Optical Counterparts of ULXs in Two Dwarf Galaxies: NGC 4861 and NGC 4449

TL;DR

This study identifies and analyzes optical counterparts of ULXs in two dwarf galaxies using HST data, confirming associations with star-forming regions and estimating properties like age and mass, while also examining archival X-ray data.

Contribution

It provides the first detailed optical counterpart analysis of ULXs in NGC 4861 and NGC 4449, including astrometry, candidate identification, and stellar property estimation.

Findings

NGC 4861 X1 associated with an HII complex

NGC 4861 X2 resides in a young star group (~400 Myr)

NGC 4449 X7 has three optical candidates with magnitudes -5.0 and -4.1

Abstract

We present the results of a search for optical candidates of Ultraluminous X-ray sources (ULXs) in two dwarf galaxies: NGC 4861 and NGC 4449 using Hubble Space Telescope {\it HST} archival data. With a precise astrometry, we confirm that NGC 4861 X1 associated with an HII complex as reported by \cite{2014MNRAS.441.1841T} and we conclude that NGC 4861 X2 resides in a young star group with 400$\pm$80 M$\odot$. We also find that NGC 4449 X7 is associated with three optical candidates within an error radius of 0$\farcs$2 at the 90$\%$ confidence level. Absolute magnitudes (M$_{v}$) of these candidates are determined as $-$5.0 and $-$4.1. The age and mass values for the three candidates by using stellar evolutionary tracks are estimated as 40-50 Myr and $\sim$8 M$\odot$, respectively. The locations of optical candidates suggest possible association with a nearby group of stars. In addition, we analyzed the previously unused archival data of {\it XMM-Newton}, {\it Chandra} and {\it Swift} where the sources detected. Although the X-ray spectral data do not allow us to discriminate between physical models, long-term data at hand are consistent with the sources being in luminous hard states.