Fabian Walter

Bio: Fabian Walter is an academic researcher from Max Planck Society. The author has contributed to research in topics: Galaxy & Star formation. The author has an hindex of 146, co-authored 999 publications receiving 83016 citations. Previous affiliations of Fabian Walter include California Institute of Technology & University of Bonn.

Comparing [CII], HI, and CO dynamics of nearby galaxies

Abstract: The HI and CO components of the interstellar medium (ISM) are usually used to derive the dynamical mass M_dyn of nearby galaxies. Both components become too faint to be used as a tracer in observations of high-redshift galaxies. In those cases, the 158 $\mu$m line of atomic carbon [CII] may be the only way to derive M_dyn. As the distribution and kinematics of the ISM tracer affects the determination of M_dyn, it is important to quantify the relative distributions of HI, CO and [CII]. HI and CO are well-characterised observationally, however, for [CII] only very few measurements exist. Here we compare observations of CO, HI, and [CII] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS and KINGFISH surveys. We find that within R_25, the average [CII] exponential radial profile is slightly shallower than that of the CO, but much steeper than the HI distribution. This is also reflected in the integrated spectrum ("global profile"), where the [CII] spectrum looks more like that of the CO than that of the HI. For one galaxy, a spectrally resolved comparison of integrated spectra was possible; other comparisons were limited by the intrinsic line-widths of the galaxies and the coarse velocity resolution of the [CII] data. Using high-spectral-resolution SOFIA [CII] data of a number of star forming regions in two nearby galaxies, we find that their [CII] linewidths agree better with those of the CO than the HI. As the radial extent of a given ISM tracer is a key input in deriving M_dyn from spatially unresolved data, we conclude that the relevant length-scale to use in determining M_dyn based on [CII] data, is that of the well-characterised CO distribution. This length scale is similar to that of the optical disk.

A Sensitive Search for [N II]205 μm Emission in a z = 6.4 Quasar Host Galaxy

Abstract: We present a sensitive search for the 3P1 → 3P0 ground-state fine structure line at 205 μm of ionized nitrogen ([N II]205μm) in one of the highest-redshift quasars (J1148+5251 at z = 6.42) using the IRAM 30 m telescope. The line is not detected at a (3σ) depth of 0.47 Jy km s^−1, corresponding to a [N II]205μm luminosity limit of L[N II] 7) using the Atacama Large Millimeter/submillimeter Array, for which the highly excited rotational transitions of CO will be shifted outside the accessible (sub-)millimeter bands.

The Superwind Galaxy NGC 4666: Gravitational Interactions and the Influence of the Resulting Starburst on the Interstellar Medium

Abstract: We present high-resolution observations of the atomic and molecular gas phase of the "superwind" galaxy NGC 4666 Deep Very Large Array (VLA) H I observations of NGC 4666 and its surroundings reveal the presence of prominent tidal arms, which provide clear evidence that the galaxy is interacting with its neighbor NGC 4668 and a newly discovered dwarf companion This interaction is also evident from a dynamical analysis, which shows that the diffuse H I envelope around NGC 4666 (radii >19 kpc) is kinematically altered with regard to the central disk This interaction has likely caused the starburst activity in NGC 4666 A cut through the H I emission distribution perpendicular to the major axis provides no evidence for the existence of an H I halo: this distribution can best be described by an inner disk and two outer spiral arms Our high-resolution Owens Valley Radio Observatory (OVRO) CO mosaic shows that the molecular gas is distributed relatively uniformly in the central ~7 kpc; the total molecular gas mass is ~10 × 1010 M☉ A kinematically interesting CO feature that can be interpreted as an expanding molecular supershell is detected near the footpoint of one of the Hα outflows at the turnover of galactic rotation About 1% of the total energy input of the starburst would be needed to create this feature A study of the CO (2 → 1) to CO (1 → 0) line-transition ratio (obtained with the IRAM 30 m and Swedish-ESO Submillimeter Telescope [SEST] telescopes) shows that there is no significant variation of the molecular gas excitation over the central galaxy disk [S(2 → 1)/S(1 → 0) 085] on scales of ~25 kpc This can be explained by the unusually uniform star formation rate across NGC 4666's disk, which sets this system apart from most other starburst systems The total amount of molecular and atomic gas is Mmol 30 × 1010 M☉, which implies that NGC 4666 can sustain many similar starburst episodes (consuming ~108 M☉ each) in the future Two new dwarf companions in the NGC 4666 group are detected, with a detection limit of 107 M☉, over an area of ~16 × 104 kpc2 We speculate that the huge H I envelope around NGC 4666 may represent a low-redshift counterpart for damped Lyα systems seen at higher redshifts

The z = 7.54 Quasar ULAS J1342+0928 Is Hosted by a Galaxy Merger

Abstract: We present ALMA 0.28"x0.20" (1.4kpc x 1.0kpc) resolution observations of the [CII] 158um line and underlying dust continuum in the host galaxy of the most distant quasar currently known, ULAS J1342+0928 at z=7.5413. Both the [CII] and continuum are detected and spatially resolved. The [CII] line is ~1.5 times more extended than the continuum emission, showing an elongated and complex structure of approximately 3.2kpc x 6.4kpc. Two separate peaks are clearly seen (>6sigma each) in three 100 km/s width [CII] channel maps. The velocity field of the [CII] gas does not show evidence of a coherent rotation field but rather chaotic motion, reminiscent of an on-going merger.

No evidence for enhanced [OIII] 88um emission in a z~6 quasar compared to its companion starbursting galaxy

Abstract: We present ALMA band 8 observations of the [OIII] 88um line and the underlying thermal infrared continuum emission in the z=6.08 quasar CFHQS J2100-1715 and its dust-obscured starburst companion galaxy (projected distance: ~60 kpc). Each galaxy hosts dust-obscured star formation at rates > 100 M_sun/yr, but only the quasar shows evidence for an accreting 10^9 M_sun black hole. Therefore we can compare the properties of the interstellar medium in distinct galactic environments in two physically associated objects, ~1 Gyr after the Big Bang. Bright [OIII] 88um emission from ionized gas is detected in both systems; the positions and line-widths are consistent with earlier [CII] measurements, indicating that both lines trace the same gravitational potential on galactic scales. The [OIII] 88um/FIR luminosity ratios in both sources fall in the upper range observed in local luminous infrared galaxies of similar dust temperature, although the ratio of the quasar is smaller than in the companion. This suggests that gas ionization by the quasar (expected to lead to strong optical [OIII] 5008A emission) does not dominantly determine the quasar's FIR [OIII] 88um luminosity. Both the inferred number of photons needed for the creation of O++ and the typical line ratios can be accounted for without invoking extreme (top-heavy) stellar initial mass functions in the starbursts of both sources.

Gaia Data Release 2. Summary of the contents and survey properties

Abstract: Context. We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system. Aims: A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results. Methods: The raw data collected with the Gaia instruments during the first 22 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into this second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products. Results: Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0.5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the GBP (330-680 nm) and GRP (630-1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14 000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia-CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent. Conclusions: Gaia DR2 represents a major achievement for the Gaia mission, delivering on the long standing promise to provide parallaxes and proper motions for over 1 billion stars, and representing a first step in the availability of complementary radial velocity and source astrophysical information for a sample of stars in the Gaia survey which covers a very substantial fraction of the volume of our galaxy.

The wide-field infrared survey explorer (wise): mission description and initial on-orbit performance

Abstract: The all sky surveys done by the Palomar Observatory Schmidt, the European Southern Observatory Schmidt, and the United Kingdom Schmidt, the InfraRed Astronomical Satellite and the 2 Micron All Sky Survey have proven to be extremely useful tools for astronomy with value that lasts for decades. The Wide-field Infrared Survey Explorer is mapping the whole sky following its launch on 14 December 2009. WISE began surveying the sky on 14 Jan 2010 and completed its first full coverage of the sky on July 17. The survey will continue to cover the sky a second time until the cryogen is exhausted (anticipated in November 2010). WISE is achieving 5 sigma point source sensitivities better than 0.08, 0.11, 1 and 6 mJy in unconfused regions on the ecliptic in bands centered at wavelengths of 3.4, 4.6, 12 and 22 micrometers. Sensitivity improves toward the ecliptic poles due to denser coverage and lower zodiacal background. The angular resolution is 6.1", 6.4", 6.5" and 12.0" at 3.4, 4.6, 12 and 22 micrometers, and the astrometric precision for high SNR sources is better than 0.15".

Planck 2018 results. VI. Cosmological parameters

Abstract: We present cosmological parameter results from the final full-mission Planck measurements of the cosmic microwave background (CMB) anisotropies, combining information from the temperature and polarization maps and the lensing reconstruction Compared to the 2015 results, improved measurements of large-scale polarization allow the reionization optical depth to be measured with higher precision, leading to significant gains in the precision of other correlated parameters Improved modelling of the small-scale polarization leads to more robust constraints on manyparameters,withresidualmodellinguncertaintiesestimatedtoaffectthemonlyatthe05σlevelWefindgoodconsistencywiththestandard spatially-flat6-parameter ΛCDMcosmologyhavingapower-lawspectrumofadiabaticscalarperturbations(denoted“base ΛCDM”inthispaper), from polarization, temperature, and lensing, separately and in combination A combined analysis gives dark matter density Ωch2 = 0120±0001, baryon density Ωbh2 = 00224±00001, scalar spectral index ns = 0965±0004, and optical depth τ = 0054±0007 (in this abstract we quote 68% confidence regions on measured parameters and 95% on upper limits) The angular acoustic scale is measured to 003% precision, with 100θ∗ = 10411±00003Theseresultsareonlyweaklydependentonthecosmologicalmodelandremainstable,withsomewhatincreasederrors, in many commonly considered extensions Assuming the base-ΛCDM cosmology, the inferred (model-dependent) late-Universe parameters are: HubbleconstantH0 = (674±05)kms−1Mpc−1;matterdensityparameterΩm = 0315±0007;andmatterfluctuationamplitudeσ8 = 0811±0006 We find no compelling evidence for extensions to the base-ΛCDM model Combining with baryon acoustic oscillation (BAO) measurements (and consideringsingle-parameterextensions)weconstraintheeffectiveextrarelativisticdegreesoffreedomtobe Neff = 299±017,inagreementwith the Standard Model prediction Neff = 3046, and find that the neutrino mass is tightly constrained toPmν < 012 eV The CMB spectra continue to prefer higher lensing amplitudesthan predicted in base ΛCDM at over 2σ, which pulls some parameters that affect thelensing amplitude away from the ΛCDM model; however, this is not supported by the lensing reconstruction or (in models that also change the background geometry) BAOdataThejointconstraintwithBAOmeasurementsonspatialcurvatureisconsistentwithaflatuniverse, ΩK = 0001±0002Alsocombining with Type Ia supernovae (SNe), the dark-energy equation of state parameter is measured to be w0 = −103±003, consistent with a cosmological constant We find no evidence for deviations from a purely power-law primordial spectrum, and combining with data from BAO, BICEP2, and Keck Array data, we place a limit on the tensor-to-scalar ratio r0002 < 006 Standard big-bang nucleosynthesis predictions for the helium and deuterium abundances for the base-ΛCDM cosmology are in excellent agreement with observations The Planck base-ΛCDM results are in good agreement with BAO, SNe, and some galaxy lensing observations, but in slight tension with the Dark Energy Survey’s combined-probe results including galaxy clustering (which prefers lower fluctuation amplitudes or matter density parameters), and in significant, 36σ, tension with local measurements of the Hubble constant (which prefer a higher value) Simple model extensions that can partially resolve these tensions are not favoured by the Planck data

The Confrontation Between General Relativity and Experiment

Abstract: The status of experimental tests of general relativity and of theoretical frameworks for analyzing them is reviewed and updated. Einstein’s equivalence principle (EEP) is well supported by experiments such as the Eotvos experiment, tests of local Lorentz invariance and clock experiments. Ongoing tests of EEP and of the inverse square law are searching for new interactions arising from unification or quantum gravity. Tests of general relativity at the post-Newtonian level have reached high precision, including the light deflection, the Shapiro time delay, the perihelion advance of Mercury, the Nordtvedt effect in lunar motion, and frame-dragging. Gravitational wave damping has been detected in an amount that agrees with general relativity to better than half a percent using the Hulse-Taylor binary pulsar, and a growing family of other binary pulsar systems is yielding new tests, especially of strong-field effects. Current and future tests of relativity will center on strong gravity and gravitational waves.

Multi-messenger Observations of a Binary Neutron Star Merger

Abstract: On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of $\sim$1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg$^2$ at a luminosity distance of $40^{+8}_{-8}$ Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Msun. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at $\sim$40 Mpc) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over $\sim$10 days. Following early non-detections, X-ray and radio emission were discovered at the transient's position $\sim$9 and $\sim$16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. (Abridged)