The Swift/UVOT catalogue of NGC4321 star forming sources: A case against density wave theory

TL;DR

This study catalogs 787 star forming regions in NGC4321 using multi-wavelength data, revealing that the spiral pattern speed varies with radius, challenging classic density wave theory.

Contribution

It provides the first detailed UV and IR catalog of star forming regions in NGC4321 and tests spiral arm theories by analyzing spatial offsets.

Findings

No significant offset between star forming regions and spiral arms.

Radial variation in pattern speed contradicts constant pattern speed models.

Star forming regions are more massive towards the galaxy center.

Abstract

We study the star forming regions in the spiral galaxy NGC4321, taking advantage of the spatial resolution (2.5 arcsec FWHM) of the Swift/UVOT camera and the availability of three UV passbands in the region 1600-3000 A, in combination with optical and IR imaging from SDSS, KPNO/Ha and Spitzer/IRAC, to obtain a catalogue of 787 star forming regions out to three disc scale lengths. We determine the properties of the young stellar component and its relationship with the spiral arms. The Ha luminosities of the sources have a strong decreasing radial trend, suggesting more massive star forming regions in the central part of the galaxy. When segregated with respect to NUV-optical colour, blue sources have a significant excess of flux in the IR at 8 micron, revealing the contribution from PAHs, although the overall reddening of these sources stays below E(B-V)=0.2 mag. The distribution of distances to the spiral arms is compared for subsamples selected according to Ha luminosity, NUV-optical colour, or ages derived from a population synthesis model. An offset is expected between these subsamples as a function of radius if the pattern speed of the spiral arm were constant - as predicted by classic density wave theory. No significant offsets are found, favouring instead a mechanism where the pattern speed has a radial dependence.