TY - JOUR A1 - Zhang, Heshou A1 - Yan, Huirong A1 - Richter, Philipp T1 - The influence of atomic alignment on absorption and emission spectroscopy JF - Monthly notices of the Royal Astronomical Society N2 - Spectroscopic observations play essential roles in astrophysics. They are crucial for determining physical parameters in our 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 ionization fraction. Synthetic observations are performed to illustrate the visibility of such effect with current facilities. A study of Photodissociation regions in rho 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. KW - magnetic fields KW - submillimetre: ISM KW - ultraviolet: ISM KW - ISM: abundances KW - ISM: lines and bands KW - techniques: spectroscopic Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty1594 SN - 0035-8711 SN - 1365-2966 VL - 479 IS - 3 SP - 3923 EP - 3935 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Worseck, Gabor A1 - Prochaska, J. Xavier A1 - McQuinn, Matthew A1 - Dall'Aglio, Aldo A1 - Fechner, Cora A1 - Hennawi, Joseph F. A1 - Reimers, Dieter A1 - Richter, Philipp A1 - Wisotzki, Lutz T1 - The end of Helium Reionization at z similar or equal to 2.7 Inferred from cosmic variance in HST/COS He II Ly alpha Absorption spectra JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters N2 - We report on the detection of strongly varying intergalactic He II absorption in HST/COS spectra of two z(em) similar or equal to 3 quasars. From our homogeneous analysis of the He II absorption in these and three archival sightlines, we find a marked increase in the mean He II effective optical depth from similar or equal to 1 at z similar or equal to 2.3 to greater than or similar to 5 at z similar or equal to 3.2, but with a large scatter of 2 less than or similar to tau(eff, He II) less than or similar to 5 at 2.7 < z < 3 on scales of similar to 10 proper Mpc. This scatter is primarily due to fluctuations in the He II fraction and the He II-ionizing background, rather than density variations that are probed by the coeval Hi forest. Semianalytic models of He II absorption require a strong decrease in the He II-ionizing background to explain the strong increase of the absorption at z greater than or similar to 2.7, probably indicating He II reionization was incomplete at z(reion) greater than or similar to 2.7. Likewise, recent three-dimensional numerical simulations of He II reionization qualitatively agree with the observed trend only if He II reionization completes at z(reion) similar or equal to 2.7 or even below, as suggested by a large tau(eff, He II) greater than or similar to 3 in two of our five sightlines at z < 2.8. By doubling the sample size at 2.7 less than or similar to z less than or similar to 3, our newly discovered He II sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized. KW - dark ages, reionization, first stars KW - diffuse radiation KW - intergalactic medium KW - quasars: absorption lines KW - quasars: individual (SDSS J092447.36+485242.8, SDSS J110155.74+105302.3) Y1 - 2011 U6 - https://doi.org/10.1088/2041-8205/733/2/L24 SN - 2041-8205 VL - 733 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Wisotzki, Lutz A1 - Bacon, R. A1 - Brinchmann, J. A1 - Cantalupo, S. A1 - Richter, Philipp A1 - Schaye, J. A1 - Schmidt, Kasper Borello A1 - Urrutia, Tanya A1 - Weilbacher, Peter Michael A1 - Akhlaghi, M. A1 - Bouche, N. A1 - Contini, T. A1 - Guiderdoni, B. A1 - Herenz, E. C. A1 - Inami, H. A1 - Kerutt, Josephine Victoria A1 - Leclercq, F. A1 - Marino, R. A. A1 - Maseda, M. A1 - Monreal-Ibero, A. A1 - Nanayakkara, T. A1 - Richard, J. A1 - Saust, R. A1 - Steinmetz, Matthias A1 - Wendt, Martin T1 - Nearly all the sky is covered by Lyman-alpha emission around high-redshift galaxies JF - Nature : the international weekly journal of science N2 - Galaxies are surrounded by large reservoirs of gas, mostly hydrogen, that are fed by inflows from the intergalactic medium and by outflows from galactic winds. Absorption-line measurements along the lines of sight to bright and rare background quasars indicate that this circumgalactic medium extends far beyond the starlight seen in galaxies, but very little is known about its spatial distribution. The Lyman-alpha transition of atomic hydrogen at a wavelength of 121.6 nanometres is an important tracer of warm (about 104 kelvin) gas in and around galaxies, especially at cosmological redshifts greater than about 1.6 at which the spectral line becomes observable from the ground. Tracing cosmic hydrogen through its Lyman-a emission has been a long-standing goal of observational astrophysics(1-3), but the extremely low surface brightness of the spatially extended emission is a formidable obstacle. A new window into circumgalactic environments was recently opened by the discovery of ubiquitous extended Lyman-alpha emission from hydrogen around high-redshift galaxies(4,5). Such measurements were previously limited to especially favourable systems(6-8) or to the use of massive statistical averaging(9,10) because of the faintness of this emission. Here we report observations of low-surface-brightness Lyman-alpha emission surrounding faint galaxies at redshifts between 3 and 6. We find that the projected sky coverage approaches 100 per cent. The corresponding rate of incidence (the mean number of Lyman-alpha emitters penetrated by any arbitrary line of sight) is well above unity and similar to the incidence rate of high-column-density absorbers frequently detected in the spectra of distant quasars(11-14). This similarity suggests that most circumgalactic atomic hydrogen at these redshifts has now been detected in emission. Y1 - 2018 U6 - https://doi.org/10.1038/s41586-018-0564-6 SN - 0028-0836 SN - 1476-4687 VL - 562 IS - 7726 SP - 229 EP - 232 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Winkel, B. A1 - Ben Bekhti, Nadya A1 - Darmstaedter, V. A1 - Floeer, L. A1 - Kerp, J. A1 - Richter, Philipp T1 - The high-velocity cloud complex Galactic center negative as seen by EBHIS and GASS I. Cloud catalog and global properties JF - Astronomy and astrophysics : an international weekly journal N2 - Using Milky Way data of the new Effelsberg-Bonn HI Survey (EBHIS) and the Galactic All-Sky Survey (GASS), we present a revised picture of the high-velocity cloud (HVC) complex Galactic center negative (GCN). Owing to the higher angular resolution of these surveys compared to previous studies (e.g., the Leiden Dwingeloo Survey), we resolve complex GCN into lots of individual tiny clumps, that mostly have relatively broad line widths of more than 15 km s(-1). We do not detect a diffuse extended counterpart, which is unusual for an HVC complex. In total 243 clumps were identified and parameterized which allows us to statistically analyze the data. Cold-line components (i.e.,Delta upsilon(fwhm) < 7.5 km s(-1)) are found in about 5% only of the identified cloudlets. Our analysis reveals that complex GCN is likely built up of several subpopulations that do not share a common origin. Furthermore, complex GCN might be a prime example for warm-gas accretion onto the Milky Way, where neutral HI clouds are not stable against interaction with the Milky Way gas halo and become ionized prior to accretion. KW - ISM: clouds KW - Galaxy: halo Y1 - 2011 U6 - https://doi.org/10.1051/0004-6361/201117357 SN - 0004-6361 VL - 533 IS - 18 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Wendt, Martin A1 - Husser, Tim-Oliver A1 - Kamann, Sebastian A1 - Monreal-Ibero, Ana A1 - Richter, Philipp A1 - Brinchmann, Jarle A1 - Dreizler, Stefan A1 - Weilbacher, Peter Michael A1 - Wisotzki, Lutz T1 - Mapping diffuse interstellar bands in the local ISM on small scales via MUSE 3D spectroscopy A pilot study based on globular cluster NGC 6397 JF - Astronomy and astrophysics : an international weekly journal N2 - Context. We map the interstellar medium (ISM) including the diffuse interstellar bands (DIBs) in absorption toward the globular cluster NGC6397 using VLT/MUSE. Assuming the absorbers are located at the rim of the Local Bubble we trace structures on the order of mpc (milliparsec, a few thousand AU). Aims. We aimed to demonstrate the feasibility to map variations of DIBs on small scales with MUSE. The sightlines defined by binned stellar spectra are separated by only a few arcseconds and we probe the absorption within a physically connected region. Methods. This analysis utilized the fitting residuals of individual stellar spectra of NGC6397 member stars and analyzed lines from neutral species and several DIBs in Voronoi-binned composite spectra with high signal-to-noise ratio (S/N). Results. 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. We detect small scale variations in Na-I and K-I as well as in several DIBs within few arcseconds, or mpc with regard to the Local Bubble. 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. KW - techniques: imaging spectroscopy KW - globular clusters: individual: NGC 6397 KW - dust, extinction KW - ISM: structure KW - ISM: lines and bands Y1 - 2017 U6 - https://doi.org/10.1051/0004-6361/201629816 SN - 1432-0746 VL - 607 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - van Loon, Jacco Th. A1 - Bailey, M. A1 - Tatton, B. L. A1 - Apellaniz, Jesus Maiz A1 - Crowther, P. A. A1 - de Koter, A. A1 - Evans, C. J. A1 - Henault-Brunet, V. A1 - Howarth, I. D. A1 - Richter, Philipp A1 - Sana, Hugues A1 - Simon Díaz, Sergio A1 - Taylor, W. A1 - Walborn, N. R. T1 - The VLT-FLAMES tarantula survey IX. - the interstellar medium seen through diffuse interstellar bands and neutral sodium JF - Astronomy and astrophysics : an international weekly journal N2 - Context. The Tarantula Nebula (a.k.a. 30 Dor) is a spectacular star-forming region in the Large Magellanic Cloud (LMC), seen through gas in the Galactic disc and halo. Diffuse interstellar bands (DIBs) offer a unique probe of the diffuse, cool-warm gas in these regions. Aims. The aim is to use DIBs as diagnostics of the local interstellar conditions, whilst at the same time deriving properties of the yet-unknown carriers of these enigmatic spectral features. Methods. Spectra of over 800 early-type stars from the Very Large Telescope Flames Tarantula Survey (VFTS) were analysed. Maps were created, separately, for the Galactic and LMC absorption in the DIBs at 4428 and 6614 angstrom and - in a smaller region near the central cluster R 136 - neutral sodium (the Na ID doublet); we also measured the DIBs at 5780 and 5797 angstrom. Results. The maps show strong 4428 and 6614 angstrom DIBs in the quiescent cloud complex to the south of 30 Dor but weak absorption in the harsher environments to the north (bubbles) and near the OB associations. The Na maps show at least five kinematic components in the LMC and a shell-like structure surrounding R 136, and small-scale structure in the Milky Way. The strengths of the 4428, 5780, 5797 and 6614 angstrom DIBs are correlated, also with Na absorption and visual extinction. The strong 4428 angstrom DIB is present already at low Na column density but the 6614, 5780 and 5797 angstrom DIBs start to be detectable at subsequently larger Na column densities. Conclusions. The carriers of the 4428, 6614, 5780 and 5797 angstrom DIBs are increasingly prone to removal from irradiated gas. The relative strength of the 5780 and 5797 angstrom DIBs clearly confirm the Tarantula Nebula as well as Galactic high-latitude gas to represent a harsh radiation environment. The resilience of the 4428 angstrom DIB suggests its carrier is large, compact and neutral. Structure is detected in the distribution of cool-warm gas on scales between one and > 100 pc in the LMC and as little as 0.01 pc in the Sun's vicinity. Stellar winds from the central cluster R 136 have created an expanding shell; some infalling gas is also detected, reminiscent of a galactic "fountain". KW - ISM: individual objects: Tarantula Nebula (30 Doradus Nebula) KW - ISM: molecules KW - ISM: kinematics and dynamics KW - ISM: lines and bands KW - ISM: structure KW - local insterstellar matter Y1 - 2013 U6 - https://doi.org/10.1051/0004-6361/201220210 SN - 0004-6361 VL - 550 IS - 9 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Tepper-Garcia, Thorsten A1 - Richter, Philipp A1 - Schaye, Joop A1 - Booth, C. M. A1 - Vecchia, Claudio Dalla A1 - Theuns, Tom A1 - Wiersma, Robert P. C. T1 - Absorption signatures of warm-hot gas at low redshift o vi JF - Monthly notices of the Royal Astronomical Society N2 - We investigate the origin and physical properties of O vi absorbers at low redshift (z = 0.25) using a subset of cosmological, hydrodynamical simulations from the OverWhelmingly Large Simulations (OWLS) project. Intervening O vi absorbers are believed to trace shock-heated gas in the warm-hot intergalactic medium (WHIM) and may thus play a key role in the search for the missing baryons in the present-day Universe. When compared to observations, the predicted distributions of the different O vi line parameters (column density, Doppler parameter, rest equivalent width W-r) from our simulations exhibit a lack of strong O vi absorbers, a discrepancy that has also been found by Oppenheimer & Dave. This suggests that physical processes on subgrid scales (e.g. turbulence) may strongly influence the observed properties of O vi systems. We find that the intervening O vi absorption arises mainly in highly metal enriched (10-1 < Z/Z(circle dot) less than or similar to 1) gas at typical overdensities of 1 < /<<>> less than or similar to 102. One-third of the O vi absorbers in our simulation are found to trace gas at temperatures T < 105 K, while the rest arises in gas at higher temperatures, most of them around T = 105.3 +/- 0.5 K. These temperatures are much higher than inferred by Oppenheimer & Dave, probably because that work did not take the suppression of metal-line cooling by the photoionizing background radiation into account. While the O vi resides in a similar region of (, T)-space as much of the shock-heated baryonic matter, the vast majority of this gas has a lower metal content and does not give rise to detectable O vi absorption. As a consequence of the patchy metal distribution, O vi absorbers in our simulations trace only a very small fraction of the cosmic baryons (< 2 per cent) and the cosmic metals. Instead, these systems presumably trace previously shock-heated, metal-rich material from galactic winds that is now mixing with the ambient gas and cooling. The common approach of comparing O vi and H i column densities to estimate the physical conditions in intervening absorbers from QSO observations may be misleading, as most of the H i (and most of the gas mass) is not physically connected with the high-metallicity patches that give rise to the O vi absorption. KW - methods: numerical KW - galaxies: formation KW - intergalactic medium KW - quasars: absorption lines KW - cosmology: theory Y1 - 2011 U6 - https://doi.org/10.1111/j.1365-2966.2010.18123.x SN - 0035-8711 VL - 413 IS - 1 SP - 190 EP - 212 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Tepper-Garcia, Thorsten A1 - Richter, Philipp A1 - Schaye, Joop A1 - Booth, C. M. A1 - Dalla Vecchia, Claudio A1 - Theuns, Tom T1 - Absorption signatures of warm-hot gas at low redshift: broad H?i Lya absorbers JF - Monthly notices of the Royal Astronomical Society N2 - We investigate the physical state of H?i absorbing gas at low redshift (z = 0.25) using a subset of cosmological, hydrodynamic simulations from the OverWhelmingly Large Simulations project, focusing in particular on broad (bHI=40 km s-1) H?i Lya absorbers (BLAs), which are believed to originate in shock-heated gas in the warm-hot intergalactic medium (WHIM). Our fiducial model, which includes radiative cooling by heavy elements and feedback by supernovae and active galactic nuclei, predicts that by z = 0.25 nearly 60?per cent of the gas mass ends up at densities and temperatures characteristic of the WHIM and we find that half of this fraction is due to outflows. The standard H?i observables (distribution of H?i column densities NH?I, distribution of Doppler parameters bHI, bHINH?I correlation) and the BLA line number density predicted by our simulations are in remarkably good agreement with observations. BLAs arise in gas that is hotter, more highly ionized and more enriched than the gas giving rise to typical Lya forest absorbers. The majority of the BLAs arise in warm-hot [log?(T/?K) similar to 5] gas at low (log?? < 1.5) overdensities. On average, thermal broadening accounts for at least 60?per cent of the BLA linewidth, which in turn can be used as a rough indicator of the thermal state of the gas. Detectable BLAs account for only a small fraction of the true baryon content of the WHIM at low redshift. In order to detect the bulk of the mass in this gas phase, a sensitivity at least one order of magnitude better than achieved by current ultraviolet spectrographs is required. We argue that BLAs mostly trace gas that has been shock heated and enriched by outflows and that they therefore provide an important window on a poorly understood feedback process. KW - methods: numerical KW - galaxies: formation KW - intergalactic medium KW - quasars: absorption lines KW - cosmology: theory Y1 - 2012 U6 - https://doi.org/10.1111/j.1365-2966.2012.21545.x SN - 0035-8711 VL - 425 IS - 3 SP - 1640 EP - 1663 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Tepper-Garcia, Thor A1 - Richter, Philipp A1 - Schaye, Joop T1 - Absorption signatures of warm-hot gas at low redshift - ne viii JF - Monthly notices of the Royal Astronomical Society N2 - At z < 1 a large fraction of the baryons is thought to reside in diffuse gas that has been shock-heated to high temperatures (10 (5)-10 (6) K). Absorption by the 770.41, 780.32 A doublet of Ne viii in quasar spectra represents a unique tool to study this elusive warm-hot phase. We have developed an analytic model for the properties of Ne viii absorbers that allows for an inhomogeneous metal distribution. Our model agrees with the predictions of a simulation from the OverWhelmingly Large Simulations project indicating that the average line-of-sight metal-filling fraction within the absorbing gas is low (c(L) similar to 0.1). Most of the Ne viii in our model is produced in low-density, collisionally ionized gas (n(H) = 10(-6)-10(-4) cm(-3), T = 10 (5)-10 (6) K). Strong Ne viii absorbers (log(10)(N-NeVIII/cm(-2))14), like those recently detected by Hubble Space Telescope/Cosmic Origins Spectrograph, are found to arise in higher density gas (n(H) greater than or similar to 10(-4) cm(-3), T approximate to 5 x 10 (5) K). Ne viii cloudlets harbour only 1 per cent of the cosmic baryon budget. The baryon content of the surrounding gas (which has similar densities and temperatures as the Ne viii cloudlets) is a factor c(-1)L higher. We conclude that Ne viii absorbers are robust probes of shock-heated diffuse gas, but that spectra with signal-to-noise ratios S/N > 100 would be required to detect the bulk of the baryons in warm-hot gas. KW - methods: analytical KW - methods: numerical KW - galaxies: formation KW - intergalactic medium KW - quasars: absorption lines KW - cosmology: theory Y1 - 2013 U6 - https://doi.org/10.1093/mnras/stt1712 SN - 0035-8711 SN - 1365-2966 VL - 436 IS - 3 SP - 2063 EP - 2081 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Sparre, Martin A1 - Whittingham, Joseph A1 - Damle, Mitali A1 - Hani, Maan H. A1 - Richter, Philipp A1 - Ellison, Sara L. A1 - Pfrommer, Christoph A1 - Vogelsberger, Mark T1 - Gas flows in galaxy mergers BT - supersonic turbulence in bridges, accretion from the circumgalactic medium, and metallicity dilution JF - Monthly notices of the Royal Astronomical Society N2 - 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 magnetohydrodynamical 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 percent) 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 percent 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 on to 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. KW - MHD KW - methods: numerical KW - galaxies: interactions KW - galaxies: starburst Y1 - 2021 U6 - https://doi.org/10.1093/mnras/stab3171 SN - 1365-2966 VL - 509 IS - 2 SP - 2720 EP - 2735 PB - Oxford Univ. Press CY - Oxford ER -