Adaptive optics imaging and optical spectroscopy of a multiple merger in a luminous infrared galaxy^★

Abstract

We present near‐infrared (NIR) adaptive optics imaging obtained with VLT/NACO and optical spectroscopy from the Southern African Large Telescope of a luminous IR galaxy (LIRG) IRAS 19115−2124. These data are combined with archival Hubble Space Telescope imaging and Spitzer imaging and spectroscopy, allowing us to study this disturbed interacting/merging galaxy, dubbed the Bird, in extraordinary detail. In particular, the data reveal a triple system where the LIRG phenomenon is dominated by the smallest of the components. One nucleus is a regular barred spiral with significant rotation, while another is highly disturbed with a surface brightness distribution intermediate to that of disc and bulge systems, and hints of remaining arm/bar structure. We derive dynamical masses in the range 3–7 × 1010 M⊙ for both. The third component appears to be a 1–2 × 1010 M⊙ mass irregular galaxy. The total system exhibits H ii galaxy‐like optical line ratios and strengths, and no evidence for active galactic nucleus (AGN) activity is found from optical or mid‐IR data. The star formation rate is estimated to be ∼190 M⊙ yr−1. We also report a search for supernovae from NIR images separated by five months and search for superstar cluster candidates. We detect outflowing gas from the Bird mostly in the range 100–300 km s−1 using Na i D absorption features. Overall, the Bird shows kinematic, dynamical and emission‐line properties typical for cool ultraluminous IR galaxies (ULIRGs). However, the interesting features setting it apart for future studies are its triple merger nature, and the location of its star formation peak – the strongest star formation, as revealed by Spitzer imaging, does not come from the two major K‐band nuclei, but from the third irregular component. This is in contrast to the conventional view that the (U)LIRG phases are powered by infalling gas to the major nuclei of the merging spiral galaxies. Aided by simulations, we discuss scenarios where the irregular component is on its first high‐speed encounter with the more massive components.

Studying the kinematic asymmetries of disks and post-coalescence mergers using a new “kinemetry” criterion

“…Our mass estimates agree with those obtained for LIRGs in Hinz & Rieke (2006) and Väisänen et al (2008), which are in the range (10 10 −10 11 ) M .…”

Shutting down or powering up a (U)LIRG? Merger components in distinctly different evolutionary states in IRAS 19115−2124 (the Bird)

“…We confirm the suggestion based on our earlier adaptive optics imaging (Väisänen et al 2008), that the dominant source of the large IR luminosity, log(LIR/L ) ≈ 11.9, is a small irregular looking component of the interaction 3 kpc away from two primary galaxy nuclei. Based on the recombination line emission maps, we find that this component, the Head of the Bird, produces more than 60% of the total IR, and SFR, of the whole galaxy system, also containing the Heart and Body components.…”

“…There are two massive nuclei in an advanced interaction with large tidal tails, with evidence of bars in the nuclear regions which are thought to facilitate the flow of gas into the central merging nuclei. Our previous observations using SALT and VLT/NACO, reported in Väisänen et al (2008), found a third kinematically separate component in the system, which Spitzer 24µm data at much lower spatial resolution suggested to be the source of the strong IR-luminosity detected by IRAS.…”

“…The details of the interaction clearly have an effect, and various more or less transient phases may be present, including offnuclear star-bursts reported by e.g. Jarrett et al (2006); Väisänen et al (2008); Inami et al (2010); Haan et al (2011), and a newly found case in IRAS 16516-0948 by Herrero-Illana et al (2017, subm.). Moreover, the possibility of multiple interactions complicate the picture.…”

Star formation in mergers with cosmologically motivated initial conditions

“…These multiple merger events are common in the LCDM universe (e.g., see Väisänen et al 2008, for an observed merging triplet), and can have important consequences for galaxy evolution. In more than 50% of all merger pairs since z < 5, a second satellite enters the common halo within five dynamical times after the first satellite entrance (MMS14).…”