WISDOM Project - XXVII. Giant molecular clouds of the lenticular galaxy NGC 1387: similarities with spiral galaxy clouds

Abstract

Molecular gas is crucial to understanding star formation and galaxy evolution, but the giant molecular clouds (GMCs) of early-type galaxies (ETGs) have rarely been studied. Here, we present analyses of the spatially resolved GMCs of the lenticular galaxy NGC 1387, exploiting high spatial resolution (0.15" or 14 pc) 12CO(2-1) line observations from the Atacama Large Millimeter/submillimeter Array. We identify 1285 individual GMCs and measure the fundamental properties (radius, velocity dispersion, and molecular gas mass) of each with a modified version of the CPROPStoo package. Unusually for an ETG, the GMCs of NGC 1387 follow scaling relations very similar to those of the Milky Way disc and Local Group galaxy clouds, and most are virialised. GMCs with large masses and radii and/or small galactocentric distances have their angular momenta aligned with the large-scale galactic rotation, while other GMCs do not. These results show that ETGs have more diversified GMC properties than previously thought. We discuss potential reasons for such diversity, and viewing-angle dependency is a plausible candidate.

Figures (17)

• Figure 1: Overview of NGC 1387 and its molecular gas content. Left: optical image from the CGS survey. Top-right: unsharp-masked HST ACS/WFC F475W image of the central region only, highlighting dust features. The red ellipses overlaid indicate the boundaries of the three regions defined in \ref{['sec:mom']} and discussed in the text. Bottom-right: the same unsharp-masked HST ACS/WFC F475W image, with the contours of the (smoothed) CO(2-1) total intensity map overlaid in purple (see \ref{['sec:co']}). A scale bar is provided in each panel.
• Figure 2: Molecular gas distribution of NGC 1387. Top: CO(2-1) zeroth (integrated-intensity), first (intensity-weighted mean line-of-sight, LoS, velocity) and second (intensity-weighted LoS velocity dispersion) moment maps. The black ellipses in the left panel indicate the boundaries of the three regions defined in \ref{['sec:mom']}. The dashed line in the middle panel shows the kinematic major axis. The synthesised beam is shown in the bottom-left corner of each panel as a black ellipse. Bottom-left: spatially integrated CO(2-1) spectrum, corrected to an infinite cleaning depth channel by channel (see \ref{['sec:mom']}). Uncertainties ($1$$\sigma$) are shown as grey shading. Bottom-right: azimuthally averaged inclination-corrected molecular gas mass surface density radial profile. Error bars indicate the $1$$\sigma$ uncertainty on the mean of each radial bin, while the blue shaded region shows the scatter (standard deviation) within each radial bin. The grey dashed vertical lines again indicate the boundaries of the three regions defined in \ref{['sec:mom']}.
• Figure 3: NGC 1387 GMCs identified by CPROPStoo, schematically overlaid on the zeroth-moment map as ellipses. There are $1079$ spatially and spectrally resolved GMCs (blue ellipses) and $206$ unresolved GMCs (green ellipses). The synthesised beam is shown in the bottom-left corner as a black filled ellipse. A scale bar is also shown in the bottom-left corner.
• Figure 4: Distributions of the fundamental GMC properties $R_\mathrm{c}$ (top-left), $\sigma_\mathrm{obs,los}$ (top-middle), $M_\mathrm{gas}$ (top-right), $\Sigma_\mathrm{gas}$ (bottom-left) and $\alpha_{\rm obs,vir}$ (bottom-right) of the NGC 1387 GMCs. Only (spatially and spectrally) resolved GMCs are included, and the different regions are shown in different colours (black for the entire sample). Gaussian fits are overlaid in matching colours. The spatial and spectral resolutions are indicated by arrows in the panels of $R_\mathrm{c}$ and $\sigma_\mathrm{obs,los}$, respectively. For $R_\mathrm{c}$, the arrow length is $\eta\sqrt{\sigma_\mathrm{maj}\sigma_\mathrm{min}}=11.4$ pc, where the standard deviation of the synthesised beam major (minor) axis is $\sigma_\mathrm{maj\,(min)}=\theta_\mathrm{maj\,(min)}/2.35$. For $\sigma_\mathrm{obs,los}$, the arrow length is the channel width of $2$ km s$^{-1}$.
• Figure 5: Molecular gas mass spectra (i.e. cumulative mass distribution functions) of the NGC 1387 GMCs. Both resolved and unresolved GMCs are included, and the different regions are shown in different colours (black for the entire sample). Best-fitting single power laws (dotted lines) and truncated power laws (dashed lines) are shown in matching colours. The nine most massive GMCs were excluded from the fit for the whole sample and the three most massive GMCs for each individual region, to avoid sensitivity to these few most massive bins. Grey horizontal error bars show the uncertainty of each GMC mass at the location where it contributes to the distribution. The grey vertical dashed line indicates our mass completeness limit (see \ref{['sec:mass']}).