Ultra Diffuse Galaxies in clusters : the peculiar gas loss of VCC 1964

TL;DR

This study investigates VCC 1964, a candidate Ultra Diffuse Galaxy in the Virgo cluster, using deep Arecibo HI data (AGES and WAVES) alongside SDSS and DESI Legacy optical data to quantify gas content and dynamics. The authors find a pronounced 9 kpc HI–optical offset, a blue, smooth optical morphology, and a very narrow HI line width that places VCC 1964 well off the baryonic Tully–Fisher relation (and the optical TFR under a K20-based correction), consistent with gas displacement by ram pressure as the galaxy enters Virgo for the first time. They explore whether the offset could arise from tides or a backsplash scenario, but favor ram-pressure stripping, though the interpretation of a potential dark matter deficit remains ambiguous because the HI is unresolved and the true dynamical state is uncertain. A direct distance measurement and more ID’d similar systems are needed to confirm the cluster-infall scenario and assess whether some cluster UDGs inherit properties from gas-rich field counterparts rather than representing a separate formation channel.

Abstract

Context : Ultra Diffuse Galaxies are low surface brightness systems which have been detected in HI in the field, where their line widths sometimes indicate significant dark matter deficits. They are rarely detected in HI in clusters, making their dynamical properties difficult to assess. The relation between field and cluster populations is unclear. Aims : Detecting UDGs entering a cluster could give important clues to their evolution, both in terms of their dynamics but also as to whether they are structurally similar - i.e. if cluster UDGs are generally the same as field UDGs except with less gas and an older stellar population. Methods : We use data from two deep Arecibo surveys, the Arecibo Galaxy Environment Survey and the Widefield Arecibo Virgo Environment Survey, to measure the gas content of the UDG-candidate VCC 1964. Optical properties are quantified from the Sloan Digital Sky Survey and DESI Legacy Surveys. Results : We find a significant 9 kpc offset between the HI and optical components of VCC 1964, no evidence of asymmetry in the HI, and only a modest deficiency level. This suggests a wholesale displacement of the gas content. The line width is 4-5 sigma deviant from the baryonic and over 6 sigma deviant from the optical forms of the Tully-Fisher relation. The optical component is blue and smooth. Conclusions : VCC 1964 is consistent with a UDG experiencing gas displacement due to ram pressure as it enters the cluster for the first time. Intriguingly, its dynamics imply a significant dark matter deficit, however we cannot rule out that this may be due to the gas being displaced out of equilibrium.

Figures (5)

• Figure 1: Renzogram of 3.5 $\sigma$ Hi contours of VCC 1964 from the combined AGES/WAVES data sets, overlaid on an LS RGB image. The white square and cross mark the positions of the Hi centroid and optical position of the galaxy, respectively. White arrows show the direction vectors to the giant ellipticals M87 and M49, the centre of the main body of the cluster and the local sub-group. The whole image is 15 across, $\approx$ 75 kpc at the assumed 17 Mpc distance. The inset images show the same close-up field of view of VCC 1964, optical LS data on the left and GALEX UV on the right.
• Figure 2: Hi spectra of VCC 1964 from AGES (blue), WAVES (red) and the combination (black). Circles and squares show the fitted W50 and W20 velocity widths, respectively.
• Figure 3: Baryonic Tully-Fisher as in AGESLeo. Filled black circles use AGES Hi data and SDSS photometry, from the Hi detected in the background of the Virgo fields and cluster members with deficiencies $<$ 0.6. Open circles are from table 1 in BTFRSample. VCC 1964 is highlighted in orange using the Hi data with (circle) and without (square) Hanning smoothing. Filled grey squares show the galaxies in HIUDGSnodm. Outwards from the solid line, the dashed and dotted lines show the 1, 2, and 4 $\sigma$ scatter.
• Figure 4: DESI Legacy Survey $g$-band image of VCC 1964 of 2.5 F.O.V., with ellipses showing different inclination estimates. Green is the result from the SMUDGES catalogue. Red shows the aperture we use for photometry on the SDSS data, using the SMUDGES fit but increasing the aperture size by a factor of 2.5 to visually estimate the limit of the galaxy. Cyan ellipses are the same size as the visual fit but with inclination angles which would give a rotation width compatible with the standard (solid) and 2 $\sigma$-scatter (dashed) of the BTFR.
• Figure 5: Optical form of the TFR following the methodology of K20. Black points are our AGES galaxies, with VCC 1964 highlighted in orange (as in figure \ref{['fig:btfr']} the orange square uses Hi data without Hanning smoothing). The solid line is the relation obtained in K20 with the dashed and dotted lines showing their estimated 1 and 2 $\sigma$ scatter. The upper dashed line shows the 6 $\sigma$ deviation.