arXiv subjects

Philipp Richter

Publications and source records attributed to Philipp Richter.

At least 37 records · Page 2Linked to original sources

arXiv2021

Molecular Gas within the Milky Way's Nuclear Wind

We report the first direct detection of molecular hydrogen associated with the Galactic nuclear wind. The Far-Ultraviolet Spectroscopic Explorer spectrum of LS 4825, a B1 Ib-II star at $l,b$ = 1.67$^{\circ}$,$-$6.63$^{\circ}$ lying $d$ = 9.9$^{+1.4}_{-0.8}$ kpc from the Sun, $\sim$1 kpc below the Galactic plane near the Galactic Center, shows two high-velocity H$_2$ components at $v_\mathrm{LSR}$ = $-79$ and $-108$ km s$^{-1}$. In contrast, the FUSE spectrum of the nearby ($\sim$0.6$^{\circ}$ away) foreground star HD 167402 at $d$=4.9$^{+0.8}_{-0.7}$ kpc reveals no H$_2$ absorption at these velocities. Over 60 lines of H$_2$ from rotational levels $J$ = 0 to 5 are identified in the high-velocity clouds. For the $v_\mathrm{LSR}$ = $-79$ km s$^{-1}$ cloud we measure total log $N$(H$_2$) $\geq$ 16.75 cm$^{-2}$, molecular fraction $f_\mathrm{H_2}$ $\geq$ 0.8%, and $T_{01}$ $\geq$ 97 and $T_{25}$ $\leq$ 439 K for the ground- and excited-state rotational excitation temperatures. At $v_\mathrm{LSR}$ = $-108$ km s$^{-1}$, we measure log $N$(H$_2$) = 16.13 $\pm$ 0.10 cm$^{-2}$, $f_\mathrm{H_2}$ $\geq$ 0.5%, and $T_{01}$ = 77$^{+34}_{-18}$ and $T_{25}$ = 1092$^{+149}_{-117}$ K, for which the excited state ortho- to para-H$_2$ is 1.0$^{+0.3}_{-0.1}$, much less than the equilibrium value of 3 expected for gas at this temperature. This non-equilibrium ratio suggests that the $-108$ km s$^{-1}$ cloud has been recently excited and has not yet had time to equilibrate. As the LS 4825 sight line passes close by a tilted section of the Galactic disk, we propose that we are probing a boundary region where the nuclear wind is removing gas from the disk.

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arXiv2021

Gas flows in galaxy mergers: supersonic turbulence in bridges, accretion from the circumgalactic medium, and metallicity dilution

In major galaxy mergers, the orbits of stars are violently perturbed, and gas is torqued to the centre, diluting the gas metallicity and igniting a starburst. In this paper, we study the gas dynamics in and around merging galaxies using a series of cosmological magneto-hydrodynamical (MHD) zoom-in simulations. We find that the gas bridge connecting the merging galaxies pre-coalescence is dominated by turbulent pressure, with turbulent Mach numbers peaking at values of 1.6-3.3. This implies that bridges are dominated by supersonic turbulence, and are thus ideal candidates for studying the impact of extreme environments on star formation. We also find that gas accreted from the circumgalactic medium (CGM) during the merger significantly contributes (27-51 per cent) to the star formation rate (SFR) at the time of coalescence and drives the subsequent reignition of star formation in the merger remnant. Indeed, 19-53 per cent of the SFR at z=0 originates from gas belonging to the CGM prior the merger. Finally, we investigate the origin of the metallicity-diluted gas at the centre of merging galaxies. We show that this gas is rapidly accreted onto the galactic centre with a time-scale much shorter than that of normal star-forming galaxies. This explains why coalescing galaxies are not well-captured by the fundamental metallicity relation.

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arXiv2021

The HI Column Density Distribution of the Galactic Disk and Halo

We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of $N$(HI)$\sim10^{14}$ cm$^{-2}$ using measurements of HI Lyman-series absorption from the Far Ultraviolet Spectroscopic Explorer (FUSE). Our results are drawn from an analysis of 25 AGN sightlines spread evenly across the sky with Galactic latitude |b|$\gtrsim 20^{\circ}$. By simultaneously fitting multi-component Voigt profiles to 11 Lyman-series absorption transitions covered by FUSE (Ly$β$-Ly$μ$) plus HST measurements of Ly$α$, we derive the kinematics and column densities of a sample of 152 HI absorption components. While saturation prevents accurate measurements of many components with column densities 17$\lesssim$log$N$(HI)$\lesssim$19, we derive robust measurements at log$N$(HI)$\lesssim$17 and log$N$(HI)$\gtrsim$19. We derive the first ultraviolet HI column density distribution function (CDDF) of the Milky Way, both globally and for low-velocity (ISM), intermediate-velocity clouds (IVCs), and high-velocity clouds (HVCs). We find that IVCs and HVCs show statistically indistinguishable CDDF slopes, with $β_{\rm IVC}=$ $-1.01_{-0.14}^{+0.15}$ and $β_{\rm HVC}=$ $-1.05_{-0.06}^{+0.07}$. Overall, the CDDF of the Galactic disk and halo appears shallower than that found by comparable extragalactic surveys, suggesting a relative abundance of high-column density gas in the Galactic halo. We derive the sky covering fractions as a function of HI column density, finding an enhancement of IVC gas in the northern hemisphere compared to the south. We also find evidence for an excess of inflowing HI over outflowing HI, with $-$0.88$\pm$0.40 M$_\odot$ yr$^{-1}$ of HVC inflow versus 0.20$\pm$0.10 M$_\odot$ yr$^{-1}$ of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.

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arXiv2021

New Evidence for Extended HeII Reionization at z>3.5 from HeII Lyman Alpha and Beta Transmission Spikes

We present new high-resolution (R~14,000) spectra of the two brightest HeII-transparent quasars in the far-UV (FUV) at z>3.5, HE2QSJ2311-1417 (z=3.70) and HE2QSJ1630+0435 (z=3.81), obtained with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). In the predominantly saturated HeII absorption spectra, both sightlines show several isolated resolved (full width at half maximum FWHM>50 km/s) transmission spikes in HeII Ly$α$ and HeII Ly$β$. The incidence of such spikes decreases with increasing redshift, but both sightlines show significant spikes at z>3.5, signaling the presence of fully ionized regions in the z>3.5 intergalactic medium (IGM). We employ an automated algorithm to compare the number of detected HeII transmission spikes to predictions from mock spectra created from the outputs of a cubic (100/h cMpc)^3 optically thin Nyx hydrodynamical simulation, assuming a range of UV background photoionization rates $Γ_\mathrm{HeII}$. From the incidence of Ly$α$ and Ly$β$ transmission spikes we infer similar photoionization rates of $Γ_\mathrm{HeII}=2.0^{+0.6}_{-0.5}\times 10^{-15}$s$^{-1}$ at 3.51 3.5 along both lines of sight, our observations provide further evidence that HeII reionization had substantially progressed at these redshifts.

astro-ph.CO↗

arXiv2021

On the relation between Lya absorbers and local galaxy filaments

The intergalactic medium (IGM) is believed to contain the majority of baryons in the universe and to trace the same dark matter structure as galaxies, forming filaments and sheets. Lya absorbers, which sample the neutral component of the IGM, have been extensively studied at low and high redshift, but the exact relation between Lya absorption, galaxies and the large-scale structure is observationally not well-constrained. In this study, we aim at characterising the relation between Lya absorbers and nearby overdense cosmological structures (galaxy filaments) at recession velocities Delta v \leq 6700 km/s by using archival observational data from various instruments. We analyse 587 intervening Lya absorbers in the spectra of 302 extragalactic background sources obtained with the COS installed on the HST. We combine the absorption-line information with galaxy data of five local galaxy filaments originally mapped by Courtois et al. (2013). Along the 91 sightlines that pass close to a filament, we identify 215 (227) Lya absorption systems (components). Among these, 74 Lya systems are aligned in position and velocity with the galaxy filaments, indicating that these absorbers and the galaxies trace the same large-scale structure. The filament-aligned Lya absorbers have a ~90 percent higher rate of incidence (dN/dz=189 for log N HI \geq 13.2) and a mildly shallower slope (-beta = -1.47) of the column density distribution function than the general Lya population at z=0, reflecting the filaments' matter overdensity. The strongest Lya absorbers are preferentially found near galaxies or close to the axis of a filament, although there is substantial scatter in this relation. Our sample of absorbers clusters more strongly around filament axes than a randomly distributed sample would do (as confirmed by a KS test), but the clustering signal is less pronounced than for the galaxies in the filaments.

astro-ph.CO↗

arXiv2020

Cloud-by-cloud, multiphase, Bayesian modeling: Application to four weak, low ionization absorbers

We present a new method aimed at improving the efficiency of component by component ionization modeling of intervening quasar absorption line systems. We carry out cloud-by-cloud, multiphase modeling making use of CLOUDY and Bayesian methods to extract physical properties from an ensemble of absorption profiles. Here, as a demonstration of method, we focus on four weak, low ionization absorbers at low redshift, because they are multi-phase but relatively simple to constrain. We place errors on the inferred metallicities and ionization parameters for individual clouds, and show that the values differ from component to component across the absorption profile. Our method requires user input on the number of phases and relies on an optimized transition for each phase, one observed with high resolution and signal-to-noise. The measured Doppler parameter of the optimized transition provides a constraint on the Doppler parameter of HI, thus providing leverage in metallicity measurements even when hydrogen lines are saturated. We present several tests of our methodology, demonstrating that we can recover the input parameters from simulated profiles. We also consider how our model results are affected by which radiative transitions are covered by observations (for example how many HI transitions) and by uncertainties in the b parameters of optimized transitions. We discuss the successes and limitations of the method, and consider its potential for large statistical studies. This improved methodology will help to establish direct connections between the diverse properties derived from characterizing the absorbers and the multiple physical processes at play in the circumgalactic medium.

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arXiv2020

Kinematics of the Magellanic Stream and Implications for its Ionization

The Magellanic Stream and the Leading Arm form a massive, filamentary system of gas clouds surrounding the Large and Small Magellanic Clouds. Here we present a new component-level analysis of their ultraviolet (UV) kinematic properties using a sample of 31 sightlines through the Magellanic System observed with the Hubble Space Telescope/Cosmic Origins Spectrograph. Using Voigt profile fits to UV metal-line absorption, we quantify the kinematic differences between the low-ion (Si II and C II), intermediate-ion (Si III), and high-ion (Si IV and C IV) absorption lines and compare the kinematics between the Stream and Leading Arm. We find that the Stream shows generally simple, single-phase kinematics, with statistically indistinguishable b-value distributions for the low-, intermediate-, and high-ion components, all dominated by narrow (b<25 km/s) components that are well aligned in velocity. In contrast, we find tentative evidence that the Leading Arm shows complex, multi-phase kinematics, with broader high ions than low ions. These results suggest that the Stream is photoionized up to C IV by a hard ionizing radiation field. This can be naturally explained by the Seyfert-flare model of Bland-Hawthorn et al. (2013, 2019), in which a burst of ionizing radiation from the Galactic Center photoionized the Stream as it passed below the south Galactic pole. The Seyfert flare is the only known source of radiation that is both powerful enough to explain the H-alpha intensity of the Stream and hard enough to photoionize Si IV and C IV to the observed levels. The flare's timescale of a few Myr suggests it is the same event that created the giant X-ray/gamma-ray Fermi Bubbles at the Galactic Center.

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arXiv2020

Hot gas in galaxy halos traced by coronal broad Lyman alpha absorbers

We explore the possibility to systematically study the extended, hot gaseous halos of low-redshift galaxies with Coronal Broad Ly alpha Absorbers (CBLAs). These are weak, thermally broadenend HI absorption lines arising from the tiny fraction of neutral hydrogen that resides in the collisionally ionized, million-degree halo gas in these galaxies. Using a semi-analytic approach, we model the spatial density and temperature distribution of hot coronal gas to predict strength, spectral shape, and cross section of CBLAs as a function of galaxy-halo mass and line-of-sight impact parameter. For virial halo masses in the range log (M/M_sun)=10.6-12.6, the characteristic logarithmic CBLA HI column densities and Doppler parameters are log N(HI)=12.4-13.4 and b(HI)=70-200 km/s, indicating that CBLAs represent weak, shallow spectral features that are difficult to detect. Yet, the expected number density of CBLAs per unit redshift in the above given mass range is dN/dz(CBLA)~3, implying that CBLAs have a substantial absorption cross-section. We compare the model predictions with a combined set of ultraviolet (UV) absorption-line spectra from HST/COS and HST/STIS that trace the halos of four low-redshift galaxies. We demonstrate that CBLAs already might have been detected in these spectra, but the complex multi-component structure and the limited signal-to-noise ratio (S/N) complicate the interpretation of these CBLA candidate systems. Our study suggests that CBLAs represent a very interesting absorber class that potentially will allow us to further explore the hot coronae of galaxies with UV spectral data.

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arXiv2019

The Mass Inflow and Outflow Rates of the Milky Way

We present new calculations of the mass inflow and outflow rates around the Milky Way, derived from a catalog of ultraviolet metal-line high velocity clouds (HVCs). These calculations are conducted by transforming the HVC velocities into the Galactic Standard of Rest (GSR) reference frame, identifying inflowing (v_GSR < 0 km/s) and outflowing (v_GSR > 0 km/s) populations, and using observational constraints on the distance, metallicity, dust content, covering fractions, and total hydrogen column density of each population. After removing HVCs associated with the Magellanic Stream and the Fermi Bubbles, we find inflow and outflow rates in cool (T~10^4 K) ionized gas of dM_in/dt >~ 0.53+/-0.17 (d/12 kpc) (Z/0.2 Z_sun)^-1 M_sun/yr and dM_out/dt >~ 0.16+/-0.06 (d/12 kpc) (Z/0.5 Z_sun)^-1 M_sun/yr. The excess of inflowing over outflowing gas suggests that the Milky Way is currently in an inflow-dominated phase, but the presence of substantial mass flux in both directions supports a Galactic fountain model, in which gas is constantly recycled between the disk and the halo. We also find that the metal flux in both directions (in and out) is indistinguishable. By comparing the outflow rate to the Galactic star formation rate, we present the first estimate of the mass loading factor (etc_HVC) of the disk-wide Milky Way wind, finding eta_HVC >~ 0.10+/-0.06 (d/12 kpc) (Z/0.5 Z_sun)^-1. Including the contributions from low- and intermediate-velocity clouds and from hot gas would increase these inflow and outflow estimates.

astro-ph.GA↗

arXiv2019

A population of high-velocity absorption-line systems residing in the Local Group

Aims. We aim to investigate the ionisation conditions and distances of Galactic high-velocity clouds (HVCs) in the Galactic halo and beyond in the direction of the Local Group (LG) barycentre and anti-barycentre, by studying spectral data of 29 extragalactic background sources obtained with Cosmic Origins Spectropgraph (COS) installed on the Hubble Space Telescope (HST). Methods. We model column-densities of low, intermediate, and high ions, such as Si II, C II, Si III, Si IV, and C IV and use this to construct a set of Cloudy ionisation models. Results. In total, we found 69 high-velocity absorption components along the 29 lines of sight. The ones in the direction of the LG barycentre span the entire range of studied velocities, 100 \lesssim |v_{LSR}|\lesssim 400 km s^-1, while the anti-barycentre sample has velocities up to about 300 km s^-1. For 49 components, we infer the gas densities. In the direction of the LG barycentre, the gas densities exhibit a large range between log n_H=-3.96 to -2.55, while in the anti-barycentre direction the densities are systematically higher, log n_H>-3.25. The barycentre absorbers can be split into two groups based on their density: a high density group with log n_H>-3.54, which can be affected by the Milky Way radiation field, and a low density group (log n_H \leq -3.54). The latter has very low thermal pressures of P/k<7.3 K cm^-3. Conclusions. Our study shows that part of the absorbers in the LG barycentre direction trace gas at very low gas densities and thermal pressures. Such properties indicate that these absorbers are located beyond the virial radius of the Milky Way. Our study also confirms results from earlier, single-sightline studies, suggesting the presence of a metal-enriched intragroup medium filling the LG near its barycentre.

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arXiv2019

The Magellanic Stream as a Probe of Astrophysics

Extending for over 200 degrees across the sky, the Magellanic Stream together with its Leading Arm is the most spectacular example of a gaseous stream in the local Universe. The Stream is an interwoven tail of filaments trailing the Magellanic Clouds as they orbit the Milky Way. Thought to be created by tidal forces, ram pressure, and halo interactions, the Stream is a benchmark for dynamical models of the Magellanic System, a case study for gas accretion and dwarf-galaxy accretion onto galaxies, a probe of the outer halo, and the bearer of more gas mass than all other Galactic high velocity clouds combined. If it survives to reach the Galactic disk, it may maintain or even elevate the Galactic star-formation rate. In this white paper, we emphasize the Stream's importance for many areas of Galactic astronomy, summarize key unanswered questions, and identify future observations and simulations needed to resolve them. We stress the importance of ultraviolet, optical, and radio spectroscopy, and the need for computational models that capture full particle and radiation treatments within an MHD environment.

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arXiv2018

The influence of atomic alignment on absorption and emission spectroscopy

Spectroscopic observations play essential roles in astrophysics. They are crucial for determining physical parameters in the universe, providing information about the chemistry of various astronomical environments. The proper execution of the spectroscopic analysis requires accounting for all the physical effects that are compatible to the signal-to-noise ratio. We find in this paper the influence on spectroscopy from the atomic/ground state alignment owing to anisotropic radiation and modulated by interstellar magnetic field, has significant impact on the study of interstellar gas. In different observational scenarios, we comprehensively demonstrate how atomic alignment influences the spectral analysis and provide the expressions for correcting the effect. The variations are even more pronounced for multiplets and line ratios. We show the variation of the deduced physical parameters caused by the atomic alignment effect, including alpha-to-iron ratio ([X/Fe]) and ionisation fraction. Synthetic observations are performed to illustrate the visibility of such effect with current facilities. A study of PDRs in $ρ$ Ophiuchi cloud is presented to demonstrate how to account for atomic alignment in practice. Our work has shown that due to its potential impact, atomic alignment has to be included in an accurate spectroscopic analysis of the interstellar gas with current observational capability.

astro-ph.GA↗

arXiv2018

The influence of magnetic fields on absorption and emission spectroscopies

Spectroscopic observations play a fundamental role in astrophysics. They are crutial to determine important physical parameters, provide information about the composition of various objects in the universe, as well as depict motions in the universe. However, spectroscopic studies often do not consider the influence of magnetic fields. In this paper, we explore the influence of magnetic fields on the spectroscopic observations using the concept of atomic alignment. Synthetic spectra are generated to show the measurable changes of the spectra due to atomic alignment. The influences of atomic alignment on absorption from DLAs, emission from H\,{\sc ii} Regions, submillimeter fine-structure lines from star forming regions are presented as examples to show this effect in diffuse gas. Furthermore, we demonstrate the influence of atomic alignment on physical parameters derived from atomic line ratios, such as the alpha-to-iron ratio([X/Fe]), interstellar temperature, and ionization rate. We conclude that Ground State Alignment (GSA) should be taken into consideration in the error budget of spectroscopic studies with high signal-to-noise(S/N) ratio.

astro-ph.IM↗

arXiv2018

COS-Weak: Probing the CGM using analogs of weak Mg II absorbers at z < 0.3

We present a sample of 34 weak metal line absorbers at $z< 0.3$ complied via the simultaneous detections ($3σ$) of the SiII$\lambda1260$ and CII$\lambda1334$ absorption lines, with $W_{r}$(SiII)$<0.2$ Å and $W_{r}$(CII)$<0.3$ Å, in archival HST/COS spectra. Our sample increases the number of known low-$z$ "weak absorbers" by a factor of $>5$. The column densities of HI and low-ionization metal lines obtained from Voigt profile fitting are used to build simple photoionization models using CLOUDY. The inferred densities and total hydrogen column densities are in the ranges of $-3.3 < \log n_{\rm H}/{\rm cm^{-3}} < -2.4$ and $16.0 < \log N_{\rm H}/{\rm cm^{-2}}<20.3$, respectively. The line of sight thicknesses of the absorbers have a wide range of $\sim$1 pc$-$50 kpc with a median value of $\sim$500 pc. The high-ionization OVI absorption, detected in 12/18 cases, always stems from a different gas-phase. Most importantly, 85% (50%) of these absorbers show a metallicity of $\rm [Si/H] > -1.0$ (0.0). The fraction of systems showing high metallicity (i.e., $\rm [Si/H]>-1.0$) in our sample is significantly higher than the HI-selected sample (Wotta et al. 2016) and the galaxy-selected sample (Prochaska et al. 2017) of absorbers probing the circum-galactic medium (CGM) at similar redshift. A search for galaxies has revealed a significant galaxy-overdensity around these weak absorbers compared to random places with a median impact parameter of 166 kpc to the nearest galaxy. Moreover, we find the presence of multiple galaxies in $\sim80$% of the cases, suggesting group environments. The observed $d\mathcal{N}/dz$ of $0.8\pm0.2$ indicates that such metal-enriched, compact, dense structures are ubiquitous in the halos of low-$z$ galaxies that are in groups. We suggest that these are transient structures that are related to outflows and/or stripping of metal-rich gas from galaxies.

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arXiv2018

Chemical Abundances in the Leading Arm of the Magellanic Stream

The Leading Arm (LA) of the Magellanic Stream is a vast debris field of H I clouds connecting the Milky Way and the Magellanic Clouds. It represents an example of active gas accretion onto the Galaxy. Previously only one chemical abundance measurement had been made in the LA. Here we present chemical abundance measurements using Hubble Space Telescope/Cosmic Origins Spectrograph Green Bank Telescope spectra of four sightlines passing through the LA, and three nearby sightlines that may trace outer fragments of the LA. We find low oxygen abundances, ranging from 4.0(+4.0,-2.0) percent solar to 12.6(+6.2,-4.1) percent solar, in the confirmed LA directions, with the lowest values found in the region known as LA III, farthest from the LMC. These abundances are substantially lower than the single previous measurement, S/H=35+/-7 percent solar (Lu et al. 1998), but are in agreement with those reported in the SMC filament of the trailing Stream, supporting a common origin in the SMC (not the LMC) for the majority of the LA and the trailing Stream. This provides important constraints for models of the formation of the Magellanic System. Finally, the HVCs in two of the three nearby sightlines show H I columns, kinematics, and oxygen abundances consistent with LA membership. This suggests that the LA is larger than traditionally thought, extending at least 20 degrees further to the Galactic northwest.

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arXiv2017

Multiple origins for the DLA at $z_\mathrm{abs}=0.313$ toward PKS 1127$-$145 indicated by a complex dust depletion pattern of Ca, Ti, and Mn

We investigate the dust depletion properties of optically thick gas in and around galaxies and its origin we study in detail the dust depletion patterns of Ti, Mn, and Ca in the multi-component damped Lyman-$α$ (DLA) absorber at $z_\mathrm{abs}=0.313$ toward the quasar PKS 1127$-$145.} We performed a detailed spectral analysis of the absorption profiles of CaII, MnII, TiII, and NaI associated with the DLA toward PKS 1127$-$145, based on optical high-resolution data obtained with the UVES instrument at the Very Large Telescope (VLT). We obtained column densities and Doppler-parameters for the ions listed above and determine their gas-phase abundances, from which we conclude on their dust depletion properties. We compared the Ca and Ti depletion properties of this DLA with that of other DLAs. One of the six analyzed absorption components shows a striking underabundance of Ti and Mn in the gas-phase, indicating the effect of dust depletion for these elements and a locally enhanced dust-to-gas ratio. In this DLA and in other similar absorbers, the MnII abundance follows that of TiII very closely, implying that both ions are equally sensitive to the dust depletion effects. Our analysis indicates that the DLA toward PKS 1127$-$145 has multiple origins. With its narrow line width and its strong dust depletion, component 3 points toward the presence of a neutral gas disk from a faint LSB galaxy in front of PKS 1127$-$145, while the other, more diffuse and dust-poor, absorption components possibly are related to tidal gas features from the interaction between the various, optically confirmed galaxy-group members. In general, the Mn/CaII ratio in sub-DLAs and DLAs possibly serves as an important indicator to discriminate between dust-rich and dust-poor in neutral gas in and around galaxies.

astro-ph.GA↗

arXiv2017

Mapping diffuse interstellar bands in the local ISM on small scales via MUSE 3D spectroscopy

We map the interstellar medium (ISM) including the diffuse interstellar bands (DIBs) in absorption toward the globular cluster NGC 6397 using VLT/MUSE. This pilot study demonstrates the power of MUSE for mapping the local ISM on very small scales which provides a new window for ISM observations. Assuming the absorbers are located at the rim of the Local Bubble we trace small-scale variations in NaI and KI as well as in several DIBs structures on the order of mpc (milliparsec, a few thousand AU). The sightlines defined by binned stellar spectra are separated by only a few arcseconds and we probe the absorption within a physically connected region. This analysis utilized the fitting residuals of individual stellar spectra of NGC 6397 member stars and analyzed lines from neutral species and several DIBs in Voronoi-binned composite spectra with high signal-to-noise ratio (S/N). We verify the suitability of the MUSE 3D spectrograph for such measurements and gain new insights by probing a single physical absorber with multiple sight lines.

astro-ph.GA↗

arXiv2016

Gas Accretion onto the Milky Way

The Milky Way is surrounded by large amounts of gaseous matter that are slowly being accreted over cosmic timescales to support star formation in the disk. The corresponding gas-accretion rate represents a key parameter for the past, present, and future evolution of the Milky Way. In this article, I discuss our current understanding of gas accretion processes in the Galaxy by reviewing past and recent observational and theoretical studies. The first part of this review deals with the spatial distribution of the different gas phases in the Milky Way halo, the origin of the gas, and its total mass. The second part discusses the gas dynamics and the physical processes that regulate the gas flow from the outer Galactic halo to the disk. From the most recent studies follows that the present-day gas accretion rate of the Milky Way is a few solar masses per year, which is sufficient to maintain the Galaxy's star-formation rate at its current level.

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