Unresolved Emission Research Articles

Water maser variability in a high-mass YSO outburst

Context. Clarifying the relationship between mass accretion and ejection history is one of the key issues in understanding high-mass star formation processes. Aims. We aim to investigate the possible relationship between the mass accretion burst event in mid-June 2015 and the jet ejection in the high-mass protostar S255 NIRS 3. Methods. The Very Long Baseline Interferometer (VLBI) monitoring observations of the 22 GHz H2O masers were carried out using VLBI Exploration of Radio Astrometry (VERA) to reveal the 3D velocity and spatial structure of the outflow/jet traced by the H2O masers in S255 NIRS 3. In addition, we conducted follow-up observations of the submillimeter continuum and the 321 GHz H2O masers with the Atacama Large Millimeter/submillimeter Array (ALMA) at Band 7. Results. We successfully measured the proper motions of the 22 GHz H2O masers associated with a bipolar outflow. The structure is almost the same as was observed in 2005 and 2010. The expansion velocity of the blueshifted bow shock traced by the 22 GHz H2O masers was measured to be 28 km s−1, corresponding to a dynamical timescale of 60 yr. The direction of the maser outflow is slightly tilted compared with the radio jet, which could suggest a more recent ejection episode during the accretion burst event. The total flux density of the 22 GHz H2O masers gradually increases from the beginning of the VLBI monitoring in early 2017 and becomes almost constant in subsequent single-dish monitoring in 2018. The brightening of the H2O masers is more prominent in the northeast outflow lobe. For the first time, we revealed extended H2O maser emission at 22 GHz in a star-forming region, which is partly resolved out by VERA and even by the most extended Very Large Array (VLA) configurations. We find that the flux variation of such an extended component is similar to that of the unresolved maser emission. The ALMA Band 7 continuum emission did not show significant variations compared with the previous observations performed five months before. We mapped the 321 GHz H2O masers in S255 NIRS 3 providing the fourth example, for this maser, of the spatial distribution in a high-mass star-forming region. Conclusions. We conclude that the bow shock structure traced by the 22 GHz H2O maser features is unlikely to originate at the interface between the radio jet powered by the recent accretion outburst and the surrounding medium. The brightening of the 22 GHz H2O masers could be due to radiative excitation by photons form the (declining) infrared (IR) outburst escaping along the cavity created by the newly ejected material. The lower ratio of the 22 GHz/321 GHz maser luminosity in the blueshifted bow shock suggests a temperature ( >1000 K), higher than for the other maser features in this region.

The MUSE Extremely Deep Field: The cosmic web in emission at high redshift

We report the discovery of diffuse extended Lyα emission from redshift 3.1 to 4.5, tracing cosmic web filaments on scales of 2.5−4 cMpc. These structures have been observed in overdensities of Lyα emitters in the MUSE Extremely Deep Field, a 140 h deep MUSE observation located in the Hubble Ultra-Deep Field. Among the 22 overdense regions identified, five are likely to harbor very extended Lyα emission at high significance with an average surface brightness of 5 × 10−20 erg s−1 cm−2 arcsec−2. Remarkably, 70% of the total Lyα luminosity from these filaments comes from beyond the circumgalactic medium of any identified Lyα emitter. Fluorescent Lyα emission powered by the cosmic UV background can only account for less than 34% of this emission at z ≈ 3 and for not more than 10% at higher redshift. We find that the bulk of this diffuse emission can be reproduced by the unresolved Lyα emission of a large population of ultra low-luminosity Lyα emitters (< 1040 erg s−1), provided that the faint end of the Lyα luminosity function is steep (α ⪅ −1.8), it extends down to luminosities lower than 1038 − 1037 erg s−1, and the clustering of these Lyα emitters is significant (filling factor < 1/6). If these Lyα emitters are powered by star formation, then this implies their luminosity function needs to extend down to star formation rates < 10−4 M⊙ yr−1. These observations provide the first detection of the cosmic web in Lyα emission in typical filamentary environments and the first observational clue indicating the existence of a large population of ultra low-luminosity Lyα emitters at high redshift.

The MUSE Deep Lensed Field on the Hubble Frontier Field MACS J0416

Context. A census of faint and tiny star forming complexes at high redshift is key to improving our understanding of reionizing sources, galaxy growth, and the formation of globular clusters. Aims. We present the MUSE Deep Lensed Field (MDLF) program, which is aimed at unveiling the very faint population of high redshift sources that are magnified by strong gravitational lensing and to significantly increase the number of constraints for the lens model. Methods. We describe Deep MUSE observations of 17.1 h of integration on a single pointing over the Hubble Frontier Field galaxy cluster MACS J0416, providing line flux limits down to 2 × 10−19 erg s−1 cm−2 within 300 km s−1 and continuum detection down to magnitude 26, both at the three sigma level at λ = 7000 Å. For point sources with a magnification (μ) greater than 2.5 (7.7), the MLDF depth is equivalent to integrating more than 100 (1000) h in blank fields, as well as complementing non-lensed studies of very faint high-z sources. The source-plane effective area of the MDLF with μ > 6.3 is < 50% of the image-plane field of view. Results. We confirm spectroscopic redshifts for all 136 multiple images of 48 source galaxies at 0.9 < z < 6.2. Within those galaxies, we securely identify 182 multiple images of 66 galaxy components that we use to constrain our lens model. This makes MACS J0416 the cluster with the largest number of confirmed constraints for any strong lens model to date. We identify 116 clumps belonging to background high-z galaxies; the majority of them are multiple images and span magnitude, size, and redshift intervals of [ − 18, −10], [ ∼ 400 − 3] parsec and 1 < z < 6.6, respectively, with the faintest or most magnified ones probing possible single gravitationally bound star clusters. The multiplicity introduced by gravitational lensing allows us, in several cases, to triple the effective integration time up to ∼51 h exposure per single family, leading to a detection limit for unresolved emission lines of a few 10−20 erg s−1 cm−2, after correction for lensing magnification. Ultraviolet high-ionization metal lines (and He IIλ1640) are detected with S/N > 10 for individual objects down to de-lensed magnitudes between 28 and 30. The median stacked spectrum of 33 sources with a median MUV ≃ −17 and ⟨z⟩ = 3.2 (1.7 < z < 3.9) shows high-ionization lines, suggesting that they are common in such faint sources. Conclusions. Deep MUSE observations, in combination with existing HST imaging, allowed us to: (1) confirm redshifts for extremely faint high-z sources; (2) peer into their internal structure to unveil clumps down to 100 − 200 pc scale; (3) in some cases, break down such clumps into star-forming complexes matching the scales of bound star clusters (< 20 pc effective radius); (4) double the number of constraints for the lens model, reaching an unprecedented set of 182 bona-fide multiple images and confirming up to 213 galaxy cluster members. These results demonstrate the power of JWST and future adaptive optics facilities mounted on the Extremely Large Telescopes (e.g., European-ELT Multi-conjugate Adaptive Optics RelaY, MAORY, coupled with the Multi-AO Imaging CamerA for Deep Observations, MICADO) or Very Large Telescope (e.g., MCAO Assisted Visible Imager and Spectrograph, MAVIS) when combined in studies with gravitational telescopes.

Size and structures of disks around very low mass stars in the Taurus star-forming region

Context. The discovery of giant planets orbiting very low mass stars (VLMS) and the recent observed substructures in disks around VLMS is challenging planet formation models. Specifically, radial drift of dust particles is a catastrophic barrier in these disks, which prevents the formation of planetesimals and therefore planets. Aims. We aim to estimate if structures, such as cavities, rings, and gaps, are common in disks around VLMS and to test models of structure formation in these disks. We also aim to compare the radial extent of the gas and dust emission in disks around VLMS, which can give us insight about radial drift. Methods. We studied six disks around VLMS in the Taurus star-forming region using ALMA Band 7 (~340 GHz) at a resolution of ~0.1″. The targets were selected because of their high disk dust content in their stellar mass regime. Results. Our observations resolve the disk dust continuum in all disks. In addition, we detect the 12CO (J = 3−2) emission line in all targets and 13CO (J = 3−2) in five of the six sources. The angular resolution allows the detection of dust substructures in three out of the six disks, which we studied by using UV-modeling. Central cavities are observed in the disks around stars MHO 6 (M 5.0) and CIDA 1 (M 4.5), while we have a tentative detection of a multi-ringed disk around J0433. We estimate that a planet mass of ~0.1 MJup or ~0.4 MSaturn is required for a single planet to create the first gap in J0433. For the cavities of MHO 6 and CIDA 1, a Saturn-mass planet (~0.3 MJup) is required. The other three disks with no observed structures are the most compact and faintest in our sample, with the radius enclosing 90% of the continuum emission varying between ~13 and 21 au. The emission of 12CO and 13CO is more extended than the dust continuum emission in all disks of our sample. When using the 12CO emission to determine the gas disk extension Rgas, the ratio of Rgas∕Rdust in our sample varies from 2.3 to 6.0. One of the disks in our sample, CIDA 7, has the largest Rgas∕Rdust ratio observed so far, which is consistent with models of radial drift being very efficient around VLMS in the absence of substructures. Conclusions. Given our limited angular resolution, substructures were only directly detected in the most extended disks, which represent 50% of our sample, and there are hints of unresolved structured emission in one of the bright smooth sources. Our observations do not exclude giant planet formation on the substructures observed. A comparison of the size and luminosity of VLMS disks with their counterparts around higher mass stars shows that they follow a similar relation.

Harnessing the Population Statistics of Subhalos to Search for Annihilating Dark Matter

Abstract The Milky Way’s dark matter halo is expected to host numerous low-mass subhalos with no detectable associated stellar component. Such subhalos are invisible unless their dark matter annihilates to visible states such as photons. One of the established methods for identifying candidate subhalos is to search for individual unassociated gamma-ray sources with properties consistent with the dark matter expectation. However, robustly ruling out an astrophysical origin for any such candidate is challenging. In this work, we present a complementary approach that harnesses information about the entire population of subhalos—such as their spatial and mass distribution in the Galaxy—to search for a signal of annihilating dark matter. Using simulated data, we show that the collective emission from subhalos can imprint itself in a unique way on the statistics of observed photons, even when individual subhalos may be too dim to be resolved on their own. Additionally, we demonstrate that, for the models we consider, the signal can be identified even in the face of unresolved astrophysical point-source emission of extragalactic and Galactic origin. This establishes a new search technique for subhalos that is complementary to established methods, and that could have important ramifications for gamma-ray dark matter searches using observatories such as the Fermi Large Area Telescope and the Cerenkov Telescope Array.

High-resolution VLA Imaging of Obscured Quasars: Young Radio Jets Caught in a Dense ISM

Abstract We present new subarcsecond-resolution Karl G. Jansky Very Large Array (VLA) imaging at 10 GHz of 155 ultraluminous (L bol ∼ 1011.7–1014.2 L ⊙) and heavily obscured quasars with redshifts z ∼ 0.4–3. The sample was selected to have extremely red mid-infrared–optical color ratios based on data from the Wide-Field Infrared Survey Explorer (WISE) along with a detection of bright, unresolved radio emission from the NRAO VLA Sky Survey (NVSS) or Faint Images of the Radio Sky at Twenty cm Survey. Our high-resolution VLA observations have revealed that the majority of the sources in our sample (93 out of 155) are compact on angular scales <0.″2 (≤1.7 kpc at z ∼ 2). The radio luminosities, linear extents, and lobe pressures of our sources are similar to young radio active galactic nuclei (e.g., gigahertz-peaked spectrum [GPS] and compact steep-spectrum [CSS] sources), but their space density is considerably lower. Application of a simple adiabatic lobe expansion model suggests relatively young dynamical ages (∼104–7 yr), relatively high ambient ISM densities (∼1–104 cm−3), and modest lobe expansion speeds (∼30–10,000 km s−1). Thus, we find our sources to be consistent with a population of newly triggered, young jets caught in a unique evolutionary stage in which they still reside within the dense gas reservoirs of their hosts. Based on their radio luminosity function and dynamical ages, we estimate that only ∼20% of classical large-scale FR I/II radio galaxies could have evolved directly from these objects. We speculate that the WISE-NVSS sources might first become GPS or CSS sources, of which some might ultimately evolve into larger radio galaxies.

Galaxy Cluster Contribution to the Diffuse Extragalactic Ultraviolet Background

Abstract The diffuse ultraviolet background radiation has been mapped over most of the sky with 2′ resolution using data from the Galaxy Evolution Explorer survey. We utilize this map to study the correlation between the UV background and clusters of galaxies discovered via the Sunyaev–Zeldovich effect in the Planck survey. We use only high Galactic latitude ( ) galaxy clusters to avoid contamination by Galactic foregrounds, and we only analyze clusters with a measured redshift. This leaves us with a sample of 142 clusters over the redshift range of 0.02 ≤ z ≤ 0.72, which we further subdivide into four redshift bins. In analyzing our stacked samples binned by redshift, we find evidence for a central excess of UV background light compared to local backgrounds for clusters with z < 0.3. We then stacked these z < 0.3 clusters to find a statistically significant excess of 12 ± 2.3 photon cm−2 s−1 sr−1 Å−1 over the median of ∼380 photon cm−2 s−1 sr−1 Å−1 measured around random blank fields. We measure the stacked radial profile of these clusters, and find that the excess UV radiation decays to the level of the background at a radius of ∼1 Mpc, roughly consistent with the maximum radial extent of the clusters. Analysis of possible physical processes contributing to the excess UV brightness indicates that non-thermal emission from relativistic electrons in the intracluster medium and faint, unresolved UV emission from cluster member galaxies and intracluster light are likely the dominant contributors.

X-Ray Monitoring of Gravitationally Lensed Radio-loud Quasars with Chandra

Abstract In this work, we calculated the sizes of unresolved X-ray emission regions in three gravitationally lensed radio-loud quasars, B 1422+231, MG J0414+0534, and Q 0957+561, using a combination of imaging and spectral analysis on the X-ray data taken from the Chandra X-Ray Observatory. We tentatively detected FeKα emission lines in MG J0414+0534 and Q 0957+561 with over 95% significance, whereas, we did not significantly detect FeKα emission in B 1422+231. We constructed differential microlensing light curves from absorption corrected count rates. We subsequently performed a microlensing analysis on the X-ray microlensing light curves to measure the X-ray source sizes in rest frame soft (0.83–3.6 keV), hard (3.6–21.8 keV), and full (0.83–21.8 keV) bands, based on either Bayesian or maximum likelihood probabilities. For B 1422+231, sizes from the two methods are consistent with each other, (Bayesian), 11.81 ± 3.75 (maximum likelihood), where . However, for MG J0414+0534 and Q 0957+561, the two methods yield substantially different results suggesting that more frequently sampled data with better signal-to-noise ratio are needed to measure the source size for these two objects. Comparing the acquired size values with the radio-quiet sample in the literature we found that our results are consistent with X-ray source size scaling approximately as R X ∝ M BH with the mass of the central supermassive black hole. Our Bayesian analysis results also indicate that radio-loud quasars tend to have larger unresolved X-ray emission sizes compared to the radio-quiet ones, suggesting a more prominent jet contribution.

A Multifrequency ALMA Characterization of Substructures in the GM Aur Protoplanetary Disk

Abstract The protoplanetary disk around the T Tauri star GM Aur was one of the first hypothesized to be in the midst of being cleared out by a forming planet. As a result, GM Aur has had an outsized influence on our understanding of disk structure and evolution. We present 1.1 and 2.1 mm ALMA continuum observations of the GM Aur disk at a resolution of ∼50 mas (∼8 au), as well as HCO+ J = 3 − 2 observations at a resolution of ∼100 mas. The dust continuum shows at least three rings atop faint, extended emission. Unresolved emission is detected at the center of the disk cavity at both wavelengths, likely due to a combination of dust and free–free emission. Compared to the 1.1 mm image, the 2.1 mm image shows a more pronounced “shoulder” near R ∼ 40 au, highlighting the utility of longer-wavelength observations for characterizing disk substructures. The spectral index α features strong radial variations, with minima near the emission peaks and maxima near the gaps. While low spectral indices have often been ascribed to grain growth and dust trapping, the optical depth of GM Aur’s inner two emission rings renders their dust properties ambiguous. The gaps and outer disk (R > 100 au) are optically thin at both wavelengths. Meanwhile, the HCO+ emission indicates that the gas cavity is more compact than the dust cavity traced by the millimeter continuum, similar to other disks traditionally classified as “transitional.”

Data from "A multi-frequency ALMA characterization of substructures in the GM Aur protoplanetary disk"

The protoplanetary disk around the T Tauri star GM Aur was one of the first hypothesized to be in the midst of being cleared out by a forming planet. As a result, GM Aur has had an outsized influence on our understanding of disk structure and evolution. We present 1.1 and 2.1 mm ALMA continuum observations of the GM Aur disk at a resolution of ~50 mas (~8 au), as well as HCO$^+$ $J=3-2$ observations at a resolution of ~100 mas. The dust continuum shows at least three rings atop faint, extended emission. Unresolved emission is detected at the center of the disk cavity at both wavelengths, likely due to a combination of dust and free-free emission. Compared to the 1.1 mm image, the 2.1 mm image shows a more pronounced shoulder near R~40 au, highlighting the utility of longer-wavelength observations for characterizing disk substructures. The spectral index $\alpha$ features strong radial variations, with minima near the emission peaks and maxima near the gaps. While low spectral indices have often been ascribed to grain growth and dust trapping, the optical depth of GM Aur's inner two emission rings renders their dust properties ambiguous. The gaps and outer disk ($R>100$ au) are optically thin at both wavelengths. Meanwhile, the HCO$^+$ emission indicates that the gas cavity is more compact than the dust cavity traced by the millimeter continuum, similar to other disks traditionally classified as transitional.

The case for strangulation in low-mass hosts: DDO 113

ABSTRACT We investigate the case for environmental quenching of the Fornax-mass satellite DDO 113, which lies only 9 kpc in projection from its host, the Large-Magellanic-Cloud-mass galaxy NGC 4214. DDO 113 was quenched about 1 Gyr ago and is virtually gas-free, while analogs in the field are predominantly star-forming and gas-rich. We use deep imaging obtained with the Large Binocular Telescope to show that DDO 113 exhibits no evidence of tidal disruption to a surface brightness of μV ∼ 29 mag arcsec−2, based on both unresolved emission and resolved stars. Mass-analogs of DDO 113 in Illustris-1 with similar hosts, small projected separations, and no significant tidal stripping first fell into their host halo 2–6 Gyr ago, showing that tidal features (or lack thereof) can be used to constrain infall times in systems where there are few other constraints on the orbit of the satellite. With the infall time setting the clock for environmental quenching mechanisms, we investigate the plausibility of several such mechanisms. We find that strangulation, the cessation of cold gas inflows, is likely the dominant quenching mechanism for DDO 113, requiring a time-averaged mass-loading factor of η = 6–11 for star-formation-driven outflows that is consistent with theoretical and observational constraints. Motivated by recent numerical work, we connect DDO 113’s strangulation to the presence of a cool circumgalactic medium (CGM) around NGC 4214. This discovery shows that the CGM of low-mass galaxies can affect their satellites significantly and motivates further work on understanding the baryon cycle in low-mass galaxies.

Searching for gamma-ray emission from galaxy clusters at low redshift

ABSTRACT We report the identification of a positive cross-correlation signal between the unresolved gamma-ray emission, measured by the Fermi Large Area Telescope, and four different galaxy cluster catalogues. The selected catalogues peak at low-redshift and span different frequency bands, including infrared, optical, and X-rays. The signal-to-noise ratio of the detected cross-correlation amounts to 3.5 in the most significant case. We investigate and comment about its possible origin, in terms of compact gamma-ray emission from AGNs inside clusters or diffuse emission from the intracluster medium. The analysis has been performed by introducing an accurate estimation of the cross-correlation power-spectrum covariance matrix, built with mock realizations of the gamma and galaxy cluster maps. Different methods to produce the mock realizations starting from the data maps have been investigated and compared, identifying suitable techniques which can be generalized to other cross-correlation studies.

A Search for Spatially Resolved Infrared Rovibrational Molecular Hydrogen Emission from the Disks of Young Stars

Abstract We present results from a survey searching for spatially resolved near-infrared line emission from molecular hydrogen gas in the circumstellar environments of nine young stars: AA Tau, AB Aur, DoAr 21, GG Tau, GM Aur, LkCa 15, LkHα 264, UY Aur, and V773 Tau. Prior high-resolution spectra of these stars showed the presence of rovibrational H2 line emission at 2.12 μm with characteristics more typical of gas located in protoplanetary disks rather than outflows. In this study, we spatially resolve the H2 emission in the eight stars for which it is detected. LkCa 15 is the only target that exhibits no appreciable H2 despite a prior detection. We find an anticorrelation between H2 and X-ray luminosities, likely indicating that the X-ray ionization process is not the dominant H2 excitation mechanism in these systems. AA Tau, UY Aur, and V773 Tau show discrete knots of H2, as typically associated with shocks in outflowing gas. UY Aur and V773 Tau exhibit spatially resolved velocity structures, while the other systems have spectrally unresolved emission consistent with systemic velocities. V773 Tau exhibits a complex line morphology indicating the presence of multiple excitation mechanisms, including red- and blueshifted bipolar knots of shock-excited outflowing gas. AB Aur, GM Aur, and LkHα 264 have centralized yet spatially resolved H2 emission consistent with a disk origin. The H2 images of AB Aur reveal spiral structures within the disk, matching those observed in ALMA CO maps. This survey reveals new insights into the structure and excitation of warm gas in the circumstellar environments of these young stars.

WISDOM project – V. Resolving molecular gas in Keplerian rotation around the supermassive black hole in NGC 0383

ABSTRACT As part of the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM), we present a measurement of the mass of the supermassive black hole (SMBH) in the nearby early-type galaxy NGC 0383 (radio source 3C 031). This measurement is based on Atacama Large Millimeter/sub-millimeter Array (ALMA) cycle 4 and 5 observations of the 12CO(2–1) emission line with a spatial resolution of 58 × 32 pc2 (0.18 arcsec × 0.1 arcsec). This resolution, combined with a channel width of 10 km s−1, allows us to well resolve the radius of the black hole sphere of influence (measured as RSOI = 316 pc = 0.98 arcsec), where we detect a clear Keplerian increase of the rotation velocities. NGC 0383 has a kinematically relaxed, smooth nuclear molecular gas disc with weak ring/spiral features. We forward model the ALMA data cube with the Kinematic Molecular Simulation (KinMS) tool and a Bayesian Markov Chain Monte Carlo method to measure an SMBH mass of (4.2 ± 0.7) × 109 M⊙, a F160W-band stellar mass-to-light ratio that varies from 2.8 ± 0.6 M⊙/L$_{\odot ,\, \mathrm{F160W}}$ in the centre to 2.4 ± 0.3 M⊙$/\rm L_{\odot ,\, \mathrm{F160W}}$ at the outer edge of the disc and a molecular gas velocity dispersion of 8.3 ± 2.1 km s−1(all 3σ uncertainties). We also detect unresolved continuum emission across the full bandwidth, consistent with synchrotron emission from an active galactic nucleus. This work demonstrates that low-J CO emission can resolve gas very close to the SMBH ($\approx 140\, 000$ Schwarzschild radii) and hence that the molecular gas method is highly complimentary to megamaser observations, as it can probe the same emitting material.

Revealing the nature of central emission nebulae in the dwarf galaxy NGC 185

Aims. In this paper we present new optical observations of the galaxy NGC 185 intended to reveal the status of supernova remnants (SNRs) in this dwarf companion of the Andromeda galaxy. Previously, it was reported that this galaxy hosts one SNR. Methods. Our deep photometric study with the 2m telescope at Rozhen National Astronomical Observatory using narrow-band Hα and [S II] filters revealed complex structure of the interstellar medium in the center of the galaxy. To confirm the classification and to study the kinematics of the detected nebulae, we carried out spectroscopic observations using the SCORPIO multi-mode spectrograph at the 6m telescope at the Special Astrophysical Observatory of the Russian Academy of Science, both in low- and high-resolution modes. We also searched the archival X-ray and radio data for counterparts of the candidate SNRs identified by our optical observations. Results. Our observations imply the presence of one more SNR, one possible H II region previously cataloged as part of an SNR, and the presence of an additional source of shock ionization in one low-brightness planetary nebula. We detected enhanced [S II]/Hα and [N II]/Hα line ratios, as well as relatively high (up to 90 km s−1) expansion velocities of the two observed nebulae, motivating their classification as SNRs (with diameters of 45 pc and 50 pc), confirmed by both photometric and spectral observations. The estimated electron density of emission nebulae is 30–200 cm−3. Archival XMM-Newton observations indicate the presence of an extended, low-brightness, soft source in projection of one of the optical SNRs, whereas the archival VLA radio image shows weak, unresolved emission in the center of NGC 185.

A GeV–TeV Measurement of the Extragalactic Background Light

Abstract The extragalactic background light (EBL) can be probed via the absorption imprint it leaves in the spectra of gamma-ray sources ( ). We recently developed a dedicated technique to reconstruct the EBL, and its evolution with redshift, from γ-ray optical depth data using a large sample of blazars detected by the Fermi Large Area Telescope. Here, we extend this data set to the TeV regime using ground-based Cherenkov observations of 38 blazars and report the first homogeneous measurement of the EBL spectral intensity covering the ultraviolet to infrared wavelengths (∼0.1–100 μm). A minimal EBL throughout the wavelength range with respect to integrated galaxy light is found, allowing little additional unresolved emission from faint or truly diffuse populations setting an upper limit of ≲4 nW m−2 sr−1 at 1.4 μm. In particular, the cosmic optical background at z = 0 is found to be . This work lays the foundation for accurate gamma-ray measurements of the EBL across its whole spectral range using a combination of GeV and TeV data.

Inferences on the timeline of reionization at z ∼ 8 from the KMOS Lens-Amplified Spectroscopic Survey

Abstract Detections and non-detections of Lyman alpha (Lyα) emission from z > 6 galaxies (<1 Gyr after the big bang) can be used to measure the timeline of cosmic reionization. Of key interest to measuring reionization’s mid-stages, but also increasing observational challenge, are observations at z > 7, where Lyα redshifts to near infra-red wavelengths. Here we present a search for z > 7.2 Lyα emission in 53 intrinsically faint Lyman Break Galaxy candidates, gravitationally lensed by massive galaxy clusters, in the KMOS Lens-Amplified Spectroscopic Survey (KLASS). With integration times of ∼7–10 h, we detect no Lyα emission with signal-to-noise ratio (S/N) > 5 in our sample. We determine our observations to be 80 per cent complete for 5σ spatially and spectrally unresolved emission lines with integrated line flux >5.7 × 10−18 erg s−1 cm−2. We define a photometrically selected sub-sample of 29 targets at z = 7.9 ± 0.6, with a median 5σ Lyα EW limit of 58 Å. We perform a Bayesian inference of the average intergalactic medium (IGM) neutral hydrogen fraction using their spectra. Our inference accounts for the wavelength sensitivity and incomplete redshift coverage of our observations, and the photometric redshift probability distribution of each target. These observations, combined with samples from the literature, enable us to place a lower limit on the average IGM neutral hydrogen fraction of $\gt 0.76 \,\, (68{{\ \rm per\ cent}}), \,\, \gt 0.46 \,\, (95{{\ \rm per\ cent}})$ at z ∼ 8, providing further evidence of rapid reionization at z ∼ 6–8. We show that this is consistent with reionization history models extending the galaxy luminosity function to $M_ \rm {\small UV}\lesssim -12$, with low ionizing photon escape fractions, $f_\textrm{esc} \lesssim 15{{\ \rm per\ cent}}$.

Properties of the Diffuse Emission around Warm Loops in Solar Active Regions

Abstract Coronal loops in active regions are the subjects of intensive investigation, but the important diffuse unresolved emission in which they are embedded has received relatively little attention. Here I measure the densities and emission measure (EM) distributions of a sample of background–foreground regions surrounding warm (2 MK) coronal loops, and introduce two new aspects to the analysis. First, I infer the EM distributions only from temperatures that contribute to the same background emission. Second, I measure the background emission co-spatially with the loops so that the results are truly representative of the immediate loop environment. The second aspect also allows me to take advantage of the presence of embedded loops to infer information about the (unresolvable) magnetic field in the background. I find that about half of the regions in my sample have narrow but not quite isothermal EM distributions with a peak temperature of 1.4–2 MK. The other half have broad EM distributions (Gaussian width >3 × 105 K), and the width of the EM appears to be correlated with peak temperature. Densities in the diffuse background are log (n/cm−3) = 8.5–9.0. Significantly, these densities and temperatures imply that the co-spatial background is broadly compatible with static equilibrium theory (RTV scaling laws) provided that the unresolved field length is comparable to the embedded loop length. For this agreement to break down, the field length in most cases would have to be substantially longer than the loop length, a factor of 2–3 on average, which for the sample in this work approaches the dimensions of only the largest active regions.

The Planet Formation Potential around a 45 Myr Old Accreting M Dwarf

Abstract Debris disk detections around M dwarfs are rare, and so far no gas emission has been detected from an M dwarf debris disk. This makes the 45 Myr old M dwarf WISE J080822.18-644357.3 a bit of a curiosity; it has a strong infrared excess at an age beyond the lifetime of a typical planet-forming disk, and also exhibits broad Hα emission consistent with active accretion from a gaseous disk. To better understand the cold gas and dust properties of this system, we obtained ALMA observations of the 1.3 mm continuum and the CO/13CO/C18O J = 2–1 emission lines. No cold CO gas is detected from this system, ruling out a gas-rich protoplanetary disk. Unresolved dust continuum emission is detected at a flux of 198 ± 15 μJy, consistent with 0.057 ± 0.006 M ⊕ worth of optically thin dust, and consistent with being generated through a collisional cascade induced by large bodies at radii <16 au. With a sufficiently strong stellar wind, dust grains released in the outer disk can migrate inward via PR drag, potentially serving as a source of grains for the strong infrared excess.

Unresolved Gamma-Ray Sky through its Angular Power Spectrum.

The gamma-ray sky has been observed with unprecedented accuracy in the last decade by the Fermi -large area telescope (LAT), allowing us to resolve and understand the high-energy Universe. The nature of the remaining unresolved emission [unresolved gamma-ray background (UGRB)] below the LAT source detection threshold can be uncovered by characterizing the amplitude and angular scale of the UGRB fluctuation field. This Letter presents a measurement of the UGRB autocorrelation angular power spectrum based on eight years of Fermi-LAT Pass 8 data products. The analysis is designed to be robust against contamination from resolved sources and noise systematics. The sensitivity to subthreshold sources is greatly enhanced with respect to previous measurements. We find evidence (with ∼3.7σ significance) that the scenario in which two classes of sources contribute to the UGRB signal is favored over a single class. A double power law with exponential cutoff can explain the anisotropy energy spectrum well, with photon indices of the two populations being 2.55±0.23 and 1.86±0.15.