[Paper Review] Nuclear activity in nearby galaxies: Mid-infrared imaging with the VLT

R. Siebenmorgen, Martin Haas+8 more|ArXiv.org|May 21, 2008

Galaxies: Formation, Evolution, Phenomena63 references16 citations

TL;DR

This study uses high-resolution mid-infrared imaging from the VLT's VISIR instrument at 0.35" spatial resolution to distinguish between active galactic nuclei (AGN) and starburst activity in nearby galaxies. It finds that AGN are consistently compact and unresolved, while starbursts show extended emission, enabling clear separation via nuclear mid-infrared surface brightness with 8m-class telescopes up to 100 Mpc.

ABSTRACT

Dust enshrouded activity can ideally be studied by mid-infrared (MIR) observations. In order to explore the AGN versus star forming origin of the nuclear MIR emission of galaxies, observations of high spatial resolution are required. Here we report on 11.3mic. observations with VISIR at the VLT, reaching 0.35'' spatial resolution (FWHM). During the scientific verification of VISIR we have observed a sample of 36 nearby galaxies having a variety of optically classified nuclear activity: 17 black hole driven active galactic nuclei (AGN), 10 starbursts (SBs) and 9 quiet spirals. 16/17 AGN are detected and unresolved, 5/10 SBs are detected and resolved with structured emission up to a few arcsec, while for 5/10 SB and all 9 quiet nuclei low upper limits are provided. The morphology of the resolved SB nuclei follows that seen at radio frequencies. The compactness of AGN and the extent of the SB nuclei is consistent with predictions from radiative transfer models and with MIR spectra of lower spatial resolution. We explore the nuclear MIR surface brightness as a quantitative measure. While AGN and SB cannot be distinguished with MIR data from 4m class telescopes, our data provide evidence that, up to a distance of 100 Mpc, AGN and SB can well be separated by means of MIR surface brightness when using 8m class telescopes.

Motivation & Objective

• To resolve the origin of nuclear mid-infrared (MIR) emission in nearby galaxies by distinguishing between AGN and starburst activity.
• To overcome contamination from extended galactic emission in low-resolution MIR data by using high spatial resolution observations.
• To test whether nuclear MIR surface brightness can reliably separate AGN from starbursts at distances up to 100 Mpc.
• To compare MIR morphologies with radiative transfer models and radio-interferometric maps for validation.
• To detect faint, low-luminosity nuclear starbursts in quiescent spirals previously missed by low-resolution surveys.

Proposed method

• Performed 11.3 µm imaging with VISIR on the VLT, achieving 0.35" FWHM spatial resolution under good seeing conditions.
• Observed a sample of 36 nearby galaxies (17 AGN, 10 starbursts, 9 quiescent spirals) at distances <100 Mpc.
• Used on-source chopping and nodding with 16" amplitudes to subtract thermal background and reduce noise.
• Applied noise filtering and specialized reduction techniques to mitigate detector artifacts such as random stripes.
• Measured nuclear MIR surface brightness from integrated flux and source extent, using 3σ upper limits for non-detections.
• Compared MIR morphologies with radio continuum maps (VLA) and radiative transfer models to validate AGN vs. starburst origins.

Experimental results

Research questions

• RQ1 Can high-resolution mid-infrared imaging resolve the true nature of nuclear MIR emission in nearby galaxies?
• RQ2 Do AGN and starburst nuclei exhibit distinct morphological signatures in mid-infrared images at 0.35" resolution?
• RQ3 Can nuclear mid-infrared surface brightness reliably separate AGN from starbursts when using 8m-class telescopes?
• RQ4 Is there evidence of faint, compact nuclear emission in quiescent spiral galaxies consistent with low-luminosity starbursts?
• RQ5 How do MIR morphologies compare with radio-interferometric maps and radiative transfer models for starburst and AGN nuclei?

Key findings

• 16 out of 17 AGN were detected and unresolved, with nuclear MIR surface brightness exceeding 20,000 L⊙/pc², consistent with AGN models.
• 5 out of 10 starbursts were resolved with extended MIR emission (up to several arcseconds), while 5 were undetected at 3σ limits below 1.6 mJy.
• The morphology of resolved starburst nuclei closely matches radio continuum emission, confirming co-spatial MIR and radio emission from compact HII regions.
• For NGC 1482, the nucleus contributes at most 25% of the total MIR flux, with the bulk of emission coming from extended star-forming regions.
• The MIR surface brightness of AGN remains high and compact, while starburst surface brightness is significantly lower and spatially extended.
• The study confirms that nuclear MIR surface brightness is a robust discriminant between AGN and starbursts when using 8m-class telescopes up to 100 Mpc.