Introduction to the data

A generic prediction of the standard cold dark matter (CDM) galaxy formation paradigm is that primary dark matter halos around big galaxies like our MW are filled with hundreds of smaller clumps of dark matter. However, the predicted number of dark matter clumps is between 500 and 1000, some 50 times larger than the entire population of known dwarf satellites orbiting our MW at that time. This discrepancy between CDM cosmology and observed dwarf galaxy counts is called the missing satellites problem. More recently focus has shifted to whether the observed distribution of MW satellites could actually be drawn from a population of dark matter sub-haloes. If the mystery behind these "Missing Satellites" and "Disk of Satellites" phenomena can be solved, the results will have a major impact on the most difficult yet deeply fundamental challenge in 21st century cosmology: understanding dark matter and its role in the formation process and evolution of galaxies over the lifetime of the Universe.

Because incompleteness in the census of dwarf satellites seriously hinders the improvement of the current cosmological model, progress can only be made if observers can provide theoreticians with the exact number of dwarf galaxies that do exist around the MW including accurate estimates of the detection limits such as star density, compactness and satellite size, distance from the MW etc. This requires a deep and systematic photometric survey of the entire space around the MW to search for the missing satellites, a technical challenge that has become feasible only recently using modern wide-field CCD cameras. There has been some progress in observations of dwarf galaxies. During the past 15 years, systematic searches of all sky digital survey data such as SDSS, DES, Pan-STARRS1 have resulted in dramatic increase of the number of known satellite galaxies (from 12 to ~50) and the discovery of a previously unseen class of extremely elusive (MV > -7) ultra-faint dwarf galaxies around the MW. A complete census of galaxies to the faintest possible limits is crucial to test this result with potentially far-reaching consequences for CDM theory and constraining the physical properties of dark matter.

Despite the recent discovery of ~40 dwarf galaxies, the total number of known dwarf satellites of the MW is still far from solving the missing satellite problem as it is ~10 times smaller than the number of dark matter halos predicted by CDM cosmology. It is intriguing to note that the detection limit of the current dwarf satellite searches is mainly determined by the surface brightness of the system (~ 31 mag arcsec^-2). In this regard, the recent discovery of the Antlia II dwarf galaxy (Torrealba et al. 2019), the lowest surface brightness system (~31.9 mag arcsec^-2), and some dwarf galaxies at the edge of detectability naturally implies that there are dwarf galaxies with even lower surface brightness yet to be discovered.

As radially pulsating low-mass horizontal branch (HB) stars in the phase of core helium burning, RR Lyrae variables are good tracers of old (> 10 Gyr) and metal-poor populations (i.e., population II). Their photometric and pulsation properties including mean magnitude, period, amplitude, and light curve, are commonly used to derive metallicity, interstellar reddening, and distance of the system. They can also provide information on the stellar structure, formation, and evolution of their host dwarf galaxies, particularly when compared with those stars in the Galactic halo or globular clusters. RR Lyrae variables can be excellent tools to detect these faint systems with old stellar population, as proven by the discovery of Antlia II, which is based on a combination of astrometry, photometry, and variability data from Gaia Data Release 2.

We have successfully performed studies of dwarf satellites using RR Lyrae variables with the wide field of view of the KMTNet. 96(Crater II), 5(Sagittarius II) and 240(Antilia II) RR Lyrae stars were detected and characterized. The detected RR Lyrae stars's data and its characteristics are given in this website.