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

We present a study of the mass distributions in the bright E0 galaxy NGC 1407and its associated group by analyzing the high quality Chandra and ROSAT X-rayspectroscopic data. In order to probe the stellar mass distribution wecalculated the B-band mass-to-light ratio profile by comparing the observedline-strength indices and multi-color photometric data with different stellarsynthesis model predictions. We find that the gas is single-phase with atemperature of ~0.7 keV within 1Re. Outside 1Re the gas temperature increasesquickly outwards to >1 keV, indicating its group origin. We reveal that theX-ray surface brightness profile shows a central excess in the innermostregion, and on both the total mass and dark matter profiles there is aflattened feature at about <1Re, which coincides with the gas temperaturetransition from the galactic level to the group level. The total mass and darkmatter distributions within 0.85Re are cuspy and can be approximated bypower-law profiles with indices of ~2, which are marginally consistent with thegeneralized NFW profiles with zeta=2. The mass in outer regions can be wellfitted by a single NFW profile, with the derived concentration parameterc=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 radiusr200=572\pm118 kpc, the inferred mass and mass-to-light ratio areM200=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 tomany clusters of galaxies. Since the obtained total mass is lower than thosegiven in the earlier galaxy kinematic works, we speculate that NGC 1400 is nota virialized member in the group's gravitational potential well.Comment: 32 pages, 6 figures, accepted for publication in Ap