A metal-rich elongated structure in the core of the group NGC 4325

¹ Núcleo de Astrofísica Teórica, Universidade Cruzeiro do Sul, Rua Galvão Bueno 868, Liberdade, CEP: 01506-000 São Paulo SP Brazil
e-mail: tflagana@gmail.com
² Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ Divisão de Astrofísica, INPE/MCT, 12227-000 São José dos Campos/SP, Brazil

Received: 15 August 2014
Accepted: 6 October 2014

Abstract

Aims. Based on XMM-Newton, Chandra, and optical DR10-SDSS data, we investigate the metal enrichment history of the group NGC 4325 (z = 0.026). To complete the analysis we used chemical evolution models and studied the optical spectrum of the central dominant galaxy through its stellar population analysis and emission line diagnostics to analyse its role in the metal enrichment of the intra-group medium.

Methods. We used X-ray 2D spectrally resolved maps to resolve structure in temperature and metallicity. We also derived gas and total masses within r₂₅₀₀ and r₅₀₀ assuming hydrostatic equilibrium and spherical symmetry. To perform stellar population analysis we applied the spectral fitting technique with STARLIGHT to the optical spectrum of the central galaxy. We simulated the chemical evolution of the central galaxy.

Results. While the temperature, pseudo-pressure, and pseudo-entropy maps showed no inhomogeneities, the spatial distribution of the metallicity shows a filamentary structure in the core of this group, which is spatially correlated with the central galaxy, suggesting a connection between the two. The analysis of the optical spectrum of the central galaxy showed no contribution by any recent AGN activity. Using the star formation history as input to chemical evolution models, we predicted the iron and oxygen mass released by supernovae (SNe) winds in the central galaxy up to the present time.

Conclusions. Comparing the predicted amount of mass released by the NGC 4325 galaxy to the ones derived through X-ray analysis we conclude that the winds from the central galaxy alone play a minor role in the IGM metal enrichment of this group inside r₂₅₀₀. The SNe winds are responsible for no more than 3% of it and of the iron mass and 21% of the oxygen mass enclosed within r₂₅₀₀. Our results suggest that oxygen has been produced in the early stages of the group formation, becoming well mixed and leading to an almost flat profile. Instead, the iron distribution is centrally peaked, indicating that this element is still being added to the IGM specifically in the core by the SNIa. A possible scenario to explain the elongated metal-rich structure in the core of the NGC 4325 is a past AGN activity, in which our results suggest an episode older than ~10⁷−10⁸ yrs and younger than 5 × 10⁸. Through the overall distribution of the galaxies, we found no signs of recent merger in the group centre that could explain the metal-rich structure.