Unveiling Long-Period Variables in M33's Central Region: Insights into Stellar Evolution and Star Formation via Near-Infrared Photometry

TL;DR

This work tackles the challenge of characterizing evolved stellar populations in M33's central region by identifying long-period variables (LPVs) using PSF photometry on near-infrared UKIRT data across $39$ epochs over $0.89\,\mathrm{deg}^2$. By combining PSF photometry from UIST and WFCAM with aperture data, the authors build a master catalog of $211{,}179$ sources and identify ~750 LPV candidates, enabling future Star-Formation History (SFH) and mass-loss analyses. The methodology leverages $J$, $H$, and $K$ bands and DAOPHOT/ALLFRAME to mitigate crowding and detect dusty AGBs, with preliminary results hinting at episodic SFH in the central kiloparsec. This catalog provides a robust basis for studying stellar feedback and chemical enrichment in the cores of spiral galaxies, contributing to models of galaxy evolution in the Local Group.

Abstract

We present an analysis of UKIRT observations obtained between 2003 and 2007 to investigate the evolved stellar populations within the central square kiloparsec of M33. Point-spread function (PSF) photometry is employed to mitigate the effects of stellar crowding and to ensure accurate measurements in this densely populated region. This method, applied to merged observations from UIST and WFCAM in the $J$, $H$, and $K$ bands, extracts $211,179$ stars by cross-matching frame-by-frame across 39 observing nights in three bands. From this, we identify approximately 750 long-period variables (LPVs), predominantly Asymptotic Giant Branch (AGB) stars, by cross-matching PSF results with aperture photometry, focusing on the UIST field for robust variability confirmation. The PSF approach proves particularly effective for resolving blended sources and detecting faint, dusty variables that might remain undetected. We also examined aperture photometry data to validate our results; however, the PSF-derived measurements provide superior depth and completeness, particularly for obscured stellar populations. The resulting master catalog provides a basis for future analyses of variability amplitudes, periods, and star-formation history (SFH), paving the way for a deeper understanding of mass-loss and the dynamical evolution of the central region of M33.

Figures (2)

• Figure 1: The central region of M33 was observed with the UIST instrument. ‌Blue circles indicate LPV stars identified through PSF photometry.
• Figure 2: An example light-curve from the central M33 region, derived from the PSF-enhanced WFCAM catalog, showing the variability of a representative LPV ($K$-band magnitude vs. epoch).