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 have studied the mass distributions in the bright E0 galaxy NGC 1407 and its associated group by analyzing Chandra and ROSAT data. To probe the stellar mass distribution, we calculated the stellar mass-to-light ratios by comparing the observed line-strength indices and multicolor photometric data with different stellar synthesis models. We find that the gas is single phase, with a temperature of ≃0.7 keV within 1Re. Outside 1Re, the gas temperature increases quickly outward 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, where the gas temperature increases rapidly. This may be a mark of the boundary between the galaxy and group halos. 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 ζ = 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 region 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 Mvir/LB = 311 ± 60 M☉/LB,☉, respectively, showing that the NGC 1407 group is an extremely dark system, similar to many clusters of galaxies.