Abstract
Abstract We present new HST F275W, F475W, and F814W imaging of the region of the Coma cluster around D100, a spiral galaxy with a remarkably long and narrow (60 × 1.5 kpc) ram pressure–stripped gas tail. We find blue sources coincident with the Hα tail, which we identify as young stars formed in the tail. We also determine they are likely to be unbound stellar complexes with sizes of ∼50–100 pc, likely to disperse as they age. From a comparison of the colors and magnitudes of the young stellar complexes with simple stellar population models, we find ages ranging from ∼1 to 50 Myr and masses ranging from 103 to ∼105 M ⊙. We find that the overall rate and efficiency of star formation are low, ∼6.0 × 10−3 M ⊙ yr−1 and ∼6 × 10−12 yr−1, respectively. The total Hα flux of the tail would correspond to a star formation rate 7 times higher, indicating that some other mechanism for Hα excitation is dominant. From an analysis of the colors, we track the progression of outside-in star formation quenching in the main body of D100, as well as its apparent companion, S0 D99. Finally, we observe that the dust extinction in the base of the tail has an outer envelope with remarkably smooth and straight edges and linear filamentary substructure strongly suggestive of magnetic fields. These features and the small amount of tail broadening strongly suggest gas cooling restricting broadening and the influence of magnetic fields inhibiting turbulence.
Highlights
One possibility is that they may be an abundance of stars formed in the interstellar medium (ISM) compressed by the galaxy rotating into the ram pressure front, such as the features seen in NGC 4921 (Kenney et al 2015)
Given that ∼72% of the u-band flux is found in the clumps of stars we identify with Hubble Space Telescope (HST), and the other 28% is found in diffuse emission throughout the tail, we assume 28% of the stars by mass in the tail are located outside our detected clumps and below the detection limits of our HST observations
Other studies of GASP galaxies, such as Poggianti et al (2019), have found that the dominant ionization mechanism for Hα excitation in these tails is photoionization from young stars. This is quite different than our results for the tail of D100, in which we find levels of star formation seven times lower than those predicted from Hα, suggesting a dominant mechanism, or mechanisms, other than photoionization from young stars is present in the tail
Summary
The recent discovery of a number of remarkable tails of gas and stars stripped from galaxies found in clusters has been an exciting area of study in the field of cluster galaxy evolution, e.g., Gavazzi et al (1995, 2001), Kenney & Koopmann (1999), Yoshida et al (2002, 2004, 2008, 2012), Oosterloo & van Gorkom (2005), Sun & Vikhlinin (2005), Cortese et al (2006, 2007), Chung et al (2007), Sun et al (2007), Kenney et al (2008, 2014), Smith et al (2010), Yagi et al (2010, 2013, 2015, 2017), Abramson et al (2011), Fossati et al (2012), Jáchym et al (2014), Ebeling et al (2014), Poggianti et al (2017), and Boselli et al (2018). In sufficiently high-mass clusters, such as Coma (∼1015 Me), star-forming galaxies can eventually be stripped of even the most strongly gravitationally bound gas at the center, completely quenching star formation (Bravo-Alfaro et al 2000; Smith et al 2010; Yagi et al 2010). Many RPS tails show star formation occurring near the body of the galaxy, a region that contains recently stripped gas (see Figure 2 of Jáchym et al 2014 for an example), as well as no clear trend in the age of stars formed with distance along the tail (e.g., Cortese et al 2007). Jáchym et al (2017) noted that while they found abundant molecular gas, they were only able to state that star formation in the tail may be present, due to the inability to distinguish between Hα arising from photoionization by hot stars and that excited by some other method
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