Tidal Tails of Minor Mergers. Ii. Comparing Star Formation in the Tidal Tails of NGC 2782

Abstract

The peculiar spiral NGC 2782 is the result of a minor merger with a mass ratio ∼ 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. However, deep UBV R and Hα narrowband images show evidence of recent star formation in the Western tail, though it lacks massive star clusters and cluster complexes. Using Herschel PACS spectroscopy, we discover 158µm [Cii] emission at the location of the three most luminous Hα sources in the Eastern tail, but not at the location of the even brighter Hα source in the Western tail. The Western tail is found to have a normal star formation efficiency (SFE), but the Eastern tail has a low SFE. The lack of CO and [Cii] emission suggests the Western tail Hii region may have a low carbon abundance and be undergoing its first star formation. The Western tail is more efficient at forming stars, but lacks massive clusters. We propose that the low SFE in the Eastern tail may be due to its formation as a splash region where gas heating is important even though it has sufficient molecular and neutral gas to make massive star clusters. The Western tail, which has lower gas surface density and does not form high mass star clusters, is a tidally formed region where gravitational compression likely enhances star formation.

Near-infrared to Mid-infrared Observations of Galaxy Mergers: NGC 2782 and NGC 7727

“…Thus, the SFR from the S7 band should be taken as an upper limit. Also note that the spatial resolution of the present observation is different from those of Knierman et al (2013) and the regions where the estimate made are not exactly the same. Figure 11 shows comparison of the SFRs thus estimated from the flux at S7 with those from Hα and [C II] for each region.…”

“…The proposed scenario requires that the destruction time scale of PAHs and the formation time scale of VSGs should be longer than the merger ages. The merger age is estimated as about 200-300 Myr for NGC 2782 (Smith 1994;Knierman et al 2013) and 1.3 Gyr for NGC 7727 (Georgakakis et al 2000). In our Galaxy, the lifetime of carbonaceous dust is estimated as 170 Myr (Serra Díaz-Cano & Jones 2008) and that of PAHs is as ∼ 100−500 Myr (Micelotta et al 2010).…”

“…In general, the ratio of the 25 µm to 12 µm increases with the incident radiation field and thus with the star-formation activity Sakon et al 2006;Onaka et al 2007a). Alternatively, the MIR continuum emission also increases with (Knierman et al 2013). The black straight dashed lines show the locations of the dust lanes seen in optical images ).…”

“…Star-formation rates for the regions indicated in Figure 3. The red squares show those estimated from the PAH bands of the present S7 image (see text for details), while the black triangles and the blue circles indicate those from Hα and [C II], respectively (Knierman et al 2013). …”

Superwind evolution: the young starburst-driven wind galaxy NGC 2782

“…Knierman et al (2013) found [C ii] emission at the location of the three most luminous Hα sources in the eastern tail, but also a lack of CO and [C ii] associated with the brighter Hα sources in the western tail. They suggested that the western tail material may be undergoing its first star formation and that the star formation efficiency (SFE) is increased by gravitational compression, due to its tidal nature.…”