Mass Distribuions In Elliptical Galaxy NGC 1407 And Its Associated Group

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 1R_e (9.0 kpc), which is typical for elliptical galaxies. 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 <1R_e, which coincides with the gas temperature transition from the galactic level to the group level. We speculate that this may be a mark of the boundary between the galaxy and group halos, as has been seen in some other cluster/group-dominating galaxies. The total mass and dark matter distributions within 0.85R_e are cuspy and can be approximated by power-law profiles with indices of～2, which are marginally consistent with the generalized NFW profiles withζ= 2. The mass in outer regions can be well fitted by a single NFW profile, and the derived concentration parameter c (18.6±1.5) is larger than the 68% upper limit for a halo at z = 0 with the given Mvir. 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±118 kpc, the inferred mass and mass-to-light ratio are M200=2.20±0.42×1013 M⊙and M_vir/L_B=311±60 M_⊙/L_B,⊙, 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.