Tidal Tails of Minor Mergers: Star Formation Efficiency in the Western Tail of NGC 2782

Abstract

While major mergers and their tidal debris are well studied, they are less common than minor mergers (mass ratios 0.3). The peculiar spiral NGC 2782 is the result of a merger between two disk galaxies with a mass ratio of ∼ 4 : 1 occurring ∼ 200 Myr ago. This merger produced a molecular and Hi rich, optically bright Eastern tail and an Hi-rich, optically faint Western tail. Non-detection of CO in the Western Tail by Braine et al. (2001) suggested that star formation had not yet begun to occur in that tidal tail. However, deep Hα narrowband images show evidence of recent star formation in the Western tail. Across the entire Western tail, we find the global star formation rate per unit area (Σ SFR ) several orders of magnitude less than expected from the total gas density. Together with extended FUV+NUV emission from Galaxy Evolution Explorer along the tail, this indicates a low global star formation efficiency in the tidal tail producing lower mass star clusters. The Hii region we observed has a local (few-kpc scale) Σ SFR from Hα that is less than that expected from the total gas density, which is consistent with other observations of tidal debris. The star formation efficiency of this Hii region inferred from the total gas density is low, but normal when inferred from the molecular gas density. These results suggest the presence of a very small, locally dense region in the Western tail of NGC 2782 or of a low metallicity and/or low pressure star forming region.

On the structure of tidally disrupted stellar debris streams

“…However, this will not necessarily be the case for the transverse structure of the stream, as self-gravity and pressure can be important at both early and late times (Kochanek 1994;Coughlin & Nixon 2015;Coughlin et al 2016). If we account for self-gravity and pressure, the transverse momentum equation becomes…”

“…On the other hand, when the tidal force of the black hole overwhelms self-gravity, H ∝ r and the entire stream evolves self-similarly. Along the full radial extent of the debris, the extremities will typically be shear-dominated, while the density in the central region is high enough to dominate over the shear of the hole (Kochanek 1994;Coughlin & Nixon 2015). We therefore expect the scaling of H to vary between the selfgravity-dominated limit and the shear-dominated limit as we traverse the extent of the stream.…”

“…Here we make a brief comparison between his findings and ours. One important result of Kochanek (1994) was that, if the impact parameter β ≡ r t /r p , where r p is the pericenter distance of the center of mass of the progenitor star, is large enough, then self-gravity will not be important for determining the evolution of the stream. Here we find a similar trend: from Figure 8, shear-dominated streams always follow ρ r 3 ∝ t −q with q < 0.…”

Can molecular clouds live long?

“…It would take place if a gas is extremely cold for excitation of CO emission or if the ratio of CO-to H 2 is very low due to dissolution of CO molecules by UV radiation (see Knierman et al 2012;Wolfire et al 2010). The efficiency of this process depends on dust-to-gas ratio as well as on the density of a cloud, so that the low dense dustpoor MCs may remain unnoticed.…”

Star formation in low density HI gas around the elliptical galaxy NGC 2865

“…Indeed the actively star-forming regions associated with the galaxy outskirts or intergalactic medium (e.g. Neff et al 2004;Lisenfeld et al 2007;Knierman et al 2012;Mullan et al 2013) have high metallicities for their luminosities, given that they are formed by gas that was pre-enriched in the "parent" galaxy (e.g. Mendes de Oliveira et al 2004;Duc et al 2007;de Mello et al 2012;TorresFlores et al 2012TorresFlores et al , 2014.…”