Probing the Mass Distributions in NGC 1407 and Its Associated Group with the X-ray Imaging Spectroscopic and Optical Photometric and Line-strength Indices Data

Abstract

We present a study of the mass distributions in the bright E0 galaxy NGC 1407 and its associated group by analyzing the high quality Chandra and ROSAT X-ray spectroscopic data. In order to probe the stellar mass distribution we calculated the B-band mass-to-light ratio profile by comparing the observed line-strength indices and multi-color photometric data with different stellar synthesis model predictions. We find that the gas is single-phase with a temperature of ~0.7 keV within 1Re. Outside 1Re the gas temperature increases quickly outwards to >1 keV, indicating its group origin. We reveal that the X-ray surface brightness profile shows a central excess in the innermost region, and on both the total mass and dark matter profiles there is a flattened feature at about <1Re, which coincides with the gas temperature transition from the galactic level to the group level. The total mass and dark matter distributions within 0.85Re are cuspy and can be approximated by power-law profiles with indices of ~2, which are marginally consistent with the generalized NFW profiles with zeta=2. The mass in outer regions can be well fitted by a single NFW profile, with the derived concentration parameter c=18.6\pm1.5. We find that the NGC 1407 group has a baryon-dominated core, while the mass in the >1Re is dominated by dark matter. At the virial radius r200=572\pm118 kpc, the inferred mass and mass-to-light ratio are M200=2.20\pm0.42E13 solarM and Mvir/LB=311\pm60 solarM/solarL,respectively, showing that the NGC 1407 group is an extremely dark system even comparable to many clusters of galaxies. Since the obtained total mass is lower than those given in the earlier galaxy kinematic works, we speculate that NGC 1400 is not a virialized member in the group's gravitational potential well.