Cloud Properties and Star Formation in M31

TL;DR

This work constructs the largest catalog of molecular clouds in M31 by applying a dendrogram to CARMA CO J=1-0 data, identifying 453 clouds and 35 complexes. It derives cloud properties including $r_{\rm c}$, $\sigma_{\rm v}^d$, $M_{\rm c}$, and $M_{\rm vir}$, finding mean values of $\sigma_{\rm v}^d = 2.8$ km s$^{-1}$, $r_{\rm c} = 22.1$ pc, and $\log_{10}(M_{\rm c}) = 5.2$, with a virial parameter distribution indicating that about 66% of clouds are gravitationally bound ($\alpha_{\rm vir}$ median = 1.4, mean = 2.0). The study demonstrates Larson-like size–velocity dispersion behavior with a slope of $0.43\pm0.05$, and a size–mass scaling of $M_{\rm c} \propto r_{\rm c}^{1.36\pm0.06}$ (shallower than Milky Way cloud complexes). It also finds a Kennicutt–Schmidt relation at ~22 pc scales with $\mathrm{SFR} \propto M_{\rm c}^{0.66\pm0.07}$ and clear offsets between cloud isosurfaces and SFR peaks, suggesting that evolutionary state modulates the KS relation at parsec scales. Overall, the results support the prevalence of MW-like, virialized clouds in M31 and provide a rich dataset for examining environment-dependent cloud evolution and star formation on small scales.

Abstract

We present a catalogue of 453 molecular clouds in M31 extracted from CO J=1-0 data observed with CARMA using a dendrogram. Our clouds have the mean values of 2.8 km s$^{-1}$, 22.1 pc and 10$^{5.2}$ M$_\odot$ for the velocity dispersion, radius and mass, respectively. The velocity dispersion shows a weak anti-correlation with the galactocentric radius. The clouds in M31 show mean and median values of 2.0 and 1.4, respectively, for their virial parameters, indicating that most of them are gravitationally bound. Our dendrogram analysis identifies 35 sources with multiple velocity components, which we classify as molecular cloud complexes. We study the size-velocity dispersion and size-mass relationships for the clouds in M31, finding the slopes of 0.43$\pm$0.05 and 1.36$\pm$0.06 for the former and the latter, respectively. Our size-velocity dispersion relationship agrees with those of Milky Way (MW) and M31 clouds. The slope of our size-mass relationship is shallower than those in clouds and cloud complexes of the MW. We find offsets between the isosurfaces of the clouds and star formation rate (SFR) peaks in M31, supporting the scenario where the evolutionary state of individual sources plays a role in the Kennicutt-Schmidt (KS) law at parsec scales. We find a slope of 0.66$\pm$0.07 for the KS law, which is slightly lower than the values of $\sim$0.8 for MW clouds.

Figures (17)

• Figure 1: Integrated intensity map of the CO J=1-0 line emission of M31 in units of K m s$^{-1}$2006AA...453..459N. The green line indicates the region mapped with CARMA.
• Figure 2: Peak intensity map of the CO J=1-0 transition in units of Kelvin. The red, black, blue, and green contours show the boundaries of 488 sources identified using a dendrogram. The extent of each source is outlined using a 3$\sigma$ threshold. This figure must be enlarged considerably to visualize details of the identified objects. A $\sigma$ of 300 mK was used for identifying sources in the regions where the majority of the sources are outlined with green contours, while a $\sigma$ of 230 mK was used for identifying sources elsewhere. The blue contours outline 35 sources with multiple velocity components identified in our dendrogram analysis. The spectra of 11 of these multiple velocity component sources and of 29 randomly selected sources (outlined with red contours) are shown in Figure \ref{['COspectra']}. All the sources with multiple components and the 29 sources outlined with red contours are labeled with a number following the numbering in Table \ref{['cloud_catalogue']}. The best-fit ellipses that follow the source isosurfaces are also indicated.
• Figure 3: Part of the dendrogram of the CO J=1-0 emission. The leaves highlighted in red are considered clouds.
• Figure 4: Top: the red ellipse is the best-fit to cloud 40 on the peak intensity map of CO J=1-0 (left panel) and on a position-velocity map (center panel) identified by the dendrogram. The position-velocity map is extracted along a path horizontal to the X axis, passing through the center of the ellipse in the left panel. The red contour corresponds to 3$\sigma$ in the left panel. The right panel shows a portion of the dendrogram, with cloud 40 highlighted in red. Medium: the same as in the top but for cloud 143. Bottom: the same as in the top but for cloud 399.
• Figure 5: Histograms showing the distribution of the $\sigma_{\rm v}^d$ (top panel), r$_{\rm c}$ (medium panel) and $M_{\rm c}$ (bottom panel). Red lines are the mean values of 2.8 km s$^{-1}$, 22.1 pc and 5.2 M$_{\odot}$ for $\sigma_{\rm v}^d$, $r_{\rm c}$, and log10(M$_{\rm c}$), respectively.