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. Based on the recent survey results we also have modeled the contribution from other group members to the stellar mass. We find that the gas is single-phase with a temperature of ≃ 0.7 keV within 1R e (1R e = 9.0 kpc), which is typical for elliptical galaxies. Outside 1R e the gas temperature increases quickly outwards to > 1 keV, indicating its group origin. With the high spatial resolution of Chandra 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 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 M vir . We find that the NGC 1407 group has a baryondominated core, while the mass in the > 1R e is dominated by dark matter. At the virial radius r 200 = 572 ± 118 kpc, the inferred mass and mass-to-light ratio are M 200 = 2.20±0.42×10 13 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.

Steepening mass profiles, dark matter and environment of X-ray bright elliptical galaxies

“…The methods employed in the literature are all parametric and therefore the shapes of the mass profile will have a systematic bias. In NGC 1407, however, the discrepancy between the outer slope of the circular velocity curve of Humphrey et al (2006) and that of our determination and the determination of Zhang et al (2007) is more significant than between the other profiles. The source of this is unclear.…”

“…The green lines are from Fukazawa et al (2006), who used Chandra observations of NGC 1399 and 4472. The blue lines are from Nagino & Matsushita (2009) who combined Chandra and XMM–Newton observations of NGC 1399, 4472 and 5846, and the pink line is from Zhang et al (2007) who combined Chandra and ROSAT data in NGC 1407. Humphrey et al (2006, 2008, 2009) (solid orange, dashed orange and dotted orange lines, respectively) used Chandra observations of NGC 1407, 4472 and 4649, respectively, and, finally, the cyan line is from Matsushita et al (2002), who used XMM–Newton observations of NGC 4486.…”

The SLUGGS survey: multipopulation dynamical modelling of the elliptical galaxy NGC 1407 from stars and globular clusters

“…On the other hand, modelling of X-ray gas in this galaxy (Humphrey et al 2006;Zhang et al 2007;Su et al 2014) found values of M/L that are lower by a factor of two. This paper is structured as follows.…”

“…The prior on the stellar mass-to-light ratio includes the values predicted by stellar population models for NGC 1407 (Zhang et al 2007;Humphrey et al 2006) in the case of a Kroupa (2001) IMF (Υ * ,B = 4 − 6.5Υ ⊙,B ) and a Salpeter (1955) IMF (Υ * ,B = 6 − 11.2Υ ⊙,B ). The large uncertainties on Υ * ,B come from systematics in the stellar population models.…”

Comparing X-Ray and Dynamical Mass Profiles in the Early-Type Galaxy NGC 4636

“…This approach has proved successful at yielding meaningful mass profiles for early-type galaxies (e.g., O'Sullivan & Ponman 2004;Fukazawa et al 2006;Humphrey et al 2006;Zhang et al 2007). The effect of the assumption of spherical symmetry has been addressed in the case of galaxy clusters, indicating that although compression and elongation along the line of sight can under-or overestimate the central mass, respectively, this is only a small effect at large radius (Piffaretti et al 2003).…”