Under Pressure: Star Clusters and the Neutral Hydrogen Medium of Tidal Tails

Abstract

Using archival data from ATCA, WSRT, and the VLA, we have analyzed the Hi emission of 22 tidal tail regions of the Mullan et al. sample of pairwise interacting galaxies. We have measured the column densities, line-of-sight velocity dispersions, and kinetic energy densities on ∼kpc scales. We also constructed a tracer of the line-of-sight velocity gradient over ∼10 kpc scales. We compared the distributions of these properties between regions that do and do not contain massive star cluster candidates (M V < -8.5; ∼ 10 4 -10 6 M ⊙ as observed in HST WFPC2 VI data). In agreement with Maybhate et al., we find that a local, ∼kpc-scale column density of log N H i 20.6 cm −2 is frequently required for detecting clustered star formation. This Hi gas also tends to be turbulent, with line-of-sight velocity dispersions σ los ≈ 10-75 km s −1 , implying high kinetic energy densities (log Σ KE > 46 erg pc −2 ). Thus high Hi densities and pressures, partly determined by the tail dynamical age and other interaction characteristics, are connected to large-scale cluster formation in tidal tails overall. Lastly, we find that the high mechanical energy densities of the gas are likely not generally due to feedback from star formation. Rather, these properties are more likely to be a cause of star formation than a result.

Gemini Follow-up of Two Massive H i Clouds Discovered with the Australian Square Kilometer Array Pathfinder

“…The surface density threshold for star formation is given by Schaye (2004) in terms of H i column density: N H,crit ∼ (3−10)×10 20 cm −2 . This value is in agreement with the column density required to detect clustered star formation as given by Mullan et al (2013).…”

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“…WALLABY will cover 75 percent of the sky (declination range of −90 • < δ < +30 • ) and is predicted to detect Hi in more than 500,000 galaxies out to a redshift of 0.26 with an angular resolution of 30 arcsec and a spectral resolution of 4 km s −1 (Duffy et al 2012, Koribalski 2012. This level of resolution is required to resolve the physical characteristics and measure the dynamical properties of tidally formed features in the nearby universe (Mullan et al 2013, Lelli et al 2015.…”

A tale of two tails: exploring stellar populations in the tidal tails of NGC 3256

“…This suggests that there may be small star formation events periodically in both tails, but the evidence of these events will rapidly fade away. Mullan et al (2013) found that tidal tails with large Hi line of sight velocity dispersion σ los and high Hi column densities were ideal locations for SCCs. Both the 3256 tails fit these criteron, and the majority of their SCCs, as determined by Knierman et al (2003) using HST WFPC2 data, lie in Hi pixels with these characteristics.…”

“…Tidal tails are signatures of galactic mergers (Schweizer 1978), illuminated by the ignition of star for-E-mail: mrodruck@psu.edu mation (Schombert et al 1990). Turbulent energy injected into the local Hi through these mergers compresses the gas, forming new stars (Renaud et al 2014); This is observationally shown in Mullan et al (2013), who were able to link the presence of Star Clusters Candidates (SCCs) to turbulent regions of Hi. While the young, in-situ formed SCCs of tidal tails have been studied in the past through imaging and spectroscopy (e.g.…”