NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast ALFA Survey

TL;DR

This study uses ALFALFA survey data to reveal a long HI tail and evidence of galaxy harassment in NGC 4254, illustrating environmental effects on galaxy morphology and interstellar medium in the Virgo cluster.

Contribution

First detailed HI mapping of NGC 4254 showing a 250 kpc tail and evidence supporting galaxy harassment as the cause of its asymmetry.

Findings

Discovery of a 250 kpc HI tail extending from NGC 4254

Identification of Virgo HI21 within the HI structure

Evidence supporting galaxy harassment as the cause of asymmetry

Abstract

We present an HI map constructed from the Arecibo Legacy Fast ALFA (ALFALFA) survey of the surroundings of the strongly asymmetric Virgo cluster Sc galaxy NGC 4254. Noted previously for its lopsided appearance, rich interstellar medium, and extradisk HI emission, NGC 4254 is believed to be entering the Virgo environment for the first time and at high speed. The ALFALFA map clearly shows a long HI tail extending ~250 kpc northward from the galaxy. Embedded as one condensation within this HI structure is the object previously identified as a "dark galaxy": Virgo HI21 (Davies et al. 2004). A body of evidence including its location within and velocity with respect to the cluster and the appearance and kinematics of its strong spiral pattern, extra-disk HI and lengthy HI tail is consistent with a picture of "galaxy harassment" as proposed by Moore et al. (1996a,b; 1998). The smoothly varying radial velocity field along the tail as it emerges from NGC 4254 can be used as a timing tool, if interpreted as resulting from the coupling of the rotation of the disk and the collective gravitational forces associated with the harassment mechanism.