We have performed a systematic study of several regions in the sky where the number of galaxies exhibiting star formation (SF) activity is greater than average. We used Kiso ultraviolet-excess galaxies (KUGs) as our SF-enhanced sample. By statistically comparing the KUG and non-KUG distributions, we discovered four KUG-rich regions with a size of ∼ 10 • × 10 • . One of these regions corresponds spatially to a filament of length ∼ 60 h −1 Mpc in the Lynx-Ursa Major region (α ∼ 9 h − 10 h , δ ∼ 42 • − 48 • ). We call this "the Lynx-Ursa Major (LUM) filament". We obtained V (RI) C surface photometry of 11 of the KUGs in the LUM filament and used these to investigate the integrated colors, distribution of SF regions, morphologies, and local environments. We found that these KUGs consist of distorted spiral galaxies and compact galaxies with blue colors. Their star formation occurs in the entire disk, and is not confined to just the central regions. The colors of the SF regions imply that active star formation in the spiral galaxies occurred 10 7−8 yr ago, while that of the compact objects occurred 10 6−7 yr ago. Though the photometric characteristics of these KUGs are similar to those of interacting galaxies or mergers, most of these KUGs do not show direct evidence of merger processes.2. KUGs are preferentially Sb-or later-type spiral galaxies. The KUG fraction changes linearly along the Hubble sequence: it is less than 10 % for E/S0 and more than 50 % for Sd/Sm.3. KUGs are biased toward less luminous galaxies. At around the knee of the luminosity function (LF) where B-luminosity L B ∼ 10 10 L ⊙ , most of the KUGs are spiral galaxies. In the fainter regime of L B < 10 9.3 L ⊙ , the dwarf population dominates.
The fraction of the blue population in a survey depends on the depth of the survey.If the survey is volume-limited and deep enough to pick up the bulk of the dwarf population, its fraction would be higher.The distribution of KUGs is inhomogeneous, not only when compared with a uniform distribution but also in comparison with the ambient galaxy distribution. Consequently there are some "KUG-rich regions". This inhomogeneity may be related to environmental effects. Some studies show that a dense environment activates star formation (e.g. Maia et al. 1994;Pastoriza et al. 1994). On the other hand, recent observational studies (e.g. Zabludoff et al. 1996(Z96)) suggest that a low-density environment can enhance star formation. Therefore, the question is unsettled.In this study we statistically analyzed the fraction of KUGs in the whole galaxy population and discovered four KUG-rich regions. Among them was a region which lies in the constellations of Lynx and Ursa Major (α ∼ 9 h − 10 h , δ ∼ 42 • − 48 • ). In this region, there is a galaxy filament with a length of ∼ 60h −1 Mpc (We use H 0 = 100h km s −1 Mpc −1 as the Hubble parameter throughout this paper). We call this structure the Lynx-Ursa Major (hereafter LUM) filament. We obtained V (RI) C surface photometry of eleven KUGs in this region in order to make a q...
