[Paper Review] Structure and star formation in disk galaxies III. Nuclear and circumnuclear H alpha emission

J. H. Knapen|ArXiv.org|Sep 2, 2004

Galaxies: Formation, Evolution, Phenomena60 citations

TL;DR

This study analyzes Hα emission in the nuclear and circumnuclear regions of 57 low-inclination spiral galaxies at ~100 pc resolution, finding that nuclear Hα peaks are more common in AGN hosts, circumnuclear emission is patchy in late-types and often ring-like in early-types, and nuclear rings—indicating recent gas inflow—strongly correlate with AGN activity, suggesting a shared fuelling mechanism despite differing spatial scales.

ABSTRACT

From H alpha images of a carefully selected sample of 57 relatively large, Northern spiral galaxies with low inclination, we study the distribution of the H alpha emission in the circumnuclear and nuclear regions. At a resolution of around 100 parsec, we find that the nuclear H alpha emission in the sample galaxies is often peaked, and significantly more often so among AGN host galaxies. The circumnuclear H alpha emission, within a radius of two kpc, is often patchy in late-type, and absent or in the form of a nuclear ring in early-type galaxies. There is no clear correlation of nuclear or circumnuclear H alpha morphology with the presence or absence of a bar in the host galaxy, except for the nuclear rings which occur in barred hosts. The presence or absence of close bright companion galaxies does not affect the circumnuclear H alpha morphology, but their presence does correlate with a higher fraction of nuclear H alpha peaks. Nuclear rings occur in at least 21% (+-5%) of spiral galaxies, and occur predominantly in galaxies also hosting an AGN. Only two of our 12 nuclear rings occur in a galaxy which is neither an AGN nor a starburst host. We confirm that weaker bars host larger nuclear rings. The implications of these results on our understanding of the occurrence and morphology of massive star formation, as well as non-stellar activity, in the central regions of galaxies are discussed.

Motivation & Objective

• To investigate the morphology of Hα emission in the nuclear and circumnuclear regions of spiral galaxies to trace massive star formation and non-stellar activity.
• To determine whether structural features like bars or companions correlate with the morphology of Hα emission in galaxy centers.
• To statistically assess the occurrence and properties of star-forming nuclear rings and their links to AGN and starburst activity.
• To test the hypothesis that gas inflow, potentially driven by bars or interactions, is linked to both nuclear star formation and AGN activity.
• To provide an unbiased statistical analysis of nuclear and circumnuclear Hα morphology using a carefully selected sample of nearby spiral galaxies.

Proposed method

• Acquired high-resolution, continuum-subtracted Hα images of 57 nearby, low-inclination spiral galaxies using the 1 m JKT and other telescopes on La Palma.
• Used spatial resolution of ~100 pc (1.5 arcsec) to classify nuclear and circumnuclear Hα emission into distinct morphological categories: peaked, patchy, ring-like, or absent.
• Defined circumnuclear regions as within 2 kpc of the galactic center to analyze emission morphology independent of nuclear features.
• Classified galaxies based on host properties: presence of bars, AGN, starbursts, and close companions, to test correlations with Hα morphology.
• Applied consistent, objective criteria for identifying nuclear rings and distinguishing them from other morphologies, avoiding bias from pre-selection of ring-hosting galaxies.
• Combined data with external databases (HyperLEDA, NED) for morphological and activity classifications to ensure sample consistency and completeness.

Experimental results

Research questions

• RQ1 What is the prevalence and morphology of Hα emission in the nuclear and circumnuclear regions of low-inclination spiral galaxies?
• RQ2 How does the presence of a bar in the host galaxy correlate with the morphology of circumnuclear or nuclear Hα emission?
• RQ3 Is there a statistical link between the occurrence of star-forming nuclear rings and non-stellar nuclear activity (AGN or starburst)?
• RQ4 How do close companions influence the morphology of circumnuclear Hα emission or the presence of nuclear Hα peaks?
• RQ5 To what extent do nuclear rings serve as indicators of recent gas inflow, and how are they related to AGN activity on different spatial scales?

Key findings

• Nuclear Hα emission is peaked in most galaxies, and this peak is significantly more common in AGN host galaxies than in non-AGN hosts.
• Circumnuclear Hα emission is patchy in late-type spirals but often absent or ring-like in early-type spirals, indicating a morphological dichotomy linked to galaxy type.
• Nuclear rings occur in 21% ± 5% of spiral galaxies, with 83% of the 12 identified rings occurring in galaxies hosting either an AGN or a starburst.
• Only two of the 12 nuclear rings were found in galaxies that are neither AGN nor starburst hosts, indicating a strong statistical link between rings and non-stellar nuclear activity.
• Nuclear rings are exclusively found in barred galaxies, confirming that bars are necessary for their formation, and weaker bars host larger rings due to orbital dynamics.
• The presence of close bright companions does not affect circumnuclear Hα morphology but correlates with a higher fraction of nuclear Hα peaks, suggesting indirect influence on central activity.