Multicolour Photometric Study of M31 Globular Clusters

TL;DR

This study provides detailed multicolour photometry of M31 globular clusters, transforming intermediate-band data to standard broad bands, and confirms the presence of a blue-tilt in the colour-magnitude diagram with high statistical significance.

Contribution

It offers new multicolour photometric data for M31 GCs and candidates, supplementing existing catalogues, and confirms the blue-tilt phenomenon with high confidence.

Findings

Identified 4 out of 61 GCs with no detectable signal in BATC images.

Confirmed the blue-tilt in M31 GCs with 99.95% confidence.

Provided transformed photometry data for previously unmeasured GCs.

Abstract

We present the photometry of 30 globular clusters (GCs) and GC candidates in 15 intermediate-band filters covering from ~3000 to ~10000 \AA using the archival CCD images of M31 observed as part of the Beijing - Arizona - Taiwan - Connecticut (BATC) Multicolour Sky Survey. We transform these intermediate-band photometric data to the photometry in the standard UBVRI broad-bands. These M31 GC candidates are selected from the Revised Bologna Catalogue (RBC V.3.5), and most of these candidates do not have any photometric data. Therefore the present photometric data are supplement to RBC V.3.5. We find that 4 out of 61 GCs and GC candidates in RBC V.3.5 do not show any signal on the BATC images at their locations. By linear fit of the distribution in colour-magnitude diagram of blue GCs and GC candidates using the data from RBC V.3.5, and in this study we find the ``blue-tilt'' of blue M31 GCs with a high confidence at 99.95% or 3.47 sigma for the confirmed GCs, and >99.99% or 4.87 sigma for GCs and GC candidates.