arXiv subjects

Michael M. Schulreich

Publications and source records attributed to Michael M. Schulreich.

6 recordsLinked to original sources

arXiv2026

The universal thickness of mid-infrared bubbles

Several hundred ring-like structures seen at $8μ$m in the Spitzer GLIMPSE surveys share a uniform morphology: the shell thickness $ΔR$ is a nearly constant fraction of the outer radius $R$, $ΔR/R\sim0.3$, across a factor of 40 in size and all exciting spectral types. This invariance is unexplained. We ask what sets this thickness and whether its near-constancy is physical or a measurement artifact. We derive analytic constraints on the location, temperature, and heating of the emitting grains, and confront the predicted scaling with radius against the GLIMPSE I and II catalogs (591 objects, 91 with kinematic distances). Grains in radiative equilibrium at the observed radii reach only 20-35 K and fall short of the required $8μ$m emissivity by more than 20 orders of magnitude; the emission must come from stochastically heated PAHs and very small grains, and therefore from the photon-dominated region, where the ring width is the far-ultraviolet penetration depth. This gives $ΔR/R=A_V^{\rm PDR}/A_V^{\rm cloud}$, independent of radius; a swept-up shell instead predicts $ΔR/R\propto(R/R_S)^{3/2}$, with $R_S$ the initial Strömgren radius. We find a pronounced angular-resolution systematic that inflates the ratio for the smallest bubbles; once this is accounted for, $ΔR/R=0.217$, with a residual dependence on physical radius of $0.03\pm0.05$, consistent with zero. A fixed ambient density is excluded at $20σ$, the swept-up shell at 9-28$σ$. The implied columns are $N_{\rm cloud}\approx8.7\times10^{21}$ cm$^{-2}$ with only 0.112 dex of scatter. The near-universal appearance of mid-infrared bubbles follows from the ratio of two nearly invariant column densities -- the dust-opacity-fixed far-ultraviolet shielding column and the Larson-like column of the parent molecular cloud -- and requires no reference to stellar winds.

astro-ph.GA↗

arXiv2026

Gas accretion onto the Milky Way: high-velocity cloud survival and the revival of the terminal-velocity paradigm

The terminal-velocity paradigm has long been used to interpret the motion and infer the distances of HVCs accreting onto the Milky Way, yet its validity under realistic Galactic conditions remains uncertain. We investigate its dynamical limits by combining analytical modelling with three-dimensional hydrodynamical simulations of clouds moving through a stratified Milky Way halo. We derive a generalized equation of motion including gravity, ram-pressure drag, a phenomenological mass-exchange model capturing mass loss and growth, and Bernoulli-driven cloud expansion. Analytical solutions for constant-property clouds provide a reference framework, while the full evolution is assessed using simulations with adiabatic physics, radiative cooling, and thermal conduction. Terminal velocity is a local equilibrium but not a global attractor: dense clouds remain quasi-ballistic over most of their trajectories and approach terminal motion only shortly before reaching the Galactic disc. Hydrodynamical effects further limit the paradigm. In adiabatic flows, instabilities rapidly disrupt the cloud, rendering the terminal-velocity description inapplicable. Radiative cooling instead promotes condensation and momentum loading, maintaining strong coupling to the background gas and restoring a terminal-velocity-like regime over extended periods, while thermal conduction mainly affects small-scale structure. Synthetic observables, including position--velocity diagrams, optical extinction, and soft X-ray emission, reproduce key features of observed HVCs such as velocity bridges and compression-driven emission. We also provide direct measurements of the effective drag coefficient for infalling clouds, finding values of order unity but strongly time-dependent. Overall, the terminal-velocity paradigm is a conditional description governed by cloud structure, mass exchange, and the ambient medium.

astro-ph.GA↗

arXiv2025

Acceleration and Transport of the Unstable Cosmic-ray Isotope $^{60}$Fe in Supernova-Enriched Environments

The unstable isotope $^{60}$Fe, with a half-life of 2.6 million years, is produced primarily in supernova explosions. The observed presence of $^{60}$Fe in cosmic rays and its detection in deep-sea crusts and sediments suggest two possible scenarios: either the direct acceleration of $^{60}$Fe from supernova ejecta or its enrichment in the circumstellar material surrounding supernova progenitors, which indicates cosmic ray production in clusters of supernovae. Focusing on the latter scenario, we consider an environment shaped by successive supernova explosions, reminiscent of the Local Bubble around the time of the most recent supernova explosion. We independently tracked the evolution of the $^{60}$Fe mass ratio within the Local Bubble using passive scalars. To investigate the spectra of protons and $^{60}$Fe, we explicitly modelled cosmic-ray acceleration and transport at the remnant of the last supernova by simultaneously solving the hydrodynamical equations for the supernova outflow and the transport equations for cosmic rays, scattering turbulence, and large-scale magnetic field, using the time-dependent acceleration code RATPaC. The main uncertainty in our prediction of the local $^{60}$Fe flux at about $pc=1$ GeV/nuc is the magnetic-field structure in the Local Bubble and the cosmic-ray diffusion beyond the approximately $100$ kyr of evolution covered by our study. We found that if the standard galactic propagation applies, the local $^{60}$Fe flux would be around 3% of that measured. If there is a sustained reduction in the diffusion coefficient at and near the Local Bubble, then the expected $^{60}$Fe flux could be up to 30% of that measured.

astro-ph.HE↗

arXiv2025

Tidal disruption events as the origin of the eROSITA and Fermi bubbles

Context: The recently discovered spherical eROSITA bubbles arise up to a latitude of $\pm$80°-85°in the X-ray regime of the Milky Way halo. Similar to the $γ$-ray Fermi bubbles, they evolve around the Galactic center, making a common origin plausible. However, the driving mechanism and evolution of both bubbles are still under debate. Aims: We investigate whether hydrodynamic energy injections at the Galactic center, such as e.g. tidal disruption events (TDEs), could have inflated both bubbles. The supermassive black hole Sagittarius A* is expected to tidally disrupt a star every 10-100 kyr, potentially leading to an outflow from the central region that drives a shock propagating into the Galactic halo due to its vertically declining density distribution, ultimately forming a superbubble that extends out of the disk similar to the eROSITA and Fermi bubbles. Methods: We model TDEs in the Galaxy using three-dimensional hydrodynamical simulations, considering different Milky Way mass models and TDE rates. We then generate synthetic X-ray maps and compare them with observations. Results: Our simulation results of a $β$-model Milky Way halo show that superbubbles, blown for 16 Myr by regular energy injections at the Galactic center that occur every 100 kyr, can have a shape, shell stability, size, and evolution time similar to estimates for the eROSITA bubbles, and an overall structure reminiscent of the Fermi bubbles. The $γ$-rays in our model would stem from cosmic ray interactions at the contact discontinuity, where they were previously accelerated by first-order Fermi acceleration at in situ shocks. Conclusions: Regular TDEs in the past 10-20 Myr near the Galactic center could have driven an outflow resulting in both, the X-ray emission of the eROSITA bubbles and the $γ$-ray emission of the Fermi bubbles.

astro-ph.HE↗

arXiv2024

Gamma-ray line emission from the Local Bubble

Deep-sea archives that include intermediate-lived radioactive $^{60}\mathrm{Fe}$ particles suggest the occurrence of several recent supernovae inside the present-day volume of the Local Bubble during the last $\sim 10$ Myr. The isotope $^{60}\mathrm{Fe}$ is mainly produced in massive stars and ejected in supernova explosions, which should always result in a sizeable yield of $^{26}\mathrm{Al}$ from the same objects. $^{60}\mathrm{Fe}$ and $^{26}\mathrm{Al}$ decay with lifetimes of 3.82 and 1.05 Myr, and emit $γ$-rays at 1332 and 1809 keV, respectively. These $γ$-rays have been measured as diffuse glow of the Milky Way, and would also be expected from inside the Local Bubble as foreground emission. Based on two scenarios, one employing a geometrical model and the other state-of-the-art hydrodynamics simulations, we estimate the expected fluxes of the 1332 and 1809 keV $γ$-ray lines, as well as the resulting 511 keV line from positron annihilation due to the $^{26}\mathrm{Al}$ $β^+$-decay. We find fluxes in the range of $10^{-6}$-$10^{-5}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$ for all three lines with isotropic contributions of 10-50%. We show that these fluxes are within reach for the upcoming COSI-SMEX $γ$-ray telescope over its nominal satellite mission duration of 2 yr. Given the Local Bubble models considered, we conclude that in the case of 10-20 Myr-old superbubbles, the distributions of $^{60}\mathrm{Fe}$ and $^{26}\mathrm{Al}$ are not co-spatial - an assumption usually made in $γ$-ray data analyses. In fact, this should be taken into account however when analysing individual nearby targets for their $^{60}\mathrm{Fe}$ to $^{26}\mathrm{Al}$ flux ratio as this gauges the stellar evolution models and the age of the superbubbles. A flux ratio measured for the Local Bubble could further constrain models of $^{60}\mathrm{Fe}$ deposition on Earth and its moon.

astro-ph.HE↗

arXiv2016

The Link Between the Local Bubble and Radioisotopic Signatures on Earth

Traces of 2-3 Myr old 60Fe were recently discovered in a manganese crust and in lunar samples. We have found that this signal is extended in time and is present in globally distributed deep-sea archives. A second 6.5-8.7 Myr old signature was revealed in a manganese crust. The existence of the Local Bubble hints to a recent nearby supernova-activity starting 13 Myr ago. With analytical and numerical models generating the Local Bubble, we explain the younger 60Fe-signature and thus link the evolution of the solar neighborhood to terrestrial anomalies.

astro-ph.EP↗