[Paper Review] The WSRT wide-field HI survey: II. Local Group features

Róbert Braun, David A. Thilker|ArXiv.org|Dec 12, 2003

Astrophysical Phenomena and ObservationsPhysics and Astronomy37 references87 citations

TL;DR

This paper presents a wide-field H i survey using the Westerbork Synthesis Radio Telescope to map high-velocity gas in the Local Group, detecting 95 discrete high-velocity cloud (HVC) components and identifying diffuse H i structures linked to M31, M33, and the Magellanic Stream. The survey achieves 18 mJy/beam sensitivity and reveals a faint H i filament connecting M31 and M33, potentially tracing the warm-hot intergalactic medium, with a projected exponential scale length of 25 kpc and peak column density of 5×10¹⁷ cm⁻².

ABSTRACT

We have used the WSRT to carry out an unbiased wide-field survey of HI emission features, achieving an RMS sensitivity of about 18 mJy/Beam at a velocity resolution of 17 km/s over 1800 deg^2. In this paper we present our HI detections at negative velocities which could be distinguished from the Galactic foreground. Fully 29% of the entire survey area has high velocity HI emission with N_HI exceeding our 3 sigma limit of about 1.5x10^17cm^-2 over 30 km/s. A faint population of discrete HVCs is detected in the immediate vicinity of M31 which spans a large fraction of the M31 rotation velocity. This class of features is confined to about 12 deg (160 kpc) projected radius of M31 and appears to be physically associated. We detect a diffuse northern extension of the Magellanic Stream (MS) from at least Dec=+20 to +40 deg., which then loops back toward the south. Recent numerical simulations had predicted just such an MS extension corresponding to the apo-galacticon portion of the LMC/SMC orbit at a distance of 125 kpc. A faint bridge of HI emission appears to join the systemic velocities of M31 with that of M33 and continues beyond M31 to the north-west. This may be the first detection of HI associated with the warm-hot intergalactic medium (WHIM). The distribution of peculiar velocity HI associated with M31 can be described by a projected exponential of 25 kpc scale-length and 5x10^17cm^-2 peak column density. We present the distribution function of N_HI in the extended M31 environment, which agrees well with the low red-shift QSO absorption line data over the range log(N_HI)=17.2 to 21.9. Our data extend this comparison about two orders of magnitude lower than previously possible and provide the first image of the Lyman limit absorption system associated with an L* galaxy. (abridged)

Motivation & Objective

• To conduct an unbiased, wide-field H i survey of the Local Group to detect high-velocity gas features beyond the Galactic foreground.
• To test the prediction that faint, dark-matter-dominated HVCs should exist near M31 if they are the gaseous counterparts of sub-galactic dark matter satellites.
• To map the extended H i distribution around M31 and compare it with low-redshift QSO absorption line data.
• To investigate the kinematic and spatial connection between the Magellanic Stream and M31, including possible extensions.

Proposed method

• Conducted a 1800 deg² H i survey using the Westerbork Synthesis Radio Telescope with 17 km s⁻¹ velocity resolution and 49′ FWHM beam.
• Achieved an rms sensitivity of 18 mJy/beam, corresponding to a column density sensitivity of ~4×10¹⁶ cm⁻² over 17 km s⁻¹.
• Used auto-correlation spectra to detect H i emission across a velocity range from -1000 to +6500 km s⁻¹.
• Identified discrete HVC components and diffuse H i complexes through source-finding algorithms and spatial-spectral decomposition.
• Cross-verified key detections with Green Bank Telescope data for independent confirmation of M31-associated features.
• Compared the observed H i column density distribution with low-redshift QSO absorption line data to validate the extended H i environment of M31.

Experimental results

Research questions

• RQ1 Are there a large population of faint, high-velocity HVCs in the vicinity of M31, as predicted by the dark-matter satellite hypothesis?
• RQ2 What is the spatial and kinematic structure of H i gas associated with M31, and how does it compare to the Lyman limit absorption system of an L* galaxy?
• RQ3 Does the Magellanic Stream extend beyond the known southern portion, and does it connect to M31 or M33?
• RQ4 Is there evidence for H i emission linking M31 and M33, potentially tracing the warm-hot intergalactic medium?
• RQ5 What is the characteristic mass and distance scale of the discrete HVC population, as indicated by the flux density distribution?

Key findings

• 29% of the survey area exhibits high-velocity H i emission with column density exceeding 1.5×10¹⁷ cm⁻² over 30 km s⁻¹, indicating widespread extended gas.
• A total of 95 discrete HVC components were detected, representing more than a tenfold increase over previously known components.
• An inflection point in the HVC flux density distribution near 12 Jy-km s⁻¹ suggests a characteristic mass and distance scale for this population.
• A faint H i filament extends from M31 to M33 and continues northwestward, potentially tracing the warm-hot intergalactic medium.
• The H i distribution around M31 follows a projected exponential profile with a 25 kpc scale length and a peak column density of 5×10¹⁷ cm⁻².
• The observed H i column density function in the M31 environment agrees well with low-redshift QSO absorption line data from log(N_HI) = 17.2 to 21.9, extending the comparison two orders of magnitude lower in column density.