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.

The Anatomy of an Extreme Starburst within 1.3 Gyr of the Big Bang Revealed by ALMA

Abstract: We present further analysis of the [C II] 158 μm fine structure line and thermal dust continuum emission from the archetype extreme starburst/active galactic nucleus (AGN) group of galaxies in the early universe, BRI 1202–0725 at z = 4.7, using the Atacama Large Millimeter Array. The group has long been noted for having a closely separated (26 kpc in projection) FIR-hyperluminous quasar host galaxy and an optically obscured submillimeter galaxy (SMG). A short ALMA test observation reveals a rich laboratory for the study of the myriad processes involved in clustered massive galaxy formation in the early universe. Strong [C II] emission from the SMG and the quasar have been reported earlier by Wagg et al. based on these observations. In this paper, we examine in more detail the imaging results from the ALMA observations, including velocity channel images, position-velocity plots, and line moment images. We present detections of [C II] emission from two Lyα-selected galaxies in the group, demonstrating the relative ease with which ALMA can detect the [C II] emission from lower star formation rate galaxies at high redshift. Imaging of the [C II] emission shows a clear velocity gradient across the SMG, possibly indicating rotation or a more complex dynamical system on a scale ~10 kpc. There is evidence in the quasar spectrum and images for a possible outflow toward the southwest, as well as more extended emission (a "bridge"), between the quasar and the SMG, although the latter could simply be emission from Lyα-1 blending with that of the quasar at the limited spatial resolution of the current observations. These results provide an unprecedented view of a major merger of gas-rich galaxies driving extreme starbursts and AGN accretion during the formation of massive galaxies and supermassive black holes within 1.3 Gyr of the big bang.

Dust and gas in luminous proto-cluster galaxies at z=4.05: the case for different cosmic dust evolution in normal and starburst galaxies

Abstract: We measure the dust and gas content of the three sub-millimeter galaxies (SMGs) in the GN20 proto-cluster at z=405 using new IRAM Plateau de Bure interferometer (PdBI) CO(4-3) and 12-33mm continuum observations All these three SMGs are heavily dust obscured, with UV-based star formation rate (SFR) estimates significantly smaller than the ones derived from the bolometric IR, consistent with the spatial offsets revealed by HST and CO imaging Based also on evaluations of the specific SFR, CO-to-H2 conversion factor and gas depletion timescale, we classify all the three galaxies as starbursts (SBs), although with a lower confidence for GN202b that might be a later stage merging event We place our measurements in the context of the evolutionary properties of main sequence (MS) and SB galaxies ULIRGs have 3-5 times larger L′CO/Mdust and Mdust/M⋆ ratios than z=0 MS galaxies, but by z∼2 the difference appears to be blurred, probably due to differential metallicity evolution SB galaxies appear to slowly evolve in their L′CO/Mdust and Mdust/M⋆ ratios all the way to z>6 (consistent with rapid enrichment of SB events), while MS galaxies rapidly increase in Mdust/M⋆ from z=0 to 2 (due to gas fraction increase, compensated by a decrease of metallicities) While no IR/submm continuum detection is available for indisputably normal massive galaxies at z>25, we show that if metallicity indeed decrease rapidly for these systems at z>3 as claimed in the literature, we should expect a strong decrease of their Mdust/M⋆, consistent with recent PdBI and ALMA upper limits We conclude that the Mdust/M⋆ ratio could be a powerful tool for distinguishing starbursts from normal galaxies at z>4

The Temperature Distribution of Dense Molecular Gas in the Center of NGC 253

Abstract: We present interferometric maps of ammonia (NH3) of the nearby starburst galaxy NGC 253. The observations have been taken with the Australia Telescope Compact Array and include the para-NH3 (1, 1) and (2, 2) and the ortho-NH3 (3, 3) and (6, 6) inversion lines. Six major complexes of dense ammonia are identified, three of them on either side of the starburst center, out to projected galactocentric radii of ~250 pc. Rotational temperatures are derived toward selected individual positions, as well as for the entire southeastern and northwestern molecular complexes. The application of radiative transfer large velocity gradient models reveals that the bulk of the ammonia molecules is embedded in a one-temperature gas phase. Kinetic temperatures of this gas are ~200 and 140 K toward the southwest and northeast, respectively. The temperatures under which ammonia was formed in the past are with 30 K also warmer toward the southwest than toward the northeast (~15-20 K). This is indicated by the ortho-ammonia-to-para-ammonia ratio, which is ~1 and 1.5-2.5 toward the southwest and northeast, respectively. Ammonia column densities in the brightest complexes are in the range of (6-11) × 1014 cm-2, which adds up to a total ammonia mass of ~20 M☉, about evenly distributed toward both sides of the nucleus. Ammonia abundances relative to H2 are ~3 × 10-8. In the southwestern complex, the ammonia abundances increase from the starburst center to larger galactocentric radii. Toward the center of NGC 253, NH3 (1, 1), (2, 2), and (6, 6) are detected in absorption against an unresolved continuum source. At the same location, however, ammonia (3, 3) is found in emission, which indicates maser activity. This would be the first detected extragalactic NH3 maser. Evidence for an expanding shell in the southwestern complex is provided. The shell, with a dynamical age of ~1.3 Myr, is centered on an X-ray point source that must be located within the dense gas of NGC 253. The shell and X-ray properties can be reproduced by the energy input of a highly obscured young stellar cluster with a mass of ~105 M☉, which also heats the dense gas. A current star formation rate of ~2.8 M☉ yr-1 is derived for the nuclear starburst in NGC 253 based on its 1.2 cm continuum emission.

COLDz: Shape of the CO Luminosity Function at High Redshift and the Cold Gas History of the Universe

Abstract: We report the first detailed measurement of the shape of the CO luminosity function at high redshift, based on >320 hr of the NSF's Karl G. Jansky Very Large Array (VLA) observations over an area of ~60 arcmin^2 taken as part of the CO Luminosity Density at High Redshift (COLDz) survey. COLDz "blindly" selects galaxies based on their cold gas content through CO(J = 1 → 0) emission at z ~ 2–3 and CO(J = 2 → 1) at z ~ 5–7 down to a CO luminosity limit of log(L’_(CO)/K km s^(−1) pc^2) ≃ 9.5. We find that the characteristic luminosity and bright end of the CO luminosity function are substantially higher than predicted by semi-analytical models, but consistent with empirical estimates based on the infrared luminosity function at z ~ 2. We also present the currently most reliable measurement of the cosmic density of cold gas in galaxies at early epochs, i.e., the cold gas history of the universe, as determined over a large cosmic volume of ~375,000 Mpc3. Our measurements are in agreement with an increase of the cold gas density from z ~ 0 to z ~ 2–3, followed by a possible decline toward z ~ 5–7. These findings are consistent with recent surveys based on higher-J CO line measurements, upon which COLDz improves in terms of statistical uncertainties by probing ~50–100 times larger areas and in the reliability of total gas mass estimates by probing the low-J CO lines accessible to the VLA. Our results thus appear to suggest that the cosmic star formation rate density follows an increased cold molecular gas content in galaxies toward its peak about 10 billion years ago, and that its decline toward the earliest epochs is likely related to a lower overall amount of cold molecular gas (as traced by CO) bound in galaxies toward the first billion years after the Big Bang.

A Chandra X-ray survey of nearby dwarf starburst galaxies: II. Starburst properties and outflows

Abstract: We present a comprehensive comparison of the X-ray properties of a sample of eight dwarf starburst galaxies observed with Chandra (IZw18, VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253, He2–10). In PaperI we presented in detail the data reduction and analysis of the individual galaxies. For the unresolved X-ray sources we find the following: point sources are in general located close to bright H ii regions, rims of superbubbles, or young stellar clusters. The number of X-ray point sources appears to be a function of the current star formation rate and the blue luminosity of the hosts. Ultraluminous X-ray sources are only found in those dwarf galaxies which are currently interacting. The power law index of the combined cumulative X-ray point source luminosity function is α = 0.24 ± 0.06, shallower than that of more massive starburst galaxies (α = 0.4 − 0.8) and of non-starburst galaxies (α ∼ 1.2). For those galaxies showing extended X-ray emission (6 out of the 8 galaxies), we derive the following: Superwinds develop along the steepest gradient of the H i distribution with volume densities of 0.02 −0.06cm 3 , pressures of 1 −3 ×10 5 Kcm 3 , thermal energies of 2 − 30 × 10 54 erg, and hot gas masses of 2 − 20 × 10 6 M⊙ (∼ 1 per cent of the H i masses. On global scales, the distribution of the X-ray emission looks remarkably similar to that seen in Hα (comparing azimuthal averages); locally however their distribution is clearly distinct in many cases – this can be explained by the different emission mechanisms (forward vs. reverse shocks). Mass-loading of order 1 to 5 is required to explain the differences between the amount of hot gas and and the modelled mass-loss from massive stars. The metallicity of the dwarf galaxies correlates with the diffuse X-ray luminosity and anti-correlates with the cooling time of the hot gas. The diffuse X-ray luminosity is also a function of the current star formation rate. The mechanical luminosities of the developing superwinds are energetic enough to overcome the gravitational potentials of their host galaxies. This scenario is supported by the overpressures of the hot gas compared to the ambient ISM. Extended H i envelopes such as tidal tails, however, may delay outflows on timescales exceeding those of the cooling time of the hot gas.

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)