Journal Club


"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.