Non-Thermal Insights on Mass and Energy Flows Through the Galactic Centre and into the Fermi Bubbles

📝 Abstract

We construct a simple model of the star-formation- (and resultant supernova-) driven mass and energy flows through the inner ~200 pc (in diameter) of the Galaxy. Our modelling is constrained, in particular, by the non-thermal radio continuum and {\gamma}-ray signals detected from the region. The modelling points to a current star-formation rate of 0.04 - 0.12 M\msun/year at 2{\sigma} confidence within the region with best-fit value in the range 0.08 - 0.12 M\msun/year which - if sustained over 10 Gyr - would fill out the ~ 10^9 M\msun stellar population of the nuclear bulge. Mass is being accreted on to the Galactic centre (GC) region at a rate ~0.3M\msun/year. The region’s star-formation activity drives an outflow of plasma, cosmic rays, and entrained, cooler gas. Neither the plasma nor the entrained gas reaches the gravitational escape speed, however, and all this material fountains back on to the inner Galaxy. The system we model can naturally account for the recently-observed ~> 10^6 ‘halo’ of molecular gas surrounding the Central Molecular Zone out to 100-200 pc heights. The injection of cooler, high-metallicity material into the Galactic halo above the GC may catalyse the subsequent cooling and condensation of hot plasma out of this region and explain the presence of relatively pristine, nuclear-unprocessed gas in the GC. The plasma outflow from the GC reaches a height of a few kpc and is compellingly related to the recently-discovered Fermi Bubbles. Our modelling demonstrates that ~ 10^9 M\msun of hot gas is processed through the GC over 10 Gyr. We speculate that the continual star-formation in the GC over the age of the Milky Way has kept the SMBH in a quiescent state thus preventing it from significantly heating the coronal gas, allowing for the continual accretion of gas on to the disk and the sustenance of star formation on much wider scales in the Galaxy [abridged].

💡 Analysis

We construct a simple model of the star-formation- (and resultant supernova-) driven mass and energy flows through the inner ~200 pc (in diameter) of the Galaxy. Our modelling is constrained, in particular, by the non-thermal radio continuum and {\gamma}-ray signals detected from the region. The modelling points to a current star-formation rate of 0.04 - 0.12 M\msun/year at 2{\sigma} confidence within the region with best-fit value in the range 0.08 - 0.12 M\msun/year which - if sustained over 10 Gyr - would fill out the ~ 10^9 M\msun stellar population of the nuclear bulge. Mass is being accreted on to the Galactic centre (GC) region at a rate ~0.3M\msun/year. The region’s star-formation activity drives an outflow of plasma, cosmic rays, and entrained, cooler gas. Neither the plasma nor the entrained gas reaches the gravitational escape speed, however, and all this material fountains back on to the inner Galaxy. The system we model can naturally account for the recently-observed ~> 10^6 ‘halo’ of molecular gas surrounding the Central Molecular Zone out to 100-200 pc heights. The injection of cooler, high-metallicity material into the Galactic halo above the GC may catalyse the subsequent cooling and condensation of hot plasma out of this region and explain the presence of relatively pristine, nuclear-unprocessed gas in the GC. The plasma outflow from the GC reaches a height of a few kpc and is compellingly related to the recently-discovered Fermi Bubbles. Our modelling demonstrates that ~ 10^9 M\msun of hot gas is processed through the GC over 10 Gyr. We speculate that the continual star-formation in the GC over the age of the Milky Way has kept the SMBH in a quiescent state thus preventing it from significantly heating the coronal gas, allowing for the continual accretion of gas on to the disk and the sustenance of star formation on much wider scales in the Galaxy [abridged].

📄 Content

Mon. Not. R. Astron. Soc. 000, 1–?? (2011) Printed 9 November 2018 (MN LATEX style file v2.2) Non-thermal Insights on Mass and Energy Flows Through the Galactic Centre and into the Fermi Bubbles R. M. Crocker1⋆ 1Max-Planck-Institut f¨ur Kernphsik, P.O. Box 103980 Heidelberg, Germany Accepted XXX. Received XXX; in original form XXX ABSTRACT We construct a simple model of the star-formation- (and resultant supernova-) driven mass and energy flows through the inner ∼200 pc (in diameter) of the Galaxy. Our modelling is constrained, in particular, by the non-thermal radio continuum and γ-ray signals detected from the region. The modelling points to a current star-formation rate of 0.04 −0.12 M⊙/year at 2σ confidence within the region with best-fit value in the range 0.08 −0.12 M⊙/year which – if sustained over 10 Gyr – would fill out the ∼109 M⊙stellar population of the nuclear bulge. Mass is being accreted on to the Galactic centre (GC) region at a rate ˙MIN ∼0.3 M⊙/year. The region’s star-formation activity drives an outflow of plasma, cosmic rays, and entrained, cooler gas. Neither the plasma nor the entrained gas reaches the gravitational escape speed, however, and all this material fountains back on to the inner Galaxy. The system we model can naturally account for the recently-observed

∼106 M⊙‘halo’ of molecular gas surrounding the Central Molecular Zone out to 100-200 pc heights. The injection of cooler, high-metallicity material into the Galactic halo above the GC may catalyse the subsequent cooling and condensation of hot plasma out of this region and explain the presence of relatively pristine, nuclear-unprocessed gas in the GC. This process may also be an important ingredient in understanding the long-term stability of the GC star-formation rate. The plasma outflow from the GC reaches a height of a few kpc and is compellingly related to the recently-discovered Fermi Bubbles by a number of pieces of evidence. These include that the outflow advects precisely i) the power in cosmic rays required to sustain the Bubbles’ γ-ray luminosity in saturation; ii) the hot gas required to compensate for gas cooling and drop-out from the Bubbles; and iii) the magnetic field required to stabilise the walls of these structures. Our modelling demonstrates that ∼109 M⊙of hot gas is processed through the GC over 10 Gyr. We speculate that the continual star-formation in the GC over the age of the Milky Way has kept the SMBH in a quiescent state thus preventing it from significantly heating the coronal gas, allowing for the continual accretion of gas on to the disk and the sustenance of star formation on much wider scales in the Galaxy. In general, our investigations explicitly reveal the GC’s important role in the Milky Way’s wider stellar ecology. Key words: cosmic rays – galaxies: star formation – Galaxy: center – ISM: jets and outflows – ISM: supernova remnants – radiation mechanisms: non-thermal 1 INTRODUCTION The inner 200 pc (in diameter) of the Milky Way features a spectacular confluence of unusual and energetic astrophysical phenomena. Within this region of the Galaxy – circumscribed by the Inner Lindblad Resonance associated with the non- axisymmetric gravitational potential of the Galactic bar ⋆E-mail: Roland.Crocker@mpi-hd.mpg.de – the distribution of stars cusps sharply into the distinct population of the so-called nuclear bulge (Serabyn & Morris 1996). Correspondingly, over the same region the current, inferred areal density of star formation, ˙Σ∗, sharply peaks to ∼200 M⊙/kpc2/yr. This is approximately three orders of magnitude higher than the mean value in the Galactic disk. With such a high ˙Σ∗, observations of the nuclei of external, star-forming galaxies tell us to expect a star-formation-driven outflow; there is much empirical and theoretical evidence arXiv:1112.6247v2 [astro-ph.GA] 26 Apr 2012 2 Roland M. Crocker et al. that such an outflow exists in the GC as we have explored in a number of recent papers (Crocker et al. 2011a; Crocker & Aharonian 2011; Crocker et al. 2011b). This paper adds significantly to that evidence. The very high star formation rate (SFR) density like- wise sustains a very high energy density in all phases of the GC ISM. Most directly, the optical and UV output of the many young, hot stars in the region is reprocessed by thick, ambient dust into a dominantly infrared photon background of ∼100 eV cm−3. Radio continuum and γ-ray observations allow one to place a robust lower limit of ∼50 µG on the typical magnetic field throughout the entire inner ∼800 pc (in diameter) of the Galaxy (Crocker et al. 2010a); modelling (Crocker et al. 2011a,b) points to a magnetic field in the inner 200 pc that is at least 100 µG. In association with and, in fact, as a necessary precondition to, the high SFR, observations reveal an enormous agglomeration of hot, dense, and highly-turbulent molecular gas of mass 3×107 M⊙(Dah- men et al. 1998; Molinari et al. 2011), 5-10% of the Milky Way’s entire H2 allocation. Thi

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