Chapter 25: Finding and Exploring Merging Pairs of Galaxies in 2MASS Using the NVO

Interacting/merging galaxies are among the most beautiful and fascinating of galaxy systems. These systems are major sites of enhanced star formation (Allam 1998, Lambas et al. 2003). Ultra Luminous InfraRed Galaxies (ULIRGs) have been shown to be associated with mergers and merger remnants (Sanders & Mirabel 1996, Genzel et al. 2001, Pihlström 2005, Conselice 2006).

Until now most catalogs of merging pairs have been selected using optical data (e.g. Vorontsov-Velyaminov 1959, 1977, Karachentsev 1972, Barton et al. 2000, Patton et al. 2002, Allam et al. 2004; which is based upon the SDSS). Since optical emission is strongly associated with star formation (via, e.g. OB associations in the blue and Hα in the red), most current catalogs of merging pairs have an inherent bias towards systems with high star formation rates.

Observations in the near infrared (NIR) more faithfully trace the underlying stellar mass distribution of galaxies, since most of the stellar mass of galaxies is tied up in low-mass, red dwarfs. Also, the NIR is less affected than the optical by absorption by interstellar dust grains.

Therefore, a catalog of merging pairs extracted from the 2MASS eXtended Source Catalog (XSC; Jarrett et al. 2000) would provide an interesting new "massselected" sample of merging pairs.

Furthermore, a merging-pairs catalog based upon the 2MASS XSC would be the first large, homogeneous, and truly all-sky catalog of such galaxy systems based upon modern detector technology. Using a very different selection algorithm, Xu et al. (2004), found 19 close galaxy pairs in a combined 2MASS/2dF catalog in the southern sky.

During the National Virtual Observatory (NVO) Summer School 2005, Allam et al. (2005) performed a pilot study by downloading 2MASS XSC data for a 125 sq. deg (2.5 deg times 50 deg) region of sky using the NVO Open SkyQuery tool. With this set of extended 2MASS objects, we used modified Karachentsev (1972) criteria to extract merging pairs of galaxies. We found 91 merging pairs in these 125 square degrees of sky, of which only ~10% appeared to be artifacts (e.g. image defects in the 2MASS XSC). Using the NVO tools and facilities (e.g. the NVO Registry, Open Sky Query, TOPCAT, VOPlot), SkyView, and the SDSS SkyServer, we explored the physical and star formation properties of these 91 merging pairs over different wavelength regimes (see http://chart.stsci.edi/twiki/pub/Main/NVOSS2Course_Notes/NVOProjectMergingPairs.ppt). Our results demonstrated the simple facilities of the NVO could quickly uncover scientifically interesting objects from a sample data set. In this paper we extend this pilot study to include all the 2MASS XSC data set. Here we compile the largest sample to date of near infrared selected galaxy pairs. A total of 19,769 pairs were found. The data we consider include 2MASS JHK photometry for the full sample. We start with a brief description of the NVO tools that were used in this study (see Sections 2&3), and the 2MASS XSC data set (see Section 4). The selection criteria and the resulting catalog are discussed in Section 5. In Section 6, we characterize various properties of the sample and in Section 7 we summarize our results.