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.

A UV Ultra-luminous Lyman Break Galaxy at Z=2.78 in NDWFS Bootes Field

Abstract: We present one of the most ultraviolet (UV) luminous Lyman Break Galaxies (LBGs) (J1432+3358) at z=2.78, discovered in the NOAO Deep Wide-Field Survey (NDWFS) Bootes field. The R-band magnitude of J1432+3358 is 22.29 AB, more than two magnitudes brighter than typical L* LBGs at this redshift. The deep z-band image reveals two components of J1432+3358 separated by 1.0" with flux ratio of 3:1. The high signal-to-noise ratio (S/N) rest-frame UV spectrum shows Lya emission line and interstellar medium absorption lines. The absence of NV and CIV emission lines, the non-detection in X-ray and radio wavelengths and mid-infrared (MIR) colors indicate no or weak active galactic nuclei (AGN) (<10%) in this galaxy. The galaxy shows broader line profile with the full width half maximum (FWHM) of about 1000 km/s and larger outflow velocity (~500 km/s) than those of typical z~3 LBGs. The physical properties are derived by fitting the spectral energy distribution (SED) with stellar synthesis models. The dust extinction, E(B-V)=0.12, is similar to that in normal LBGs. The star formation rates (SFRs) derived from the SED fitting and the dust-corrected UV flux are consistent with each other, ~300 Msun/yr, and the stellar mass is 1.3e11 Msun. The SFR and stellar mass in J1432+3358 are about an order of magnitude higher than those in normal LBGs. The SED-fitting results support that J1432+3358 has a continuous star formation history with the star formation episode of 630 Myr. The morphology of J1432+3358 and its physical properties suggest that J1432+3358 is in an early phase of 3:1 merger process. The unique properties and the low space number density (~1e-7 Mpc^{-3})are consistent with the interpretation that such galaxies are either found in a short unobscured phase of the star formation or that small fraction of intensive star-forming galaxies are unobscured.

The Violent Interstellar Medium of Nearby Dwarf Galaxies

Abstract: High resolution HI observations of nearby dwarf galaxies (most of which are situated in the M81 group at a distance of about 3.2 Mpc) reveal that their neutral interstellar medium (ISM) is dominated by hole-like features most of which are expanding. A comparison of the physical properties of these holes with the ones found in more massive spiral galaxies (such as M31 and M33) shows that they tend to reach much larger sizes in dwarf galaxies. This can be understood in terms of the galaxy's gravitational potential. The origin of these features is still a matter of debate. In general, young star forming regions (OB-associations) are held responsible for their formation. This picture, however, is not without its critics and other mechanisms such as the infall of high velocity clouds, turbulent motions or even gamma ray bursters have been recently proposed. Here I will present one example of a supergiant shell in IC 2574 which corroborates the picture that OB associations are indeed creating these structures. This particular supergiant shell is currently the most promising case to study the e®ects of the combined effects of stellar winds and supernova explosions which shape the neutral interstellar medium of (dwarf) galaxies.

The First Luminous Quasars and Their Host Galaxies

Abstract: High-Redshift quasars probe the growth of early supermassive black holes (SMBHs) in the universe. In the next decade, new observations will push the frontier to the first luminous quasars at z > 9, probe fainter quasar populations that trace earlier phases of BH growth, and connect SMBH growth with the rise of the earliest massive galaxies.

VLA Imaging of the Intriguing H I Cloud HIJASS J1021+6842 in the M81 Group

Abstract: We present VLA H i 21 cm observations of HIJASS J10216842, which has been discovered in the direction of the M81 Group. Our synthesis imaging reveals that the H i is distributed over a larger angular extent and velocity range than the single-dish discovery observations. Assuming that HIJASS J10216842 is at the distance of the M81 Group, we detect M, of H i distributed over as much as 30 kpc, i.e., substantially larger 8 1.5 # 10 than the biggest dwarf galaxies in the same group. At the depth of our imaging, the H i appears to be confined to at least seven clouds. Peak H i column densities are ∼ atoms cm 2 , which is well below the canonical 20 1.8 # 10 star formation threshold of ∼10 21 atoms cm 2 and therefore consistent with the fact that no optical counterpart has as yet been identified. A gradient in velocity is observed across the extent of the detected H i; assuming that the object is gravitationally bound we derive a dynamical mass of M, and a dark-to-luminous mass 9 7 # 10 ratio of 110. Alternatively, a tidal origin may also result in the observed velocity gradient, which would lead to a considerably lower dynamical mass. Given the above properties and the absence of evidence of a stellar population, HIJASS J10216842 is unique amongst the other systems in the M81 Group. Subject headings: galaxies: clusters: general — galaxies: dwarf — galaxies: formation — galaxies: individual (HIJASS J10216842) — radio lines: galaxies

Studying the first galaxies with ALMA

Abstract: We discuss observations of the first galaxies, within cosmic reionization, at centimeter and millimeter wavelengths. We present a summary of current observations of the host galaxies of the most distant QSOs (z∼6). These observations reveal the gas, dust, and star formation in the host galaxies on kpc-scales. These data imply an enriched ISM in the QSO host galaxies within 1 Gyr of the big bang, and are consistent with models of coeval supermassive black hole and spheroidal galaxy formation in major mergers at high redshift. Current instruments are limited to studying truly pathologic objects at these redshifts, meaning hyper-luminous infrared galaxies (L FIR ∼1013 L ⊙). ALMA will provide the one to two orders of magnitude improvement in millimeter astronomy required to study normal star forming galaxies (i.e. Ly-α emitters) at z∼6. ALMA will reveal, at sub-kpc spatial resolution, the thermal gas and dust—the fundamental fuel for star formation—in galaxies into cosmic reionization.

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)