Merger-induced Tidal Features
Our hierarchical paradigm of structure formation is underpinned by galaxy merging. The merger process is thought to strongly influence stellar mass build up, black-hole growth and morphological transformation over cosmic time. However, while the role of mergers has been explored from a theoretical point of view, a comprehensive empirical exploration remains missing. This is largely because no single survey conducted so far offers the opportunity to uniformly study galaxy merging from low to high redshift. Current surveys are either wide and shallow or narrow and deep, and studies based on these datasets therefore have to trade off between depth and volume.
Most importantly, since low-mass galaxies far outnumber their massive counterparts, the overwhelming majority of mergers involve low mass ratios (i.e. are 'minor mergers'). Since the surface-brightness of merger-induced tidal features is a strong function of merger mass ratio, minor mergers produce faint tidal features that are largely invisible in past wide area surveys. The resultant heterogeneity in the datasets used, our inability to trace the minor merger process, and the strong cosmic variance suffered by small galaxy samples, has made it difficult to put strong statistical observational constraints on theoretical predictions of galaxy mergers.
LSST has the depth, volume, and wavelength coverage needed to perform a uniform study of mergers involving L* galaxies out to z ~ 2, and a statistical study of bright galaxy mergers out to z ~ 5. The wide area coverage of LSST will be critical for addressing the effects of cosmic variance on measures of the merger rate, which can vary by a factor of two or more even on projected scales of a square degree, and to quantify the role of mergers (and minor mergers in particular) on star formation, black-hole growth and morphological transformation over the lifetime of the Universe.
Fig 1: Examples of standard-depth, multi-colour images (left) from the SDSS and their deeper r-band Stripe 82 counterparts (right) of minor-merger remnants (Kaviraj 2014). Examples are shown of early-type (row 1, left), Sa (row 1, middle), Sb (row 1, right), Sc (row 2, left) and Sd (row 2, middle) galaxies. Extended tidal debris is visible in the Stripe 82 imaging but not in the shallow standard-depth SDSS imaging. For comparison, single LSST visits will be as deep as the SDSS Stripe 82 and the final LSST 10-year survey will offer images that are more than 4 mags deeper than the Stripe 82.
Ongoing mergers can be detected by identifying systems where the merging progenitors are in close proximity to each other i.e. 'close pairs'. LSST’s six-band photometry will result in photometric redshift accuracies, for massive galaxies, of about ~0.03 (1+z). This is comparable to or better than those used in other studies for the identification of close galaxy pairs, and will allow for the selection of merging galaxies with a wide range in colour. While current surveys detect only a few hundred galaxy pairs per square degree at 0.1 < z < 1.0, LSST should be able to observe more than a million close pairs out to z ~ 1.
Post-merger systems exhibit longer-lived low-surface-brightness shells and extended tidal features (Fig 1). While these features typically contribute a few percent of the total galaxy light, with surface brightnesses fainter than 25 mag per arcsec^2, they should be readily visible in the deep LSST imaging. Scaling from the small deep surveys like the CFHTLS-Deep survey, LSST should detect on the order of 15 million galaxies undergoing such tidal interactions.
Fig 2: Low surface brightness tidal streams surrounding NGC5907 (Martınez-Delgado et al. 2008). At z = 0.1, a semi-major axis of 50 kpc corresponds to 27". A dwarf galaxy of absolute magnitude M(R) = −16 stretched uniformly around a circular stream of radius 27" with a half-light width of 2 kpc will have a mean surface brightness of μ(R) = 29.2, 0.07% of the mean dark-sky brightness. The ability to detect and measure the parameters of such streams depends critically on the flatness of the LSST sky background and the ability to model it.
Detailed studies of the tidal streams around galaxies (Fig 2) will have a bearing on a variety of interesting issues. The streams are heated by interaction with dark matter sub-halos within the larger galaxy halo. Statistical studies of the widths of tidal streams may provide constraints on the clumpiness of dark matter halos. The shapes of tidal streams also provide constraints on the shapes of galaxy halos. This can be studied statistically using large samples of tidal streams revealed by the deep images from LSST. By the time LSST begins observing, we expect that hundreds of individual galaxies will have been targeted for deep study with other facilities. LSST, however, will allow us to create a deep, unbiased statistical survey of thousands more galaxies.