Abstract
Passive early-type galaxies dominate cluster cores at z ≲ 1.5. At higher redshift, cluster core galaxies are observed to have on-going star-formation, which is fueled by cold molecular gas. We measured the molecular gas reservoir of the central region around the radio-loud active galactic nucleus (AGN) in the cluster CARLA J1103 + 3449 at z = 1.44 using NOEMA. The AGN synchrotron emission dominates the continuum emission at 94.48 GHz, and we measured its flux at the AGN position and at the position of two radio jets. Combining our measurements with published results over the range 4.71–94.5 GHz, and assuming Ssynch ∝ ν−α, we obtain a flat spectral index of α = 0.14 ± 0.03 for the AGN core emission, and a steeper index of α = 1.43 ± 0.04 and α = 1.15 ± 0.04 at positions close to the western and eastern lobes, respectively. The total spectral index is α = 0.92 ± 0.02 over the range 73.8 MHz–94.5 GHz. We detect two CO(2–1) emission lines, both blueshifted with respect to the AGN. Their emission corresponds to two regions, ~17 kpc southeast and ~14 kpc southwest of the AGN, not associated with galaxies. In these two regions, we find a total massive molecular gas reservoir of Mgastot = 3.9 ± 0.4 × 1010 M⊙, which dominates (≳60%) the central total molecular gas reservoir. These results can be explained by massive cool gas flows in the center of the cluster. The AGN early-type host is not yet quenched; its star formation rate is consistent with being on the main sequence of star-forming galaxies in the field (star formation rate ~30–140 M⊙ yr−1), and the cluster core molecular gas reservoir is expected to feed the AGN and the host star formation before quiescence. The other confirmed cluster members show star formation rates at ~2σ below the field main sequence at similar redshifts and do not have molecular gas masses larger than galaxies of similar stellar mass in the field.
Highlights
We focus on the molecular gas that can be detected by CO emission and molecular gas upper limits that can be inferred from the CO emission, with the caveat that this might not trace all the molecular gas in the galaxies that we study
On the continuum emission map at the observed frequency of νobs = 94.48 GHz, we detect an extended source in the cluster central region, with the brightest peak at the position of the active galactic nucleus (AGN) (>26σ, Fig. 4, white contours)
Comparing the NOEMA continuum emission with radio observations at 4.71 GHz from Best et al (1999) (Fig. 4, blue contours), the NOEMA continuum emission visually corresponds to the radio jets
Summary
At redshifts z < 1.5 galaxy cluster cores are dominated by red, quenched, early-type galaxies, while blue, star-forming, latetype galaxies are mostly found in the field (e.g., Dressler 1980; Balogh et al 1998, 2004; Postman et al 2005; Mei et al 2009; Rettura et al 2011; Lemaux et al 2012, 2019; Wagner et al 2015; Tomczak et al 2019). Other observations show a reversal of the star formation–density relation and of ongoing star formation in cluster cores at z > 1.5, with a much more varied galaxy population compared to clusters at lower redshifts Other cluster cores at z 1.5 present equal percentages of quiescent and star-forming galaxies (Fassbender et al 2011; Tadaki et al 2012; Zeimann et al 2012; Mei et al 2012; Noirot et al 2016). At higher redshifts (z ∼ 3–4), protoclusters show high star formation and star-burst activity (Umehata et al 2015; Lewis et al 2018; Miller et al 2018; Oteo et al 2018; Kubo et al 2019; Hill et al 2020; Ivison et al 2020; Long et al 2020; Toshikawa et al 2020)
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