Disk galaxies seen at high angles to the line of sight, the so-called edge-on galaxies, are the only extragalactic objects where it is possible to study the vertical distribution of stars and gas. Thus the edge-on galaxies provide a unique opportunity to directly study the three-dimensional distribution of matter in galaxies. The vertical structure of galactic disks and the vertical velocity dispersion are determined by the gravitational potential, which depends on the mass distribution. The smaller the vertical velocity dispersion of stars, the thinner the disk. The thickness of a self-gravitating collisionless disk is determined by the dynamic heating caused by warping instabilities. In addition, interaction with nearby galaxies, major and minor mergings also cause the dynamical heating of the disk. The massive galactic halo stabilizes the stellar disk and leads to the existence of very flat systems. The thickness of the disk reflects the relative contribution of the dark matter halo to the total mass of the galaxy.
Very thin disk galaxies are an ideal laboratory for studying the formation and evolution of galactic disks. Many authors note ( Karachentsev 1989, Kautsch 2009, Shao et al. 2015, Bizyaev et al. 2017 ) that bulgeless galaxies are located outside the high density regions. An obvious explanation is the lack of tidal perturbations of isolated disks from close neighbors. Kormendy in a book “Secular Evolution in Disk Galaxies” stresses that the significant number of massive galaxies without bulges represents a big problem for modern theories of galaxy formation, since numerous minor merger events lead to the growth of galaxy bulges. Stellar disks with extremely low velocity dispersion make an even bigger problem (Bizyaev et al. 2017). Sachdeva et al. (2015) compared the properties of disk galaxies at redshift z~0.9 from the GOODS survey observed with HST with similar galaxies from the SDSS survey. They found that the stellar mass of galaxies increased more than twice over the last 8 billion years. A significant increase in the mass and size of galaxies shows that the accretion of external matter is the dominant mode of growth for galaxies. The evolution of galaxy size is faster in denser regions, and classical bulges are formed mostly due to minor merging. At the same time, the existence of superthin stellar disks points to alternative ways of galaxy formation in the least dense regions of the Universe. The role of the cosmological environment in the galaxy formation and evolution is an understudied problem in modern astrophysics.
Late-type galaxies without bulges consisting of a simple disk are ubiquitous in the local Universe. It is believed that they have not experienced recent major mergers. At the same time, signs of interactions are visible in the pure disk galaxies. For example, many disks have warps (Reshetnikov & Combes 1998). Multiple minor mergings are typical for nearby low-mass galaxies (Bournaud et al. 2007). The disturbances should heat thin disks (Kazantzidis et al. 2008) and lead to the growth of bulges (Naab & Burkert 2003). The hierarchical clustering predicts that only a small number of simple disks should survive the evolution in the Universe. Internal secular evolution also plays an important role in transformations of galaxy disks. For example, the bar instability leads to a redistribution of matter in the disk and to the formation of a pseudobulge (Kormendy & Kennicutt 2004). This makes thin and ultra-thin disks an ideal laboratory for the comparison between the theory and observations, and for studying mechanisms of secular evolution.
In framework of the study of edge-on galaxies, this database combines photometric and spectral observations of edge-on galaxies, as well as literature data, The data on kinematics, multicolor photometry, star formation, and gas content allow us to obtain the structural and evolutionary parameters of the disks of galaxies.