[Paper Review] Chandra studies of the X-ray gas properties of galaxy groups

Ming Sun, G. Mark Voit+4 more|arXiv (Cornell University)|May 15, 2008

Galaxies: Formation, Evolution, Phenomena3 references20 citations

TL;DR

This study presents a systematic Chandra analysis of 43 nearby galaxy groups, deriving robust X-ray gas properties to at least $r_{2500}$ and to $r_{500}$ for 11 groups. It confirms a universal temperature profile beyond 0.15$r_{500}$, reveals low intrinsic scatter in entropy at $r_{2500}$ (~10%), and shows that group gas fractions are low and scatter significantly within $r_{2500}$, explaining the large scatter in luminosities, while scaling relations remain well-behaved down to $2 \times 10^{13}$ h⁻¹ M⊙.

ABSTRACT

We present a systematic analysis of 43 galaxy groups (kT_500=0.7-2.7 keV or M_500=10^13-10^14 h^-1 M_solar, 0.012r_2500 for all 43 groups. For 23 groups, gas properties can be robustly derived to or extrapolated to r_500. We show that in spite of the large variation in T profiles inside 0.15 r_500, the T profiles of these groups are similar at >0.15 r_500 and are consistent with a "universal temperature profile". We present the entropy-T relations at six characteristics radii (30 kpc - r_500), for 43 groups from this work and 14 clusters from Vikhlinin et al. (2008). Despite large scatter in the entropy values at <0.15 r_500, the intrinsic scatter from r_2500 is much smaller and remains the same (~10%) to r_500. We also present scaling relations for the gas fraction. It appears that the average gas fraction between r_2500 and r_500 has no temperature dependence, ~ 0.12 for 1 - 10 keV systems. The group gas fractions within r_2500 are generally low and have large scatter. This work shows that the difference of groups from hotter clusters stems from the difficulty of compressing group gas to inside r_2500. The large scatter of the group gas fraction within r_2500 causes large scatter in the group entropy around the center and may be responsible for the large scatter of the luminosities. Nevertheless, the groups appear more regular and more like clusters beyond r_2500, from the results on gas fraction and entropy. Therefore, mass proxies can be extended into low mass systems. The M-T and M-Y relations derived in this work are indeed well behaved down to at least 2E13 h^-1 M_solar.

Motivation & Objective

• To systematically analyze X-ray gas properties in 43 nearby galaxy groups using high-resolution Chandra data.
• To determine the extent to which gas properties—temperature, entropy, and gas fraction—can be reliably measured to $r_{500}$ and $r_{2500}$.
• To investigate whether galaxy groups follow the same scaling relations as massive clusters, especially the $K-T$ and $M-T$ relations, and to assess the role of baryonic physics in shaping these relations.
• To quantify the intrinsic scatter in entropy and gas fraction and link it to physical processes like AGN feedback and cooling in low-mass systems.

Proposed method

• Utilized archival Chandra X-ray data with robust background subtraction and modeling to derive surface brightness and temperature profiles.
• Traced gas properties to $r_{2500}$ for all 43 groups, and to $r_{500}$ for 11 groups using spectral fitting and radial binning.
• Applied hydrostatic equilibrium modeling to estimate total mass and concentration $c_{500}$, with caution in the central regions due to potential non-equilibrium conditions.
• Combined with 14 clusters from Vikhlinin et al. (2008) to construct $K-T$ relations at six radii: 30 kpc, 0.15$r_{500}$, $r_{2500}$, $r_{1500}$, $r_{1000}$, and $r_{500}$.
• Used extrapolation and profile fitting to estimate $r_{500}$ and $M_{500}$ for groups where direct measurement was not possible.
• Accounted for contamination from nearby sources and extended structures by excluding regions within 1.4 times $r_{500}$ of neighboring groups.

Experimental results

Research questions

• RQ1 To what extent do X-ray gas temperature profiles in galaxy groups exhibit a universal shape beyond 0.15$r_{500}$?
• RQ2 What is the intrinsic scatter in entropy at $r_{2500}$, and how does it evolve to $r_{500}$ in groups and clusters?
• RQ3 How do gas fractions in groups vary with temperature and radius, and what explains the large scatter in $r_{2500}$ gas fractions?
• RQ4 Are the $M_{500}-T_{500}$ and $M_{500}-Y_{\rm X,500}$ scaling relations well-behaved down to $2 \times 10^{13}$ h⁻¹ M⊙?
• RQ5 What physical mechanisms—such as AGN feedback or galactic winds—can explain the observed entropy excess and scatter in group properties?

Key findings

• Despite large scatter in temperature profiles within 0.15$r_{500}$, the temperature profiles of galaxy groups are consistent with a universal profile beyond that radius.
• The intrinsic scatter in entropy at $r_{2500}$ is low (~10%) and remains constant to $r_{500}$, indicating a stable thermal state in the outer group regions.
• The entropy excess at $r_{500}$ is confirmed in both groups and clusters, but its magnitude is smaller than previously reported by ROSAT and ASCA.
• The average gas fraction between $r_{2500}$ and $r_{500}$ is ~0.12 and shows no temperature dependence for 1–10 keV systems.
• Group gas fractions within $r_{2500}$ are generally low and exhibit large scatter, which correlates with the large scatter in group luminosities.
• The $M_{500}-T_{500}$ and $M_{500}-Y_{\rm X,500}$ scaling relations are well-behaved down to $2 \times 10^{13}$ h⁻¹ M⊙, supporting the use of mass proxies in low-mass systems.