@phdthesis{oai:nagoya.repo.nii.ac.jp:00012603,
 author = {Yamauchi, Chisato and ヤマウチ, チサト},
 month = {Mar},
 note = {The formation processes of early-type galaxies is a long-standing issue of great importance to galaxy evolution. One of hypothesis is that early-type galaxies form via mergers of gaseous disc galaxies or of many dwarf galaxies (e.g., Toomre 1977). The merger hypothesis is supported by some observational studies, such as the dynamical disturbances of observed ellipticals such as shell structures (e.g., Malin&Carter 1983; Schweizer&Seitzer 1988)and kinematically decoupled cores (e.g., Kormendy 1984; Bender& Surma 1992) of elliptical galaxies. Theoretically, ancestral equal-mass galaxies are completely destroyed so as to form a spheroid that looks more like an elliptical galaxy, and merging of two unequal-mass spirals are finally transformed into one S0 galaxy that has a flattened oblate spheroid (e.g., Bekki 1998), with a centralized violent starburst (e.g., Mihos& Heanquist 1996). More recently, hierarchical galaxy formation models in cold dark matter cosmologies have postulated that massive galaxies have complex assembly histories and were built up gradually through mergers and periods of star formation (White& Frenk 1991). In these models properties such as mass, star formation rate, and morphology are transient, depending largely on the merger history and the time elapsed since the most recent merger (e.g., Kauffmann, White & Guiderdoni 1993).  E+A galaxies have been understood as post-starburst galaxies based on the observed strong Balmer absorption lines and the absence of Hα or [OII] emission lines. Since E+A galaxies are very rare and difficult to find, for more than 20 years, most of the previous work is focused on the global properties of E+A galaxies to reveal the origin of E+A galaxies. Recent studies suggest merger/interaction origin of E+A galaxies, therefore, the E+A phase might be very important stage of galaxy formation scenario. However, the two transitions, “from merger/interaction galaxy to E+A galaxy” and “from E+A galaxy to early-type galaxy”, are still uncertain. One of the best way to ascertain the two transitions is the investigation of internal properties, such as age and metallicity gradients, of E+A galaxies along the evolutionary sequence. Previous work often lacked a statistical significance, and did not study them along the evolutionary sequence since E+A galaxies are extremely rare. We thenfocus oninternal propertiesof E+A galaxies,two-dimensional (2D) color and radial color gradients, and study them along the evolutionary sequence using clean E+A sample in huge galaxy catalog.  In this study, we investigate the radial color gradient and 2D color property of 22 E+A galaxies with 5.5˚A < Hδ EW < 8.5˚A and normal 49 early-type galaxies as a control sample at the redshift of < 0.2in the Second Data Release of the Sloan Digital Sky Survey. We found that substantial number of E+A galaxies exhibit positive slopes of radial color gradient (bluer gradients toward the center) which are seldom seen in normal early-type galaxies. We found irregular ‘Color Morphologies’ – asymmetrical and clumpy pattern – at the center of g−r and r−i 2D colormap of E+A galaxies with positive slope of color gradient. Kolomogorov-Smirnov two-sample tests show that g−r and r−i color gradient distributions of E+A galaxies are different from that of  early-type galaxies with a more than 99.99% significance level. We also found a tight correlation between radial color gradients and colors, and between radial color gradients and 4000˚A break in the E+A sample; E+A galaxies which exhibit bluer color or weaker D4000 tend to have positive slopes of radial color gradient. We compare the GISSEL model and E+A’s observational quantities, Hδ EW, D4000 and u−g color, and found that almost all our E+A galaxies are placed under a single evolution track. Therefore, these results are interpreted as E+A galaxies evolve from Hδ EW ∼ 8˚A  to Hδ EW ∼ 5˚A their color gradient changes from positive to negative, and the irregular 2D colormap becomes smoother, during the time scale of ∼300 Myr.  Our results favor a hypothesis that E+A galaxies are post-starburst galaxies caused by the merger/interaction and underwent a centralized violent starburst, and make the two transitions, “from merger/interaction galaxy to E+A galaxy” and “from E+A galaxy to early-type galaxy”, more plausible in one of the evolutionary paths of early-type galaxies., 名古屋大学博士学位論文 学位の種類:博士(理学) (課程) 学位授与年月日　平成17年3月25日},
 school = {名古屋大学, Nagoya University},
 title = {Radial and 2D Color Properties of E+A Galaxies},
 year = {2005}
}