Detailed photometric analysis of young star groups in the galaxy NGC 300

Abstract

Aims. The purpose of this work is to understand the global characteristics of the stellar populations in NGC 300. In particular, we focused our attention on searching young star groups and study their hierarchical organization. The proximity and orientation of this Sculptor Group galaxy make it an ideal candidate for this study. Methods. The research was conducted using archival point spread function (PSF) fitting photometry measured from images in multiple bands obtained with the Advanced Camera for Surveys of the Hubble Space Telescope (ACS/HST). Using the path linkage criterion (PLC), we cataloged young star groups and analyzed them from the observation of individual stars in the galaxy NGC 300. We also built stellar density maps from the bluest stars and applied the SExtractor code to identify overdensities. This method provided an additional tool for the detection of young stellar structures. By plotting isocontours over the density maps and comparing the two methods, we could infer and delineate the hierarchical structure of the blue population in the galaxy. For each region of a detected young star group, we estimated the size and derived the radial surface density profiles for stellar populations of different color (blue and red). A statistical decontamination of field stars was performed for each region. In this way it was possible to build the colormagnitude diagrams (CMD) and compare them with theoretical evolutionary models. We also constrained the present-day mass function (PDMF) per group by estimating a value for its slope. Results. The blue population distribution in NGC 300 clearly follows the spiral arms of the galaxy, showing a hierarchical behavior in which the larger and loosely distributed structures split into more compact and denser ones over several density levels. We created a catalog of 1147 young star groups in six fields of the galaxy NGC 300, in which we present their fundamental characteristics. The mean and the mode radius values obtained from the size distribution are both 25 pc, in agreement with the value for the Local Group and nearby galaxies. Additionally, we found an average PDMF slope that is compatible with the Salpeter value.

OUP accepted manuscript

“…These values are in good agreement with the OB association size found in other galaxies, using automatic search methods and HST images. Rodríguez et al (2016) found a value of 25 pc for the mode and the average radius of young star groups in NGC 300; although these values are lower than ours, we must consider that they used values of radius equal to 1σ instead of 2σ as in this work. Bresolin et al (1998) studied the OB associations in seven spiral galaxies, NGC 925, NGC 2090, NGC 2541, NGC 3351, NGC 3621, NGC 4548, and M101, finding size distribution peaks between 25 and 45 pc and an average radius between 25 and 60 pc.…”

“…6). We also noticed that, in a similar study for the NGC 300 galaxy, Rodríguez et al (2016) adopted p = 10 stars; however, for NGC 253 we are leading with a more distant galaxy and there is probably a blending effect with the individual objects in the ACS images used. Therefore, we considered p = 8 objects as a reasonable value.…”

“…We compared the young groups that we detected with those identified in NGC 300, which were studied by Rodríguez et al (2016) using similar observational data to those in the present work.…”

“…11 we plot the number of star members versus the size of the groups in each galaxy. To be consistent with Rodríguez et al (2016) we adopt in this figure a radius of 1σ (the radial standard deviation of the group member location). We performed a quadratic fit to the data, in order to compare the numbers of members detected in groups of the same size in both galaxies.…”

“…The distribution and properties of these systems are useful to understand the most recent history of the host galaxy as well as the star-formation process in different environments. For this reason, in recent decades several works have focused on performing global studies and assembling catalogues of young stars groups over the nearest galaxies: LMC (Gouliermis et al 2003), SMC (Battinelli 1991), M 31 (Bianchi et al 2012), M 33 (Chandar, Bianchi & Ford E-mail: jimenaro@fcaglp.unlp.edu.ar 1999; Bedin et al 2005), NGC 6347 (Massi et al 2015), NGC 300 (Pietrzyński et al 2001;Rodríguez, Baume & Feinstein 2016), NGC 7793 (Pietrzyński et al 2005), M 81 (Nantais et al 2010), and M 101 (Bresolin, Kennicutt & Stetson 1996), to name just a few.…”