[Paper Review] The WSRT wide-field HI survey: I. The background galaxy sample

Róbert Braun, David A. Thilker+1 more|arXiv (Cornell University)|May 21, 2003

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy22 references17 citations

TL;DR

This paper presents the first wide-field H i survey using the Westerbork Synthesis Radio Telescope (WSRT) to detect neutral hydrogen emission from external galaxies in the northern sky, achieving 18 mJy/beam sensitivity over 1800 deg². It identifies 155 external galaxies at positive LGSR velocities, with 23 detected in H i for the first time, and finds evidence for diffuse H i on 100 kpc scales or low-mass companions, reconciling the H i mass function with HIPASS after correcting for galaxy density variations.

ABSTRACT

We have used the Westerbork array to carry out an unbiased wide-field survey for HI emission features, achieving an RMS sensitivity of about 18 mJy/Beam at a velocity resolution of 17 km/s over 1800 deg^2 and between -1000 < V_Hel

Motivation & Objective

• To conduct an unbiased, wide-field H i survey in the northern sky to detect emission from external galaxies and high-velocity clouds.
• To test the prediction of a population of faint compact high-velocity clouds (CHVCs) near M31 using improved sensitivity.
• To measure the H i mass function (HIMF) in a large survey volume and compare it with existing surveys like HIPASS and ADBS.
• To assess the impact of large-scale galaxy density variations on HIMF normalization and shape.
• To identify and characterize H i-detected galaxies, including those without optical counterparts or redshifts.

Proposed method

• Conducted a wide-field H i survey using the WSRT with an effective beam FWHM of 49′, achieving 18 mJy/beam sensitivity and 17 km s⁻¹ velocity resolution.
• Acquired both auto- and cross-correlation data to improve dynamic range and sensitivity across 1800 deg² centered on M31.
• Performed source detection on H i spectra with significance thresholds >8σ and cross-identified candidates within 30′ of DSS images.
• Classified detections as 'confused' if companions were present within 30′ and 400 km s⁻¹, and assessed positional offsets between optical and H i centroids.
• Used a discrete formulation of density correction to account for varying galaxy number density with distance in the survey volume.
• Fitted the H i mass function using maximum-likelihood estimation and compared results with HIPASS and ADBS surveys after correcting for survey geometry and density effects.

Experimental results

Research questions

• RQ1 Does the WSRT survey detect a significant population of faint CHVCs near M31, as predicted by theoretical models?
• RQ2 To what extent do large-scale galaxy density variations affect the normalization and shape of the H i mass function in wide-area surveys?
• RQ3 Are there systematic differences in H i flux measurements between large-beam surveys (like this one) and smaller-beam surveys, and what might explain them?
• RQ4 How many H i-detected galaxies are newly discovered, and what fraction are confused by nearby companions?
• RQ5 What is the relationship between H i flux excess in large beams and the presence of diffuse or undetected gas in massive galaxy environments?

Key findings

• The survey detected 155 external galaxies at positive LGSR velocities with H i fluxes exceeding 8σ significance, of which 23 were detected in H i for the first time.
• All but one H i detection have plausible optical associations within a 30′ radius in DSS images, though several lack published redshifts or were not previously cataloged.
• 23% of detections were classified as 'confused' due to nearby companions within 30′ and 400 km s⁻¹, indicating complex group environments.
• A few unconfused galaxies showed H i centroid offsets >10 kpc from their optical centers, suggesting possible tidal distortions or undetected gas-rich companions.
• An excess of H i flux in large-beam surveys relative to smaller-beam surveys was observed as a function of distance and gas mass, possibly indicating diffuse H i on 100 kpc scales or a population of undetected low-mass companions.
• After applying a discrete density correction, the derived H i mass function parameters (log(M∗) = 9.79 ± 0.06, Θ∗ = 86 ± 21 × 10⁻⁴ Mpc⁻³ dex⁻¹, α = -1.30 ± 0.08) showed good agreement with HIPASS BGC results, resolving prior discrepancies linked to galaxy density variations.