Transformation of a Virgo Cluster Dwarf Irregular Galaxy by Ram Pressure Stripping: Ic3418 and Its Fireballs

Abstract

We present optical imaging and spectroscopy and HI imaging of the Virgo Cluster galaxy IC 3418, which is likely a "smoking gun" example of the transformation of a dwarf irregular into a dwarf elliptical galaxy by ram pressure stripping. IC 3418 has a spectacular 17 kpc length UV-bright tail comprised of knots, head-tail, and linear stellar features. The only Hα emission arises from a few HII regions in the tail, the brightest of which are at the heads of head-tail UV sources whose tails point toward the galaxy ("fireballs"). Several of the elongated tail sources have Hα peaks outwardly offset by ∼80-150 pc from the UV peaks, suggesting that gas clumps continue to accelerate through ram pressure, leaving behind streams of newly formed stars which have decoupled from the gas. Absorption line strengths, measured from Keck DEIMOS spectra, together with UV colors, show star formation stopped 300±100 Myr ago in the main body, and a strong starburst occurred prior to quenching. While neither Hα nor HI emission are detected in the main body of the galaxy, we have detected 4×10 7 M ⊙ of HI from the tail with the VLA. The gas consumption timescale in the tail is relatively long, implying that most of the stripped gas does not form stars but joins the ICM. The velocities of tail HII regions, measured from Keck LRIS spectra, extend only a small fraction of the way to the cluster velocity, suggesting that star formation does not happen in more distant parts of the tail. Stars in the outer tail have velocities exceeding the escape speed, but some in the inner tail should fall back into the galaxy, forming halo streams. One likely fallback stream is identified. 7 In this paper we will follow the majority of recent authors and refer to the more massive early type dwarf galaxies as dEs, and lower mass ones as dSphs. However some authors (Kormendy & Bender 2012) prefer the name Spheroidal (Sph) for more massive early type dwarfs, since they are distinct systems from giant Ellipticals, and dEs (or Sphs) and dSphs seem to be part of the same family. dE can be interpreted as "early type dwarf".

Gas loss in simulated galaxies as they fall into clusters

“…A clear trend is observed inward of two to three cluster virial radii: the fraction of gas-rich galaxies decreases monotonically with decreasing cluster-centric distance. This trend is in good agreement with observations that the HI gas depletion increases with decreasing cluster-centric distance inward of about two virial radii (15,18,20,24,(48)(49)(50)(51)(52). Intriguingly, observations indicate that the distribution of star formation rates of cluster galaxies begins to change, compared with the field population, at a cluster-centric radius of about three virial radii.…”

“…14-24 and references therein). Recent detailed observations of ongoing gas depletion from galaxies in clusters are presented by the Very Large Array survey of neutral hydrogen (HI) in Virgo galaxies showing long tails of stripped gas behind galaxies, where the gas clumps in the tails are accelerated by ram pressure, leaving behind streams of new stars (22,24). Long tails of HI and ionized gas are observed in galaxies in several nearby clusters, consistent with the description given here (e.g., refs.…”

Spectacular tails of ionized gas in the Virgo cluster galaxy NGC 4569

“…We should recall, however, that these condensed regions of star formation appear in the simulations whenever the gas is allowed to cool and the contribution of the different heating processes is underestimated. The presence of several condensed regions in another Virgo cluster galaxy, IC 3418 (Hester et al 2010;Fumagalli et al 2011;Kenney et al 2014), where the conditions of the intracluster medium are expected to be similar to those encountered by NGC 4569, clearly indicates that the process of formation of these extraplanar HII regions is still far from being understood.…”

“…The smallest resolved features in the Hα+[NII] image have a filametary structure of thickness larger than 200 pc. We note that the NUV image of the galaxy and its surrounding regions was obtained with a very long exposure (16 993 s) and it is thus very sensitive to extraplanar star-forming regions, such as those observed around M49 (Arrigoni- Battaia et al 2012) or VCC 1217 (Hester et al 2010;Fumagalli et al 2011;Kenney et al 2014). The Hα+[NII] surface brightness of the extraplanar HII region which are associated with these two galaxies is ∼2 × 10 −17 erg cm −2 s −1 arcsec −2 , thus well above the A68, page 8 of 17 Table 2.…”

“…The Virgo cluster has another major quality: owing to its proximity, the angular resolution of multifrequency data is comparable to that obtained in high-resolution simulations. The comparison of the kinematic and spectrophotometric properties of several bright Virgo galaxies has indeed been crucial for the identification of the perturbing process (Vollmer et al 1999(Vollmer et al , 2000(Vollmer et al , 2005(Vollmer et al , 2006(Vollmer et al , 2008a(Vollmer et al , 2008b(Vollmer et al , 2012Vollmer 2003;Kenney et al 2004;Boselli et al 2005Abramson et al 2011;Kenney et al 2014;Abramson & Kenney 2014;Cortes et al 2015).…”