CONCERTO: Extracting the power spectrum of the [C_II] emission line

¹ Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
e-mail: mathilde.van-cuyck@lam.fr
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁴ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
⁵ Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France
⁶ Univ. Grenoble Alpes, CNRS, LPSC/IN2P3, 38000 Grenoble, France
⁷ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁸ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden

Received: 28 February 2023
Accepted: 20 May 2023

Abstract

Context. CONCERTO is the first experiment to perform a [C_II] line intensity mapping (LIM) survey on the COSMOS field to target z > 5.2. Measuring the [C_II] angular power spectrum allows us to study the role of dusty star-forming galaxies in the star formation history during the epochs of Reionization and post-Reionization. The main obstacle to this measurement is the contamination by bright foregrounds: the dust continuum emission and atomic and molecular lines from foreground galaxies at z ≲ 3.

Aims. We evaluate our ability to retrieve the [C_II] signal in mock observations of the sky using the Simulated Infrared Dusty Extragalactic Sky (SIDES), which covers the mid-infrared to millimetre range. We also measure the impact of field-to-field variance on the residual foreground contamination.

Methods. We compared two methods for dealing with the dust continuum emission from galaxies (i.e. the cosmic infrared background fluctuations): the standard principal component analysis (PCA) and the asymmetric re-weighted penalized least-squares (arPLS) method. For line interlopers, the strategy relies on masking low-redshift galaxies using the instrumental beam profile and external catalogues. As we do not have observations of CO or deep-enough classical CO proxies (such as L_IR), we relied on the COSMOS stellar mass catalogue, which we demonstrate to be a reliable CO proxy for masking. To measure the angular power spectrum of masked data, we adapted the P of K EstimatoR (POKER) from cosmic infrared background studies and discuss its use on LIM data.

Results. The arPLS method achieves a reduction in the cosmic infrared background fluctuations to a sub-dominant level of the [C_II] power at z ∼ 7, a factor of > 70 below our fiducial [C_II] model. When using the standard PCA, this factor is only 0.7 at this redshift. The masking lowers the power amplitude of line contamination down to 2 × 10⁻² Jy² sr⁻¹. This residual level is dominated by faint undetected sources that are not clustered around the detected (and masked) sources. For our [C_II] model, this results in a detection at z = 5.2 with a power ratio [C_II]/(residual interlopers) = 62 ± 32 for a 22% area survey loss. However, at z = 7, [C_II]/(residual interlopers) = 2.0 ± 1.4, due to the weak contrast between [C_II] and the residual line contamination. Thanks to the large area covered by SIDES-Uchuu, we show that the power amplitude of line residuals varies by 12–15% for z = 5.2 − 7, which is less than the field-to-field variance affecting [C_II] power spectra.

Conclusions. We present an end-to-end simulation of the extragalactic foreground removal that we ran to detect the [C_II] at high redshift via its angular power spectrum. We show that cosmic infrared background fluctuations are not a limiting foreground for [C_II] LIM. On the contrary, the CO and [C_I] line contamination severely limits our ability to accurately measure the [C_II] angular power spectrum at z ≳ 7.