HST Imaging of Dust Structures and Stars in the Ram Pressure Stripped Virgo Spirals NGC 4402 and NGC 4522: Stripped From the Outside in With Dense Cloud Decoupling

Abstract

We describe and constrain the origins of ISM structures likely created by ongoing ICM ram pressure stripping in two Virgo Cluster spirals, NGC 4522 and NGC 4402, using HST BVI images of dust extinction and stars, as well as supplementary HI, Hα, and radio continuum images. With a spatial resolution of ∼10 pc in the HST images, this is the highest-resolution study to date of the physical processes that occur during an ICM-ISM ram pressure stripping interaction, ram pressure stripping's effects on the multi-phase, multi-density ISM, and the formation and evolution of rampressure-stripped tails. In dust extinction, we view the leading side of NGC 4402 and the trailing side of NGC 4522, so we see distinct types of features in both. In both galaxies, we identify some regions where dense clouds are decoupling or have decoupled and others where it appears that kpc-sized sections of the ISM are moving coherently. NGC 4522 has experienced stronger, more recent pressure and has the "jellyfish" morphology characteristic of some ram pressure stripped galaxies. Its stripped tail extends up from the disk plane in continuous upturns of dust and stars curving up to ∼2 kpc above the disk plane. On the other side of the galaxy, there is a kinematically and morphologically distinct extraplanar arm of young, blue stars and ISM above a mostly-stripped portion of the disk, and between it and the disk plane are decoupled dust clouds that have not been completely stripped. The leading side of NGC 4402 contains two kpc-scale linear dust filaments with complex substructure that have partially decoupled from the surrounding ISM. NGC 4402 also contains long dust ridges, suggesting that large parts of the ISM are being pushed out at once. Both galaxies contain long ridges of polarized radio continuum emission indicating the presence of large-scale ordered magnetic fields. We propose that magnetic fields could bind together gas of different densities, causing nearby gas of different densities to be stripped at the same rate and creating the large, coherent dust ridges and upturns. A number of factors likely play roles in determining what types of structures form as a result of ram pressure, including ram pressure strength and history, the location within the galaxy relative to the leading side, and pre-existing substructure in the ISM that may be bound together by magnetic fields during stripping.

Molecular Gas Dominated 50 kpc Ram Pressure Stripped Tail of the Coma Galaxy D100*

“…Such gas lumps decouple from the surrounding diffuse gas and may fall back to the galaxy potential. The same phenomenon, although observed spatially rather than kinematically, has been observed in high resolution optical images of other cluster spirals, which show dense decoupled clouds spatially offset from more diffuse dust which is further downstream (Crowl et al 2005;Abramson & Kenney 2014;Abramson et al 2016;Kenney et al 2015). The velocity field of the stripped ISM may have a large range at any distance.…”

“…The D100 tail was also covered by Herschel observations, but only upper limits on FIR emission in the tail can be obtained (will be done elsewhere; Sivanadam et al, in prep.). HST images of the Virgo galaxies NGC 4522 and NGC 4402 that are being stripped clearly show dust being removed from the disk along with the gas (Abramson et al 2016).…”

OMEGA–OSIRIS mapping of emission-line galaxies in A901/2–V. The rich population of jellyfish galaxies in the multicluster system Abell 901/2

“…These galaxies can have active star formation and/or host AGN activity. Given that jellyfish galaxies have been found to strongly emit in Hα (Smith et al 2010;Vulcani et al 2016;Abramson et al 2016;Bellhouse et al 2017;Sheen et al 2017) it is a reasonable starting point to search for them in OMEGA. Three of us (ACS, BRP and FRO) visually inspected the HST/F606W images searching for visual morphological features of gas stripping.…”

Galaxy cluster mergers as triggers for the formation of jellyfish galaxies: case study of the A901/2 system

“…However, the number of jellyfish galaxies found in single systems is usually small. The numbers range from 21 in Coma (Smith et al 2010, Yagi et al 2010, 3 in Virgo (Abramson et al 2016, Kenney et al 2014, Kenney & Koopmann 1999, 1 in A3627 (Sun et al 2006) and 5 in A2744 (Rawle et al 2014). Nevertheless, the rich population of ∼70 jellyfish galaxy candidates found in the A901/2 system (Roman-Oliveira, et al 2019) indicates that clusters in interaction may be an ideal environment to search for these galaxies.…”