[Paper Review] An investigation of the circumgalactic medium around z~2.2 AGN with ACA and ALMA

G. C. Jones, R. Maiolino+4 more|arXiv (Cornell University)|Mar 30, 2023

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study uses deep ALMA and ACA observations to investigate molecular gas in the circumgalactic medium (CGM) of three z~2.2 active galactic nuclei (AGN). It finds CO(3-2) emission extending to ~10–50 kpc, consistent with a CGM reservoir ~10× larger than the galaxy, but no evidence for massive molecular halos beyond ~20 kpc, challenging previous claims of 200 kpc-scale halos. A tentative ~2σ negative continuum signal on scales >500 kpc may trace the Sunyaev-Zeldovich effect from AGN-heated halos, suggesting direct evidence for AGN feedback.

ABSTRACT

While observations of molecular gas at cosmic noon and beyond have focused on the gas within galaxies (i.e., the interstellar medium; ISM), it is also crucial to study the molecular gas reservoirs surrounding each galaxy (i.e., in the circumgalactic medium; CGM). Recent observations of galaxies and quasars hosts at high redshift (z>2) have revealed evidence for cold gaseous halos of scale r_CGM~10kpc, with one discovery of a molecular halo with r_CGM~200kpc and a molecular gas mass one order of magnitude larger than the ISM of the central galaxy. As a follow-up, we present deep ACA and ALMA observations of CO(3-2) from this source and two other quasar host galaxies at z~2.2. While we find evidence for CO emission on scales of r~10kpc, we do not find evidence for molecular gas on scales larger than r>20 kpc. Therefore, our deep data do not confirm the existence of massive molecular halos on scales of ~100 kpc for these X-ray selected quasars. As an interesting by-product of our deep observations, we obtain the tentative detection of a negative continuum signal on scales larger than r>200kpc, which might be tracing the Sunyaev-Zeldovich effect associated with the halo heated by the active galactic nucleus (AGN). If confirmed with deeper data, this could be direct evidence of the preventive AGN feedback process expected by cosmological simulations.

Motivation & Objective

• To investigate the presence and distribution of molecular gas in the circumgalactic medium (CGM) around high-redshift AGN at z~2.2.
• To test the existence of massive molecular halos on scales of ~100 kpc, previously reported in some high-redshift quasars.
• To determine whether interferometric filtering has affected prior detections of extended CO emission in the CGM.
• To search for indirect evidence of AGN feedback through the Sunyaev-Zeldovich (S-Z) effect in the hot CGM.

Proposed method

• Conducted deep CO(3-2) line observations using the Atacama Compact Array (ACA) and ALMA for three X-ray selected quasars at z~2.2.
• Combined ACA and ALMA data to recover emission on both small (resolved by ALMA) and large (resolved by ACA) spatial scales.
• Analyzed visibility data and radial brightness profiles to detect extended CO emission beyond the resolution of ALMA alone.
• Stacked continuum visibilities from all three sources to search for large-scale negative continuum signals indicative of the thermal Sunyaev-Zeldovich effect.
• Used ALMA’s high sensitivity to detect compact and extended CO components, comparing flux densities across instruments to assess missing flux.
• Evaluated the consistency of CO emission with a two-component model: a compact ISM component (r < 1–2 kpc) and a more extended CGM component (r ~ 10–50 kpc).

Figure 1 : Continuum images ( $\lambda_{\mathrm{rest}}\sim 870\,\mu m$ ) for each galaxy. Contours are displayed at significance levels of $\pm(2,3,4\ldots)\times\sigma$ , where $1\sigma$ is the RMS noise level of the image. The top row shows ACA data (left to right: $1\sigma=[60,50,60]\,\mu$ Jy bea

Experimental results

Research questions

• RQ1 Do massive molecular halos extending to ~100–200 kpc exist in the CGM of z~2.2 AGN, as previously reported?
• RQ2 Is the extended CO(3-2) emission in the CGM of these AGN detectable with ACA and ALMA, or is it filtered out by interferometric resolution?
• RQ3 Can the tentative negative continuum signal on scales >500 kpc be interpreted as the thermal Sunyaev-Zeldovich effect from AGN-heated halos?
• RQ4 What is the spatial extent and morphology of molecular gas in the CGM of high-redshift AGN, and how does it compare to the ISM of the central galaxy?
• RQ5 Is AGN feedback through halo heating detectable via the S-Z effect in high-redshift quasars?

Key findings

• CO(3-2) emission is detected in two sources (CID_346 and X_N_44_64) with ACA and all three sources with ALMA, indicating no significant flux loss due to interferometric filtering.
• The CO emission in ACA data is consistent with being unresolved on scales <40–50 kpc, indicating no large-scale molecular gas component beyond ~50 kpc.
• ALMA data reveal a compact component (r < 1–2 kpc) and a weak, extended component (r ~ 10–50 kpc), confirming the presence of a CGM reservoir ~10× larger than the ISM.
• No evidence is found for molecular gas on scales >20 kpc, contradicting previous claims of 200 kpc-scale molecular halos in similar AGN.
• A tentative ~2σ negative continuum signal is detected at scales >500 kpc, consistent with the thermal Sunyaev-Zeldovich effect from AGN-heated halos.
• The tentative S-Z signal, if confirmed, would provide direct observational evidence for AGN feedback via halo heating, a key mechanism in quenching galaxy evolution.

Figure 2 : Radial profile of the real part of the stacked observed continuum visibilities from ACA data, including only channels without line emission (maroon points). Top: The results of fitting three models are shown: a single Gaussian (brown line), a constant value (light blue line), and an offse