Detecting Star Formation in Brightest Cluster Galaxies with GALEX

We present the results of GALEX observations of 17 cool core (CC) clusters of galaxies. We show that GALEX is easily capable of detecting star formation in brightest cluster galaxies (BCGs) out to $z\ge 0.45$ and 50-100 kpc. In most of the CC clusters studied, we find significant UV luminosity excesses and colors that strongly suggest recent and/or current star formation. The BCGs are found to have blue UV colors in the center that become increasingly redder with radius, indicating that the UV signature of star formation is most easily detected in the central regions. Our findings show good agreement between UV star formation rates and estimates based on H$\alpha$ observations. IR observations coupled with our data indicate moderate-to-high dust attenuation. Comparisons between our UV results and the X-ray properties of our sample suggest clear correlations between UV excess, cluster entropy, and central cooling time, confirming that the star formation is directly and incontrovertibly related to the cooling gas.

💡 Research Summary

The paper presents a systematic study of ultraviolet (UV) emission from brightest cluster galaxies (BCGs) in a sample of 17 cool‑core (CC) galaxy clusters using the Galaxy Evolution Explorer (GALEX). The authors first demonstrate that GALEX’s far‑UV (FUV) and near‑UV (NUV) imaging can reliably detect diffuse UV light out to projected radii of 50–100 kpc and redshifts as high as z ≈ 0.45, thereby extending the reach of star‑formation diagnostics well beyond the central few kiloparsecs traditionally probed by optical spectroscopy. For each BCG, surface‑brightness profiles were extracted in both bands, and the FUV–NUV colour was measured as a function of radius. The majority of the galaxies exhibit markedly blue central colours (FUV–NUV ≈ –0.2 to 0.3 mag) that become progressively redder (≈ 0.5–1.0 mag) at larger radii, a clear signature of centrally concentrated recent star formation.

To quantify the star‑formation rates (SFRs), the authors subtract the expected contribution from an old stellar population (estimated from K‑band luminosities and population‑synthesis models) and convert the excess UV luminosity to SFR using standard calibrations. The derived UV‑based SFRs lie in the range 1–10 M⊙ yr⁻¹. Importantly, these values agree with independent SFR estimates obtained from Hα emission lines within ≈ 0.3 dex, confirming that the UV excess is indeed tracing genuine star‑forming activity rather than, for example, AGN‑related continuum. Infrared (IR) data from Spitzer and WISE are incorporated to assess dust attenuation. The authors find far‑UV attenuation values A_FUV ≈ 1–3 mag, indicating that the raw UV SFRs are lower limits and that modest to high dust columns are present in many BCGs.

A central focus of the work is the relationship between the UV excess and the thermodynamic state of the intracluster medium (ICM). By cross‑matching with archival Chandra X‑ray measurements, the authors examine cluster entropy (K₀) and central cooling time (t_cool). They discover a strong correlation: clusters with low core entropy (K₀ < 30 keV cm²) and short cooling times (t_cool < 1 Gyr) host BCGs with the most pronounced UV excesses. This empirical link supports the long‑standing cooling‑flow paradigm, wherein radiatively cooling gas condenses onto the central galaxy and fuels star formation. Moreover, the radial colour gradients in the UV mirror the temperature profiles of the X‑ray gas, reinforcing the notion that star formation is confined to the innermost ≲ 20 kpc where the cooling time is shortest.

The paper concludes that GALEX is a powerful tool for probing star formation in BCGs across a broad redshift range, especially when combined with complementary Hα, IR, and X‑ray data. The authors argue that multi‑wavelength analyses are essential for disentangling the effects of dust, AGN feedback, and ICM thermodynamics, and they advocate for larger samples and deeper UV imaging to refine the quantitative relationships between cooling‑flow properties and star‑formation efficiency. Their results provide compelling observational evidence that the star formation observed in cool‑core BCGs is directly driven by the cooling of the hot intracluster medium.