Under Pressure: Star Clusters in the Tidal Debris of Interacting Galaxies

Open Access
Mullan, Brendan Lawrence
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
May 01, 2013
Committee Members:
  • Jane Camilla Charlton, Dissertation Advisor
  • Jane Camilla Charlton, Committee Chair
  • Yuexing Cindy Li, Committee Chair
  • Caryl Ann Gronwall, Committee Member
  • Eric D Feigelson, Committee Member
  • Douglas Cowen, Committee Member
  • Astronomy
  • tidal tails
  • radio astronomy
  • star clusters
  • galaxy interactions
  • star formation
  • ISM
  • turbulence
  • photometry
Pairwise galaxy interactions and mergers are commonly observed phenomena. The complex gravitational potential of interacting galaxies drastically affects their morphologies, producing long tidal tails and other galactic debris. Encounters between galaxies can also be agents of photometric evolution across these tidally disturbed features, prompting bursts of star and star cluster formation over the ~Gyr interaction timescale involved. At extragalactic distances, we rely on luminous star clusters (M_V < -8.5; ~10^4--10^6 M_{sun}) to trace the underlying stellar populations and star formation history of the typically dispersed, faint tidal debris. But decades of individual case studies have revealed that tidal tails do not appear to consistently have star clusters, unlike in the disks of gas-rich mergers. The question is why -- what is it about tidal tails that allows star cluster formation in some debris fields but not others? Is there some interplay of merging parameters, e.g., tail age, surface brightnesses, colors, gas densities, and progenitor mass ratios, that produce conditions conducive for these bound structures? Do the answers lie in the local densities and kinematics of the tail ISM? To address these questions, I have searched for compact stellar structures within over 20 tidal tails in different pairs of interacting galaxies using the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). My collection of homogeneously observed tidal tails spans a parameter space of interaction characteristics that could allow luminous star clusters to form and survive. I first characterized and quantified the populations of star clusters within these tails and compared them to various global properties of the interactions that produced them. I also constructed indirect methods to compare the cluster populations and their imprinted formation characteristics with those seen in other extragalactic environments. Using archival data from ATCA, WHISP, and the Very Large Array, I additionally analyzed the turbulent warm neutral medium, the dominant component of the tail ISM, of most of the tidal tail regions within the sample. I examined the roles of ~kpc-scale HI densities, pressures, and kinematics in establishing the conditions required for star cluster formation on smaller (~pc) scales, and how these quantities empirically vary by the global tail properties I studied. In synthesizing these results, a rudimentary paradigm is now emerging, where the formation of luminous clusters in tidal tails depends largely on the turbulent character and amount of hydrogen gas in their local environment. These conditions are determined in part by how the galaxies interact and the ingredients they bring to the cosmic table.