HI observations of the starburst galaxy NGC 2146

Abstract

Abstract. NGC 2146 is a peculiar spiral galaxy which is currently undergoing a major burst of star formation and is immersed in a extended H I structure that has morphological and kinematical resemblence to a strong tidal interaction. This paper reports aperture synthesis observations carried out in the 21 cm line with the Very Large Array (VLA ) of two fields positioned to optimally cover the H I streams to the north and south of the galaxy, along with a 300 ft total power spectral mapping program to recover the low surface brightness extended emission. The observations reveal elongated streams of neutral hydrogen towards both the north and the south of the optical galaxy extending out up to 6 Holmberg radii. The streams are not in the principle plane of rotation of the galaxy, but instead are suggestive of a tidal interaction between NGC 2146 and a LSB companion that was destroyed by the encounter and remains undetected at optical wavelengths. Part of the southern stream is turning back to fall into the main galaxy, where it will create a long-lived warp in the H I disk of NGC 2146. Analysis of the trajectory of the outlying gas suggests that the closest encounter took place about 0.8 billion years ago and that infall of debris will continue for a similar time span.

NGC 2146's starburst region and extended structure

“…In the case where the present-day expansion velocity of v exp ≈ 35 km s −1 acted for most of the time, and from the dimension of the H ii ring of R III ≈ 10 kpc, we arrive at a time t in ≈ R III /v exp ≈ 10 kpc/35 km s −1 ≈ 300 Myr. Such an age is closer to a starburst occurring 100 Mr after a merger, as predicted by Mihos & Hernquist (1996) and Barnes & Hernquist (1996), than the 1000 Myr of an encounter postulated by Taramopoulos et al (2001). However, the apparent coincidence or discrepancy rests entirely on the assumption of the inclination of the ring and the derived expansion.…”

“…Contrary to the far-evolved merger hypothesis, Taramopoulos et al (2001) propose from their observation of a Northern and Southern H i tail (∼150 kpc extension to the South) that the most active part of an encounter has occurred ∼1000 Myr ago between NGC 2146 and a gas-rich, slow rotating, low-surface-brightness (LSB) galaxy. The remaining part of this galaxy is perhaps found ∼70 kpc south of NGC 2146…”

“…and of luminosity L OB ≈ 10 5 L , then the present total mass of OB stars of the starburst region is on the order of M OB ≈ 10 7 M . Young et al (1988); (2) 2MASS: www.irsa.ipac.caltech.edu; (3) Cabrera-Lavers & Garzon (2004); (4) Draper et al (1995); (5) Lisenfeld et al (1996); (6) Casper & Shane (1996); (7) Taramopoulos et al (2001). measured H i mass is concentrated in the center region.…”

“…The starburst galaxy NGC 2146 of type Sb II pec (Sandage & Tammann 1987), at a distance of 14.5 Mpc (with 1 ≈ 70 pc), is large and massive and has some outstanding features: a distorted outer shape, although three spiral arms seem to be distinguishable (de Vaucouleurs 1950;Burbidge et al 1959;Benvenuti et al 1975); H i tails, at least at the southern side of an extension up to ∼150 kpc (Caspers & Shane 1986;Taramopoulos et al 2001); a large amount of dust concentrated near the center, with part of it visible as a prominent dust lane and dust "fingers" and polarisation of the visible light (Benvenuti et al 1975;Sandage & Bedke 1994;Draper et al 1995); it has relatively high infrared luminosity L IR = 6 × 10 10 L (Sanders et al 1986); a central condensation, a weak ring, and extended and warped emission of molecular gas (CO) inside which the starburst occurs (Jackson & Ho 1988;Young et al 1988); two H 2 O kilo-masers in the starburst region (Tarchi et al 2002), along with radio supernovae, supernova remnants, and ultra-dense H ii regions (Kronberg & Biermann 1981;Tarchi et al 2000); strong nonthermal continuum radiation produced to a large extent by supernova explosions (Lisenfeld et al 1996); X-ray point sources and diffuse X-ray emission (Armus et al 1995;Della Ceca et al 1999;Inui et al 2005); a cone-like outflow along the minor axis (Armus et al 1995;Greve et al 2000); and maybe a stellar bar (Cabrera-Lavers & Garzon 2004). In short, NGC 2146 is a distorted, dusty, and molecule-rich galaxy with a strong central starburst and a superwind driven out along the minor axis.…”

The Westerbork SINGS survey

“…This galaxy is a prototypical merger, evidenced by tidal H i structures and a warped optical disk. The streamer seen in the neutral hydrogen towards the South continues into a very extended tidal tail (Taramopoulos et al 2001). The merger has resulted in vigorous star formation throughout this galaxy, resulting in a brightness comparable to that of M 82.…”