[Paper Review] GMRT Observations of NGC 3079

J. Irwin, D. J. Saikia|Sep 13, 2003

Astrophysics and Cosmic Phenomena18 citations

TL;DR

This study presents high-resolution GMRT observations of NGC 3079 at 326, 615, and 1281 MHz, revealing extended radio structures including two giant radio loops (up to 11 kpc from the nucleus), a 4.8 kpc-high halo, and HI-associated extensions. The findings suggest episodic nuclear outflows with average velocities of ~100 km s⁻¹, and indicate that NGC 3079 may be creating its own local intergalactic medium via outflows.

ABSTRACT

We present new observations at three frequencies (326 MHz, 615 MHz, and 1281 MHz) of the radio lobe spiral galaxy, NGC 3079, using the Giant Metrewave Radio Telescope. These observations are consistent with previous data obtained at other telescopes and reveal the structure of the nuclear radio lobes in exquisite detail. In addition, new features are observed, some with HI counterparts, showing broad scale radio continuum emission and extensions. The galaxy is surrounded by a radio halo that is at least 4.8 kpc in height. Two giant radio extensions/loops are seen on either side of the galaxy out to $\sim$ 11 kpc from the major axis, only slightly offset from the direction of the smaller nuclear radio lobes. If these are associated with the nuclear outflow, then the galaxy has experienced episodic nuclear activity. Emission along the southern major axis suggests motion through a local IGM (not yet detected) and it may be that NGC 3079 is itself creating this local intergalactic gas via outflows. We also present maps of the minimum energy parameters for this galaxy, including cosmic ray energy density, electron diffusion length, magnetic field strength, particle lifetime, and power.

Motivation & Objective

• To investigate the origin and structure of extended radio emission in NGC 3079, a nearby spiral galaxy with prominent nuclear radio lobes.
• To determine whether large-scale radio features are linked to episodic nuclear activity or AGN outflows.
• To examine the association of radio structures with HI gas and assess their kinematic and energetic implications.
• To compute minimum energy parameters such as magnetic field strength, cosmic ray energy density, and particle lifetime across the galaxy.
• To explore the possibility that NGC 3079 is creating its own local intergalactic medium (IGM) through outflows.

Proposed method

• Multi-frequency radio continuum observations were conducted using the Giant Metrewave Radio Telescope (GMRT) at 326 MHz, 615 MHz, and 1281 MHz.
• Data were processed using AIPS software after pre-editing with the GMRED tool to flag bad data points and outliers in UV data.
• Spectral index maps were generated over a restricted region to analyze frequency-dependent emission properties.
• Minimum energy parameters—including cosmic ray energy density, electron diffusion length, magnetic field strength, particle lifetime, and total power—were computed along the line of sight.
• Radio contours were superimposed on HI and optical images to assess spatial correlations between radio, neutral hydrogen, and optical structures.
• The galaxy’s distance (16.5 Mpc) and inclination (84°) were used to convert angular scales to physical distances.

Experimental results

Research questions

• RQ1 Are the large-scale radio extensions in NGC 3079 associated with episodic nuclear outflows from the active galactic nucleus?
• RQ2 What is the kinematic and energetic relationship between the radio structures and the surrounding HI gas?
• RQ3 Can the observed radio halo and extended features be explained by particle diffusion and minimum energy conditions?
• RQ4 Is the offset emission at the southern tip of the major axis indicative of motion through a local intergalactic medium?
• RQ5 To what extent is NGC 3079 responsible for creating its own local IGM via outflows?

Key findings

• Two giant radio extensions, one on each side of the galaxy, extend up to 11 kpc from the major axis and are slightly offset from the direction of the nuclear lobes, suggesting a nuclear origin.
• A prominent eastern radio loop, best seen at 615 MHz, extends to 11 kpc from the disk and is spatially offset from a direct extension of the nuclear lobe.
• A northern loop with a diameter of 3.8 kpc and a projected distance of 7.2 kpc from the major axis is associated with a large HI arc, indicating a disk-origin for this feature.
• A radio halo extends at least 4.8 kpc in height perpendicular to the disk, following the galaxy’s major axis and indicating widespread emission.
• Emission at the southern tip of the major axis is offset from the optical axis, suggesting motion through a broader-scale IGM, possibly created by the galaxy’s own outflows.
• The particle lifetime estimate of 10⁸ years implies an average outflow speed of ~100 km s⁻¹ for material reaching the outermost radio features at 11 kpc.