arXiv subjects

E. Koch

Publications and source records attributed to E. Koch.

13 recordsLinked to original sources

arXiv2025

M^3D: Mosaicking M33 with MUSE datacubes I. Unveiling the diversity of HII regions in M33 with MUSE

We present new VLT/MUSE observations of a 3 $\times$ 8 arcmin$^2$ mosaic along the southern major axis of the Local Group galaxy M33. These data provide an unprecedented view of the galaxy's interstellar medium (ISM) and allow us to resolve ionised nebulae at a spatial scale of $\approx$5 pc. We identified and catalogued 131 HII regions, down to H\alpha luminosities of $\approx 5\times$10$^{35}$ erg s$^{-1}$, one order of magnitude fainter than previous surveys on nearby galaxies beyond the Local Group, and we compared these regions with the spatial distribution of ionising stars and embedded star clusters. For each region, we extracted the corresponding integrated optical spectra and measured the intensity of key optical emission lines (H\beta, [OIII], [NII], H\alpha, [SII], [SIII]), other weaker optical lines when detectable, and Paschen lines to characterise the physical properties of the ioinised gas, such as density, dust attenuation, and metallicity. Our spatially resolved line ratio and flux maps reveal a remarkable diversity in ionisation properties, from dust-obscured regions hosting young stellar objects to highly ionised bubbles exhibiting high [OIII]/H\beta ratios. Our data reveal a diversity of ionisation fronts, ranging from well-defined to partial to absent. The radial profiles we obtained indicate the presence of both optically thin (density-bounded) HII regions permitting the escape of ionising photons and fully ionised, optically thick (ionisation-bounded) HII regions. The richness of this MUSE mosaic offers an unprecedented view of the ionised ISM at $\approx$5 pc resolution, providing direct insight into how stellar feedback shapes its environment.

astro-ph.GA↗

arXiv2022

Cloud-scale Radio Surveys of Star Formation and Feedback in Triangulum Galaxy M 33: VLA Observations

Studying the interplay between massive star formation and the interstellar medium (ISM) is paramount to understand the evolution of galaxies. Radio continuum (RC) emission serves as an extinction-free tracer of both massive star formation and the energetic components of the interstellar medium. We present a multi-band radio continuum survey of the local group galaxy M 33 down to ~30 pc linear resolution observed with the Karl G. Jansky Very Large Array (VLA). We calibrate the star-formation rate surface density and investigate the impact of diffuse emission on this calibration using a structural decomposition. Separating the thermal and nonthermal emission components, the correlation between different phases of the interstellar medium and the impact of massive star formation are also investigated. Radio sources with sizes <~ 200 pc constitute about 36% (46%) of the total RC emission at 1.5 GHz (6.3 GHz) in the inner 18' x 18' (or 4kpc x 4kpc) disk of M 33. The nonthermal spectral index becomes flatter with increasing star-formation rate surface density, indicating the escape of cosmic ray electrons {from their birth places}. The magnetic field strength also increases with star-formation rate following a bi-modal relation, indicating that the small-scale turbulent dynamo acts more efficiently at higher luminosities and star-formation rates. Although the correlations are tighter in star-forming regions, the nonthermal emission is correlated also with the more quiescent molecular gas in the ISM. An almost linear molecular star-formation law exists in M 33 when excluding diffuse structures. Massive star formation amplifies the magnetic field and increases the number of high-energy cosmic ray electrons, which can help the onset of winds and outflows.

astro-ph.GA↗

arXiv2022

Variations in the $\Sigma_{\rm SFR} {-} \Sigma_{\rm mol} {-} \Sigma_{\rm \star}$ plane across galactic environments in PHANGS galaxies

There exists some consensus that stellar mass surface density ($\Sigma_{*}$) and molecular gas mass surface density ($\Sigma_{\rm mol}$) are the main quantities responsible for locally setting the star formation rate. This regulation is inferred from locally resolved scaling relations between these two quantities and the star formation rate surface density ($\Sigma_{\rm SFR}$). However, the universality of these relations is debated. Here, we probe the interplay between these three quantities across different galactic environments at a spatial resolution of 150 pc. We perform a hierarchical Bayesian linear regression to find the best set of parameters $C_{*}$, $C_{\rm mol}$, and $C_{\rm norm}$ that describe the star-forming plane conformed by these quantities, such that $\log \Sigma_{\rm SFR} = C_{*} \log \Sigma_{*} + C_{\rm mol} \log \Sigma_{\rm mol} + C_{\rm norm}$, and explore variations in the determined parameters across galactic environments, focusing our analysis on the $C_{*}$ and $C_{\rm mol}$ slopes. We find signs of variations in the posterior distributions of $C_{*}$ and $C_{\rm mol}$ across different galactic environments. Bars show the most negative value of $C_{*}$, a sign of longer depletion times, while spiral arms show the highest $C_{*}$ among all environments. We conclude that systematic variations in the interplay of $\Sigma_{*}$, $\Sigma_{\rm mol}$ and $\Sigma_{\rm SFR}$ across galactic environments exist at a spatial resolution of 150 pc, and we interpret these variations as produced by an additional mechanism regulating the formation of stars that is not captured by either $\Sigma_{*}$ or $\Sigma_{\rm mol}$. We find that these variations correlate with changes in the star formation efficiency across environments, which could be linked to the dynamical state of the gas that prevents it from collapsing and forming stars, or to changes in the molecular gas fraction.

astro-ph.GA↗

arXiv2008

On the mechanism for orbital-ordering in KCuF3

The Mott insulating perovskite KCuF3 is considered the archetype of an orbitally-ordered system. By using the LDA+dynamical mean-field theory (DMFT) method, we investigate the mechanism for orbital-ordering (OO) in this material. We show that the purely electronic Kugel-Khomskii super-exchange mechanism (KK) alone leads to a remarkably large transition temperature of T_KK about 350 K. However, orbital-order is experimentally believed to persist to at least 800 K. Thus Jahn-Teller distortions are essential for stabilizing orbital-order at such high temperatures.

cond-mat.str-el↗

arXiv2006

Static vs. dynamical mean field theory of Mott antiferromagnets

Studying the antiferromagnetic phase of the Hubbard model by dynamical mean field theory, we observe striking differences with static (Hartree-Fock) mean field: The Slater band is strongly renormalized and spectral weight is transferred to spin-polaron side bands. Already for intermediate values of the interaction $U$ the overall bandwidth is larger than in Hartree-Fock, and the gap is considerably smaller. Such differences survive any renormalization of $U$. Our photoemission experiments for Cr-doped V$_2$O$_3$ show spectra qualitatively well described by dynamical mean field theory.

cond-mat.str-el↗

arXiv2006

Electron-phonon interaction and antiferromagnetic correlations

We study effects of the Coulomb repulsion on the electron-phonon interaction (EPI) in a model of cuprates at zero and finite doping. We find that antiferromagnetic correlations strongly enhance EPI effects on the electron Green's function with respect to the paramagnetic correlated system, but the net effect of the Coulomb interaction is a moderate suppression of the EPI. Doping leads to additional suppression, due to reduced antiferromagnetic correlations. In contrast, the Coulomb interaction strongly suppresses EPI effects on phonons, but the suppression weakens with doping.

cond-mat.str-el↗

arXiv2004

PANIC: A Near-infrared Camera for the Magellan Telescopes

PANIC (Persson's Auxiliary Nasmyth Infrared Camera) is a near-infrared camera designed to operate at any one of the f/11 folded ports of the 6.5m Magellan telescopes at Las Campanas Observatory, Chile. The instrument is built around a simple, all-refractive design that reimages the Magellan focal plane to a plate scale of 0.125'' pixel^{-1} onto a Rockwell 1024x1024 HgCdTe detector. The design goals for PANIC included excellent image quality to sample the superb seeing measured with the Magellan telescopes, high throughput, a relatively short construction time, and low cost. PANIC has now been in regular operation for over one year and has proved to be highly reliable and produce excellent images. The best recorded image quality has been ~0.2'' FWHM.

astro-ph↗

arXiv2003

Field-doping of C60 crystals: Polarization and Stark splitting

We investigate the possibility of doping C60 crystals by applying a strong electric field. For an accurate description of a C60 field-effect device we introduce a multipole expansion of the field, the response of the C60 molecules, and the Stark splitting of the molecular levels. The relevant response coefficients and splittings are calculated ab initio for several high symmetry orientations. Using a group theoretic analysis we extend these results to arbitrary orientations of the C60 molecules with respect to the external field. We find that, surprisingly, for the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively, the two leading multipole components lift the degeneracy of the molecular level in the same way. Moreover the relative signs of the splittings turn out to be such that the splittings add up when the external field induces charge into the respective level. That means that when charge carriers are put into a level, its electronic structure is strongly modified. Therefore, in general, in C60 field-effect devices charge is not simply put into otherwise unchanged bands, so already because of this their physics should be quite different from that of the alkali-doped fullerenes.

cond-mat↗

arXiv1999

Decay properties of the density matrix and Wannier functions for interacting systems

For non-interacting electrons the one-particle density matrix and the related Wannier functions characterize a material as insulating or metallic. Introducing many-body Wannier functions, we show that this characterization can be carried over to interacting systems. In particular, we find that an exponential decay of the density matrix characterizes not only band insulators but also Mott insulators. The properties of the many-body Wannier functions differ, however, from those of the Wannier functions of a non-interacting systems.

cond-mat↗

arXiv1999

Metal-insulator transitions: Influence of lattice structure, Jahn-Teller effect, and Hund's rule coupling

We study the influence of the lattice structure, the Jahn-Teller effect and the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb). The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60 (bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases) the value Uc of the Coulomb integral at which the metal-insulator transition occurs. There is an important partial cancellation between the Jahn-Teller effect and the Hund's rule coupling.

cond-mat.str-el↗

arXiv1997

Discrete Hubbard-Stratonovich transformations for systems with orbital degeneracy

A discrete Hubbard-Stratonovich transformation is presented for systems with an orbital degeneracy $N$ and a Hubbard Coulomb interaction without multiplet effects. An exact transformation is obtained by introducing an external field which takes $N+1$ values. Alternative approximate transformations are presented, where the field takes fewer values, for instance two values corresponding to an Ising spin.

cond-mat.str-el↗

arXiv1997

Pauli susceptibility of A3C60 (A=K, Rb)

The Pauli paramagnetic susceptibility of A3C60 (A= K, Rb) compounds is calculated. A lattice quantum Monte Carlo method is applied to a multi-band Hubbard model, including the on-site Coulomb interaction U. It is found that the many-body enhancement of the susceptibility is of the order of a factor of three. This reconciles estimates of the density of states from the susceptibility with other estimates. The enhancement is an example of a substantial many-body effect in the doped fullerenes.

cond-mat.str-el↗

arXiv1997

Important role of alkali atoms in A4C60

We show that hopping via the alkali atoms plays an important role for the t1u band of A4C60 (A=K, Rb), in strong contrast to A3C60. Thus the t1u band is broadened by more than 40 % by the presence of the alkali atoms. The difference between A4C60 and A3C60 is in particular due to the less symmetric location of the alkali atoms in A4C60.

cond-mat.str-el↗