[View Abstracts by Session]

Session: Extragalactic Astronomy and Astrophysics

1. Name: Dr. Panos Charalampopoulos (University of Turku)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Modeling the polarized light of Tidal Disruption Events
   Abstract:
   Tidal disruption events (TDEs) occur in the nuclei of galaxies, when a star is tidally destroyed by a supermassive black hole (SMBH). TDEs are being observed across the electromagnetic spectrum and their properties are highly diverse, making the origin of the optical luminosity a mystery and leading to different scenarios for the underlying emission mechanism. Even though most of them are observed in the optical regime, polarimetric observations of optical TDEs have been very scarce until now. More recently, polarimetric observations of TDEs started to take place and the first sample study of polarization from optical TDEs was published. The nascent field of optical TDE polarimetry calls for theoretical interpretation and in this talk I will present such a work, using the 3-D Monte Carlo radiative transfer code POSSIS and the collision-induced outflow (CIO) TDE emission scenario, in order to model the continuum polarization levels of TDEs. We find that the polarization depends strongly on the optical depths at the central regions and the viewing angle. Finally, by comparing our model predictions to polarization observations of a few TDEs, we attempt to constrain their observed viewing angles and we show that multi-epoch polarimetric observations can become a key factor in constraining the viewing angle of TDEs.
2. Name: Dr. Maria Chira (IAASARS, NOA)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Forward-modeling the cosmic accretion history
   Abstract:
   I will discuss a class of statistical, forward-modeling methodologies that are based on the continuity equation of supermassive black holes (SMBHs) to reconstruct the accretion history of the Universe. A byproduct of this procedure are predictions on several AGN ensemble properties (e.g. duty cycles, variability etc.), which when compared to observables, can be used to constrain model parameters (e.g., radiative efficiency of SMBHs, eddington ratio distribution of AGN) and obtain further insight into the evolution of AGN. I will present preliminary results on the active and dormant Black Hole Mass Functions (BHMFs) based on the latest observed AGN luminosity functions. I will also show how the model above can be extended with analytical expressions that describe the flux variability of active black holes (excess variance) to make predictions on the observed ensemble variability of AGN populations. Comparison with observations can constrain models of the AGN variability (Power Spectrum Density; PSD) and provide information on the nature of the observed variations.
3. Name: Mr. Charalampos Daoutis (IA-FORTH & Univ. of Crete)
   Coauthors: Kyritsis Elias (University of Crete - I.A FORTH)
   Zezas Andreas (University of Crete - I.A FORTH)
   Kouroumpatzakis Konstantinos (Astronomical Institute, Czech Academy of Sciences)
   Type: Oral
   Title: A versatile classification tool for galactic activity using optical and infrared colors
   Abstract:
   The activity classification of galaxies is usually performed using fluxes of optical emission lines (e.g., emission-line ratios). So far, these diagnostics have been applied to a wide range of galaxy samples with great success. However, they suffer from two major disadvantages. The first is that the measurement of the flux of an emission line can often be challenging. The second is that the use of emission lines prohibits the applicability of these diagnostics to dormant systems, i.e., the passive galaxies. In this work, we define a new diagnostic tool by training a machine learning algorithm to characterize galactic activity (or the lack thereof) using as discriminating features two infrared colors, (W1-W2) and (W2-W3; WISE survey), and one optical color, g-r (SDSS or PanSTARRS). This new diagnostic classifies galaxies into five activity classes: star-forming, active galactic nucleus (AGN), LINER, composite, and passive. The overall accuracy achieved is ~81% while the achieved completeness is ~81% for the star-forming and ~85% for the passive galaxies. Besides the excellent scores achieved for the star-forming and passive galaxies, we also find that it offers significantly improved results for the AGNs in the local Universe compared to other commonly used mid-IR diagnostics which tend to miss the lower-luminosity AGN. Furthermore, another advantage of this new classification tool is its ability to identify very blue, metal poor, star-forming galaxies that are often misclassified as AGNs (AGN imposters) by the majority of the available mid-IR diagnostics.
4. Name: Mr. Charalampos Daoutis (IA-FORTH & Univ. of Crete)
   Coauthors: Kyritsis Elias (Institute of Astrophysics - FORTH, Univ. of Crete)
   Zezas Andreas (University of Crete - I.A FORTH)
   Kouroumpatzakis Konstantinos (Astronomical Institute, Czech Academy of Sciences)
   Bonfini Paolo (University of Crete)
   Type: Poster
   Title: A holistic approach in optical galactic activity diagnostics
   Abstract:
   The activity classification of galaxies is an important but difficult topic in observational astrophysics. Today, well-known galactic activity classification methods are based primarily on the use of characteristic optical emission-line ratios. However, the use of emission lines does not allow a unifying optical diagnostic for passive and active galaxies. To address this limitation, we introduce a new activity diagnostic based on the equivalent with (EW) of the Hα, OIII[λ5007], and NII[λ6584] lines and the D4000 continuum break. Using these data we build a machine-learning model that is able to discriminate between the different excitation mechanisms in galaxies. Besides the fact that this diagnostic allows us to include the class of passive galaxies, the use of the D4000 allows us to discriminate between composite-like spectra resulting from mixed activity galaxies (i.e., star-forming and AGN) or from older stellar populations (e.g., post-AGB stars). Finally, we find that this new diagnostic can be reduced to a 2-dimensional plot of the D4000 against the log(EW(OIII[λ5007])2), while maintaining good accuracy. This offers a more direct and detailed characterization of the galactic activity than any other existing method. Since this diagnostic is based on equivalent widths: it does not require the removal of the stellar continuum, it can be applied to a wider number of galaxies, and it includes all possible types of galactic activity under one unified scheme.
5. Name: Prof. Kalliopi Dasyra (National & Kapodistrian Univ. of Athens)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Linking molecular clouds in outflows to star formation
   Abstract:
   The change in the star formation rate associated with the compression or dissipation of molecular gas in galactic outflows has long been advocated. We present measurements of the changes in the internal and in the external pressure of molecular clouds that are caused by the passage of black hole jets. We find that both enhancement and suppression of star formation can simultaneously happen, as both compression of some clouds and evaporation of other clouds is seen in the VLT MUSE and ALMA data. Radiative-transfer and level population modelling of these data provided the temperature and density of the gas needed for the pressure gradient calculations, and revealed that several jet-ISM interaction mechanisms can lead to these gradients, including mechanical heating and cosmic rays. Approved observations with the MIRI instrument on board the Webb will enable us to determine the pressure of intermediate gas layers, shedding more light on the dynamical fate of dense clouds in outflows. Stellar population synthesis modelling for the determination of the stellar ages along the jet is underway and will benefit from JWST NISpec data.
6. Name: Mr. Georgios Dimopoulos (Universidad Diego Portales)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Exploring the circumnuclear environment of SMBHs using ray-tracing simulations in the X-ray band
   Abstract:
   Supermassive black holes at the center of galaxies tend to accumulate mass in high accretion rate and produce a copious amount of radiation. A significant part of the emitted radiation is in X-rays and therefore, the study of X-rays that originate from the very center of the AGN can give us useful information about the physics that take place. We are using the RefleX platform, a ray-tracing code that generates X-ray spectra (Paltani&Ricci, 2017), in order to develop models that take into account the interactions between the X-rays and the surrounding material. In my presentation, I will introduce the new models we have developed featuring more complex and realistic geometries than the models that are currently available. The first one, includes a toroidal dusty component along with a polar medium perpendicular to the plane of rotation of the SMBH. For the second model, we take advantage of the capability of RefleX to handle complex geometries and therefore, we include, besides the torus and the polar medium, the accretion disk and broad-line region. Moreover, some examples of the models applied on real data will be illustrated. Complex models will play a crucial role in the coming years with the advent of high-resolution X-ray instruments, such as the one on XRISM.
7. Name: Ms. Eleftheria Drigga (NOA, AUTH)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Properties and Evolution of Active Galactic Nuclei in Galaxy Clusters
   Abstract:
   There is compelling evidence that the presence of AGN is closely linked to the large-scale environment, and that AGN in clusters are strongly affected by their environment, albeit in a complicated way. As the most massive self-gravitating entities of the universe, clusters are ideal laboratories to investigate the impact of dense environments on AGN demographics. The AGN fraction in cluster galaxies was found to depend on the distance from the cluster centre, the mass of the cluster and the redshift. Interestingly, in contrast to the lack of AGN in the centres of massive clusters, a number of studies have found an excess of X-ray detected AGN in cluster outskirts, supporting the presence of an in-falling population. Several studies have suggested that these observational results are due to ram pressure stripping by the hot ICM, galaxy mergers and interactions, which may play an important role in AGN triggering and evolution. In this talk, I will focus on recent results from the XXL survey and other X-ray or SZ selected cluster samples that span a wide redshift range. These results provide observational evidence of the physical mechanisms that drive AGN and galaxy evolution within clusters.
8. Name: Dr. Natascha M. Forster Schreiber (MPE Garching)
   Coauthors: No coauthors were included.
   Type: Oral
   Title: Star-forming Galaxies at Cosmic Noon and Beyond
   Abstract:
   Lookback studies have assembled a fairly complete census of galaxies over 85% of cosmic time and established that the bulk of stars, which today reside in massive ellipticals and spirals, formed rapidy at z~1-3, "cosmic noon". Most of this star formation took place in massive gas-rich, turbulent disks, which already followed tight scaling relations in their global properties, and in which dense bulges, fast growing central black holes, and galactic winds were ubiquitous. I will discuss observational results that unveiled the regulated evolution of galaxies since z~3 and recent insights into the galaxy-internal physical processes at play, with a focus on spatially and spectrally resolved studies of the kinematics, gas, and star formation. I will outline exciting new developments in the quest to unlock the drivers of galaxy evolution at and beyond cosmic noon with the newest cutting-edge ground- and space-based instrumentation, paving the way for science with next-generation large aperture telescopes like the ELT in the next decade.
9. Name: Mr. Markos Georganopoulos (UMBC)
   Coauthors: No coauthors were included.
   Type: Poster
   Title: The first hard X-ray spectrum of a kpc-scale jet with NuSTAR
   Abstract:
   High-resolution imaging with Chandra and HST has revealed that many powerful jets emit strongly in the optical/UV and X-rays on kpc to Mpc scales. In the most powerful jets, the optical and X-ray have spectral properties which show they are not a continuation of the radio synchrotron emission. These extra high-energy emission components, despite dominating the energetic output, are currently unidentified. Understanding the emission mechanism in jets is the only way to get a proper accounting of the energy they carry and the impact on their environments. We present a deep (400 ks) NuSTAR observation of Pictor A, the only large-scale resolved jet target which can be easily resolved and detected separately from the AGN core. We find that the X-ray spectrum continues into the NuSTAR band with virtually the same spectral index as that seen in deep Chandra observations, with no evidence of a cut-off. We confirm strong variability in the hotspot as seen by Chandra, and some hints of the same in the NuSTAR observations. We interpret the full suite of observations as indicating a synchrotron origin for the X-ray emission, from a distinct electron energy distribution from that which produces the radio emission.
10. Name: Dr. Ioannis Georgantopoulos (National Observatory of Athens)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: X-ray Active Galactic Nuclei at high redshift
    Abstract:
    X-ray observations provide the most efficient means to detect Active Galactic Nuclei that is accretion into supermassive black holes. This is because X-rays unlike the optical wavelengths are hardly affected by intrinsic obscuration. It appears that Active Galactic Nuclei evolve in parallel with the star-formation density in galaxies up to redshifts of z=2-3 (the cosmic noon epoch). Here, we provide the most accurate yet estimate of the X-ray luminosity function at even higher redshifts in the Early Universe (3>z>6) using the largest sample so far (>600) of high-redshift AGNs selected in the Chandra deep fields, Legacy-COSMOS and XMM-XXL northern fields. One third of the X-ray sources have available spectroscopic information, while we derived the X-ray spectral parameters for all the sources in our sample in order to determine the intrinsic obscuration. Our estimated luminosity function is in agreement with an exponential decline model similar to what is witnessed at optical wavelengths, while the derived BHAD lies well below the star-formation density. Comparison with a number of cosmological galaxy simulations is presented.
11. Name: Dr. Nectaria Gizani (HOU, SST)
    Coauthors: Veldes George (University of Thessaly, Dt of Physics, Heron Laboratory)
    Type: Oral
    Title: On the Hellenic radio telescope THERMOpYlae
    Abstract:
    By April 2018 the advent of the Hellenic radio telescope with the historic name THERMOpYlae has been announced as a joint project between the Hellenic Open University, School of Science and Technology and the University of Thessaly, Heron Laboratory. The project involved the conversion of a redundant 32m telecommunication dish into the Hellenic Radio Telescope. The necessary infrastructure is provided by the Hellenic Telecommunication Company OTE via memorandum of understanding with the universities. This was the first initiative of Greece obtaining a scientific Instrument, capable for single dish as well as Very Long Baseline Interferometric observations. We report on the work in progress.
12. Name: Mr. John Kallimanis (National & Kapodistrian Univ. of Athens)
    Coauthors: Dasyra Kalliopi (Assistant Professor, National and Kapodistrian University of Athens)
    Type: Poster
    Title: The Influence of Shocks on the Jet-impacted Gas Emission of IC5063
    Abstract:
    AGN-driven outflows affect radiation emissions of the interstellar medium they disturb. The propagation of shock waves constitutes one of the ways that AGN feedback affects the gas in galaxies. By fitting the observed lines in VLT MUSE data, we derive physical quantities (electron density, temperature), via line ratio diagnostics for regions of the galaxy IC5063 that are impacted by the jet of its black hole. We model the emission using the MAPPINGS shock simulations and thus derive shock physical parameters, in regions where the influence of shocks is present. The jets of IC5063 are oriented so that they interact with with certain parts of the galaxy 's disk. Thus, the presence of both shocked and unshocked ISM is deduced. The use of IFU observations, with its enhanced spatial resolution, provides finer details of the structure of the ISM of the galaxy. Predictions for JWST data of the warm H2 emission will be made, which will provide further insight on the physics of the ISM emission following a jet-ISM interaction.
13. Name: Prof. Antonios Katsianis (SYSU / SJTU)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: The star formation of galaxies for the last 13 billion years.
    Abstract:
    In this talk I present the evolution of the star formation rates (SFRs) and stellar masses of galaxies for the last 13 billion years of the Universe both in observations and cosmological simulations. First, I employ cosmological simulations combined with radiative transfer and demonstrate that the adopted methodology / indicator (e.g. IR, UV, Ha, SED) to obtain the observed galaxy SFRs and stellar masses are bound to heavily affect the derived properties something that I further show by comparing results from different observational studies. In addition, I demonstrate that state-of-the-art simulations (EAGLE, TNG, Simba, semi-analytic models) are found to suffer from severe limitations mostly connected to resolution effects and the adopted feedback prescriptions which have to be reconsidered. Last, I demonstrate that the observed star formation rate density can be described by only two parameters and a function that resembles a Gamma distribution, like numerous other physical processes in Nature (from Economy to Biology) while it has a plateau from z = 1-4 and not a strong peak at z = 2 (Katsianis et al. 2021). I proceed to demonstrate that Dark matter halo growth follows the same Gamma growth pattern using an analysis of Mass accretion Histories (Katsianis et al. 2023). I will present where we agree and disagree as community via Polls from the EAS 2021 and how these opinions evolved in the last 2 years.
14. Name: Ms. Olga Kefallinou (National & Kapodistrian Univ. of Athens)
    Coauthors: No coauthors were included.
    Type: Poster
    Title: Models of non-thermal gamma radiation of blazars
    Abstract:
    Blazars are active galactic nuclei (AGNs) with their relativistic jets pointed toward the observer. Due to the orientation of the jet, the flux across the electromagnetic spectrum, from radio waves to very high energy gamma-rays, is strongly enhanced by relativistic Doppler boosting. According to leptonic models for the high-energy emission from blazars, relativistic electrons in the inner jets inverse-Compton scatter photons from a variety of sources. The high energy radiation is produced via inverse Compton (IC) scattering of low-energy photon fields that can be either synchrotron emission (synchrotron self-Compton - SSC) or can originate outside the jet. The present project provides an analysis on the beaming of γ-ray radiation in a relativistic moving blob by comparing the External Compton (EC) emission and the Synchrotron-Self-Compton (SSC) emission. Based on these radiation models, and using the ratio of peak synchrotron to EC or SSC fluxes (Compton Dominance) parameters of a sample of blazar sources, such as the magnetic field and jet powers, are derived, knowing the observable physical quantities.
15. Name: Dr. Konstantinos Kolokythas (North-West University)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: En route to the SKA era: MeerKAT's sharp new view on galaxy clusters
    Abstract:
    Radio observations of galaxy clusters are powerful tools for the detection of diffuse cluster-scale synchrotron emission, which carries information about the cluster formation history. Observations using Square Kilometre Array (SKA) precursor and pathfinder instruments are nowadays opening up a new window on diffuse cluster radio sources and challenge our simple classification scheme (radio halos, mini-halos, and radio relics), making clear the need for an update of our current knowledge. Towards this direction, the unique combination of the MeerKAT radio telescope's unprecedented high sensitivity to low surface brightness emission, dense uv-coverage, and high resolution is key to the re-discovery of galaxy clusters and in-depth comprehension of the diffuse structures formation mechanism. In this talk, I will summarize the results from the MeerKAT’s Galaxy Cluster Legacy Survey (MGCLS), a program of long-track observations of 115 galaxy clusters at 1.28 GHz spread out over the Southern sky, focusing on the diffuse radio emission detected in galaxy clusters. New results will be highlighted from a few important well-known cluster systems in order to reveal both the much-improved radio images compared to previous radio observations, as well as the new discoveries that open up new areas of investigation in cluster formation and evolution that set the path to the SKA era.
16. Name: Ms. Mado Kourakou (National & Kapodistrian Univ. of Athens)
    Coauthors: Audibert Anelise (Instituto de Astrofísica de Canarias)
    Dasyra Kalliopi (National & Kapodistrian Univ. of Athens)
    Papachristou Michalis (National & Kapodistrian Univ. of Athens)
    Fernández-Ontiveros Juan Antonio (Centro de Estudios de Física del Cosmos de Aragón)
    Ruffa Ilaria (Cardiff Hub for Astrophysics Research & Technology, Cardiff University)
    Bisigello Laura ( Università di Padova)
    Combes Françoise (Observatoire de Paris)
    Salomé Philippe (Observatoire de Paris)
    Gruppioni Carlotta (INAF - Osservatorio di Astrofisica e Scienza dello Spazio di Bologna)
    Polkas Markos (Donostia International Physics Center)
    Type: Oral
    Title: Stellar mass function of ARC radiogalaxies
    Abstract:
    The stellar or gaseous mass function of galaxies is an important tool of galactic evolution and of cosmological simulations of galaxy formation. A gas mass function of radio galaxies from the local universe up to z=2.5, was recently presented by Audibert et al. (2022), showing an increase in the mass of molecular gas of the CO-detected radiogalaxies with redshift, highlighting that star formation is more common in high-z radio galaxies than previously thought. The catalog used in that work was the ALMA Radiosource Catalogue (ARC) - a survey of 120 radiogalaxies used as ALMA (Atacama Large Millimeter/submillimeter Array) calibrators or calibrators, which is representative of the NVSS 1.4GHz survey at flux threshold of 0.4Jy. To better understand the evolutionary stage of those galaxies, their relation to the main sequence of star formation needs to be found. The estimation of the stellar mass of these radiogalaxies in the last ~11 billion years is needed for this purpose. We use an own code of bayesian probabilistic Spectral Energy Distribution (SED) fitting to infer the stellar mass. First results will be presented in that direction.
17. Name: Dr. Konstantinos Kouroumpatzakis (Astronomical Institute, Czech Academy of Sciences)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Infrared star-formation rate and stellar mass calibrations and their dependence on stellar population age and extinction
    Abstract:
    The stellar mass (M⋆) and the star formation rate (SFR) are among the most important features of galaxies. Measuring these fundamental properties accurately is critical for understanding the present state of galaxies, their history, and future evolution. Infrared (IR) photometry is widely used to measure the M⋆ and SFR of galaxies because of its wide all-sky coverage and the fact that the near-IR traces the continuum emission of the majority of their stellar populations (SPs), and the mid/far-IR traces the dust emission powered by star-forming activity. This work uses an MCMC and a machine learning framework to explore the dependence of the IR emission of galaxies on their extinction, and the age of their SPs. It provides accurate and precise IR-photometry SFR and M⋆ calibrations that account for both effects and it quantifies the scatter of these and other IR-based calibrations. The calibrations for the SFR are using a combination of the WISE bands 1 and 3, or the JWST NIR-F200W and MIRI-F2100W. The M⋆ calibrations are based on the WISE band-1, or the JWST NIR-F200W, and the optical u−r or g−r colors. The proposed calibrations show robust estimations while minimizing the scatter and biases throughout a wide range of SFRs and stellar masses. The SFR calibration offers better results, especially in dust-free or passive galaxies where the contributions of old SPs or biases from the lack of dust are significant. Similarly, the M⋆ calibration yields significantly better results for dusty and high-SFR galaxies where dust emission can otherwise bias the estimations.
18. Name: Ms. Evgenia Koutsoumpou (National & Kapodistrian Univ. of Athens )
    Coauthors: Dasyra Kalliopi (National and Kapodistrian University of Athens)
    Type: Poster
    Title: Investigating the Impact of Black Hole Jets on the Interstellar Medium of IC 5063
    Abstract:
    We study massive outflows triggered by the accretion of matter towards the central supermassive black hole and their impact on the interstellar medium (ISM) of IC 5063. The wind outflows that have been detected in different phases of the gas surrounding IC 5063, render it ideal for our research. We use the astrochemical radiative transfer code, CLOUDY to model the atomic and molecular cloud areas of IC 5063 and examine the effects of cosmic rays and Far UV radiation on these clouds. Furthermore, we analyze the physical properties of the gas, such as density, temperature, and pressure by use of line diagnostics for optical spectrum emission lines and molecular emission lines, in order to gain a better understanding of the dynamical stability of the clouds. We also intend to include forthcoming observations from the James Webb Space Telescope into our work.
19. Name: Mr. Elias Kyritsis (IA-FORTH & Univ. of Crete)
    Coauthors: Zezas Andreas (Institute of Astrophysics -FORTH/ Physics Department - UoC)
    Haberl Frank (Max-Planck-Institut fuer Extraterrestrische Physik (MPE))
    Wilms Joern (Dr Karl Remeis Observatory)
    Sasaki Manami (Dr Karl Remeis Observatory)
    Basu-Zych Antara (University of Maryland)
    Vulic Neven (Goddard Space Flight Center (GSFC/NASA))
    Hornschemeier Ann (Goddard Space Flight Center (GSFC/NASA))
    Weber Phillipp (Dr Karl Remeis Observatory)
    Type: Oral
    Title: The first unbiased survey of star-forming galaxies with eROSITA: Scaling relations and a population of X-ray luminous starbursts
    Abstract:
    eROSITA, the first 0.2-8.0 keV all-sky X-ray survey, provides the first unbiased census of X-ray galaxies in the local Universe. We combine the first scan of eROSITA (eRASS:1) with the HECATE catalog of nearby galaxies (D ≤ 200Mpc) resulting in a sample of ∼20000 highly-secure star-forming galaxies. This allows us to investigate the Lx-SFR-M∗-Metallicity relation with less biased and larger sample than any other previous study. Our analysis reveals a population of star-forming galaxies with (Lx ≥ 1040 erg/s) and significant deviations from the standard scaling relations. By stacking the X-ray emission of the galaxies in (SFR,M∗,D) bins we find that: i) the average scaling relations resulting from this unbiased sample are still significantly higher than those currently used, ii) there is a strong anti-correlation between metallicity, age and X-ray luminosity. Based on these results we provide new calibration of the scaling relations, we quantify the scatter, and we investigate the nature of the most X-ray luminous galaxies which tend to be young low-metallicity starburst galaxies serving as analogs for high-z galaxies.
20. Name: Mr. Elias Kyritsis (IA-FORTH & Univ. of Crete)
    Coauthors: Kovlakas Konstantinos (Institute of Space Sciences (IEEC-CSIC))
    Kouroumpatzakis Konstantinos (Astronomical Institute Czech Academy of Sciences)
    Zezas Andreas (Institute of Astrophysics -FORTH/ Physics Department - UoC)
    Charalampos Daoutis (Institute of Astrophysics -FORTH/ Physics Department - UoC)
    Type: Poster
    Title: The Heraklion Extragalactic Catalogue (HECATE v2.0): A value-added catalogue and its applications
    Abstract:
    The Heraklion Extragalactic CATalogue is a reference catalog which contains all known nearby galaxies in the local Universe (D ≲ 200 Mpc). By incorporating and homogenizing data from HyperLEDA, NED, and infrared/optical surveys, the catalog provides a broad array of information such as positions, sizes, distances, morphological classifications, Star Formation Rate (SFR), stellar mass (M⋆), metallicity, and activity classifications. In the updated version (v2.0) of the catalogue, we increase the completeness of SFR, and M⋆ from ∼46% and ∼65%, to ∼76% and ∼90%, respectively. We use a combination of new multi-band calibrations and standard monochromatic mid-infrared diagnostics. In addition, we calculate the gas-phase metallicity of the galaxies, using the standard emission line-based calibrations, and the stellar mass-metallicity relation. As a result, the completeness of the metallicity is increased from ∼30% to ∼90%. Finally, by applying a new galactic activity classifier based on a machine learning model using mid-IR/optical colors, we increase the activity classification in the catalog from ∼30% to ∼60%. This catalog can have several astrophysical applications such as the identification of the hosts of gravitational-wave (GW) events, demographics of different source populations, and the connection between the X-ray luminosity of galaxies with their stellar population parameters using large statistical samples.
21. Name: Dr. Giorgos Leloudas (DTU Space)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Searching for the electromagnetic counterparts of gravitational waves
    Abstract:
    The simultaneous discovery of gravitational waves and electromagnetic radiation from the merger of two neutron stars in 17 August 2017 was a landmark event with deep implications in various scientific fields ranging from astrophysics, nuclear physics, relativity and cosmology. This achievement has not been repeated since, highlighting the challenges in responding to gravitational wave events in real time, and searching for faint candidate counterparts in large areas of the sky, like for a needle in a haystack. In this talk, I will present several ongoing efforts to repeat this breakthrough during the O4 observing run (start: May 2023). I will primarily focus on the large European ENGRAVE collaboration, presenting our goals, strategy, modus operandi and (if all goes well) example searches from the first month of O4 (or past cases from O3). I will also present BlackGEM, a new array of telescopes in Chile, on a special mission to hunt for the counterparts of gravitational waves.
22. Name: Mr. Nikolaos Mandarakas (IA-FORTH & Univ. of Crete)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Optical Polarimetry of nearby galaxies
    Abstract:
    It has been proposed that measuring the integrated optical polarization of galaxies affected by the cosmic shear can shrink significantly the uncertainties and provide constraints in the measured dark matter parameters. This is based on the following two arguments: 1) The position angle (or electric vector position angle - EVPA) of the integrated galactic polarization can be a proxy of the orientation of its major axis, prior to lensing. 2) Gravitational lensing does not alter the position angle of the polarization of light, contrary to the observed position angle of the galaxy. Therefore, if we measure the polarization angle and the position angle of lensed galaxies, we could directly infer the original position angle, before lensing, and thus, the rotation on the position angle produced by weak lensing. This would tremendously constrain the derived dark matter parameters. However, there are not enough observational evidence for the first argument. I have performed a 3-filter study of integrated optical polarization of nearby galaxies using the RoboPol instrument. I am going to present preliminary results, focusing on the correlation between EVPA and galactic orientation, as well as discuss the polarization levels observed in those galaxies and other potential correlations. Apart from the weak-lensing aspect, this kind of study can help understand what are the potential mechanisms that produce polarization in galaxies, and how morphological characteristics may affect it.
23. Name: Dr. Alexandros Maragkoudakis (NASA Ames Research Center)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Spectral variations among different scenarios of PAH processing or formation
    Abstract:
    Utilizing the NASA Ames PAH IR Spectroscopic Database, we examine the variations in the spectral characteristics and intensities of polycyclic aromatic hydrocarbons (PAHs) in two different scenarios of PAH processing (or formation): (1) small PAHs are being destroyed (or equivalently large PAHs are being formed, referred to as SPR case) and (2) large PAHs are being destroyed (or equivalently small PAHs are being formed, referred to as LPR case). The variation in the PAH band intensities as a function of the average number of carbon atoms ⟨Nc⟩ has the highest dynamic range in the SPR case, suggesting that smaller PAHs have higher impact on the PAH band strengths. PAH plateaus show overall declining emission with ⟨Nc⟩, and their higher dynamic range in the SPR case also suggests that smaller PAHs are mainly contributing to the plateau emission. The 7.7/(11.0 + 11.2) μm PAH band ratio presents the least amount of variance with the lowest dynamic range, rendering this ratio as the better choice for tracing PAH charge. The 3.3/(11.2 + 11.0) μm PAH band ratio is the only ratio that has both a monotonic variance and fully separated values among the SPR and LPR scenarios, highlighting its efficiency as PAH size tracer but also allowing the characterization of the dominant scenario of processing or formation in a given region or source. We present new PAH charge-size diagnostic diagrams, which can provide insights into the average, maximum, or minimum NC within astrophysical sources.
24. Name: Prof. Eileen Meyer (University of Maryland Baltimore County)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: The Unsolved Problem of Jets from Black Holes
    Abstract:
    One of the first things that astronomers usually determine for any new astrophysical source class is the emission process generating the radiation -- thermal, synchrotron, inverse Compton, Bremsstrahlung, etc. Identifying these processes is critical to using the EM observations to probe the physical environments of very distant objects. Despite the fact that jets from black holes were first understood to exist over 40 years ago, we are still in ignorance about many primary aspects of these systems -- including the radiation mechanism at high energies, the particle makeup of the jets, and how particles are accelerated, possibly to energies as high as 100 TeV and hundreds of kpc from the central engine. I will discuss how this mystery first really got going with the launch of Chandra in 1999, and show how high-resolution observations with observatories like Hubble and ALMA have continued to add pieces to the puzzle -- sometimes seemly solving a problem, and other times opening a new one. I will conclude with some perspectives on how new observatories, like the Chandra successors AXIS and Lynx, can help us solve these long-open questions in jet physics.
25. Name: Dr. Konstantinos Migkas (Leiden University)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Cross-calibration of eROSITA cluster temperatures with other X-ray telescopes
    Abstract:
    Galaxy cluster gas temperatures are a crucial part of cluster astrophysics and cosmology. They are an excellent proxy of total cluster mass, while they are also crucial for tests of cosmic isotropy with clusters. However, it has been shown that cluster temperature measurements can significantly vary between different X-ray telescopes. As those temperatures determine our view on galaxy cluster physics, mass estimates, and cosmological parameters, it is of utter importance to cross-calibrate X-ray instruments to avoid systematic biases. In this work, we present for the first time the cross-calibration of eROSITA against XMM-Newton and Chandra, using the first all-sky survey of eROSITA. We measured and compared core and core-excised temperatures of 51 common clusters between eROSITA and XMM-Newton and 127 clusters between eROSITA and Chandra, using the same spectral extraction regions. The comparison takes place in the full, soft, and hard X-ray bands, to better understand the origin of any discrepancies. Overall, significant deviations between the temperature measurements are observed across all instruments. For the full X-ray band and average-sided clusters, eROSITA measures 10% and 25% lower temperatures than XMM-Newton and Chandra respectively. The agreement is better for the soft band and/or galaxy groups and worse for the hard band and/or massive clusters. The results of this work will be essential for future cluster studies that wish to combine eROSITA data with other X-ray instruments.
26. Name: Dr. Christos Panagiotou (MIT)
    Coauthors: Kara Erin (MIT)
    Papadakis Iossif (University of Crete)
    Dovciak Michal (Astronomical Institute of the Academy of Science)
    Kammoun Elias (IRAP, Universite de Toulouse)
    Type: Oral
    Title: Reproducing the UV/optical variability of AGN
    Abstract:
    AGN continuum reverberation mapping is based on the broadly accepted idea that the X-ray corona irradiates the accretion disk, causing a delayed UV/optical response. However, in recent years, the results of several multi-wavelength studies pointed at a discrepancy between observations and the underlying theory, detecting low X-ray/UV correlations and larger than expected interband time lags. In order to further examine the assumption of disk X-ray reprocessing, we developed a physical model to reproduce the UV/optical power spectra of AGN assuming the thermal reprocessing of the X-rays in the disk. We apply our model to a sample of sources and we found that the observed power spectra and time lags can be well explained. Expanding on this result, we show that the low X-ray/UV correlation is, in fact, expected if the corona is dynamic. This is simply due to the underlying assumptions in the definition of the cross correlation function statistic. Our results suggest that X-ray disk reprocessing accounts for the full variability properties of the observed AGN disk emission, reconciling observations with theoretical predictions; while our model offers a novel way to probe the inner region of AGN.
27. Name: Dr. Christos Panagiotou (MIT)
    Coauthors: De Kishalay (MIT)
    Masterson Megan (MIT)
    Kara Erin (MIT)
    Calzadilla Michael (MIT)
    Eilers Anna-Christina (MIT)
    Frostig Danielle (MIT)
    Karambelkar Viraj (Caltech)
    Kasliwal Mansi (Caltech)
    Lourie Nathan (MIT)
    Meisner Aaron (NOIRLAB)
    Simcoe Robert (MIT)
    Stein Robert (Caltech)
    Zolkower Jeffry (Caltech)
    Type: Poster
    Title: A luminous dust-obscured Tidal Disruption Event candidate in a star forming galaxy at 42 Mpc
    Abstract:
    Tidal Disruption Events (TDEs) offer a unique opportunity to study the nature of dormant supermassive black holes and the physics of accretion. While the vast majority of TDEs to date has been identified by wide-field sky surveys in the optical and X-ray bands, recent studies indicate that a considerable fraction of TDEs may be dust obscured, and thus preferentially detected in the infrared (IR) wavebands. In this talk, I will present the discovery of a luminous mid-IR nuclear flare identified in a systematic transient search of archival images from the NEOWISE mid-IR survey. The transient event took place in the nearby galaxy NGC 7392, at a distance of around 42 Mpc; yet, no optical or X-ray flare was detected. We interpreted the transient as the nearest TDE detected in the last decade, which was missed at other wavelengths due to dust obscuration, hinting at the existence of a large population of TDEs that have been historically overlooked. Unlike most previously detected TDEs, the transient was discovered in a star forming galaxy, corroborating earlier suggestions that dust obscuration suppresses significantly the detection of TDEs in these environments. Moreover, ongoing analysis of the NEOWISE archive suggests that MIR-detected TDEs are very common, nearly doubling the number of detected TDEs in our cosmic backyard. Our results demonstrate that the study of IR-detected TDEs is critical in order to obtain a complete understanding of the physics of TDEs and to conclude whether TDEs occur preferentially in a particular class of galaxies.
28. Name: Mr. Evangelos-Dimitrios Paspaliaris (NOA & AUTh)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Star-forming early-type galaxies and quiescent late-type galaxies in the local Universe.
    Abstract:
    There is a general consensus that late-type galaxies undergo intense star-formation activity while early-type galaxies are mostly inactive. We question this general conception and investigate the existence of star-forming early-type galaxies and quiescent late-type galaxies in the local Universe. Our sample consists of 2,209 morphologically classified galaxies in the GAMA survey. The separation into star-forming and quiescent galaxies is based on the dominant ionising process, making use of established criteria based on the WHα width and the [NII/Hα] ratio. Performing a spectral energy distribution fitting analysis we derived the physical properties of these galaxies, such as the stellar mass, dust mass, star-formation rate, and we also estimated the unattenuated and the dust-absorbed stellar emission, for both the young (≤ 200 Myr) and old (> 200 Myr) stellar populations. In order to explore the differences from their “typical” counterparts, we compared their average spectral energy distributions, their physical and structural properties, as well as the density of their local environment. We found that about 47% of E/S0 galaxies in our sample show ongoing star-formation activity and 8% of late-type galaxies are quiescent. Moreover, the unattenuated shape of the average SED of the star-forming galaxies and the scaling relations of their physical properties are very similar, for all morphological types. Concerning their structural parameters, quiescent galaxies tend to show larger values of the Sérsic index and larger effective radii (compared to star-forming galaxies). Finally, we find that star-forming galaxies preferably reside in lower density environments compared to the quiescent ones, which exhibit a higher percentage of sources being members of groups.
29. Name: Dr. Panos Patsis (RCAAM, Academy of Athens)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: THE GAS INFLOW IN THE CENTRAL KILOPARSEC OF GALACTIC BARS
    Abstract:
    We discuss the gas inflow in barred spiral galaxies from the scale of kiloparsecs towards the central parsecs. It is now well known that the presence of bars in galactic potentials is crucial in triggering such inflow. However, the clear picture we have about the gas flow in bars ends at nuclear rings formed at an Inner Lindblad Resonance (ILR). In our study we investigate the flow in the zone inside the ILR by means of Lagrangian and Eulerian hydrodynamical models. High resolution models using the grid-based algorithm RAMSES, allow us to get a detailed picture of the gas dynamics down to about a few pc from the galaxy center. We explore the conditions under which the gas may reach the central regions of the galaxy feeding nuclear activity, as well as the properties of the model that block this process. We evaluate the role of the hydrodynamical properties of the gas in comparison with the role of the gravitational forces determined by the underlying potential at the central regions and we discuss the presence of trailing and leading nuclear spirals in the models.
30. Name: Prof. Giulia Rodighiero (University of Padova)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Constraints on early galaxy formation from the JWST 
    Abstract:
    In this presentation I will provide an update about the most recent findings on distant galaxies from the James Webb Space Telescope. One year after the release of the first public images, a wealth of high-redshift candidate sources have been reported, with striking spectroscopic confirmations beyond z~10. I will review the main implications from the literature about the assembly of early galaxies at Cosmic Dawn. I will also highlight some results from our group related to the presence of distant dusty objects currently unexplained by theoretical models.
31. Name: Mr. Charalampos Sinnis (National & Kapodistrian Univ. of Athens)
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Numerical investigation of instabilities in relativistic magnetized jets
    Abstract:
    One special trait of astrophysical jets is their enhanced structural stability, as their total length is several times their initial radii. Although we know from previous studies that instabilities develop along their flow, it is not fully understood how they affect the outflow properties. From linear stability analysis, a specific mode has emerged which shows really small growth timescales, comparable to the jet's radius light crossing time. Such small timescales indicate that this mode prevails over almost any other counterpart that have been studied in the literature so far, with a typical value for the timescales of the other modes to be at least ten times higher. This could mean that this "hyper--unstable" solution is capable of changing the initial jet state, even before the rest of the solutions have started to develop. Hence, the study of the non--linear stages of this special mode is imperative. In order to do so, we present relativistic MHD numerical simulations of unperturbed magnetized jet configurations which are able to trigger the "hyper--unstable" solution. Most notably, we observe the formation of a spine--sheath type jet accompanied by vortices that propagate for large distances along the jet. It is really important that we observe the transformation of the initial state of the jet to a new modified configuration which seems to be smooth and stable without any sign of turbulence.
32. Name: Dr. Georgios Vernardos (City University of New York (CUNY))
    Coauthors: No coauthors were included.
    Type: Oral
    Title: Gravitational lensing in the era of big surveys: current status and challenges
    Abstract:
    Gravitational lensing of galaxies is a field with major and timely scientific applications: measuring cosmological parameters, testing dark matter and galaxy evolution models, measuring the IMF at cosmological distances, and zooming-in on the heart of quasars, to name a few. The rate of discovery of new galaxy scale lenses has been almost exponential in the last few years. All-sky surveys such as Euclid and LSST – both starting operations in 2023 - are expected not only to increase the sample size from hundreds to tens of thousands but also to provide pristine high-quality data to perform ground-breaking measurements with. This will come at the cost of extending current image and time-series processing techniques to handle this surge of data and understanding their systematics. Inevitably, Artificial Intelligence is a promising path to achieve this. In this presentation, I will review the current state of the field of galaxy-galaxy lensing from discovery to measurement of target properties. Then, I will present the challenges that need to be met in order to maximize the scientific outcomes in the context of Euclid, LSST and beyond.
33. Name: Prof. Andreas Zezas (Univ. of Crete & IA-FORTH)
    Coauthors: Zhang William (NASA GSFC)
    Hornschemeier Ann (NASA GSFC)
    Hodges-Kluck Edmund ( NASA GSFC)
    STAR-X Team ()
    Type: Oral
    Title: The STAR-X mission: the next frontier in wide-area and monitoring surveys
    Abstract:
    The Survey and Time-domain Astrophysics Research eXplorer (STAR-X) is a MIDEX-class mission selected for a NASA Phase-A study. It features a combination of wide-field, sensitive, high spatial resolution X-ray and UV telescopes, and the ability for cadenced surveys and fast response observations. I will present the capabilities and science goals of STAR-X, focusing on the wide-area surveys and time-domain observations. The wide area surveys will give a sensitive census of AGN and galaxy populations, 10-50 times deeper than the eROSITA survey, reaching the sensitivity of the Chandra Deep Field surveys. STAR-X will also provide new insights in our understanding of transient and explosive events, such as tidal disruption events, supernovae, and compact object mergers, by providing measurements of their X-ray and UV emission at very early times. I will discuss the scientific leaps in our understanding of these phenomena and the cosmological evolution of populations of AGN and galaxies that are enabled by STAR-X.