra Assessment II. Properties of Molecular Clouds in the LMC In the Milky Way, molecular clouds follow “Larson’s Laws”: empirical scaling relations between basic physical cloud properties such as size, linewidth and mass [6]. One goal of extragalactic molecular cloud studies is to determine whether these laws apply in other galaxies, or whether the physical properties of molecular clouds are sensitive to differences in the galactic environment. This work requires large samples of extragalactic molecular clouds, observed with sufficient resolution and sensitivity to obtain precise measurements of their properties. In the regions of the LMC that MAGMA has surveyed to date, we have identified 113 molecular clouds. The clouds follow similar - but not identical - relations to Milky Way clouds (Figure 2 & Table 1). X-factor is not very sensitive to variations in radiation field or metallicity <X CO > = 2.3 × 10 20 cm -2 (K km s -1 ) -1 <X CO > = 3.2 ± 1.1 × 10 20 cm -2 (K km s -1 ) -1 Greater dynamical mass of large GMCs due to accreting atomic gas, or mass- dependent variations in the X-factor log M v = 1.6 + 0.81 log L log M v = (-0.1±0.2) + (1.2±0.1) log L Stronger interstellar radiation field erodes low-density envelopes of LMC clouds, steepening the mass spectrum dN/dM ∝ M -1.5 ±0.1 dN/dM ∝ M -1.8±0.1 Variations in amplitude of interstellar turbulence lead to different constants of proportionality in the size-linewidth relation log σ = -0.28 + 0.55 log R log σ = (-0.6±0.1) + (0.7±0.1) log R Possible Explanation Milky Way MAGMA-LMC Table 1 A comparison between the physical properties of molecular clouds in the LMC and the inner Milky Way. The relations for the Milky Way clouds are taken from references [7-10]. References: [1] Kim et al. ApJ 148 (2003) [2] Meixner et al. AJ 132 (2006) [3] Stanimirovic et al. MNRAS 302 (1999) [4] Bolatto et al. ApJ 655 (2007) [5] Fukui et al. PASJ 53 (2001) [6] Larson MNRAS 194 (1981) [7] Solomon et al. ApJ 319 (1987) [8] Rosolowsky ApJ 654 (2007) [9] Strong & Mattox A&A 308 (1996) [10] Rosolowsky PASP 117 (2005) [11] Staveley-Smith et al. MNRAS 299 (2003) [12] Bergin et al. ApJ 612 (2004) [13] Blitz Conf. Proc. The Evolution of the Interstellar Medium (1990) [14] Elmegreen & Elmegreen ApJ 320 (1987) [15] Kim et al. ApJS 171 (2007) [16] Dickey et al. A&A 289 (1994) [17] Marx-Zimmer et al. A&A 354 (2000) Figure 4 (Below) Average radial profile of the HI velocity dispersion (top), peak brightness (middle) and column density (bottom) for 38 molecular clouds in the MAGMA survey. The flattening of the HI peak brightness curve motivates the brightness threshold, T c = 40K, that we use to identify the HI emission associated with molecular clouds. A. Hughes (Swinburne U./ATNF), T. Wong (U. Illinois), E. Muller (Nagoya U.), J. Pineda (JPL/Caltech), J. Ott (NRAO) & the MAGMA collaboration For more information about the MAGMA project, please contact Tony Wong ([email protected] ). A B Figure 3 (Top) Integrated intensity map of the HI emission surrounding a MAGMA molecular cloud. The white contours show the 12 CO integrated intensity in steps of +2 K km s -1 . (Bottom) 3D visualisation snapshot of another molecular cloud and surrounding HI gas. The CO emission is pink, the HI emission is shown in progressively lighter shades of blue, indicating steps of +10K. Cloud A M CO = 1.2 × 10 5 M M HI = 3.0 × 10 5 M Cloud B M CO = 1.1 × 10 5 M M HI = 1.4 × 10 5 M B Figure 1 An integrated intensity map of the 12 CO J=(1→0) emission from molecular clouds in the LMC by the MAGMA survey. The blue rectangles indicate the regions that have been surveyed to date. The black contours outline the integrated 12 CO emission (W CO =0.8 K km s -1 ) across the whole LMC, as observed by NANTEN [5]. Figure 2 (Left) Size-linewidth relation for the 113 LMC molecular clouds observed by MAGMA. The dotted line is the relation for GMCs in the inner Milky Way [7,8]. The solid line represents the relation for the MAGMA clouds (see Table 1). (Middle) Plot of virial mass as a function of CO luminosity for the MAGMA clouds. The thin dotted lines indicate constant values of X CO = 0.4, 4.0 and 40 x 10 20 cm -2 (K km s -1 ) -1 . The thick dotted line represents the standard Galactic X CO value [9]. The solid line is the relation for the MAGMA clouds (see Table 1). (Right) Cumulative mass distribution for MAGMA clouds. The solid line is our best-fitting truncated power law to clouds with masses greater than 10 4.2 M . I. MAGMA: The MAGellanic Mopra Assessment MAGMA is a high-resolution mapping survey by the ATNF Mopra Telescope of the 12 CO J=(1→0) and 13 CO J=(1→0) emission in the Magellanic Clouds. The data has comparable resolution to the best existing surveys of dust and atomic gas in the Magellanic Clouds [e.g. 1-4], probing spatial scales almost four times smaller than the typical size of a giant molecular cloud in the Milky Way. In the Large Magellanic Cloud (LMC), MAGMA will map all 114 molecular clouds detected by the NANTEN telescope with total flux greater than 7000 K km s -1 pc 2 and peak brightness greater than 1 K km s -1 [5]. In the Small Magellanic Cloud (SMC), MAGMA is mapping molecular clouds in a northern region of the galaxy that has never been mapped at high resolution. More details and first results from MAGMA in the SMC are presented in Erik Muller’s poster. MAGMA’s observations in the LMC are almost 50% complete (see Figure 1). In this poster, we present an overview of the physical properties of LMC molecular clouds observed by MAGMA to date. We also discuss the properties of the atomic gas surrounding a small sample of isolated molecular clouds in the LMC.
Transcript of Properties of LMC Molecular Clouds from MAGMA, the ... · MAGMA’s observations in the LMC are...
III. Properties of the Atomic Gas surrounding LMC Molecular CloudsThe interstellar medium of the LMC appears to be dominated by atomic gas (e.g [11]). CO emission frommolecular clouds caps the high-density peaks and ridges of the pervasive HI emission, which often showscomplex line profiles with multiple velocity components. In some cases where the HI line profiles arerelatively simple, however, it is possible to isolate the HI emission that is associated with an individualmolecular cloud. These HI+CO complexes are interesting because they provide test cases for theories ofmolecular cloud formation. Models in which atomic gas contracts under self-gravity, gradually becomingopaque to FUV radiation and allowing H2 molecules to form on dust grain surfaces suggest that molecularclouds might lie at the core of HI envelopes. Models of turbulence in the interstellar gas, on the other hand,suggest that molecular clouds form rapidly behind shock fronts (e.g. [12]).We have isolated the HI emission associated with seven molecular clouds in the MAGMA survey. Toidentify this emission, we generate a map of the local HI peak brightness for the velocity range ±ΔV/2around the radial velocity of the molecular cloud, where ΔV is the FWHM CO linewidth. A 3-D mask isgenerated from the 2-D map by incorporating all pixels with significant (>5σ) HI emission that arecontiguous with regions in the 2-D map that lie above a brightness threshold, Tc=40K (Figure 4, see also[13,14]). Example images of the atomic gas surrounding two molecular clouds are shown in Figure 3.Assuming that the HI emission is optically thin, we find that the atomic gas surrounding a molecular cloud istypically ~1.2 to 3 times more massive than the molecular cloud itself. Dynamical mass estimates for theseatomic gas structures are considerably higher, suggesting that they are far from self-gravitating (see also[15]). We note that our mass estimates derived from the HI integrated intensity are probably lower limits,since our identification method imposes a brightness threshold on the HI emission, and the atomic gassurrounding a molecular cloud should have significant optical depth. For the HI+CO complexes to be self-gravitating, the atomic gas would require optical depths of τ=5-9, higher than the values obtained by HIabsorption experiments in the LMC (τ=0.5-1.5, [16,17]).
Properties of LMC Molecular Clouds from MAGMA,the Magellanic Mopra Assessment
II. Properties of Molecular Clouds in the LMCIn the Milky Way, molecular clouds follow “Larson’s Laws”: empirical scaling relations betweenbasic physical cloud properties such as size, linewidth and mass [6]. One goal of extragalacticmolecular cloud studies is to determine whether these laws apply in other galaxies, or whetherthe physical properties of molecular clouds are sensitive to differences in the galacticenvironment. This work requires large samples of extragalactic molecular clouds, observed withsufficient resolution and sensitivity to obtain precise measurements of their properties. In theregions of the LMC that MAGMA has surveyed to date, we have identified 113 molecular clouds.The clouds follow similar - but not identical - relations to Milky Way clouds (Figure 2 & Table 1).
X-factor is not very sensitive to variationsin radiation field or metallicity
<XCO> = 2.3 × 1020
cm-2 (K km s-1)-1<XCO> = 3.2 ± 1.1 × 1020
cm-2 (K km s-1)-1
Greater dynamical mass of large GMCsdue to accreting atomic gas, or mass-dependent variations in the X-factor
log Mv = 1.6 + 0.81 log Llog Mv = (-0.1±0.2) + (1.2±0.1) log L
Stronger interstellar radiation field erodeslow-density envelopes of LMC clouds,
steepening the mass spectrumdN/dM ∝ M-1.5 ±0.1dN/dM ∝ M-1.8±0.1
Variations in amplitude of interstellarturbulence lead to different constants of
proportionality in the size-linewidth relationlog σ = -0.28 + 0.55 log Rlog σ = (-0.6±0.1) + (0.7±0.1) log R
Possible ExplanationMilky WayMAGMA-LMC
Table 1 A comparison between the physical properties of molecularclouds in the LMC and the inner Milky Way. The relations for the Milky
Way clouds are taken from references [7-10].
References: [1] Kim et al. ApJ 148 (2003) [2] Meixner et al. AJ 132 (2006) [3] Stanimirovic et al. MNRAS 302 (1999) [4] Bolatto et al. ApJ 655 (2007) [5] Fukui et al.PASJ 53 (2001) [6] Larson MNRAS 194 (1981) [7] Solomon et al. ApJ 319 (1987) [8] Rosolowsky ApJ 654 (2007) [9] Strong & Mattox A&A 308 (1996) [10] RosolowskyPASP 117 (2005) [11] Staveley-Smith et al. MNRAS 299 (2003) [12] Bergin et al. ApJ 612 (2004) [13] Blitz Conf. Proc. The Evolution of the Interstellar Medium (1990)[14] Elmegreen & Elmegreen ApJ 320 (1987) [15] Kim et al. ApJS 171 (2007) [16] Dickey et al. A&A 289 (1994) [17] Marx-Zimmer et al. A&A 354 (2000)
Figure 4 (Below) Average radial profile of the HI velocitydispersion (top), peak brightness (middle) and columndensity (bottom) for 38 molecular clouds in the MAGMAsurvey. The flattening of the HI peak brightness curvemotivates the brightness threshold, Tc = 40K, that we use toidentify the HI emission associated with molecular clouds.
A. Hughes (Swinburne U./ATNF), T. Wong (U. Illinois), E. Muller (Nagoya U.), J. Pineda (JPL/Caltech), J. Ott (NRAO)& the MAGMA collaboration
For more information about the MAGMA project, please contact Tony Wong ([email protected]).
A
B
Figure 3 (Top) Integratedintensity map of the HI emissionsurrounding a MAGMA molecularcloud. The white contours showthe 12CO integrated intensity insteps of +2 K km s-1. (Bottom) 3Dvisualisation snapshot of anothermolecular cloud and surroundingHI gas. The CO emission is pink,the HI emission is shown inprogressively lighter shades ofblue, indicating steps of +10K.Cloud AMCO = 1.2 × 105 M
MHI = 3.0 × 105 M
Cloud BMCO = 1.1 × 105 M
MHI = 1.4 × 105 MB
Figure 1 An integrated intensity map of the 12CO J=(1→0)emission from molecular clouds in the LMC by the MAGMAsurvey. The blue rectangles indicate the regions that have beensurveyed to date. The black contours outline the integrated 12COemission (WCO=0.8 K km s-1) across the whole LMC, asobserved by NANTEN [5].
Figure 2 (Left) Size-linewidth relation for the 113 LMC molecular clouds observed by MAGMA. Thedotted line is the relation for GMCs in the inner Milky Way [7,8]. The solid line represents the relation forthe MAGMA clouds (see Table 1). (Middle) Plot of virial mass as a function of CO luminosity for theMAGMA clouds. The thin dotted lines indicate constant values of XCO = 0.4, 4.0 and 40 x 1020 cm-2
(K km s-1)-1. The thick dotted line represents the standard Galactic XCO value [9]. The solid line is therelation for the MAGMA clouds (see Table 1). (Right) Cumulative mass distribution for MAGMA clouds.The solid line is our best-fitting truncated power law to clouds with masses greater than 104.2 M.
I. MAGMA: The MAGellanic Mopra AssessmentMAGMA is a high-resolution mapping survey by the ATNF Mopra Telescope of the 12CO J=(1→0)and 13CO J=(1→0) emission in the Magellanic Clouds. The data has comparable resolution to thebest existing surveys of dust and atomic gas in the Magellanic Clouds [e.g. 1-4], probing spatialscales almost four times smaller than the typical size of a giant molecular cloud in the Milky Way.In the Large Magellanic Cloud (LMC), MAGMA will map all 114 molecular clouds detected by theNANTEN telescope with total flux greater than 7000 K km s-1 pc2 and peak brightness greaterthan 1 K km s-1 [5]. In the Small Magellanic Cloud (SMC), MAGMA is mapping molecular clouds ina northern region of the galaxy that has never been mapped at high resolution. More details andfirst results from MAGMA in the SMC are presented in Erik Muller’s poster.MAGMA’s observations in the LMC are almost 50% complete (see Figure 1). In this poster, wepresent an overview of the physical properties of LMC molecular clouds observed by MAGMA todate. We also discuss the properties of the atomic gas surrounding a small sample of isolatedmolecular clouds in the LMC.
