Suzaku Measurements of Hot Halo Emission at Outskirts for Two Poor Galaxy Groups: NGC 3402 and NGC 5129

摘要

We present $Suzaku$ off-center observations of two poor galaxy groups, NGC3402 and NGC 5129, with temperatures below 1 keV. Through spectraldecomposition, we measure their surface brightnesses and temperatures out to330 and 680 times the critical density of the universe for NGC 3402 and NGC5129, respectively. These quantities are consistent with extrapolations fromexisting inner measurements of the two groups. With the refined X-rayluminosities, both groups prefer $L_X-T$ relations without a break in the groupregime. Furthermore, we measure the electron number densities and hydrostaticmasses at these radii. We find that the electron number density profilesrequire three $eta$ model components, with nearly flat slopes in the 3$^{rd}$$eta$ component for both groups. However, we find the effective slope in theoutskirts to be $eta_{out}$ = 0.59 and 0.49 for NGC 3402 and NGC 5129,respectively. Adding the gas mass measured from the X-ray data and stellar massfrom group galaxy members, we measure baryon fractions of $f_b$ = 0.113 $pm$0.013 and 0.091 $pm$ 0.006 for NGC 3402 and NGC 5129, respectively. Combiningother poor groups with well measured X-ray emission to the outskirts, we findan average baryon fraction of $f_{b,ave}$ = 0.100 $pm$ 0.004 for X-ray brightgroups with temperatures between 0.8$-$1.3 keV, extending existing constraintsto lower mass systems.