Discovery of a Population of X-ray luminous starbursts with eROSITA
The eROSITA (extended ROentgen Survey with an Imaging Telescope Array) survey, the first all-sky X-ray survey in the 0.2-8.0 keV band, revolutionized our understanding of the X-ray emission of galaxies by providing an unbiased census of their populations.
Two members of the Department of Physics (Elias Kyritsis, PhD student, and Andreas Zezas, affiliated faculty) participate in the eROSITA Nearby galaxies Working Group led by Dr. Frank Haberl and currently by Dr. Chandreyee Maitra at the Max-Planck Institute for Extraterrestrial Physics in Garching, Germany. This group also includes researchers from Dr. Karl Remeis-Observatory, Bamberg, (Germany), and the NASA Goddard Space Flight Center, (USA).
In a recent study, led by Elias Kyritsis, the team used X-ray data from the first all-sky scan of the eROSITA survey (eRASS1) to study for the first time a robust and unbiased statistical sample of star-forming galaxies. The X-ray emission from normal galaxies, i.e. those without active galactic nuclei (AGN), originates primarily from X-ray binary populations (XRBs) and diffuse hot gas. To connect the emission from XRBs with the stellar populations of the host galaxy, Elias Kyritsis also led the development of the updated value-added galaxy catalogue HECATEv2. This all-sky galaxy catalogue provides a wealth of information for all known galaxies within 200 Mpc (z<0.05) such as: accurate distances and sizes, optical/mid-IR photometry, stellar population parameters (star-formation rate -SFR-; stellar mass -M★-; Metallicity) based on multi-wavelength indicators, and robust activity classifications based on spectroscopic and IR/optical photometric diagnostics.
In this way, the team studied the X-ray emission from the largest sample of normal galaxies so far (~18.000 objects), as a function of their SFR, M★, Metallicity, and stellar population age. Their results revealed a subpopulation of dwarf metal-poor star-burst galaxies hosting extremely luminous XRB populations up to ~100 times more luminous than previous studies. Based on these findings, the researchers rederived the scaling relations between X-ray luminosity and stellar population parameters by using the most unbiased sample of normal galaxies so far.
The discovery of these luminous galaxies challenges our understanding of XRB populations, as they suggest the presence of a large population of extremely luminous objects. This implies either extremely high accretion rates, the existence of very massive stellar black holes, or even the presence of Intermediate Mass Black Holes. In fact, the study of these systems is crucial for probing the population of black holes and neutron stars forming at the end of the stellar life, and for understanding the evolution of binary stellar systems. Such systems can be the progenitors of the most energetic phenomena in the Universe: γ-ray bursts and compact object mergers leading to the production of gravitational wave radiation. These updated scaling relations can be used to constrain the evolutionary parameters of XRBs by testing different evolution scenaria through binary population synthesis. Additionally, this previously unrecognized XRB population, can be used to improve our understanding of the cosmic evolution of galaxies. In fact, the majority of these X-ray luminous galaxies are dwarf starburst galaxies, which serve as local analogues of high-redshift galaxies, allowing us to place better constraints on the evolutionary paths of galaxies and compact objects through cosmic history.
(b) Right: The LX/SFR as a function of the specific SFR (an indication of the intensity of star-forming activity) for the population of the X-ray luminous star-forming galaxies revealed by the eRASS1 survey. The color code shows the gas metallicity. As it is shown there is a subpopulation of extremely X-ray luminous star-forming galaxies, with low metallicities and higher sSFRs. The red dashed line indicates one of the scaling relations (Lehmer et al. 2016).
