[Paper Review] The Herschel Virgo Cluster Survey. XVII. SPIRE point-source catalogs and number counts

C. Pappalardo, G. J. Bendo+20 more|Research Explorer (The University of Manchester)|Oct 7, 2014

Galaxies: Formation, Evolution, Phenomena76 references14 citations

TL;DR

This paper presents three independent SPIRE point-source catalogs from the Herschel Virgo Cluster Survey at 250, 350, and 500 µm using an iterative source extraction method with timeline-based photometry and Gaussian source subtraction. The catalogs reveal steepening number counts below 200 mJy, indicating strong evolution in faint submillimeter galaxies, with models underperforming at bright fluxes due to incomplete Rayleigh-Jeans SED modeling.

ABSTRACT

We present three independent catalogs of point-sources extracted from SPIRE images at 250, 350, and 500 micron as a part of the Herschel Virgo Cluster Survey (HeViCS). The source positions are determined by estimating the likelihood to be a real source for each peak on the maps and the flux densities are estimated using the sourceExtractorTimeline, a timeline-based point source fitter. Afterwards, each source is subtracted from the maps, removing a Gaussian function in every position with the full width half maximum equal to that estimated in sourceExtractorTimeline. This procedure improves the robustness of our algorithm in terms of source identification. The HeViCS catalogs contain about 52000, 42200, and 18700 sources selected at 250, 350, and 500 micron above 3sigma and are ~ 75%, 62%, and 50% complete at flux densities of 20 mJy at 250, 350, 500 micron, respectively. We then measured source number counts at 250, 350, and 500 micron and we also cross-correlated the catalogs with the Sloan Digital Sky Survey to investigate the redshift distribution of the nearby sources. From this cross-correlation, we select ~2000 sources with reliable fluxes and a high signal-to-noise ratio, finding an average redshift z~0.3+/-0.22. The number counts at 250, 350, and 500 micron show an increase in the slope below 200 mJy, indicating a strong evolution in number of density for galaxies at these fluxes. In general, models tend to overpredict the counts at brighter flux densities, underlying the importance of studying the Rayleigh-Jeans part of the spectral energy distribution to refine the theoretical recipes of the models. Our iterative method for source identification allowed the detection of a family of 500 micron sources that are not foreground objects belonging to Virgo and not found in other catalogs.

Motivation & Objective

• To produce robust, complete, and reliable point-source catalogs from Herschel SPIRE maps in the Virgo Cluster field.
• To measure source number counts at 250, 350, and 500 µm to probe the evolution of submillimeter galaxies.
• To assess the reliability and completeness of the catalogs through artificial source injection and permutation methods.
• To cross-identify sources with SDSS to determine redshift distributions and distinguish low- vs. high-redshift populations.
• To evaluate the performance of semi-analytical models against observed number counts and identify discrepancies in SED modeling.

Proposed method

• Source detection via iterative peak likelihood estimation using the SourceExtractorSussextractor task.
• Photometry estimation with the sourceExtractorTimeline, a timeline-based point-source fitter that models source profiles with Gaussian functions.
• Source subtraction via Gaussian removal at each detected source position to reduce confusion and improve detection of fainter sources.
• Completeness and flux accuracy assessed by injecting artificial sources into the timeline and measuring recovery rates.
• Catalog reliability evaluated using a permutation method to estimate false detection rates.
• Cross-correlation with SDSS DR10 to assign optical counterparts and redshifts to a subsample of sources.

Figure 1: HeViCS image at 250 $\mu$ m with detected sources (red points). The blue squares indicate regions with the full coverage.

Experimental results

Research questions

• RQ1 What is the completeness and reliability of SPIRE point-source catalogs in the Virgo Cluster field at 250, 350, and 500 µm?
• RQ2 How do the number counts of submillimeter sources evolve across the SPIRE bands, and what do they reveal about galaxy evolution?
• RQ3 What is the redshift distribution of the detected sources, and how do they split between optically detected and undetected populations?
• RQ4 Why do current semi-analytical models fail to reproduce the observed number counts at bright fluxes?
• RQ5 To what extent does the Rayleigh-Jeans part of the SED influence the observed number counts and model discrepancies?

Key findings

• The catalogs contain approximately 52,000, 42,200, and 18,700 sources at 250, 350, and 500 µm above 3σ, with completeness of ~75%, ~62%, and ~50% at 20 mJy, respectively.
• Number counts show a steepening slope below 200 mJy, indicating strong evolution in the number density of faint submillimeter galaxies.
• The slope of the number counts becomes steeper from 250 µm to longer wavelengths, suggesting increasing contribution from high-redshift, actively star-forming galaxies.
• Below 40 mJy, counts flatten and decrease due to confusion noise, limiting reliable source detection at very faint fluxes.
• Models consistently overpredict the number counts at bright fluxes, highlighting the need for improved Rayleigh-Jeans SED modeling in theoretical frameworks.
• Sources without optical counterparts in SDSS show faster evolution than those with counterparts, and their SEDs are consistent with higher redshifts (z ~ 0.3 ± 0.22) and cooler dust temperatures.

Figure 2: V2 field region before (left panel) and after (right panel) background subtraction.