HerMES: deep number counts at 250, 350, and 500 microns in the COSMOS and GOODS-N fields and the build-up of the cosmic infrared background

Abstract: ABRIGED Herschel/SPIRE has provided confusion limited maps of deep fields at 250, 350, and 500um, as part of the HerMES survey. Due to confusion, only a small fraction of the Cosmic Infrared Background can be resolved into individually-detected sources. Our goal is to produce deep galaxy number counts and redshift distributions below the confusion limit, which we then use to place strong constraints on the origins of the cosmic infrared background and on models of galaxy evolution. We individually extracted the bright SPIRE with a method using the positions, the flux densities, and the redshifts of the 24um sources as a prior, and derived the number counts and redshift distributions of the bright SPIRE sources. For fainter SPIRE sources, we reconstructed the number counts and the redshift distribution below the confusion limit using the deep 24um catalogs associated with photometric redshift and information provided by the stacking of these sources into the deep SPIRE maps. Finally, by integrating all these counts, we studied the contribution of the galaxies to the CIB as a function of their flux density and redshift. Through stacking, we managed to reconstruct the source counts per redshift slice down to ~2 mJy in the three SPIRE bands, which lies about a factor 10 below the 5sigma confusion limit. None of the pre-existing population models are able to reproduce our results at better than 3sigma. Finally, we extrapolate our counts to zero flux density in order to derive an estimate of the total contribution of galaxies to the CIB, finding 10.1, 6.5, and 2.8 nW/m2/sr at 250, 350, and 500um, respectively. These values agree well with FIRAS absolute measurements, suggesting our number counts and their extrapolation are sufficient to explain the CIB. Finally, combining our results with other works, we estimate the energy budget contained in the CIB between 8 and 1000um: 26 nW/m2/sr.

An Overview of HerMES: Understanding Deep Number Counts at Submillimeter Wavelengths in the COSMOS and GOODS-N Fields

The paper discusses the analysis conducted within the Herschel Multi-tiered Extragalactic Survey (HerMES) that focuses on deep number counts at submillimeter wavelengths of 250 μm, 350 μm, and 500 μm. The emphasis lies on the COSMOS and GOODS-N fields as part of an effort to elucidate the contributions to the cosmic infrared background (CIB) from galaxies and their evolution over cosmic timescales. Using the Herschel Space Observatory's Spectral and Photometric Imaging Receiver (SPIRE), the authors attempt to probe the nature of the CIB down to the confusion limit by leveraging deep galaxy counts and redshift distributions.

Methodological Approach

The investigation applies a novel approach combining direct source extraction and statistical analyses such as stacking to surpass the limitations posed by source confusion at submillimeter wavelengths. The authors employ ancillary 24 μm data along with photometric redshift catalogs to more reliably pinpoint source extraction above the confusion limit. Extending beyond this threshold, a stacking analysis is performed to statistically reconstruct number counts of fainter sources, systematically accounting for clustering biases that could distort the final interpretation. Through this, the authors are able to explore sub-populations down to approximately 2 mJy, which is substantially lower than typical limitations.

Key Findings

The analysis reveals several notable insights into the submillimeter cosmos:

• Deep Number Counts: The authors successfully reconstruct deep number counts that fall below the traditional confusion limit, enabling them to detect the peak of the Euclidean-normalized counts and observe a power-law behavior extending toward fainter flux densities.

• Redshift Distribution: The study uncovers significant contributions to the CIB originating largely at redshifts around 1.0 to 1.5, which also supports cosmic star formation history in alignment with these epochs.

• Cosmic Infrared Background Contribution: The extrapolation of counts derives an estimate for the contribution of galaxy populations to the CIB. The cumulative values closely match FIRAS measurements, underlining the efficiency of the survey methods in capturing the integral picture of the CIB.

Implications for Galaxy Evolution Models

The paper presents refined observational constraints that challenge existing galaxy evolution models to reconcile discrepancies highlighted by the new number counts and redshift distributions. Models such as those proposed by Bethermin et al. and other contemporaries, despite their alignment with some aspects of the findings, show deviations that emphasize the necessity for refined models considering greater detail in the underlying astrophysical processes.

Conclusions and Prospects for Future Research

The study not only provides a comprehensive analysis of the submillimeter sky through the HerMES survey but also establishes foundational data that warrant further exploration and theoretical advances in understanding galaxy formation, evolution, and their interplay with the cosmic background. The challenge now lies in integrating these empirical findings with theoretical models, ensuring enhanced accuracy in predicting cosmic phenomena and refining our understanding of the universe's evolution over billions of years. Future developments may further incorporate larger datasets, improved models, and multi-wavelength approaches to exhaustively elucidate the role of submillimeter galaxies in cosmic evolution.