The deepest Herschel-PACS far-infrared survey: number counts and infrared luminosity functions from combined PEP/GOODS-H observations (1303.4436v2)

Abstract: We present results from the deepest Herschel-PACS (Photodetector Array Camera and Spectrometer) far-infrared blank field extragalactic survey, obtained by combining observations of the GOODS (Great Observatories Origins Deep Survey) fields from the PACS Evolutionary Probe (PEP) and GOODS-Herschel key programmes. We describe data reduction and the construction of images and catalogues. In the deepest parts of the GOODS-S field, the catalogues reach 3-sigma depths of 0.9, 0.6 and 1.3 mJy at 70, 100 and 160 um, respectively, and resolve ~75% of the cosmic infrared background at 100um and 160um into individually detected sources. We use these data to estimate the PACS confusion noise, to derive the PACS number counts down to unprecedented depths and to determine the infrared luminosity function of galaxies down to LIR=10^11 Lsun at z~1 and LIR=10^12 Lsun at z~2, respectively. For the infrared luminosity function of galaxies, our deep Herschel far-infrared observations are fundamental because they provide more accurate infrared luminosity estimates than those previously obtained from mid-infrared observations. Maps and source catalogues (>3-sigma) are now publicly released. Combined with the large wealth of multi-wavelength data available for the GOODS fields, these data provide a powerful new tool for studying galaxy evolution over a broad range of redshifts.

• The paper combines PEP and GOODS-H data to achieve unprecedented FIR depth and resolve a major fraction of the cosmic infrared background.
• It employs advanced PSF-fitting methods and reaches sensitivity limits as low as 0.6 mJy at 100 µm for robust source extraction.
• The study identifies significant evolution in infrared luminosity functions, offering key insights into dust-obscured star formation and galaxy growth.

Overview of the Herschel-PACS Far-Infrared Survey

The paper titled "The deepest Herschel-PACS far-infrared survey: number counts and infrared luminosity functions from combined PEP/GOODS-H observations" utilizes the Herschel space observatory's powerful instrumentation to provide an in-depth analysis of extragalactic sources in the far-infrared (FIR) spectrum. By combining observations from the Herschel/PACS Evolutionary Probe (PEP) and the GOODS-Herschel key programs, a comprehensive survey reaching unprecedented depth was achieved, allowing detailed insights into the cosmic infrared background and galaxy evolution.

Survey Methodology and Data

The research capitalizes on data from the PACS instrument aboard Herschel, primarily focusing on the GOODS-North and GOODS-South fields, leveraging comprehensive data from 70, 100, and 160 µm wavelengths. The deepest regions within the GOODS-S field achieved sensitivity limits as low as 0.6 mJy at 100 µm and 1.3 mJy at 160 µm, resolving about 75% of the cosmic infrared background in these bands into distinct sources. This comprehensive survey employs advanced methods of data processing, including PSF-fitting for source extraction, both with position priors and in a blind mode, to generate high-reliability catalogs of sources.

Results and Findings

The results notably include detailed number counts and an analysis of the infrared luminosity function (IRLF), critical for understanding the distribution and evolution of star-forming galaxies over a range of redshifts:

• Number Counts: The survey establishes differential number counts down to unprecedented FIR depths. At 100 and 160 µm, a significant fraction of cosmic infrared background was resolved. The data, representing fainter flux densities compared to previous work, aligns well with most existing models, although discrepancies in the faint-end slope present challenges to several models of galaxy evolution.
• Infrared Luminosity Functions: By breaking through prior limitations posed by mid-infrared data and confusion noise, this survey provides more accurate luminosity functions down to $L_{\rm IR} = 10^{11}\, \rm{L}_{\odot}$ at $z \approx 1$ and $L_{\rm IR} = 10^{12}\, \rm{L}_{\odot}$ at $z \approx 2$. The survey identifies an evolution in $L_{\rm IR}$ and galaxy number counts indicating a substantial increase in infrared luminous galaxy populations over cosmological time, consistent with a substantial degree of star formation obscured by dust at these epochs.

Implications and Future Work

The implications of this research are significant for understanding galaxy formation and evolution. The accurate characterization of the FIR emission from galaxies advances our understanding of the star formation history and the processes governing galaxy growth. Given that a considerable portion of star formation is enshrouded by dust, surveys like this are vital for a complete picture of galaxy evolution.

The paper indicates paths for future research, particularly in refining theoretical models to accommodate the steep faint-end slopes observed. Additionally, it underscores the importance of multi-wavelength surveys to grasp the complex interplay of dust-obscured star formation, AGN activity, and their cosmic evolution.

These comprehensive results from the combination of PEP and GOODS-Herschel observations enhance our comprehension of the FIR universe, laying a robust foundation for future infrared astronomy, particularly in the era of upcoming observatories like the James Webb Space Telescope. The publicly released catalogues, rich with multi-wavelength data from the GOODS fields, are poised to be an invaluable asset for continued exploration of cosmic history.