Discrepant Mass Estimates in the Cluster of Galaxies Abell 1689

Abstract

We present a new mass estimate of a well-studied gravitational lensing cluster, Abell 1689, from deep Chandra observations with a total exposure of 200 ks. Within r = 200 h −1 kpc, the X-ray mass estimate is systematically lower than that of lensing by 30-50%. At r > 200 h −1 kpc, the mass density profiles from X-ray and weak lensing methods give consistent results. The most recent weak lensing work suggest a steeper profile than what is found from the X-ray analysis, while still in agreement with the mass at large radii. Fitting the total mass profile to a Navarro-Frenk-White model, we find M 200 = (1.16 +0.45 −0.27 ) × 10 15 h −1 M with a concentration, c 200 = 5.3 +1.3 −1.2 , using non-parametric mass modeling. With parametric profile modeling we find M 200 = (0.94 +0.11 −0.06 ) × 10 15 h −1 M and c 200 = 6.6 +0.4−0.4 . This is much lower compared to masses deduced from the combined strong and weak lensing analysis. Previous studies have suggested that cooler small-scale structures can bias X-ray temperature measurements or that the northern part of the cluster is disturbed. We find these scenarios unlikely to resolve the central mass discrepancy since the former requires 70-90% of the space to be occupied by these cool structures and excluding the northern substructure does not significantly affect the total mass profiles. A more plausible explanation is a projection effect. Assuming that the gas temperature and density profiles have a prolate symmetry, we can bring the X-ray mass estimate into a closer agreement with that of lensing. We also find that the previously reported high hardband to broad-band temperature ratio in A1689, and many other clusters observed with Chandra, may be resulting from the instrumental absorption that decreases 10-15% of the effective area at ∼ 1.75 keV. Caution must be taken when analyzing multiple spectral components under this calibration uncertainty.

SUZAKUOBSERVATION OF A1689: ANISOTROPIC TEMPERATURE AND ENTROPY DISTRIBUTIONS ASSOCIATED WITH THE LARGE-SCALE STRUCTURE

“…Therefore, in our case of Abell 1689, we expect that the PSF effect is at most ∼ 0.1 keV in the cluster outskirts outside 6 ′ , which is much smaller than the statistical errors. The effect inside 6 ′ would be also negligible because our temperature profile is consistent with the Chandra result (Peng et al 2009) as described above. In order to see the other effect, the residual signals from the excluded point sources in 10.…”

“…′ 4 for the five annular regions. Although the temperature profile is a projected one obtained by the two-dimensional spectral analysis, it coincides well with a three-dimensional temperature profile obtained with Chandra data (Peng et al 2009) in the overlapping region. The temperature is ∼ 10 keV within 4.…”

“…′ 0 (Peng et al 2009), we can say that the systematic effects of the Suzaku PSF are safely within the statistical errors in this region. In our 10.…”

“…′ 0 (Peng et al 2009, Model 3 in table 9). In contrast to the temperature profile, the electron densities of the four offset directions are consistent with one another within errors in any circular annulus.…”

Discovery of diffuse radio emission in the galaxy cluster A1689

“…The authors state that their result could support the presence of colder, denser, and more luminous small-scale structures that could be responsible for a bias in the observed temperature (Kawahara et al 2007). Peng et al (2009) refute this view and show that X-ray and lensing estimates can be made consistent by considering a prolate distribution for the gas temperature and density with the major axis aligned with the line-of-sight.…”