Abstract
Abstract Nearby, low-metallicity dwarf starburst galaxies hosting active galactic nuclei (AGNs) offer the best local analogs to study the early evolution of galaxies and their supermassive black holes (BHs). Here we present a detailed multiwavelength investigation of star formation and BH activity in the low-metallicity dwarf–dwarf galaxy merger Mrk 709. Using Hubble Space Telescope Hα and continuum imaging combined with Keck spectroscopy, we determine that the two dwarf galaxies are likely in the early stages of a merger (i.e., their first pass) and discover a spectacular ∼10 kpc long string of young massive star clusters (t ≲ 10 Myr; M ⋆ ≳ 105 M ⊙) between the galaxies triggered by the interaction. We find that the southern galaxy, Mrk 709 S, is undergoing a clumpy mode of star formation resembling that seen in high-redshift galaxies, with multiple young clusters/clumps having stellar masses between 107 and 108 M ⊙. Furthermore, we present additional evidence for a low-luminosity AGN in Mrk 709 S (first identified by Reines et al. using radio and X-ray observations), including the detection of the coronal [Fe x] optical emission line. The work presented here provides a unique glimpse into processes key to hierarchical galaxy formation and BH growth in the early universe.
Highlights
Dwarf galaxies form the largest subset of galaxies in the Universe (e.g., Binggeli et al 1988), and their mergers play a crucial role in hierarchical galaxy formation (e.g., Stierwalt et al 2017)
We have presented a study of the low-metallicity dwarf-dwarf galaxy merger Mrk 709 with the goal of investigating star formation and active galactic nucleus (AGN) activity
In addition to radio and X-ray evidence for an AGN, we detect [Fe X]λ 6374 in the Sloan Digital Sky Survey (SDSS) spectrum of Mrk 709 S. [Fe X] is a known AGN coronal line (Penston et al 1984; Netzer 2013), and has a high-ionization potential of 262.1 eV (Oetken 1977). This coronal line can be photoionized by the hard AGN continuum (e.g., Nussbaumer & Osterbrock 1970; Korista & Ferland 1989; Oliva et al 1994; Pier & Voit 1995), or more likely in Mrk 709 S, mechanically excited by out-flowing winds caused by radio jets (Wilson & Raymond 1999)
Summary
Dwarf galaxies form the largest subset of galaxies in the Universe (e.g., Binggeli et al 1988), and their mergers play a crucial role in hierarchical galaxy formation (e.g., Stierwalt et al 2017). Dwarf-dwarf mergers trigger intense star formation, and lead to the formation of blue compact dwarfs (BCDs; Stierwalt et al 2017; Paudel et al 2018; Kado-Fong et al 2020; Zhang et al 2020) that have strongly starbursting spectra and physical sizes of 1 kpc (Bekki 2008). These low-mass, lowmetallicity BCDs can host super star clusters (SSCs; Johnson et al 2000; Reines et al 2008b), which have properties expected of the progenitors of globular clusters (e.g., O’Connell et al 1994).
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