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

Abstract

We present Suzaku off-center observations of two poor galaxy groups, NGC 3402 and NGC 5129, with temperatures below 1 keV. Through spectral decomposition, we measured their surface brightnesses and temperatures out to 530 and 1430 times the critical density of the universe for NGC 3402 and NGC 5129, respectively. These quantities are consistent with extrapolations from existing inner measurements of the two groups. With the refined bolometric X-ray luminosities, both groups prefer $L_X$-$T$ relations without a break in the group regime. Furthermore, we have determined the electron number densities and hydrostatic masses at these radii. We found that the surface brightness and electron number density profiles require two $\beta$ model components, as well as the indication that a third $\beta$ model may be needed for NGC 3402. Adding the gas mass measured from the X-ray data and stellar mass from group galaxy members, we computed baryon fractions of $f_b$ = 0.0693 $\pm$ 0.0068 and $f_b$ = 0.095 $\pm$ 0.014 for NGC 3402 and NGC 5129, respectively. Combining other poor groups with well-measured X-ray emission to the outskirts, we found an average baryon fraction extrapolated to $r_{500}$ of $\overline{f_{b,500}}$ = 0.0912 $\pm$ 0.0050 for X-ray-bright groups with temperatures between 0.8 and 1.3 keV, extending existing constraints to lower-mass systems and indicating that significant baryon losses exist below approximately $r_{500}$.

44 pages, 13 figures and 8 tables

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