arXiv subjects

N. Arimoto

Publications and source records attributed to N. Arimoto.

57 records · Page 4Linked to original sources

arXiv1998

Evolution of the colour-magnitude relation of early-type galaxies in distant clusters

We present a thorough quantitative analysis of the evolution of the colour-magnitude relation for early-type galaxies in 17 distant clusters with redshifts 0.31 < z < 1.27 using the Kodama & Arimoto (1997) evolutionary model for elliptical galaxies. The model is calibrated to reproduce the colour- magnitude relation for Coma ellipticals at z ~ 0 and gives the evolution of the slope and zero-point as a function of redshift. We find no significant differences between the colour-magnitude relations of the clusters in our sample. The slopes can be reproduced by a single model sequence in which all elliptical galaxies are assumed to be equally old (the maximum age difference allowed for the brightest 3 magnitudes is only 1 Gyr) and to have mean stellar metallicities which vary as a function of galaxy luminosity. The zero-points of the colour-magnitude relations constrain the epoch of major star formation in early-type galaxies to z_f>2-4. This study provides two important constraints for any model of the formation of rich clusters: the uniformity of the ages of the stellar populations in the early-type galaxies and the universality of the metallicity sequence of these galaxies as a function of galaxy mass.

astro-ph↗

arXiv1997

Clustering of red Galaxies near the Radio-loud Quasar 1335.8+2834 at z=1.1

We have obtained new deep optical and near-infrared images of the field of the radio-loud quasar 1335.8+2834 at $z=1.086$ where an excess in the surface number density of galaxies was reported by Hutchings et al. [AJ, 106, 1324] from optical data. We found a significant clustering of objects with very red optical-near infrared colors, $4 \lesssim R-K \lesssim 6$ and $3 \lesssim I-K \lesssim 5$ near the quasar. The colors and magnitudes of the reddest objects are consistent with those of old (12 Gyr old at z=0) passively-evolving elliptical galaxies seen at $z=1.1$, clearly defining a `red envelope' like that found in galaxy clusters at similar or lower redshifts. This evidence strongly suggests that the quasar resides in a moderately-rich cluster of galaxies (richness-class $\geq 0$). There is also a relatively large fraction of objects with moderately red colors ($3.5 < R-K < 4.5$) which have a distribution on the sky similar to that of the reddest objects. They may be interpreted as cluster galaxies with some recent or on-going star formation.

astro-ph↗

arXiv1996

The Luminosity-Metallicity Relation for Bulges of Spiral Galaxies

Optical integrated spectra of bulges have been obtained for 28 spiral galaxies. By applying an appropriate aperture size for each galaxy, the unavoidable contamination of disk starlight has been carefully minimized and set to $\sim$ 1/7 of the total stellar light. The sample covers a wide range in bulge luminosity and morphology. The Mg$_2$ index shows a tight correlation with the bulge intrinsic luminosity, similar - and extended at fainter magnitudes - to the relationship known for ellipticals. Other features such as CaIIK, CN, G-band, and TiO show similar trend with the bulge luminosity. On the contrary, the Fe5270 and Fe5335 iron lines show a constant equivalent width - within some dispersion - for all bulges. A simple application of a stellar population synthesis model suggests that magnesium should be most enhanced with respect to iron in the brightest bulges. Concerning the structural parameters, bulges do occupy the same region in the fundamental plane as ellipticals. In conclusion, the present study favors the idea that the bulk of stars in bulges have formed with only moderate or negligible influence of the disk material, likely by very similar processes as those driving the formation of elliptical galaxies.

astro-ph↗