A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE). XIX. The discovery of a spectacular 230 kpc Halpha tail following NGC 4569 in the Virgo cluster

TL;DR

Ram pressure stripping in galaxy clusters mobilizes the ISM, forming extended Hα tails that trace the infall and interaction history of galaxies within the ICM. This study combines deep VESTIGE Hα imaging with follow-up spectroscopy from APO, MMT (Binospec), and CFHT (SITELLE) to detect and kinematically characterize an ionized-gas tail trailing NGC 4569 in the Virgo cluster, extending the known tail to at least 230 kpc and revealing a coherent velocity gradient toward the cluster velocity. A drag-based deceleration framework, with $d\upsilon/dt = - b \upsilon^{2}$ and $b = \rho_{\rm ICM}/(\rho_{\rm cloud} h)$, relates traveled distance to velocity via $l = \ln(\upsilon_{0,rad}/\upsilon_{\rm rad})/b$ and projected distance $l_{\rm sky} = (\xi - 1 - \ln \xi) \sin\theta / b$ (with $\xi = \upsilon_{0,rad}/\upsilon_{\rm rad}$), yielding a characteristic timescale $\tau = 5.5 \sin\theta /(b \upsilon_{0,rad})$ and a representative age of order $\sim$0.5 Gyr for plausible geometries. The results show that stripped ionized gas can decelerate and mix with the ICM over large distances, making these clumps valuable multi-phase tracers of cluster dynamics and the cloud-crushing physics in the ICM.

Abstract

Context. Galaxies fly inside galaxy clusters and ram pressure by the ICM can remove a large amount of the ISM from the galaxy, and deposit the gas in the ICM. The ISM decoupled from the host galaxy leaves a long trail following the moving galaxy. Such long trails track the galaxy motion and can be detected with sensitive data in Halpha. Aims. We study the Halpha tail trailing NGC 4569 in the Virgo cluster. Methods. The initial discovery was made with the deep Halpha imaging data with CFHT, from the VESTIGE project. The follow-up spectroscopic observations were made with APO/DIS, MMT/Binospec and CFHT/SITELLE. Results. Besides the known 80 kpc Halpha tail downstream of NGC 4569, the deep Halpha imaging data allow the Halpha tail detected to at least 230 kpc from the galaxy. More importantly, the Halpha clumps implied from the imaging data are confirmed with the spectroscopic data. The Halpha clumps show a smooth radial velocity gradient across about 1300 km/s, eventually reaching the velocity of the cluster. Conclusions. This discovery, for the first time, demonstrates the full deceleration process of the stripped ISM. This discovery also showcases the potential with wide-field Halpha survey on galaxy clusters to discover intracluster optical emission-line clouds originated from cluster galaxies. These clouds provide kinematic tracers to the infall history of cluster galaxies and the turbulence in the ICM. They are also excellent multi-phase objects to study the classical cloud crushing problem and other relevant important physical processes.

Figures (4)

• Figure 1: Upper: The net H$\alpha$ image showing the extended optical emission-line clumps/filaments trailing NGC 4569. Bright stars and their scattered halos are masked. The black box in the dashed line is the SITELLE field shown in Fig. \ref{['fig:sitelle']}. Three zoom-ins are also shown. The direction towards M87 is also shown at the lower right corner. Note that the clumps/filaments around NGC 4531 have very different velocities from NGC 4531. Because of the various instrumental artifacts, spectroscopic follow-ups are required. Lower Left: The same net H$\alpha$ image as the one shown in the upper panel, with velocities of 94 clumps from Binospec overlaid. See Fig. \ref{['fig:sitelle']} for velocities at the front part of the tail from SITELLE. Lower Right: The velocities (and 1-$\sigma$ uncertainty) of 94 clumps from Binospec in black + the kernel density estimation (KDE) of the SITELLE velocities (as shown in Fig. \ref{['fig:sitelle']}) vs. distance from the nucleus of NGC 4569. The KDE shows the probability density function of the SITELLE velocities, with yellow as the highest density. For reference, NGC 4569 is at -235 km s$^{-1}$ and NGC 4531 is at 90 km s$^{-1}$. The velocity of the Virgo cluster A is 955 km s$^{-1}$ from Boselli14, while Kashibadze20 gave a velocity of 1070 km s$^{-1}$ for the Virgo cluster. The observed clumps show a smooth velocity gradient from NGC 4569 and there is no evidence for their connection with NGC 4531. The end of the detected H$\alpha$ tail is nearly at rest (along the line of sight) with respect to the Virgo cluster mean velocity. There are a few clumps at distance $\sim$ 100 kpc with velocities of 50 - 100 km s$^{-1}$. They are in three groups and are marked in small blue circles in the top panel. Despite their similar velocities to NGC 4531's, there is no spatial evidence for the connection.
• Figure 2: Binospec spectra on four slit positions, from one of the brightest regions in the top, a region with median brightness in the 2nd row, to two spectra with about the least significant detections in our sample. Our best-fit model is shown in the red line. Only the portion around [N ii], H$\alpha$ and [S ii] is shown. The shown spectra are unbinned on the spectral axis with a 0.62Å sampling. The central coordinate of each position (in brackets) and the best-fit velocity are shown in the upper right corner.
• Figure 3: The SITELLE velocity map in the region shown in Fig. \ref{['fig:f1']}, overlaid on the net H$\alpha$ image in the grey scale. The big green ellipse in the dashed line shows the D25 aperture of NGC 4569, while the small green ellipse in the dotted line is 1/3 of the size of the big green ellipse. NGC 4569's nucleus is marked by a green cross. One can observe the disk rotation close to the nucleus and the truncated H$\alpha$ disk Boselli16. The red ellipse in the dashed line shows the D25 aperture of IC 3583 with a velocity of 1121 km s$^{-1}$. Its interaction with NGC 4569 was excluded by Boselli16.
• Figure 4: The [N ii]$\lambda$6584Å / H$\alpha$ ratios for 88 clumps and the [S ii]$\lambda\lambda$6716, 6731 / H$\alpha$ ratios for 49 clumps trailing NGC 4569 vs. the distance from the nucleus of NGC 4569. The dashed lines show the median ratios. The shown uncertainty is 1-$\sigma$.