August 19, 2015

Bubbles and Filaments at the Heart of the Milky Way

From the analysis of all available data collected by the satellite XMM-Newton in the range of the X-rays, an international team has established the most detailed map of the hot gas of the central region of the Galaxy.

Bubbles, lobes and filaments are listed in this study, some for the first time. This gas, with a temperature of several millions of degrees, traces episodes of intense activity in this complex region of the sky. Supernovae, stellar winds from massive stars and in particular outbursts from the central super-massif black hole of the Galaxy, Sgr A*, are mentioned in order to explain the diffuse X-ray emission detected by the European observatory XMM-Newton.

This work, carried out with the contribution of researchers of the Atroparticule et Cosmologie laboratory of Paris, of the Service d’Astrophysique of CEA-Irfu of Saclay and of the CSNSM of Orsay, with the support of CNES, has led to a compilation of an atlas of diffuse X-ray sources of the central regions of the Galaxy. This database will be extremely useful for future complementary studies, in the X-ray range and at other wavelengths. These results are published in the MNRAS journal and are the subject of an ESA press release.

Credits: ESA.

The central regions of the Milky Way represent one of the richest laboratory for modern astrophysics due of the large variety of the objects present there, and because of this they have been surveyed many time in particular in the X-ray domain between 0.1 and 10 keV. This energy range is particularly adapted to trace the presence of an essential, still little understood, component of the diffuse emission, the hot gas.  With the purpose of drawing a detailed map of the diffuse structures of the region at different scales and in particular of its hot gas, the scientists have used all the useful data collected with the XMM-Newton satellite since 1999. They have exploited in particular a complete survey of the zone performed in 2012 within the framework of an XMM large observation project.

Between the most interesting structures that have been identified, the scientists have characterized some super-bubbles of several tens of light-years in size, cavities created by the successive explosions of stars over periods of several tens of thousands of years.

These measurements have allowed the researchers to evaluate the average rate of supernova explosion in the central regions and therefore compute the star formation rate, an essential parameters to describe the general activity of the region.

Carte du gaz chaud des regions centrales de la galaxie.png
Carte du gaz chaud des régions centrales de la Galaxie obtenu par l’amalgame de plus d’une centaine d’observations XMM-Newton entre 0.5 et 12 keV. Elle couvre une région de 1000 années-lumière en longitude galactique (axe horizontal) et de 500 années-lumière en latitude galactique (axe vertical). Le temps d’exposition sur chaque zone est typiquement de 200 ksec excepté dans une zone de 125 années-lumière autour du centre galactique où il atteint 1.5 million de secondes. Le code de couleur indique l’énergie des photons X détectés: rouge (0.5-2 keV), vert (2- 4.5 keV) et bleu (4.5-12 keV). De très nombreuses structures diffuses sont visibles sur cette carte ainsi qu’un ensemble de sources ponctuelles très intenses, principalement des étoiles compactes (trous noirs ou étoiles à neutrons) en couple avec une étoile normale.

Credits: ESA/XMM-Newton/G. Ponti et al. 2015.

The derived images also show two oval structures, quasi-symmetrical lobes located on both sides of Sgr A*. These two lobes have typical size of ten light years and their intersection at the position of Sgr A* suggests that they were originated there. The strong concentration in this region of young stars and of their powerful stellar winds could explain the observed structures but episodes of activity linked to the central super-massif black hole are also not excluded since traces of Sgr A* activity in the past have been discovered recently by the same research team.

Several other weak structures have been discovered during this very deep X-ray survey of the Galactic Center and in particular a thin filament of 15 light years. At larger scales, a thin V-structure located north of the central part could make the base of a component much larger (several thousands of light years) discovered by the gamma-ray Fermi satellite and called “Fermi-bubbles”. 

Carte du gaz chaud des regions centrales de la galaxie plasma.png
Carte similaire à la précédente, obtenue à l’énergie de raies d’émission caractéristiques du plasma chaud. L’insert à gauche est un zoom de la région centrale marqué par un carré blanc, sur lequel sont indiquées les structures diffuses principales. Les 2 ellipses rouges sont des lobes situés au nord et au sud du trou noir central super-massif Sgr A*. Les ellipses en jaune indiquent les structures associées à des super-bulles. Les traits verts indiquent une large structure qui évoque une expansion à grande échelle vers le nord galactique.

Credits: ESA/XMM-Newton/G. Ponti et al. 2015.

This work has allowed the scientists to establish a detailed catalogue of diffuse X-ray structures of the central region of the Galaxy. Now available to the science community it constitutes a precious tool for future studies, in the X-ray domain but also at other wavelengths, that will be carried out in order to better understand the origin of the different components of the diffuse emission of this very complex region of the sky.

article references

The XMM-Newton view of the central degrees of the Milky Way, G. Ponti, M. R. Morris, R. Terrier, F. Haberl, R. Sturm, M. Clavel, S. Soldi, A. Goldwurm, P. Predehl, K. Nandra, G. Belanger, R. S. Warwick and V. Tatischeff, MNRAS, 19/8/2015.