Imaging the disk-halo interface of NGC 891: a 2.7 kpc-thick molecular gas disk

Abstract

Halos surrounding spiral galaxies act as the bridges connecting the galactic disk and the intergalactic medium (IGM). They host a significant fraction of the baryonic mass in the Universe, and feedback from star formation (SF) or active galactic nuclei (AGN) likely plays an important role in regulating this vertical baryonic component. Despite its importance, the contribution of extraplanar molecular gas remains poorly understood. We aim to characterize the vertical extent and the kinematics of molecular gas traced by CO(2-1) emission in the nearby (D = 9.5 Mpc) spiral galaxy NGC 891, one of the best studied edge-on galaxies. We also compare our results with HI, H$α$-traced DIG and dust maps from the literature. Our analysis is based on new CO(2-1) observations of NGC 891 obtained with the IRAM 30m telescope. We mapped two 6 kpc $\times$ 6 kpc regions on the northeastern side and the area surrounding the galactic center. We apply a careful method to estimate and remove the residual contribution of the error beam to the CO cube. The vertical extent of the molecular gas is best described by a two-component Gaussian fit, consisting of a bright thin disk component (deconvolved FWHM $\simeq$ 360 pc) and a fainter thick disk component (deconvolved FWHM $\simeq$ 1.1 kpc). Statistically significant CO(2-1) emission is detected up to 1.3-1.4 kpc above the disk midplane. We estimate that the thick molecular disk component contains up to 27% of the total molecular gas mass of the galaxy. Our results demonstrate that SF-driven feedback in a non-starburst galaxy can lift significant amounts of molecular gas to large vertical distances. We interpret the presence of extraplanar molecular gas in NGC 891 in the framework of a galactic fountain scenario, in which material is expelled from star-forming regions and transported toward the outer halo.

Figures (17)

• Figure 1: A comparison of the observed CO integrated intensity map (in white contours: from $3\sigma$ to 600$\sigma$, logarithmically spaced by 0.26 dex, with $\sigma=0.28$K km s$^{-1}$) with the fraction of clean emission (dimensionless, in color scale) corrected for the error beam contribution, obtained after the two-step iterative process described in Sect \ref{['Error-beam']}. The beam size (11.2$^{\prime\prime} \times$ 11.2 $^{\prime\prime}$) is represented by the blue-filled circle at the bottom-left corner of the figure.
• Figure 2: Spectrum extracted at ($\Delta x$, $\Delta z$) = (75$^{\prime\prime}$, --30$^{\prime\prime}$). The different curves show the initial spectrum before the correction (black) and the result obtained after the first (red curve) and second (blue curve) iteration of the error beam estimation algorithm described in Sect \ref{['Error-beam']}.
• Figure 3: CO moment maps. From top to bottom: the velocity-integrated intensities ( panel a)), the intensity-weighted velocities ( panel b) and the velocity dispersions ( panel c) derived from the CO(2--1) line in the mapped region. Contour levels for the CO intensities increase logarithmically from 3$\sigma$ to 600$\sigma$ in steps of 0.26 dex, with $\sigma=0.28~$K km s$^{-1}$. Mean-velocity contours range from -180 to 180 km s$^{-1}$ in steps of 20 km s$^{-1}$. Dashed lines indicate negative contour values. Dispersion contours go from 10 to 100 km s$^{-1}$ in steps of 10 km s$^{-1}$. The 30 m beam size (11.2$^{\prime\prime}$) is indicated as a filled circle in the bottom left corner of each panel. A vertical thickness of 2.7 kpc is represented as horizontal dashed lines, as visual reference.
• Figure 4: The centroids (blue histogram) and deconvolved $FWHM$ thickness values (orange histogram) derived from the single-Gaussian fits of the out-of-plane molecular gas distribution across the major axis of the galaxy. The dashed lines show the estimated mean values of $\Delta z$ and $FWHM$. The error bars are derived from the covariance of the parameters from the Gaussian fit.
• Figure 5: Comparison of the average out-of-plane distribution of CO (<$I_{\rm CO}$>($\Delta z$)) with the fits of the single-Gaussian ( upper panels) and double-Gaussian ( lower panels) decomposition models described in Sect. \ref{['CO-decomposition']}. The blue curve shows the observed profile <$I_{\rm CO}$>($\Delta z$). The green and yellow curves in the lower left panel show the fit of the thin and thick molecular disk components of the double-Gaussian fit, respectively. The black dashed curves represent the sum of all the components in both models. The upper and lower right panels display the residuals of the fits. Horizontal dashed lines indicate the $\pm3\sigma$ levels with $\sigma$ = 58 mK km s$^{-1}$.