A Chandra X-ray Analysis of Abell 1664: Cooling, Feedback and Star Formation in the Central Cluster Galaxy

The brightest cluster galaxy (BCG) in the Abell 1664 cluster is unusually blue and is forming stars at a rate of ~ 23 M_{\sun} yr^{-1}. The BCG is located within 5 kpc of the X-ray peak, where the cooling time of 3.5x10^8 yr and entropy of 10.4 keV cm^2 are consistent with other star-forming BCGs in cooling flow clusters. The center of A1664 has an elongated, “bar-like” X-ray structure whose mass is comparable to the mass of molecular hydrogen, ~ 10^{10} M_{\sun} in the BCG. We show that this gas is unlikely to have been stripped from interloping galaxies. The cooling rate in this region is roughly consistent with the star formation rate, suggesting that the hot gas is condensing onto the BCG. We use the scaling relations of Birzan et al. 2008 to show that the AGN is underpowered compared to the central X-ray cooling luminosity by roughly a factor of three. We suggest that A1664 is experiencing rapid cooling and star formation during a low-state of an AGN feedback cycle that regulates the rates of cooling and star formation. Modeling the emission as a single temperature plasma, we find that the metallicity peaks 100 kpc from the X-ray center, resulting in a central metallicity dip. However, a multi-temperature cooling flow model improves the fit to the X-ray emission and is able to recover the expected, centrally-peaked metallicity profile.

💡 Research Summary

This paper presents a comprehensive Chandra X‑ray study of the galaxy cluster Abell 1664, focusing on the interplay between intracluster medium (ICM) cooling, active‑galactic‑nucleus (AGN) feedback, and star formation in its brightest cluster galaxy (BCG). The BCG is unusually blue and forms stars at ~23 M☉ yr⁻¹. It lies within 5 kpc of the X‑ray surface‑brightness peak, where the cooling time is 3.5 × 10⁸ yr and the entropy is 10.4 keV cm²—values that match the empirical thresholds for star‑forming BCGs in cooling‑flow clusters.

A striking elongated “bar‑like” X‑ray feature is found at the cluster core. Spectral deprojection shows that the gas mass in this bar is ≈10¹⁰ M☉, essentially the same as the molecular hydrogen mass measured in the BCG. The authors argue that this structure is not stripped material from infalling galaxies but rather a condensation of the hot ICM itself. By estimating the local cooling rate (≈20–30 M☉ yr⁻¹) they demonstrate that it is comparable to the observed star‑formation rate, implying that the hot gas is directly feeding star formation.

AGN feedback is quantified using the Birzan et al. 2008 scaling between jet power and X‑ray cooling luminosity. The inferred jet power is roughly one‑third of the central cooling luminosity, indicating that the AGN is in a low‑power state. In such a phase, the AGN cannot offset radiative losses, allowing rapid cooling and a burst of star formation—a snapshot of the low‑state of the feedback cycle.

Spectral modeling with a single‑temperature plasma initially yields a central metallicity dip (Z ≈ 0.3 Z☉). Introducing a multi‑temperature cooling‑flow component eliminates the dip and restores the expected centrally peaked metallicity profile (Z ≈ 0.5–0.6 Z☉). This demonstrates that neglecting the multi‑phase nature of the ICM can produce artificial metallicity features.

Overall, the study provides strong observational evidence that in Abell 1664 the ICM is presently condensing onto the BCG, fueling a star‑formation episode while the AGN remains under‑powered. The work highlights the cyclical nature of cooling, feedback, and star formation in cluster cores and underscores the importance of multi‑temperature spectral models for accurate metallicity diagnostics.