arXiv subjects

J. B. Jensen

Publications and source records attributed to J. B. Jensen.

13 recordsLinked to original sources

arXiv2026

Euclid: Calibrating distances from surface brightness fluctuations with Early Release Observations of the Fornax cluster

Surface brightness fluctuations (SBF), the pixel-to-pixel variations in flux that arise from the statistical distribution of stars in galaxy images, provide a powerful tool to measure redshift-independent distances from photometric data alone. This method is particularly important in the era of large imaging surveys, such as those carried out during the Euclid mission. Here we present the first application of SBF to Euclid data, using the FAST-SBF code to measure stellar fluctuation amplitudes in the $I_\mathrm{E}$ band for a sample of galaxies in the Early Release Observations (ERO) of the Fornax galaxy cluster. Although the Euclid data reduction pipeline is not optimized for SBF measurements, extensive testing suggests that we are able to extract robust results. We calibrate the absolute fluctuation magnitude $\overline{M}_\mathrm{IE}$ as a function of the $(I_\mathrm{E}{-}H_\mathrm{E})$ colour for 15 galaxies in Fornax, then test this relation on galaxies in two other ERO fields: the Perseus cluster and dwarf satellite candidates around NGC 6744. Overall, we find reasonable agreement with distances in the literature and good agreement in cases where FAST-SBF indicates the measurements are robust. Finally, we compare our results against Stellar Population Tools (SPoT) simple stellar population models, and discuss the possibility of using SBF with Euclid to probe the underlying stellar populations of the galaxies.

astro-ph.GA↗

arXiv2019

Probing the Time Domain with High Spatial Resolution

Two groundbreaking new facilities will commence operations early in the 2020s and thereafter define much of the broad landscape of US optical-infrared astronomy in the remaining decade. The Large Synoptic Survey Telescope (LSST), perched atop Cerro Pachon in the Chilean Andes, will revolutionize the young field of Time Domain Astronomy through its wide-field, multi-band optical imaging survey. At the same time, the James Webb Space Telescope (JWST), orbiting at the Sun-Earth L2 Lagrange point, will provide stunningly high-resolution views of selected targets from the red end of the optical spectrum to the mid-infrared. However, the spatial resolution of the LSST observations will be limited by atmospheric seeing, while JWST will be limited in its time-domain capabilities. This paper highlights the scientific opportunities lying between these two landmark missions, i.e., science enabled by systems capable of astronomical observations with both high cadence in the time domain and high resolution in the spatial domain. The opportunities range from constraining the late phases of stellar evolution in nearby resolved populations to constraining dark matter distributions and cosmology using lensed transient sources. We describe a system that can deliver the required capabilities.

astro-ph.IM↗

arXiv2018

A Precise Distance to the Host Galaxy of the Binary Neutron Star Merger GW170817 Using Surface Brightness Fluctuations

The joint detection of gravitational waves and electromagnetic radiation from the binary neutron star (BNS) merger GW170817 has provided unprecedented insight into a wide range of physical processes: heavy element synthesis via the $r$-process; the production of relativistic ejecta; the equation of state of neutron stars and the nature of the merger remnant; the binary coalescence timescale; and a measurement of the Hubble constant via the "standard siren" technique. In detail, all of these results depend on the distance to the host galaxy of the merger event, NGC4993. In this paper we measure the surface brightness fluctuation (SBF) distance to NGC4993 in the F110W and F160W passbands of the Wide Field Camera 3 Infrared Channel on the Hubble Space Telescope (HST). For the preferred F110W passband we derive a distance modulus of $m{-}M=33.05\pm0.08\pm0.10$ mag, or a linear distance $d=40.7\pm1.4\pm1.9$ Mpc (random and systematic errors, respectively); a virtually identical result is obtained from the F160W data. This is the most precise distance to NGC4993 available to date. Combining our distance measurement with the corrected recession velocity of NGC4993 implies a Hubble constant $H_0=71.9\pm 7.1~km~s^{-1}~Mpc^{-1}$. A comparison of our result to the GW-inferred value of $H_0$ indicates a binary orbital inclination of $i\,{\gtrsim}\,137~°$. The SBF technique can be applied to early-type host galaxies of BNS mergers to ${\sim\,}100$ Mpc with HST and possibly as far as ${\sim\,}300$ Mpc with the James Webb Space Telescope, thereby helping to break the inherent distance-inclination degeneracy of the GW signals at distances where many future BNS mergers are likely to be detected.

astro-ph.GA↗

arXiv2009

GOALS: The Great Observatories All-Sky LIRG Survey

The Great Observatories All-sky LIRG Survey (GOALS) combines data from NASA's Spitzer, Chandra, Hubble and GALEX observatories, together with ground-based data into a comprehensive imaging and spectroscopic survey of over 200 low redshift Luminous Infrared Galaxies (LIRGs). The LIRGs are a complete subset of the IRAS Revised Bright Galaxy Sample (RBGS). The LIRGs targeted in GOALS span the full range of nuclear spectral types defined via traditional optical line-ratio diagrams as well as interaction stages. They provide an unbiased picture of the processes responsible for enhanced infrared emission in galaxies in the local Universe. As an example of the analytic power of the multi-wavelength GOALS dataset, we present data for the interacting system VV 340 (IRAS F14547+2449). Between 80-95% of the total far-infrared emission (or about 5E11 solar luminosities) originates in VV 340 North. While the IRAC colors of VV 340 North and South are consistent with star-forming galaxies, both the Spitzer IRS and Chandra ACIS data indicate the presence of a buried AGN in VV 340 North. The GALEX far and near-UV fluxes imply a extremely large infrared "excess" (IRX) for the system (IR/FUV = 81) which is well above the correlation seen in starburst galaxies. Most of this excess is driven by VV 340 N, which alone has an IR excess of nearly 400. The VV 340 system seems to be comprised of two very different galaxies - an infrared luminous edge-on galaxy (VV 340 North) that dominates the long-wavelength emission from the system and which hosts a buried AGN, and a face-on starburst (VV 340 South) that dominates the short-wavelength emission.

astro-ph.CO↗

arXiv2009

Magnetic cooling at Risoe DTU

Magnetic refrigeration at room temperature is of great interest due to a long-term goal of making refrigeration more energy-efficient, less noisy and free of any environmentally hostile materials. A refrigerator utilizing an active magnetic regenerator (AMR) is based on the magnetocaloric effect, which manifests itself as a temperature change in magnetic materials when subjected to a varying magnetic field. In this work we present the current state of magnetic refrigeration research at Risoe DTU with emphasis on the numerical modeling of an existing AMR test machine. A 2D numerical heat-transfer and fluid-flow model that represents the experimental setup is presented. Experimental data of both no-heat load and heat load situations are compared to the model. Moreover, results from the numerical modeling of the permanent magnet design used in the system are presented.

cond-mat.mtrl-sci↗

arXiv2006

The Brightest Stars in M32: Comparing Predictions from Spectra with the Resolved Stellar Content

Broad- and narrow-band images covering the 1 - 4um wavelength interval are used to investigate the properties of the brightest AGB stars in the Local Group galaxy M32. The brightest AGB stars near the center of M32 have peak M_L' brightnesses and K-L' colors that are similar to those of luminous AGB stars in the Galactic disk, while the density of bright AGB stars per unit visible and near-infrared surface brightness is found to be constant out to projected major axis distances of 1 kpc. The J-K color distribution of bright AGB stars throughout much of the galaxy is consistent with that of a single population of AGB stars, the majority of which are long period variables, having a common metallicity and age. Thus, these data do not support spectroscopic studies that find an age gradient in M32. The well-mixed AGB content of M32 is consistent with tidal stirring. The stellar content of M32 is compared with that of the M31 bulge. While the peak K-band brightnesses of AGB stars in the two systems agree to within a few tenths of a magnitude, M32 contains more bright AGB stars per unit integrated brightness than the outer bulge of M31.

astro-ph↗

arXiv2004

Ultra-large bandwidth hollow-core guiding in all-silica Bragg fibers with nano-supports

We demonstrate a new class of hollow-core Bragg fibers that are composed of concentric cylindrical silica rings separated by nanoscale support bridges. We theoretically predict and experimentally observe hollow-core confinement over an octave frequency range. The bandwidth of bandgap guiding in this new class of Bragg fibers exceeds that of other hollow-core fibers reported in the literature. With only three rings of silica cladding layers, these Bragg fibers achieve propagation loss of the order of 1 dB/m.

physics.optics↗

arXiv2003

A Major SGR-like Outburst and Rotation Glitch in the No-Longer-So-Anomalous X-ray Pulsar 1E 2259+586

We report a major outburst from the Anomalous X-ray Pulsar 1E 2259+586, in which over 80 X-ray bursts were detected in four hours using the Rossi X-ray Timing Explorer. The bursts range in duration from 2 ms to 3 s and have fluences in the 2-10 keV band that range from 3 x 10^{-11} to 5 x 10^{-9} erg cm^{-2}. We simultaneously observed increases of the pulsed and persistent X-ray emission by over an order of magnitude relative to quiescent levels. Both decayed significantly during the course of our 14 ks observation. Correlated spectral hardening was also observed, with the spectrum softening during the observation. In addition, we observed a pulse profile change, in which the amplitudes of the two peaks in the pulse profile were swapped. The profile relaxed back to its pre-outburst morphology after ~6 days. The pulsar also underwent a sudden spin-up (df/f = 4 x 10^{-6}), followed by a large (factor of \~2) increase in spin-down rate which persisted for >18 days. We also observed, using the Gemini-North telescope, an infrared enhancement, in which the K_s (2.15 um) flux increased, relative to that measured in a observation made in 2000, by a factor of ~3, three days post-outburst. The IR counterpart then faded by a factor of ~2 one week later. In addition, we report an upper limit of 50 uJy on radio emission at 1.4 GHz two days post-outburst. The X-ray properties of this outburst are like those seen only in Soft Gamma Repeaters. This conclusively unifies Anomalous X-ray Pulsars and Soft Gamma Repeaters, as predicted uniquely by the magnetar model.

astro-ph↗

arXiv2000

The brightness of SN 1991T and the uniformity of decline-rate and colour corrected absolute magnitudes of supernovae Ia

We present a distance to NGC 4527, the host galaxy of the type Ia SN 1991T, measured by surface brightness fluctuations. This supernova has been labelled ``peculiar'' both on the grounds of its spectroscopic behaviour and its apparent overluminosity with respect to other supernovae. The distance modulus to NGC 4527 and thus to SN 1991T is 30.26+-0.09. This relatively short distance largely removes the discrepancy with other Ia supernovae having similar light-curve characteristics and also removes the motivation for interpreting SN 1991T as a super-Chandrasekhar explosion. However, the reddening uncertainty results in significant uncertainty of the absolute magnitudes.

astro-ph↗

arXiv2000

Bias and temperature dependence of the 0.7 conductance anomaly in Quantum Point Contacts

The 0.7 (2e^2/h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, for a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at 0.85 (2e^2/h) at finite bias. Varying the gate voltage the transition between the 1.0 and the 0.85 (2e^2/h) plateaus occurs for definite bias voltages, which defines a gate voltage dependent energy difference $Δ$. This energy difference is compared with the activation temperature T_a extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find Δ= k_B T_a which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge Δbelow the chemical potential becomes thermally depopulated as the temperature is increased.

cond-mat.mes-hall↗

arXiv1999

The Hosts of Ultraluminous Infrared Galaxies

Complete samples of ultraluminous infrared galaxies have been imaged at R-band and K-band from Mauna Kea. Here we present a preliminary analysis of the host galaxy magnitudes and the 1-D radial profiles for a subset of objects in the IRAS 1-Jy sample of ULIGs (z < 0.3), and compare these properties with recently published data for "low-z" QSOs. ULIGs in the 1-Jy sample reside in luminous hosts, with mean luminosities 2.7L* at K-band (range 0.7-11 L*) and 2.2L* at R-band (range 0.5-9 L*), values that are remarkably similar in the mean and range for the hosts of low-z QSOs. Approximately one-third of ULIGs have single nuclei and radial profiles that are closely approximated by a r{1/4}-law over the inner 2-10 kpc galactocentric radius. These "E-like" hosts have half-light radii, and surface brightness similar to QSO hosts at R-band, but systematically smaller half-light radii than QSOs at K-band.

astro-ph↗

arXiv1998

Activated Behavior of the 0.7 2(e^2)/h Conductance Anomaly in Quantum Point Contacts

The 0.7 conductance anomaly in the quantized conductance of trench etched GaAs quantum point contacts is studied experimentally. The temperature dependence of the anomaly measured with vanishing source-drain bias reveals the same activated behavior as reported earlier for top-gated structures. Our main result is that the zero bias, high temperature 0.7 anomaly found in activation measurements and the finite bias, low temperature 0.9 anomaly found in transport spectroscopy have the same origin: a density dependent excitation gap.

cond-mat.mes-hall↗

arXiv1998

Temperature dependence of the ``0.7'' 2(e^2)/h quasi plateau in strongly confined quantum point contacts

We present new results of the ``0.7'' 2(e^2)/h structure or quasi plateau in some of the most strongly confined point contacts so far reported. This strong confinement is obtained by a combination of shallow etching and metal gate deposition on modulation doped GaAs/GaAlAs heterostructures. The resulting subband separations are up to 20 meV, and as a consequence the quantized conductance can be followed at temperatures up to 30 K, an order of magnitude higher than in conventional split gate devices. We observe pronounced quasi plateaus at several of the lowest conductance steps all the way from their formation around 1 K to 30 K, where the entire conductance quantization is smeared out thermally. We study the deviation of the conductance from ideal integer quantization as a function of temperature, and we find an activated behavior, exp(-T_a/T), with a density dependent activation temperature T_a of the order of 2 K. We analyze our results in terms of a simple theoretical model involving scattering against plasmons in the constriction.

cond-mat.mes-hall↗