arXiv subjects

K. Kouroumpatzakis

Publications and source records attributed to K. Kouroumpatzakis.

8 recordsLinked to original sources

arXiv2026

HECATEv2: An all-sky galaxy catalogue for multimessenger astrophysics

We present HECATEv2, the second release of the Heraklion Extragalactic Catalogue (HECATE), an all-sky, value-added galaxy catalogue comprising 204733 galaxies from the HyperLEDA database with recession velocity <14000 km/s (D~200 Mpc). This release focuses on qualitative upgrades of the provided information while maintaining the same parent galaxy sample as HECATEv1. Improvements include a new cosmology-based distance framework, expanded and homogenised optical and mid-infrared photometry from SDSS-DR17/NSA, PS1-DR2, and AllWISE, and new quality-control flags for stellar contamination, incorrect photometry, and coordinate inconsistencies. We also extend the galaxy-size coverage and derive stellar population parameters for a substantially larger fraction of the sample. Star-formation rates (SFR) and stellar masses (Mstar) are now available for >70% of galaxies using updated mid-IR/optical calibrations that account for stellar population age and dust attenuation, while gas-phase metallicities are derived for ~90%. Activity classifications are provided for >50% of galaxies based on spectroscopic and/or photometric diagnostics, and supermassive black hole masses for ~86%. In terms of L$_{B}$,L$_{Ks}$,SFR, and Mstar, HECATEv2 is among the most complete local-Universe catalogues with spectroscopic redshifts. We also provide spatial completeness maps as a function of distance and luminosity, highlighting variations across the sky. Compared to other catalogues (e.g. GLADE+, NED-LVS), HECATEv2 offers broader (optical, near- and far-IR photometry, metallicity, activity classifications) or comparable (mid-IR photometry, SFR, Mstar) coverage, making it a robust reference for studies of SMBH-host galaxy connections, gravitational-wave and high-energy transient hosts, population analyses, and rare galaxy subpopulations.

astro-ph.GA↗

arXiv2026

An automated activity classification tool for optical galaxy spectra

Reliable, versatile galaxy activity diagnostics are essential for understanding galaxy evolution. Traditional methods frequently necessitate extensive preprocessing, such as starlight subtraction and emission line deblending (e.g., Hα and [N II]), which can introduce substantial biases and uncertainties due to their model-dependent nature. In this work we developed an automated, diagnostic tool capable of distinguishing between star-forming (SF), active galactic nuclei (AGN), low-ionization nuclear emission-line regions (LINERs), composite, and passive galaxies. We developed a diagnostic tool based on a support vector machine trained on data from optical emission-line ratios and color selection criteria. From literature studies and exploring combinations of discriminatory feature schemes, we found that the equivalent widths of Hβ, [O III]λ5007, and Hα+[N II]λ6548,84 as key diagnostic features. Additionally, galaxies classified as AGN can be distinguished into broad- and narrow-line AGN by measuring the full quarter at the half-maximum of Hα and [N II] complex. We have developed a diagnostic tool that encompasses all activities of galaxies while achieving high performance scores across all of them. Our diagnostic achieves overall accuracy of 83% and recall of 79% for SF, 94% for AGN, 85% for LINER, 77% for composite, and 96% for passive galaxies. Our diagnostic tool significantly improves upon existing diagnostics as it eliminates the need for preprocessing (i.e., starlight subtraction or flux calibration) and spectral line fitting, includes all activity classes under one scheme, and distinguishes the two main AGN types. In addition, omitting starlight subtraction does not significantly reduce performance. Furthermore, Its narrow wavelength requirement enables use across a wide redshift range, making it ideal for high-z studies.

astro-ph.GA↗

arXiv2025

Gas excitation in galaxies and active galactic nuclei with He IIλ4686 and X-ray emission

The origin of He II emission in galaxies remains a debated topic, requiring ionizing photons with energies exceeding 54 eV. While massive stars, such as Wolf-Rayet stars, have been considered potential sources, their UV flux often fails to fully explain the observed He II emission. Recent studies suggest that X-ray binaries (XRBs) might contribute significantly to this ionization. We explore the relationship between X-ray and $\rm He~II \lambda4686$ emission in a statistically significant sample of galaxies, investigating whether X-ray sources, including active galactic nuclei (AGNs) and XRBs, serve as the primary mechanism for He II ionization across different galactic environments. We cross-matched a sample of known well-detected He II galaxies with the Chandra Source Catalog, yielding 165 galaxies with X-ray and $\rm He~II \lambda4686$ detections. The sources were classified into star-forming galaxies (SFGs) and AGNs based on the BPT diagram and a classification scheme defined for He II galaxies. We find a strong, linear correlation between X-ray and He II luminosity across AGNs and SFGs spanning over seven orders of magnitude. AGNs generally exhibit higher He II/H$β$ flux ratios, stronger extinction, and harder X-ray spectra. The O32 ratio of SFGs is tightly correlated with the H$β$ equivalent width ($\rm EW_{Hβ}$) but not with the He II/H$β$ ratio, suggesting a different excitation mechanism. We derive an O32--$\rm EW_{Hβ}$ line above which only AGNs of our sample reside. The tight correlation between X-ray and He II luminosity supports X-rays as the primary driver of He II excitation. While AGNs have one common ionization source, the central black hole, in SFGs low-energy species are mainly excited by UV emission related to star-forming activity, however, high-energy species like He II require the presence of XRBs.

astro-ph.GA↗

arXiv2024

From seagull to hummingbird: New diagnostic methods for resolving galaxy activity

Context. A major challenge in astrophysics is classifying galaxies by their activity. Current methods often require multiple diagnostics to capture the full range of galactic activity. Furthermore, overlapping excitation sources with similar observational signatures complicate the analysis of a galaxy's activity. Aims. This study aims to create an activity diagnostic tool that overcomes the limitations of current emission line diagnostics by identifying the underlying excitation mechanisms in mixed-activity galaxies (e.g., star formation, active nucleus, or old stellar populations) and determining the dominant ones. Methods. We use the random forest machine-learning algorithm, trained on three main activity classes -- star-forming, AGN, and passive -- that represent key gas excitation mechanisms. This diagnostic employs four distinguishing features: the equivalent widths of [O iii] $λ$5007, [N ii] $λ$6584, H$α$, and the D4000 continuum break index. Results. The classifier achieves near-perfect performance, with an overall accuracy of ~ 99% and recall scores of ~ 100% for star-forming, ~ 98% for AGN, and ~ 99% for passive galaxies. These exceptional scores allow for confident decomposition of mixed activity classes into the primary gas excitation mechanisms, overcoming the limitations of current classification methods. Additionally, the classifier can be simplified to a two-dimensional diagnostic using the D4000 index and log$_{10}$(EW([O iii])$^{2}$) without significant loss of diagnostic power. Conclusions. We present a diagnostic for classifying galaxies by their primary gas excitation mechanisms and deconstructing the activity of mixed-activity galaxies into these components. This method covers the full range of galaxy activity. Aditionally, D4000 index serves as an indicator for resolving the degeneracy among various activity components.

astro-ph.GA↗

arXiv2024

X-ray observations of Blueberry galaxies

Compact star-forming galaxies were dominant galaxy types in the early Universe. Blueberry galaxies (BBs) represent their local analogues being very compact and having intensive star formation. Motivated by high X-ray emission recently found in other analogical dwarf galaxies, called Green Peas, we probe into the X-ray properties of BBs to determine if their X-ray emission is consistent with the empirical laws for star-forming galaxies. We performed the first X-ray observations of a small sample of BBs with the XMM-Newton satellite. Spectral analysis for detected sources and upper limits measured via Bayesian-based analysis for very low-count measurements were used to determine the X-ray properties of our galaxy sample. Clear detection was obtained only for 2 sources, with one source exhibiting an enhanced X-ray luminosity to the scaling relations. For the remaining 5 sources, only an upper limit was constrained, suggesting BBs to be rather underluminous as a whole. Our analysis shows that the large scatter cannot be easily explained by the stochasticity effects. While the bright source is above and inconsistent at almost the 99% confidence level, the upper limits of the two sources are below the expected distribution. These results indicate that the empirical relations between the star formation rate, metallicity, and X-ray luminosity might not hold for BBs with uniquely high specific star formation rates. One possible explanation could be that the BBs may not be old enough to have a significant X-ray binary population. The high luminosity of the only bright source can be then caused by an additional X-ray source, such as a hidden active galactic nucleus or more extreme ultraluminous X-ray sources.

astro-ph.GA↗

arXiv2024

Blueberry galaxies up to 200 Mpc and their optical and infrared properties

Dwarf highly star-forming galaxies (SFGs) dominated the early Universe and are considered the main driver of its reionization. However, direct observations of these distant galaxies are mainly confined to rest-frame ultraviolet and visible light, limiting our understanding of their complete properties. Therefore, it is still paramount to study their local analogs, the green pea (GP) and blueberry (BB) galaxies. This work aims to expand our knowledge of BBs by identifying a new sample that is closer and in the southern sky. In addition to the already known BBs, this new sample will allow for a statistically significant study of their properties probed by visible and infrared (IR) light. By utilizing the HECATE catalog, which provides photometry and characterization of galaxies, along with data from Pan-STARSS and SDSS, this study selects and analyzes a new sample of BBs. We employed spectral energy distribution fitting to derive homogeneous measurements of star-formation rates and stellar masses. Additionally, we measured emission-line fluxes, including $\rm HeII~λ4686$, through spectral fitting. Through this work, we identified 48 BBs, of which 40 were first recognized as such, with the nearest at 19~Mpc. 14 of the BBs are in the south sky. The BBs tend to be extremely IR red in both WISE $W1-W2$ and $W2-W3$ colors, distinguishing them from typical SFGs. Dwarf SFGs with higher specific star-formation rates tend to have redder IR colors. Blueberry galaxies stand out as the most intensely star-forming sources in the local Universe among dwarf galaxies. They exhibit unique characteristics, such as being intrinsically bluer in visible light, redder in the infrared, and less massive. They also have higher specific star-formation rates, equivalent widths, lower metallicities, and the most strongly ionized interstellar medium compared to typical SFGs and GPs.

astro-ph.GA↗

arXiv2023

Optical polarization from colliding stellar stream shocks in a tidal disruption event

A tidal disruption event (TDE) occurs when a supermassive black hole rips apart a passing star. Part of the stellar material falls toward the black hole, forming an accretion disk that in some cases launches a relativistic jet. We performed optical polarimetry observations of a TDE, AT 2020mot. We find a peak linear polarization degree of $25\pm4$%, consistent with highly polarized synchrotron radiation, as is typically observed from relativistic jets. However, our radio observations, taken up to 8 months after the optical peak, do not detect the corresponding radio emission expected from a relativistic jet. We suggest that the linearly polarized optical emission instead arises from shocks that occur during accretion disk formation, as the stream of stellar material collides with itself.

astro-ph.HE↗

arXiv2019

Search for AGN counterparts of unidentified Fermi-LAT sources with optical polarimetry: Demonstration of the technique

The third Fermi-LAT catalog (3FGL) presented the data of the first four years of observations from the Fermi Gamma-ray Space Telescope mission. There are 3034 sources, 1010 of which still remain unidentified. Identifying and classifying gamma-ray emitters is of high significance with regard to studying high-energy astrophysics. We demonstrate that optical polarimetry can be an advantageous and practical tool in the hunt for counterparts of the unidentified gamma-ray sources (UGSs). Using data from the RoboPol project, we validated that a significant fraction of active galactic nuclei (AGN) associated with 3FGL sources can be identified due to their high optical polarization exceeding that of the field stars. We performed an optical polarimetric survey within $3σ$ uncertainties of four unidentified 3FGL sources. We discovered a previously unknown extragalactic object within the positional uncertainty of 3FGL J0221.2+2518. We obtained its spectrum and measured a redshift of $z=0.0609\pm0.0004$. Using these measurements and archival data we demonstrate that this source is a candidate counterpart for 3FGL J0221.2+2518 and most probably is a composite object: a star-forming galaxy accompanied by AGN. We conclude that polarimetry can be a powerful asset in the search for AGN candidate counterparts for unidentified Fermi sources. Future extensive polarimetric surveys at high galactic latitudes (e.g., PASIPHAE) will allow the association of a significant fraction of currently unidentified gamma-ray sources.

astro-ph.HE↗