Journal Club


"Cosmology intertwined: A review of the particle physics, astrophysics, and cosmology associated with the cosmological tensions and anomalies"

Chandra Shekhar Saraf (NCAC, Warsaw)

The standard Λ Cold Dark Matter (ΛCDM) cosmological model provides a good description of a wide range of astrophysical and cosmological data. However, there are a few big open questions that make the standard model look like an approximation to a more realistic scenario yet to be found. In this paper, we list a few important goals that need to be addressed in the next decade, taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant H₀, the σ₈-S₈ tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the 5.0 σ tension between the Planck CMB estimate of the Hubble constant H₀ and the SH0ES collaboration measurements. After showing the H₀ evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the Planck CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density Ωₘ, and the amplitude or rate of the growth of structure (σ₈ , fσ₈). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the H₀-S₈ tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals. Finally, we give an overview of upgraded experiments and next-generation space missions and facilities on Earth that will be of crucial importance to address all these open questions.

Abdalla, E. et al., Journal of High Energy Astrophysics (2022)


"Spin-down driven neutron star to quark star conversion"

Prasad R (IISER Bhopal)

The existence of quark stars is an open problem in astrophysics, and their formation is possible in several astrophysical scenarios via the quark-hadron phase transition. We addressed the spin-down induced phase transition scenario, wherein magnetic braking drives neutron stars from their birth (Keplerian rotation) to later stages of life (slow spin). The central density is found to rise during the slowing down stages, and on reaching a critical phase transition density, the neutron star transits to a hybrid star branch, and a quark core is seeded. The further slowing down results in the growth of the quark core. We computed the mass and size of the quark core during different stages of evolutionary history. The phase transition onset leads to an anomalous change in the magnetic braking index. Also, it can excite the star's f-mode oscillations, leading to burst-type gravitational wave signals in the range of present detectors. The other emissions could be neutrino bursts and GRBs. Detection of these signals and their sky localization may help in finding the quark/hybrid stars formed via phase transition events. This talk is based on:


"Evidence of structural discontinuities in the inner core of red-giant stars"

Christian Eze (NCAC, Warsaw)

Red giants are stars in the late stages of stellar evolution. Because they have exhausted the supply of hydrogen in their core, they burn the hydrogen in the surrounding shell . Once the helium in the core starts fusing, the star enters the clump phase, which is identified as a striking feature in the color-magnitude diagram. Since clump stars share similar observational properties, they are heavily used in astrophysical studies, as probes of distance, extinction through the galaxy, galaxy density, and stellar chemical evolution. In this work, we perform the detailed observational characterization of the deepest layers of clump stars using asteroseismic data from Kepler. We find evidence for large core structural discontinuities in about 6.7% of the stars in our sample, implying that the region of mixing beyond the convective core boundary has a radiative thermal stratification. These stars are otherwise similar to the remaining stars in our sample, which may indicate that the building of the discontinuities is an intermittent phenomenon.

Vrard, Mathieu et al. "Evidence of structural discontinuities in the inner core of red-giant stars". Nature Communications 13. (2022)


"Multiple stellar population in globular clusters"

Gergely Hajdu (NCAC, Warsaw)

We use images collected with the near-infrared camera (NIRCam) on board the James Webb Space Telescope and with the Hubble Space Telescope (HST) to investigate multiple populations at the bottom of the main sequence (MS) of 47 Tucanae. The F115W vs. F115W-F322W2 CMD from NIRCam shows that, below the knee, the MS stars span a wide color range, where the majority of M-dwarfs exhibit blue colors, and a tail of stars are distributed toward the red. A similar pattern is observed from the F160W vs. F110W-F160W CMD from HST, and multiple populations of M-dwarfs are also visible in the optical F606W vs. F606W-F814W CMD. The NIRCam CMD shows a poorly-populated sequence of faint MS stars that we tentatively associate with a population of very low-mass stars. We introduce a chromosome map of M-dwarfs that reveals an extended first population and three main groups of second-population stars. By combining isochrones and synthetic spectra with appropriate chemical composition, we simulate colors and magnitudes of different stellar populations in the NIRCam filters (at metallicities [Fe/H]=-1.5 and [Fe/H]=-0.75) and identify the photometric bands that provide the most efficient diagrams to investigate the multiple populations in globular clusters. Models are compared with the observed CMDs of 47 Tucanae to constrain M-dwarfs' chemical composition. Our analysis suggests that the oxygen range needed to reproduce the colors of first- and second-population M-dwarfs is similar to that inferred from spectroscopy of red giants, challenging the proposal that the chemical variations are due to mass transfer phenomena in proto-clusters.


"An Upper Palaeolithic Proto-writing System and Phenological Calendar"

Fatima Kayanikhoo (NCAC, Warsaw)

In at least 400 European caves such as Lascaux, Chauvet and Altamira, Upper Palaeolithic Homo sapiens groups drew, painted and engraved non-figurative signs from at least ∼42,000 BP and figurative images (notably animals) from at least 37,000 BP. Since their discovery ∼150 years ago, the purpose or meaning of European Upper Palaeolithic non-figurative signs has eluded researchers. Despite this, specialists assume that they were notational in some way. Using a database of images spanning the European Upper Palaeolithic, we suggest how three of the most frequently occurring signs—the line <|>, the dot <•>, and the —functioned as units of communication. We demonstrate that when found in close association with images of animals the line <|> and dot <•> constitute numbers denoting months, and form constituent parts of a local phenological/meteorological calendar beginning in spring and recording time from this point in lunar months. We also demonstrate that the sign, one of the most frequently occurring signs in Palaeolithic non-figurative art, has the meaning . The position of the within a sequence of marks denotes month of parturition, an ordinal representation of number in contrast to the cardinal representation used in tallies. Our data indicate that the purpose of this system of associating animals with calendar information was to record and convey seasonal behavioural information about specific prey taxa in the geographical regions of concern. We suggest a specific way in which the pairing of numbers with animal subjects constituted a complete unit of meaning—a notational system combined with its subject—that provides us with a specific insight into what one set of notational marks means. It gives us our first specific reading of European Upper Palaeolithic communication, the first known writing in the history of Homo sapiens.

20000-year Ice Age drawings mystery
An Upper Palaeolithic Proto-writing System and Phenological Calendar

20000-year Ice Age drawings mystery
An Upper Palaeolithic Proto-writing System and Phenological Calendar


"Perpendicular arms, triangle or circle: the new shapes of gravitational wave detectors"

Lami Suleiman (NCAC, Warsaw)

The third generation of Gravitational Wave detectors includes the space based triangular detector LISA and the two ground based detectors Cosmic Explorer and Einstein telescope. If Cosmic Explorer is a brute force improvement on the LIGO/Virgo perpendicular arms configuration, Einstein telescope is exploring much more “exotic” features for signal improvement. For this Journal Club, I will first discuss the underground and triangle shape configurations in discussion for Einstein Telescope. Then, I will present two papers that propose the detection of Gravitational Waves in LHC like particles accelerators.

Articles for the discussion:

  • “Detection of gravitational waves in circular particle accelerators” Phys. Rev. D 102, 122006 (2020) Rao Suvrat, Brüggen Marcus, Liske Jochen
  • “Erratum: Detection of gravitational waves in circular particle accelerators” Phys. Rev. D 102, 122006 (2020)
  • “Detection of gravitational waves in circular particle accelerators II. Response analysis and parameter estimation using synthetic data” eprint arXiv:2301.08331


"Neutron star planets: Atmospheric processes and irradiation"

Amedeo Romagnolo (NCAC, Warsaw)

Of the roughly 3000 neutron stars known, only a handful have sub-stellar companions. The most famous of these are the low-mass planets around the millisecond pulsar B1257+12. New evidence indicates that observational biases could still hide a wide variety of planetary systems around most neutron stars. We consider the environment and physical processes relevant to neutron star planets, in particular the effect of X-ray irradiation and the relativistic pulsar wind on the planetary atmosphere. We discuss the survival time of planet atmospheres and the planetary surface conditions around different classes of neutron stars, and define a neutron star habitable zone based on the presence of liquid water and retention of an atmosphere. Depending on as-yet poorly constrained aspects of the pulsar wind, both Super-Earths around B1257+12 could lie within its habitable zone.

Patruno, A., & Kama, M. (2017) Astronomy & Astrophysics, Volume 608



"Reading M87's DNA: A Double Helix Revealing a Large-scale Helical Magnetic Field"

Krzysztof Nalewajko (NCAC, Warsaw)

We present unprecedented high-fidelity radio images of the M87 jet. We analyzed Jansky Very Large Array broadband full-polarization radio data from 4 to 18 GHz. The observations were taken with the most extended configuration (A configuration), which allows the study of the emission of the jet up to kiloparsec scales with a linear resolution of ~10 pc. The high sensitivity and resolution of our data allow us to resolve the jet width. We confirm a double-helix morphology of the jet material between ~300 pc and ~1 kpc. We found a gradient of the polarization degree with a minimum at the projected axis and maxima at the jet edges and a gradient in the Faraday depth with opposite signs at the jet edges. We also found that the behavior of the polarization properties along the wide range of frequencies is consistent with internal Faraday depolarization. All of these characteristics strongly support the presence of a helical magnetic field in the M87 jet up to 1 kpc from the central black hole, although the jet is most likely particle-dominated at these large scales. Therefore, we propose a plausible scenario in which the helical configuration of the magnetic field has been maintained to large scales thanks to the presence of Kelvin-Helmholtz instabilities.

Pasetto, A., et al. (2021). ApJL, 923(1)


"Tests of Hierarchical Accretion in the Virgo Cluster"

Aikaterini-Niovi Triantafyllaki (Tartu Observatory, Estonia)

Large concentrations of mass are now understood to be the products of a Hubble time’s worth of merging and accretion. This history is preserved in the outer regions of galaxies' halos, where the dynamical scales are longer. This makes it possible to preserve fossil records of these events in the form of longlasting substructures imprinted in the physical properties of their stellar populations. In practice, this information is often hidden at surface brightness values below the sky. Planetary Nebulae (PNe) can solve this observational challenge: owing to their strong [OIII] emission line— they are easily detected— PNe offer a unique tool to investigate low surface brightness regions and gather detailed observational proof of the structures' evolution. In order to search for hierarchical processes, I analysed data from the Virgo Planetary Nebula Survey (VPNS) with the aim to study physical properties of its PN population and how they relate to the cluster properties as well as tracing variations in metallicity as a consequence of the presence of accretion events. As a result, the different values of the PN α-parameter are consistent with a gradient from more metal rich stars in the galaxies' centres towards more metal poor populations of stars at large radii until the galaxy stellar population mixes with the IC component, consistent with a late built-up of the galaxies' halos. By comparing the α-parameter values of galaxies subject to environmental effects with those in close regions of intracluster (IC) regions, this work has shown that the Virgo intracluster light (ICL) is built up over time as a consequence of the tidal forces acting on both late- and small early-type galaxies. This, in turn, causes the ICL to be characterised by different metallicity values, especially in the north-west region where the IC component is highly unrelaxed. With this work I also provided evidence for an unknown accretion event in the halo of the central galaxy, M87, that has caused an important modification of the metallicity of its outer stellar populations.

Aikaterini-Niovi Triantafyllaki & Alessia Longobardi (in preparation)


"A candidate runaway supermassive black hole identified by shocks and star formation in its wake"

Gergely Hajdu (NCAC, Warsaw)

The interaction of a runaway supermassive black hole (SMBH) with the circumgalactic medium (CGM) can lead to the formation of a wake of shocked gas and young stars behind it. Here we report the serendipitous discovery of an extremely narrow linear feature in HST/ACS images that may be an example of such a wake. The feature extends 62 kpc from the nucleus of a compact star-forming galaxy at z=0.964. Keck LRIS spectra show that the [OIII]/Hβ ratio varies from ~1 to ~10 along the feature, indicating a mixture of star formation and fast shocks. The feature terminates in a bright [OIII] knot with a luminosity of 1.9x1041 ergs/s. The stellar continuum colors vary along the feature, and are well-fit by a simple model that has a monotonically increasing age with distance from the tip. The line ratios, colors, and the overall morphology are consistent with an ejected SMBH moving through the CGM at high speed while triggering star formation. The best-fit time since ejection is ~39 Myr and the implied velocity is v~1600 km/s. The feature is not perfectly straight in the HST images, and we show that the amplitude of the observed spatial variations is consistent with the runaway SMBH interpretation. Opposite the primary wake is a fainter and shorter feature, marginally detected in [OIII] and the rest-frame far-ultraviolet. This feature may be shocked gas behind a binary SMBH that was ejected at the same time as the SMBH that produced the primary wake.

van Dokkum, P. et al., eprint arXiv:2302.04888 (2023)


"Exploring the correlations between galaxy properties and environment in the cosmic web using marked statistics"

Unnikrishnan Sureshkumar (University of the Witwatersrand, Johannesburg, South Africa)

Galaxies live in dark matter haloes and hence the galaxy properties are majorly defined by the properties of the haloes. Thus the environmental dependence of dark matter halo properties prompts a correlation between galaxy properties and the environment. In this talk, I will discuss the results from our works (arXiv:2102.04177 and arXiv:2201.10480) that explored how luminosities in optical to mid-infrared bands, stellar mass, and star formation rate are correlated with the environment. We use a set of stellar mass-selected and 3.4 μm luminosity-selected galaxies from the Galaxy and Mass Assembly (GAMA) survey. We utilize the galaxy two-point correlation functions and marked correlation functions to investigate the environmental correlations. I will also discuss the impact of various selection effects on the galaxy clustering measurements. Additionally, I will show the results of our ongoing work with data from simulations.


"Reproducing the iron emission of type 1 AGN"

Dragana Ilic (University of Belgrade)

"Changing looks of the nuclei of Seyfert galaxies NGC 1566, NGC 3516 and NGC 2617"

Victor Oknyansky (University of Haifa)