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.

Physical Constraints on the Extended Interstellar Medium of the z = 6.42 Quasar J1148+5251: [C ii]158 μm, [N ii]205 μm, and [O i]146 μm Observations

Abstract: We report new Northern Extended Millimeter Array observations of the [C ii]158 μm, [N ii]205 μm, and [O i]146 μm atomic fine structure lines (FSLs) and dust continuum emission of J1148+5251, a z = 6.42 quasar, which probe the physical properties of its interstellar medium (ISM). The radially averaged [C ii]158 μm and dust continuum emission have similar extensions (up to θ=2.51−0.25+0.46arcsec , corresponding to r=9.8−2.1+3.3kpc , accounting for beam convolution), confirming that J1148+5251 is the quasar with the largest [C ii]158 μm-emitting reservoir known at these epochs. Moreover, if the [C ii]158 μm emission is examined only along its NE–SW axis, a significant excess (>5.8σ) of [C ii]158 μm emission (with respect to the dust) is detected. The new wide-bandwidth observations enable us to accurately constrain the continuum emission, and do not statistically require the presence of broad [C ii]158 μm line wings that were reported in previous studies. We also report the first detection of the [O i]146 μm and (tentatively) [N ii]205 μm emission lines in J1148+5251. Using FSL ratios of the [C ii]158 μm, [N ii]205 μm, [O i]146 μm, and previously measured [C i]369 μm emission lines, we show that J1148+5251 has similar ISM conditions compared to lower-redshift (ultra)luminous infrared galaxies. CLOUDY modeling of the FSL ratios excludes X-ray-dominated regions and favors photodissociation regions as the origin of the FSL emission. We find that a high radiation field (103.5–4.5 G 0), a high gas density (n ≃ 103.5–4.5 cm−3), and an H i column density of 1023 cm−2 reproduce the observed FSL ratios well.

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

Abstract: Most galactic nuclei are now believed to harbour supermassive black holes. Studies of stellar motions in the central few light-years of our Milky Way Galaxy indicate the presence of a dark object with a mass of about 2.6 million solar masses. This object is spatially coincident with Sagittarius A* (Sgr A*), the unique compact radio source located at the dynamical centre of our Galaxy. By analogy with distant quasars and nearby active galactic nuclei (AGN), Sgr A* is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole. However, Sgr A* is much fainter than expected in all wavebands, especially in X-rays, casting some doubt on this model. Recently, we reported the first strong evidence of X-ray emission from Sgr A*. Here we report the discovery of rapid X-ray flaring from the direction of Sgr A*. These data provide compelling evidence that the X-ray emission is coming from accretion onto a supermassive black hole at the Galactic Centre, and the nature of the variations provides strong constraints on the astrophysical processes near the event horizon of the black hole.

Co-evolution of massive black holes and their host galaxies at high redshift: discrepancies from six cosmological simulations and the key role of JWST

Abstract: The James Webb Space Telescope will have the power to characterize high-redshift quasars at z > 6 with an unprecedented depth and spatial resolution. While the brightest quasars at such redshift (i.e., with bolometric luminosity Lbol > 10 erg/s) provide us with key information on the most extreme objects in the Universe, measuring the black hole (BH) mass and Eddington ratios of fainter quasars with Lbol = 10 −10 erg/s opens a path to understand the build-up of more normal BHs at z > 6. In this paper, we show that the Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA large-scale cosmological simulations do not agree on whether BHs at z > 4 are overmassive or undermassive at fixed galaxy stellar mass with respect to the MBH −M? scaling relation at z = 0 (BH mass offsets). Our conclusions are unchanged when using the local scaling relation produced by each simulation or empirical relations. We find that the BH mass offsets of the simulated faint quasar population at z > 4, unlike those of bright quasars, represent the BH mass offsets of the entire BH population, for all the simulations. Thus, a population of faint quasars with Lbol = 10 45 − 10 erg/s observed by JWST can provide key constraints on the assembly of BHs at high redshift. Moreover, this will help constraining the highredshift regime of cosmological simulations, including BH seeding, early growth, and co-evolution with the host galaxies. Our results also motivate the need for simulations of larger cosmological volumes down to z ∼ 6, with the same diversity of sub-grid physics, in order to gain statistics on the most extreme objects at high redshift.

Spatially Resoved 12CO(2-1)/12CO(1-0) in the Starburst Galaxy NGC 253: Assessing Optical Depth to Constrain the Molecular Mass Outflow Rate

Abstract: We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of 12CO(1-0) and 12CO(2-1) in the central 40" (680 pc) of the nuclear starburst galaxy NGC 253, including its molecular outflow. We measure the ratio of brightness temperature for CO(2-1)/CO(1-0), r_21, in the central starburst and outflow-related features. We discuss how r_21 can be used to constrain the optical depth of the CO emission, which impacts the inferred mass of the outflow and consequently the molecular mass outflow rate. We find r_21 less than or equal to 1 throughout, consistent with a majority of the CO emission being optically-thick in the outflow, as it is in the starburst. This suggests that the molecular outflow mass is 3-6 times larger than the lower limit reported for optically thin CO emission from warm molecular gas. The implied molecular mass outflow rate is 25-50 solar masses per year, assuming that conversion factor for the outflowing gas is similar to our best estimates for the bulk of the starburst. This is a factor of 9-19 times larger than the star formation rate in NGC 253. We see tentative evidence for an extended, diffuse CO(2-1) component.

High Sensitivity Array Observations of the z = 4.4 QSO BRI 1335−0417

Abstract: We present sensitive phase-referenced VLBI results on the radio continuum emission from the z = 4.4 QSO BRI 1335-0417. The observations were carried out at 1.4 GHz using the High Sensitivity Array. Our sensitive VLBI image at 189 × 113 mas (1.25 × 0.75 kpc) resolution shows continuum emission in BRI 1335-0417 with a total flux density of 208 ± 46 μJy, consistent with the flux density measured with the VLA. The size of the source at FWHM is 255 × 138 mas (1.7 × 0.9 kpc), and the derived intrinsic brightness temperature is ~3.5 × 104 K. No continuum emission is detected at the full VLBI resolution (32 × 7 mas, 211 × 46 pc), with a 4 σ point-source upper limit of 34 μJy beam-1, or an upper limit to the intrinsic brightness temperature of 5.6 × 105 K. The highest angular resolution with at least a 4.5 σ detection of the radio continuum emission is 53 × 27 mas (0.35 × 0.18 kpc). At this resolution the image shows a continuum feature in BRI 1335-0417 with a size of 64 × 35 mas (0.42 × 0.23 kpc) at FWHM and an intrinsic brightness temperature of ~2 × 105 K. The extent of the observed continuum sources at 1.4 GHz and the derived brightness temperatures show that the radio emission (and thus, presumably, the far-IR emission) in BRI 1335-0417 is powered by a major starburst with a massive star formation rate of order a few thousand M⊙ yr-1. Moreover, the absence of any compact high brightness temperature source suggests that there is no radio-loud AGN in this z = 4.4 QSO.

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)