Flows of hot plasma connecting the Milky Way centre to the corona, halo and beyond

Project Information

Hot Milk

Grant agreement ID: 865637

DOI

10.3030/865637

EC signature date 19 December 2019

Start date 1 March 2020

End date 28 February 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 989 375,00

EU contribution

€ 1 989 375,00

Coordinated by

ISTITUTO NAZIONALE DI ASTROFISICA
Italy

Periodic Reporting for period 4 - Hot Milk (Flows of hot plasma connecting the Milky Way centre to the corona, halo and beyond)

Reporting period: 2024-09-01 to 2026-02-28

Summary of the context and overall objectives of the project

The modern theory of galaxy formation dictates that a halo of hot plasma should be present around all Milky Way-like galaxies, forming the so-called hot circum-galactic medium (CGM)
Indeed, we expect that in the early Universe, due to the collapse of the dark matter halo, plasma within the gravitational potential of galaxies (hundreds of kiloparsecs) has been shock heated to temperatures of millions of Kelvin.
Because of its long cooling time, we expect that such plasma is still hot, therefore emitting in the X-ray band and containing the majority of the normal (baryonic) mass of the galaxy.
The theory predicts that galaxies should grow through the slow re-condensation of the CGM, with a slow flow of matter from the CGM becoming the building blocks for galaxy growth.
In opposition, recent progress in the study of active galactic nuclei and starburst galaxies have demonstrated that outflows from the centers and discs of these galaxies can power outflows of hot plasma which can replenish the CGM with energy, entropy, metals and particles.
Therefore, galaxy evolution is deeply linked to this cycle of baryons between the center and disc of galaxies with their CGM.

Because of the lack of adequate X-ray instruments, despite its fundamental role in galaxy evolution theory, the observational evidence of hot CGM around galaxies was very scarce.
In 2019 the eROSITA telescope onboard the SRG space observatory opened its X-ray eyes and started an unprecedented all sky survey.
By using the eROSITA data of the all sky survey, we aim to provide a characterisation of the hot plasma around the Milky Way, to characterise the hot CGM around the Magellanic Clouds (the closest satellite galaxies of the Milky Way), as well as to measure the average properties of the hot CGM around stacks of galaxies, in the nearby Universe.
This will allow us to place solid observational constraints on the properties of the hot CGM, therefore to check whether the way we think galaxies form and grow-over-cosmic-times is correct or not.

During the first part of the Hot Milk project, we have contributed to the creation of the first eROSITA image of the entire X-ray Universe, which we have already released to the public.
So far, the survey has accumulated four complete all sky surveys.
These eROSITA all sky images represent more than one order of magnitude improvement in sensitivity and more than a factor of 20 improvement in energy resolution in the soft X-ray band (0.3-2 keV), compared with the previous all sky survey (ROSAT).
Additionally, eROSITA provides the first-ever true imaging survey in the hard X-ray band (2-8 keV).

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

While producing these all sky maps, we have discovered two gigantic features emitting X-ray radiation, which we called the eROSITA bubbles (Predehl et al. 2020).
These features can be clearly observed in the eROSITA maps.
They appear as two bubbles originating from the Galactic center, with an extension of ~30 degrees on the Galactic plane and an extent of ~80-85 degrees in Galactic latitudes.
We have estimated that the bubbles are inflated by the outflow from the Galactic center and that they extend to about 15 kiloparsec above the disc.
Therefore, they possess a size comparable to the one of the entire galaxy.
This is clear evidence that they must be key players within the Milky Way echo-system.
Some were surprised by the discovery of these gigantic features.
As described in the Hot Milk project, supported by ERC, we had predicted that an X-ray coverage of the Galactic outflow would lead to exciting scientific results.
However, I have to admit that I was also surprised to observe that the thermal energy inside the eROSITA bubbles is about ten times larger than predicted.
This is remarkable, because it demonstrates that activity at the Galactic center can have an influence on the CGM even stronger than previously thought, with significant implications for the growth of the Milky Way.

During the Hot Milk project, we have also studied the base of the Galactic outflow, on a scale of few hundred parsecs from the Milky Way center, where the Galactic center chimneys are located (Ponti et al. 2019; Heywood et al. 2019).
We discovered that the X-ray, radio, and infrared emissions are deeply interconnected, affecting one another and forming coherent features on scales of hundreds of parsecs, therefore indicating a common physical link associated with the GC outflow (Ponti et al. 2021).
These multi-wavelength observations corroborate the idea that the chimneys represent the channel connecting the quasi-continuous, but intermittent, activity at the GC with the base of the Galactic outflow.
In particular, the prominent edges and shocks observed in the radio and mid-infrared bands testify to the most powerful, more recent outflows from the central parsecs of the Milky Way.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

To probe the physical properties of the hot CGM towards the outer Galaxy, we have also studied the soft X-ray emission along a line of sight away from the Galactic outflow and the Galactic disc (Ponti+2022a,b).
We first separated the different contributions to the soft X-ray background into distinct components.
These are, in order of distance from us: solar wind charge exchange (which is confined within the Heliosphere <<1pc); local hot bubble (~200pc); Galactic corona (<10kpc); CGM (~200kpc); extragalactic cosmic X-ray background with possible contribution from the warm hot intergalactic medium (Ponti+2022a,b).
Then, we have measured the properties of the hot CGM (i.e. its temperature, emission measure, metallicity, etc.), finding a surprisingly low metallicity.
Additionally, we have found further evidence for the presence of a hotter (kT~0.7keV) thermal component, which we associate to the emission from the Galactic corona (with a significant contribution from faint stars).

We are now constraining the physical properties of the X-ray emitting plasma associated with each of the Galactic components, with the aim to constrain their 3-dimensional geometry.
We aim at developing a simplified model of the density distribution of the hot plasma composing the CGM, the Galactic corona and the Galactic outflow.
This will then provide us with a high quality and high resolution template of the hot plasma within Milky Way-like galaxy, which can be used to understand galaxies in the nearby Universe.

Additionally, working on a relatively small survey accumulated during the performance-verification phase, we have demonstrated that, by stacking tens of thousands of galaxies, we can place constraints on the average CGM properties around galaxies (Comparat+2022). This is very relevant to test predictions of galaxy evolution theory and to check whether the CGM of the Milky Way is atypical or not.

Main panel: The X-ray universe a seen by the eROSITA X-ray telescope. Left panel: Zoom in with XMM.