[Paper Review] X-ray source populations in galaxies

G. Fabbiano|CERN Bulletin|Nov 16, 2005

Astrophysical Phenomena and ObservationsPhysics and Astronomy1 references177 citations

TL;DR

This review synthesizes X-ray source populations in galaxies using high-resolution, sensitive X-ray observations, revealing that low-mass X-ray binaries (LMXBs) dominate in old stellar systems like E and S0 galaxies, while high-mass X-ray binaries (HMXBs) trace recent star formation. The study identifies ultraluminous X-ray sources (ULXs) and very soft X-ray sources as key components, with ULXs possibly hosting intermediate-mass black holes or stellar-mass black holes in extreme states.

ABSTRACT

Today's sensitive, high-resolution X-ray observations allow the study of populations of X-ray sources, in the luminosity range of Galactic X-ray binaries, in galaxies as distant as 20-30 Mpc. The traditional astronomical tools of photometric diagrams and luminosity functions are now applied to these populations, providing a direct probe of the evolved binary component of different stellar populations. The study of the X-ray populations of E and S0 galaxies has revamped the debate on the formation and evolution of low-mass X-ray binaries (LMXBs) and on the role of globular clusters in these processes. While overall stellar mass drives the amount of X-ray binaries in old stellar populations, the amount of sources in star forming galaxies is related to the star formation rate. Short-lived, luminous, high-mass binaries (HMXBs) dominate these young populations. The most luminous sources in these systems are the debated ULXs, which have been suggested to be ~100-1000 Msol black holes, but could alternatively include a number of binaries with stellar mass black holes. Very soft sources have also been discovered in many galaxies and their nature is currently being debated. Observations of the deep X-ray sky, and comparison with deep optical surveys, are providing the first evidence of the X-ray evolution of galaxies.

Motivation & Objective

• To characterize the X-ray source populations in galaxies across different morphological types, especially E and S0 galaxies.
• To investigate the formation and evolution mechanisms of low-mass X-ray binaries (LMXBs) in old stellar populations.
• To determine the connection between X-ray source luminosity functions and star formation rates in star-forming galaxies.
• To explore the nature of ultraluminous X-ray sources (ULXs) and very soft X-ray sources in external galaxies.
• To use deep X-ray and optical surveys to trace the X-ray evolution of galaxies over cosmic time.

Proposed method

• Application of photometric diagrams and luminosity functions to X-ray source populations in galaxies observed with Chandra and XMM-Newton.
• Use of high-resolution X-ray imaging to resolve individual X-ray sources in galaxies up to 20–30 Mpc distance.
• Comparison of X-ray source counts and luminosity functions with stellar population synthesis models to infer binary evolution pathways.
• Analysis of X-ray spectra to classify sources as LMXBs, HMXBs, ULXs, or very soft X-ray sources.
• Cross-identification of X-ray sources with optical and infrared counterparts from deep multiwavelength surveys.
• Use of deep X-ray sky surveys to study the cosmic evolution of X-ray source populations in galaxies.

Experimental results

Research questions

• RQ1 What drives the number and luminosity distribution of X-ray sources in old stellar populations such as E and S0 galaxies?
• RQ2 How do high-mass X-ray binaries (HMXBs) relate to recent star formation in galaxies?
• RQ3 What is the nature of ultraluminous X-ray sources (ULXs), and do they host intermediate-mass black holes?
• RQ4 What are the physical properties and origins of very soft X-ray sources detected in external galaxies?
• RQ5 How do X-ray source populations evolve over time, and what do deep X-ray and optical surveys reveal about this evolution?

Key findings

• The number of X-ray sources in old stellar populations is primarily driven by total stellar mass, with LMXBs dominating the population.
• In star-forming galaxies, the X-ray source count is strongly correlated with the star formation rate, indicating a dominant contribution from short-lived, luminous HMXBs.
• Ultraluminous X-ray sources (ULXs) exceed the Eddington limit for stellar-mass black holes and may host intermediate-mass black holes (~100–1000 M☉), though stellar-mass black holes in super-Eddington states remain a viable alternative.
• Very soft X-ray sources have been detected in multiple galaxies, but their nature remains uncertain, with possible explanations including quiescent black holes or highly absorbed systems.
• Deep X-ray surveys combined with optical data reveal evidence of X-ray evolution in galaxies, supporting the idea that X-ray source populations trace the star formation and merger history of galaxies.
• The luminosity function of X-ray sources in galaxies shows a bimodal distribution, reflecting the distinct evolutionary pathways of LMXBs and HMXBs.