Extraplanar emission in isolated edge-on late-type galaxies.II. The H$α$ kinematics

TL;DR

This work uses high-resolution Hα kinematics from Fabry-Perot observations of 14 isolated, nearly edge-on late-type galaxies to derive corrected rotation curves and measure vertical rotation lags. The envelope-tracing method is employed to extract RCs and quantify a lag ΔV/Δz ≈ 32.0 ± 10.6 km s$^{-1}$ kpc$^{-1}$ in seven galaxies, with lag gradients indicating possible internal or CGM-driven origins. Comparison with HI data and Tully–Fisher relations confirms the reliability of the kinematic measurements, while the lag shows no robust correlation with global galaxy properties, though a tentative link to tidal strength is noted. The results suggest that extended disc gas and eDIG in these isolated systems are likely influenced by interactions with the Circumgalactic Medium (CGM) rather than solely by internal processes, underscoring CGM's role in shaping extraplanar gas dynamics in low-density environments.

Abstract

Isolated galaxies are rare yet invaluable for studying secular evolution, as their physical properties can remain largely unaffected by external influences for several billion years, primarily shaped by internal evolutionary processes. This study focuses on a representative sample of nearly edge-on ($i\geq80^{\circ}$) late-type galaxies selected from the Catalogue of Isolated Galaxies (CIG). We analyse the H$α$ kinematics derived from Fabry-Perot data and integrate these findings with a comprehensive examination of the UV, optical, and FIR properties of these galaxies to study their dynamic evolutionary processes. We investigate the individual kinematics by computing rotation curves and dynamical masses for each galaxy in the sample. The accuracy of our kinematic results is confirmed through comparisons with HI data and by applying the B- and K-band Tully-Fisher relationships. Among the galaxies studied, we observed a rotational lag along the $z$-axis in half of the total sample (7 out of 14 cases), with an average lag ($ΔV/Δz=32.0\pm10.6\,{\rm km\,s^{-1}\,pc^{-1}}$) consistent with previous research findings. Notably, not all galaxies exhibiting measurable lag display morphological extraplanar components, leading us to conclude that this cannot serve as a definitive marker for the extraplanar Diffuse Ionized Gas (eDIG) component. While we found no significant correlations between rotation lag and overall galaxy properties, there seems to be a potential correlation with tidal strength. Based on the kinematic characteristics observed in our sample, we suggest that that the extended disc gas likely originates from interactions with the Circumgalactic Medium (CGM) rather than arising internally within the galaxies themselves.

Figures (21)

• Figure 1: The R-band map from the Digitized Sky Survey (DSS) of galaxies in our sample. The ellipse tracing the $3\sigma$ surface brightness of the K-band image is overlaid on the maps to represent the stellar disc formed by the old stellar population. The contour of the lowest H$\alpha$ emission level corresponding to a signal-to-noise ratio SNR$\geq$5 is shown, illustrating the distribution of ionized gas relative to the old stellar disc and the optical emission.
• Figure 2: Position-velocity diagram (PVD) along the kinematic major axis ($z=0$) of the galaxy CIG 201, derived from the Voronoi-tessellated H$\alpha$ data cube including only emission with SNR$\geq5\sigma$. The ET method is applied to extract the rotation curve from the PVD. The maximum intensity ($I_{\rm max}$) is traced by a dotted line crossing the centre of the PVD and approximated by an $\arctan$ function. The minimum intensity ($I_{\rm min}$), corresponding to the $5\sigma$ emission contour, is traced by the dashed line and also fitted with an $\arctan$ function. The envelope intensity ($I_{\rm env}$), computed using the relation \ref{['equation:I_env']} with $\eta=0.3$, is shown by the filled blue circles (approaching side) and empty red circles (receding side). The solid black line represents the best $\arctan$ fit to these $I_{\rm env}$ points. Contours corresponding to $I_{\rm min}$ (purple dashed line contour) and $I_{\rm env}$ (cyan solid line contour) are superimposed onto the PVD. The upper-left inset displays the location of the pseudo-slit overlaid on the H$\alpha$ monochromatic map of CIG 201.
• Figure 3: CIG 201 (UGC 3979). Example of the layout of the graphs and maps used for the kinematic analysis of each galaxy. Top left: (a) 2MASS K$_s$-band image, (b) H$\alpha$ monochromatic map, (c) radial velocity map and, (d) velocity dispersion map, with the ellipse fitted to the 3$\sigma$ surface brightness level of the 2MASS K$_s$-band image and the photometric (star, $\star$) and kinematic (diamond, $\diamond$) centres overlaid. Top right: PVDs along the major axis at various heights from the galaxy's kinematic major axis. The major-axis PVD shows the $I_{env}$ derived using $\eta = 0.3$, represented by filled blue circles for the receding side and empty red circles for the approaching side, these points were used to fit the $\arctan$ function (solid line). The $I_{env}$ using $\eta = 0.3$ at different heights from the kinematic major axis are plotted: dash-dotted lines correspond to heights of $\pm 3.0$ arcsec and dotted lines to heights of $\pm 6.1$ arcsec. Middle: PVDs along the minor axis at increasing distances from the kinematic center. The kinematic major axis (black line) and pseudo-slits (gray lines) are shown on the H$\alpha$ map. Bottom left: rotation curves derived from the intensity peak (filled squares for approaching, empty squares for receding) and the ET method (blue filled circles for approaching, red empty circles for receding). For the ET method, the inclination-corrected $\arctan$ fitting to I$_{\rm env}$ (with $\eta=0.3$) for the major-axis PVD is shown as a solid line, while the shaded area corresponds to the range $\eta = [0.2, 0.5]$. To measure the lag in rotation, inclination-corrected dashed and dotted lines from the upper panels are also included. Bottom middle: OLS fit of the relation between maximum rotation velocities at different heights and the height ($z$), with the confidence region shaded. Bottom right: Lag in rotation vs. galactic radius, the slope of this plot could indicate the origin of the extraplanar component (see the text).
• Figure 4: Comparison of the location of our isolated galaxy sample (circles) with the general galaxy population from the GHASP sample, in the Tully-Fisher relations for the B-band epinat-2008-ii and the NIR torres-flores-2011. The solid line represents the fit to the GHASP, with the dashed lines outlining the uncertainty range (see the text). The consistency between the isolated galaxy sample and the GHASP relations serves to validate the accuracy of the rotation velocity measurements for the isolated galaxies.
• Figure 5: Comparison between the maximum rotation velocities derived from Hi Jones2018 and H$\alpha$ emission for galaxies in our sample. The red line represents the OLS best-fitting function ($V_{\rm max,H\alpha} = 1.08 \pm 0.13\,V_{\rm max, \ion{H}{i}}$) and the shaded area indicates the confidence region. As a reference, the dashed line traces the one-to-one Hi and H$\alpha$$V_{\rm max}$ relation (slope $m=1$). We highlight three galaxies (triangles) which did not exhibit a morphological extraplanar component in [][]sardaneta-2024 and now they do not show any particular trend in this plot. The slight differences between H$\alpha$ and Hi velocities in these isolated galaxies are likely due to internal effects.