Luminosity Relation Research Articles

Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. XIII. Ultraviolet Time Lag of Hβ Emission in Mrk 142

We performed a rigorous reverberation-mapping analysis of the broad-line region (BLR) in a highly accreting (L/L Edd = 0.74–3.4) active galactic nucleus, Markarian 142 (Mrk 142), for the first time using concurrent observations of the inner accretion disk and the BLR to determine a time lag for the Hβ λ4861 emission relative to the ultraviolet (UV) continuum variations. We used continuum data taken with the Niel Gehrels Swift Observatory in the UVW2 band, and the Las Cumbres Observatory, Dan Zowada Memorial Observatory, and Liverpool Telescope in the g band, as part of the broader Mrk 142 multiwavelength monitoring campaign in 2019. We obtained new spectroscopic observations covering the Hβ broad emission line in the optical from the Gemini North Telescope and the Lijiang 2.4 m Telescope for a total of 102 epochs (over a period of 8 months) contemporaneous to the continuum data. Our primary result states a UV-to-Hβ time lag of days in Mrk 142 obtained from light-curve analysis with a Python-based running optimal average algorithm. We placed our new measurements for Mrk 142 on the optical and UV radius–luminosity relations for NGC 5548 to understand the nature of the continuum driver. The positions of Mrk 142 on the scaling relations suggest that UV is closer to the “true” driving continuum than the optical. Furthermore, we obtain = 6.32 ± 0.29 assuming UV as the primary driving continuum.

The Far Side of the Galactic Bar/Bulge Revealed through Semi-regular Variables

The Galactic bulge is critical to our understanding of the Milky Way. However, due to the lack of reliable stellar distances, the structure and kinematics of the bulge/bar beyond the Galactic center have remained largely unexplored. Here, we present a method to measure distances of luminous red giants using a period–amplitude–luminosity relation anchored to the Large Magellanic Cloud, with random uncertainties of 10%–15% and systematic errors below 1%–2%. We apply this method to data from the Optical Gravitational Lensing Experiment to measure distances to 190,302 stars in the Galactic bulge and beyond out to 20 kpc. Using this sample, we measure a distance to the Galactic center of R 0 = 8108 ± 106stat ± 93sys pc, consistent with direct measurements of stars orbiting Sgr A*. We cross-match our distance catalog with Gaia DR3 and use the subset of 39,566 overlapping stars to provide the first constraints on the Milky Way’s velocity field (V R , V ϕ , V z ) beyond the Galactic center. We show that the V R quadrupole from the bar’s near side is reflected with respect to the Galactic center, indicating that the bar is bisymmetric and aligned with the inner disk. We also find that the vertical height V Z map has no major structure in the region of the Galactic bulge, which is inconsistent with a current episode of bar buckling. Finally, we demonstrate with N-body simulations that distance uncertainty plays a factor in the alignment of the major and kinematic axes of the bar, necessitating caution when interpreting results for distant stars.

Theoretical Period–Radius and Period–Luminosity Relations for Mira Variables with Solar Metallicity

Evolutionary sequences of AGB stars with initial masses on the main sequence MZAMS =1.5 M , 2 M and 3 M were computed for the initial metallicity Z = 0.014. Selected models ofevolutionary sequences with envelopes under thermal equilibrium were used as initial conditions forcalculation of nonlinear stellar pulsations. The hydrodynamic models of each evolutionary sequence areshown to concentrate along the continuous line in the period–radius and period–luminosity diagrams.The theoretical period–radius and period–luminosity relations differ from one another for different main–sequence star masses because the stellar luminosity of AGB stars depends on the degenerate carboncore mass which increases with increasing MZAMS. In hydrodynamic models of evolutionary sequencesMZAMS = 2 M and MZAMS = 3 M the periods of the first overtone pulsators are 86 d ≤ Π ≤ 123 dand 174 d ≤ Π ≤ 204 d, whereas all models of the evolutionary sequence MZAMS = 1.5 M oscillate inthe fundamental mode. Fairly regular radial oscillations exist in stars with pulsation periods Π 500 d.In models with longer periods the amplitude rapidly increases with increasing Π and oscillations becomeirregular.

LIMpy: A Semianalytic Approach to Simulating Multiline Intensity Maps at Millimeter Wavelengths

Mapping of multiple lines such as the fine-structure emission from [C ii] (157.7 μm), [O iii] (52 and 88.4 μm), and rotational emission lines from CO are of particular interest for upcoming line intensity mapping (LIM) experiments at millimeter wavelengths, due to their brightness features. Several upcoming experiments aim to cover a broad range of scientific goals, from detecting signatures of the epoch of reionization to the physics of star formation and its role in galaxy evolution. In this paper, we develop a semianalytic approach to modeling line strengths as functions of the star formation rate (SFR) or infrared luminosity based on observations of local and high-z galaxies. This package, LIMpy (Line Intensity Mapping in Python), estimates the intensity and power spectra of [C ii], [O iii], and CO rotational transition lines up to the J levels (1–0) to (13–12) based both on analytic formalism and on simulations. We develop a relation among halo mass, SFR, and multiline intensities that permits us to construct a generic formula for the evolution of several line strengths up to z ∼ 10. We implement a variety of star formation models and multiline luminosity relations to estimate the astrophysical uncertainties on the intensity power spectrum of these lines. As a demonstration, we predict the signal-to-noise ratio of [C ii] detection for an EoR-Spec-like instrument on the Fred Young Submillimeter Telescope. Furthermore, the ability to use any halo catalog allows the LIMpy code to be easily integrated into existing simulation pipelines, providing a flexible tool to study intensity mapping in the context of complex galaxy formation physics.

The compactness of ultra-faint dwarf galaxies: A new challenge?

So far, numerical simulations of ultra-faint dwarf galaxies (UFDs) have failed to properly reproduce the observed size–luminosity relation. In particular, no hydrodynamical simulation run has managed to form UFDs with a half-light radius as small as 30 pc, as seen in observations of several UFD candidates. We tackle this problem by developing a simple but numerically clean and powerful method in which predictions of the stellar content of UFDs from ΛCDM cosmological hydrodynamical simulations are combined with very high-resolution dark-matter-only runs. This method allows us to trace the buildup history of UFDs and to determine the impact of the merger of building-block objects on their final size. We find that, while no UFDs more compact than 20 pc can be formed, slightly larger systems are only reproduced if all member stars originate from the same initial mini-halo. However, this imposes that (i) the total virial mass is smaller than 3 × 108 M⊙, and (ii) the stellar content prior to the end of the reionisation epoch is very compact (< 15 pc) and strongly gravitationally bound, which is a challenge for current hydrodynamical numerical simulations. If initial stellar building blocks are larger than 35 pc, the size of the UFD will extend to 80 pc. Finally, our study shows that UFDs keep strong imprints of their buildup history in the form of elongated or extended stellar halos. Those features can erroneously be interpreted as tidal signatures.

The SN 2023ixf Progenitor in M101. I. Infrared Variability

Observational evidence points to a red supergiant (RSG) progenitor for SN 2023ixf. The progenitor candidate has been detected in archival images at wavelengths (≥0.6 μm) where RSGs typically emit profusely. This object is distinctly variable in the infrared (IR). We characterize the variability using pre-explosion mid-IR (3.6 and 4.5 μm) Spitzer and ground-based near-IR (JHK s ) archival data jointly covering 19 yr. The IR light curves exhibit significant variability with rms amplitudes in the range 0.2–0.4 mag, increasing with decreasing wavelength. From a robust period analysis of the more densely sampled Spitzer data, we measure a period of 1091 ± 71 days. We demonstrate using Gaussian process modeling that this periodicity is also present in the near-IR light curves, thus indicating a common physical origin, which is likely pulsational instability. We use a period–luminosity relation for RSGs to derive a value of M K = −11.58 ± 0.31 mag. Assuming a late M spectral type, this corresponds to at T eff = 3200 K and to at T eff = 3500 K. This gives an independent estimate of the progenitor’s luminosity, unaffected by uncertainties in extinction and distance. Assuming the progenitor candidate underwent enhanced dust-driven mass loss during the time of these archival observations, and using an empirical period–luminosity–based mass-loss prescription, we obtain a mass-loss rate of around (2–4) × 10−4 M ⊙ yr−1. Comparing the above luminosity with stellar evolution models, we infer an initial mass for the progenitor candidate of 20 ± 4 M ⊙, making this one of the most massive progenitors for a Type II SN detected to date.

Crowded No More: The Accuracy of the Hubble Constant Tested with High-resolution Observations of Cepheids by JWST

High-resolution James Webb Space Telescope (JWST) observations can test confusion-limited Hubble Space Telescope (HST) observations for a photometric bias that could affect extragalactic Cepheids and the determination of the Hubble constant. We present JWST NIRCAM observations in two epochs and three filters of >320 Cepheids in NGC 4258 (which has a 1.5% maser-based geometric distance) and in NGC 5584 (host of SN Ia 2007af), near the median distance of the SH0ES HST SN Ia host sample and with the best leverage among them to detect such a bias. JWST provides far superior source separation from line-of-sight companions than HST in the near-infrared to largely negate confusion or crowding noise at these wavelengths, where extinction is minimal. The result is a remarkable >2.5× reduction in the dispersion of the Cepheid period–luminosity relations, from 0.45 to 0.17 mag, improving individual Cepheid precision from 20% to 7%. Two-epoch photometry confirmed identifications, tested JWST photometric stability, and constrained Cepheid phases. The P–L relation intercepts are in very good agreement, with differences (JWST−HST) of 0.00 ± 0.03 and 0.02 ± 0.03 mag for NGC 4258 and NGC 5584, respectively. The difference in the determination of H0 between HST and JWST from these intercepts is 0.02 ± 0.04 mag, insensitive to JWST zero-points or count rate nonlinearity thanks to error cancellation between rungs. We explore a broad range of analysis variants (including passband combinations, phase corrections, measured detector offsets, and crowding levels) indicating robust baseline results. These observations provide the strongest evidence yet that systematic errors in HST Cepheid photometry do not play a significant role in the present Hubble Tension. Upcoming JWST observations of >12 SN Ia hosts should further refine the local measurement of the Hubble constant.

Z-GAL: A NOEMA spectroscopic redshift survey of bright Herschel galaxies

The z-GAL survey observed 137 bright Herschel-selected targets with the IRAM Northern Extended Millimeter Array, with the aim to measure their redshift and study their properties. Several of them have been resolved into multiple sources. Consequently, robust spectroscopic redshifts have been measured for 165 individual galaxies in the range 0.8 < z < 6.5. In this paper we analyse the millimetre spectra of the z-GAL sources, using both their continuum and line emission to derive their physical properties. At least two spectral lines are detected for each source, including transitions of 12CO, [CI], and H2O. The observed 12CO line ratios and spectral line energy distributions of individual sources resemble those of local starbursts. In seven sources the para-H2O (211−202) transition is detected and follows the IR versus H2O luminosity relation of sub-millimetre galaxies. The molecular gas mass of the z-GAL sources is derived from their 12CO, [CI], and sub-millimetre dust continuum emission. The three tracers lead to consistent results, with the dust continuum showing the largest scatter when compared to 12CO. The gas-to-dust mass ratio of these sources was computed by combining the information derived from 12CO and the dust continuum and has a median value of 107, similar to star-forming galaxies of near-solar metallicity. The same combined analysis leads to depletion timescales in the range between 0.1 and 1.0 Gyr, which place the z-GAL sources between the ‘main sequence’ of star formation and the locus of starbursts. Finally, we derived a first estimate of stellar masses – modulo possible gravitational magnification – by inverting known gas scaling relations: the z-GAL sample is confirmed to be mostly composed by starbursts, whereas ∼25% of its members lie on the main sequence of star-forming galaxies (within ±0.5 dex).

WALLABY pre-pilot survey: ultra-diffuse galaxies in the Eridanus supergroup

ABSTRACT We present a pilot study of the atomic neutral hydrogen gas (H i) content of ultra-diffuse galaxy (UDG) candidates. In this paper, we use the pre-pilot Eridanus field data from the Widefield ASKAP L-band Legacy All-sky Blind Survey to search for H i in UDG candidates found in the Systematically Measuring Ultra-diffuse Galaxies survey (SMUDGes). We narrow down to 78 SMUDGes UDG candidates within the maximum radial extents of the Eridanus subgroups for this study. Most SMUDGes UDGs candidates in this study have effective radii smaller than 1.5 kpc and thus fail to meet the defining size threshold. We only find one H i detection, which we classify as a low-surface-brightness dwarf. Six putative UDGs are H i-free. We show the overall distribution of SMUDGes UDG candidates on the size–luminosity relation and compare them with low-mass dwarfs on the atomic gas fraction versus stellar mass scaling relation. There is no correlation between gas-richness and colour indicating that colour is not the sole parameter determining their H i content. The evolutionary paths that drive galaxy morphological changes and UDG formation channels are likely the additional factors to affect the H i content of putative UDGs. The actual numbers of UDGs for the Eridanus and NGC 1332 subgroups are consistent with the predicted abundance of UDGs and the halo virial mass relation, except for the NGC 1407 subgroup, which has a smaller number of UDGs than the predicted number. Different group environments suggest that these putative UDGs are likely formed via the satellite accretion scenario.

First light and reionization epoch simulations (FLARES) IX: the physical mechanisms driving compact galaxy formation and evolution

ABSTRACT In the First Light And Reionization Epoch Simulations (Flares) suite of hydrodynamical simulations, we find the high-redshift (z > 5) intrinsic size–luminosity relation is, surprisingly, negatively sloped. However, after including the effects of dust attenuation, we find a positively sloped UV observed size–luminosity relation in good agreement with other simulated and observational studies. In this work, we extend this analysis to probe the underlying physical mechanisms driving the formation and evolution of the compact galaxies driving the negative size–mass/size–luminosity relation. We find the majority of compact galaxies (R1/2, ⋆ < 1 pkpc, which drive the negative slope of the size–mass relation, have transitioned from extended to compact sizes via efficient centralized cooling, resulting in high specific star formation rates in their cores. These compact stellar systems are enshrouded by non-star-forming gas distributions as much as 100 times larger than their stellar counterparts. By comparing with galaxies from the Eagle simulation suite, we find that these extended gas distributions ‘turn on’ and begin to form stars between z = 5 and 0 leading to increasing sizes, and thus the evolution of the size–mass relation from a negative to a positive slope. This explicitly demonstrates the process of inside-out galaxy formation in which compact bulges form earlier than the surrounding discs.

Clumping and X-rays in cooler B supergiant stars

Context. B supergiants (BSGs) are evolved stars with effective temperatures between ~10 to ~30 kK. Knowing the properties of these objects is important to understand massive star evolution. Located on the cool end of the line-driven wind regime, the study of their atmospheres can help us to understand the physics of their winds and phenomena such as the bi-stability jump. Aims. Despite being well-studied stars, key UV features of their spectra have so far not been reproduced by atmosphere models for spectral types later than B1. In this study, we aim to remedy this situation by performing quantitative spectral analyzes that account for the effects of X-rays and clumping in the wind. In addition, we also briefly investigate the evolutionary status of our sample stars based on the stellar parameters we obtained. Methods. We determined photospheric and wind parameters via quantitative spectroscopy using atmosphere models computed with CMFGEN and PoWR. These models were compared to high-resolution UV and optical spectra of four BSGs: HD206165, HD198478, HD53138, and HD164353. We further employed GENEC and MESA tracks to study the evolutionary status of our sample. Results. When including both clumping and X-rays, we obtained a good agreement between synthetic and observed spectra for our sample stars. For the first time, we reproduced key wind lines in the UV, where previous studies were unsuccessful. To model the UV spectra, we require a moderately clumped wind (fV∞ ≳ 0.5). We also infer a relative X-ray luminosity of about 10−7.5 to 10−8, which is lower than the typical ratio of 10−7. Moreover, we find a possible mismatch between evolutionary mass predictions and the derived spectroscopic masses, which deserves deeper investigation as this might relate to the mass-discrepancy problem present in other types of OB stars. Conclusions. Our results provide direct spectroscopic evidence that both X-rays and clumping need to be taken into account to describe the winds of cool BSGs. However, their winds seem to be much less structured than in earlier OB-type stars. Our findings are in line with observational X-rays and clumping constraints as well as recent hydrodynamical simulations. The evolutionary status of BSGs seems to be diverse with some objects potentially being post-red supergiants or merger products. The obtained wind parameters provide evidence for a moderate increase of the mass-loss rate around the bi-stability jump.

Metallicities of Classical Cepheids in the Inner Galactic Disk

Metallicity gradients refer to the sloped radial profiles of the metallicities of gas and stars and are commonly seen in disk galaxies. A well-defined metallicity gradient of the Galactic disk is observed particularly well with classical Cepheids, which are good stellar tracers thanks to their period–luminosity relation, allowing precise distance estimation and other advantages. However, the measurement of the inner-disk gradient has been impeded by the incompleteness of previous samples of Cepheids and the limitations of optical spectroscopy in observing highly reddened objects. Here we report the metallicities of 16 Cepheids measured with high-resolution spectra in the near-infrared YJ bands. These Cepheids are located at 3–5.6 kpc in Galactocentric distance, R GC, and reveal the metallicity gradient in this range for the first time. Their metallicities are mostly between 0.1 and 0.3 dex in [Fe/H] and more or less follow the extrapolation of the metallicity gradient found in the outer part, R GC > 6.5 kpc. The gradient in the inner disk may be shallower or even flat, but the small sample does not allow the determination of the slope precisely. More extensive spectroscopic observations would also be necessary for studying minor populations, if any, with higher or lower metallicities that were reported in previous literature. In addition, the 3D velocities of our inner-disk Cepheids show a kinematic pattern that indicates noncircular orbits caused by the Galactic bar, which is consistent with the patterns reported in recent studies on high-mass star-forming regions and red giant branch stars.

The Iron Yield of Core-collapse Supernovae

We present a systematic analysis of 191 stripped-envelope supernovae (SE SNe), aimed at computing their 56Ni masses from the luminosity in their radioactive tails () and/or in their maximum light, and the mean 56Ni and iron yields of SE SNe and core-collapse SNe. Our sample consists of SNe IIb, Ib, and Ic from the literature and from the Zwicky Transient Facility Bright Transient Survey. To calculate luminosities from optical photometry, we compute bolometric corrections using 49 SE SNe with optical and near-IR photometry, and develop corrections to account for the unobserved UV and IR flux. We find that the equation of Khatami & Kasen for radioactive 56Ni-powered transients with a single free parameter does not fit the observed peak time–luminosity relation of SE SNe. Instead, we find a correlation between , peak time, peak luminosity, and decline rate, which allows for measuring individual 56Ni masses to a precision of 14%. Applying this method to the whole sample, we find, for SNe IIb, Ib, and Ic, mean 56Ni masses of 0.066 ± 0.006, 0.082 ± 0.009, and 0.132 ± 0.011 M ⊙, respectively. After accounting for their relative rates, for SE SNe as a whole, we compute mean 56Ni and iron yields of 0.090 ± 0.005 and 0.097 ± 0.007 M ⊙, respectively. Combining these results with the recent Type II SN mean 56Ni mass derived by Rodríguez et al., core-collapse SNe, as a whole, have mean 56Ni and iron yields of 0.055 ± 0.006 and 0.058 ± 0.007 M ⊙, respectively. We also find that radioactive 56Ni-powered models typically underestimate the peak luminosity of SE SNe by 60%–70%, suggesting the presence of an additional power source contributing to the luminosity at peak.

High-resolution Spectroscopic Metallicities of Milky Way Cepheid Standards and Their Impact on the Leavitt Law and the Hubble Constant

Milky Way Cepheid variables with accurate Hubble Space Telescope photometry have been established as standards for primary calibration of the cosmic distance ladder to achieve a percent-level determination of the Hubble constant (H 0). These 75 Cepheid standards are the fundamental sample for investigation of possible residual systematics in the local H 0 determination due to metallicity effects on their period–luminosity relations. We obtained new high-resolution (R ∼ 81,000), high-signal-to-noise (S/N ∼ 50–150) multiepoch spectra of 42 out of 75 Cepheid standards using the ESPaDOnS instrument at the 3.6 m Canada–France–Hawaii Telescope. Our spectroscopic metallicity measurements are in good agreement with the literature values with systematic differences up to 0.1 dex due to different metallicity scales. We homogenized and updated the spectroscopic metallicities of all 75 Milky Way Cepheid standards and derived their multiwavelength (GVIJHK s ) period–luminosity–metallicity and period–Wesenheit–metallicity relations using the latest Gaia parallaxes. The metallicity coefficients of these empirically calibrated relations exhibit large uncertainties due to low statistics and a narrow metallicity range (Δ[Fe/H] = 0.6 dex). These metallicity coefficients are up to 3 times better constrained if we include Cepheids in the Large Magellanic Cloud and range between −0.21 ± 0.07 and −0.43 ± 0.06 mag dex−1. The updated spectroscopic metallicities of these Milky Way Cepheid standards were used in the Cepheid–supernovae distance ladder formalism to determine H 0 = 72.9 ± 1.0 km s−1 Mpc−1, suggesting little variation (∼0.1 km s−1 Mpc−1) in the local H 0 measurements due to different Cepheid metallicity scales.

Estimation of the Galactocentric Distance of the Sun from Cepheids Close to the Solar Circle

Based on Cepheids located near the solar circle, we have determined the Galactocentric distance of the Sun R0 and the Galactic rotation velocity at the solar distance V0. For our analysis we used a sample of -200 classical Cepheids from the catalogue by Skowron et al. (2019), where the distances to them were determined from the period–luminosity relation. For these stars the proper motions and line-of-sight velocities were taken from the Gaia DR3 catalogue. The values of R0 found lie within the range 7.8–8.3 kpc, depending on the heliocentric distance of the sample stars, on the adopted solar velocity relative to the local standard of rest, and on whether or not the perturbations caused by the Galactic spiral density wave are taken into account. The dispersion of the R0 estimates is -2 kpc. Similarly, the values of V0 lie within the range 240–270 km s-1 with a dispersion of the estimates of 70–90 km s-1. We consider the following estimates to be the final ones: R0=8,24+0,20 kpc and V0=268+8 km s-1 found by taking into account the perturbations from the Galactic spiral density wave.

Zwicky Transient Facility and Globular Clusters: The gr-band Period–Luminosity Relations for Mira Variables at Maximum Light and their Applications to Local Galaxies

Based on 14 Miras located in seven globular clusters, we derived the first gr-band period–luminosity (PL) at maximum light for the large-amplitude Mira variables using the multiyear light-curve data collected from the Zwicky Transient Facility (ZTF). Since Miras are red variables, we applied a color-term correction to subsets of ZTF light curves, and found that such corrections do not have a large impact on period determinations. We applied our derived PL relations to the known extragalactic Miras in five local galaxies (Sextans, Leo I, Leo II, NGC 6822 and IC 1613), and determined their Mira-based distances. We demonstrated that our PL relations can be applied to short-period (≲300 days) Miras, including those in the two most distant galaxies (NGC 6822 and IC 1613) in our sample even when only a portion of the light curves around maximum light have detections. We have also shown that the long-period extragalactic Miras do not follow the PL relations extrapolated to longer periods. Hence, our derived PL relations are only applicable to the short-period Miras, which will be discovered in abundance in local galaxies within the era of Vera C. Rubin Observatory’s Legacy Survey of Space and Time.

The Seoul National University AGN Monitoring Project. IV. Hα Reverberation Mapping of Six AGNs and the Hα Size–Luminosity Relation

The broad-line region (BLR) size–luminosity relation has paramount importance for estimating the mass of black holes in active galactic nuclei (AGNs). Traditionally, the size of the Hβ BLR is often estimated from the optical continuum luminosity at 5100 Å, while the size of the Hα BLR and its correlation with the luminosity is much less constrained. As a part of the Seoul National University AGN Monitoring Project, which provides 6 yr photometric and spectroscopic monitoring data, we present our measurements of the Hα lags of high-luminosity AGNs. Combined with the measurements for 42 AGNs from the literature, we derive the size–luminosity relations of the Hα BLR against the broad Hα and 5100 Å continuum luminosities. We find the slope of the relations to be 0.61 ± 0.04 and 0.59 ± 0.04, respectively, which are consistent with the Hβ size–luminosity relation. Moreover, we find a linear relation between the 5100 Å continuum luminosity and the broad Hα luminosity across 7 orders of magnitude. Using these results, we propose a new virial mass estimator based on the Hα broad emission line, finding that the previous mass estimates based on scaling relations in the literature are overestimated by up to 0.7 dex at masses lower than 107 M ⊙.

The Progenitors of Superluminous Type Ia Supernovae

Recent observations of type Ia supernovae (SNe Ia) have discovered a subclass of “super-Chandrasekhar” SNe Ia (SC SNe Ia) whose high luminosities and low ejecta velocities suggest that they originate from the explosions of white dwarfs (WDs) with masses that exceed the Chandrasekhar mass limit. Different models have been proposed to explain the progenitors of these explosions, including a “magnetized WD” model and a “WD merger” model. To test the robustness of these models, we conduct a 1D numerical parameter survey of WD explosions using these models as initial conditions. We follow the explosions using the hydrodynamics code Castro and then use the radiation transport code SuperNu to create light curves and spectra for the models. We find that while both classes of models fall within the range of SC SNe Ia observations on the light-curve width–luminosity relation, only the WD merger models reproduce the observed low ejecta velocities. The light curves of our merger models are more similar photometrically to observations than our magnetized models. Given this, we discuss possible explanations for the brightest SC SNe Ia observations that cannot be reproduced with our WD merger models. This study provides the basis for future SC SNe Ia observations and higher-dimensional numerical models.

Minor Mergers Are Not Enough: The Importance of Major Mergers during Brightest Cluster Galaxy Assembly

We investigate the roles of major and minor mergers during brightest cluster galaxy (BCG) assembly using surface brightness profiles, line indices, and fundamental plane relations. Based on our own sample and consistently reanalyzed Sloan Digital Sky Survey data, we find that BCGs and luminous normal ellipticals (LNEs) have similar central velocity dispersions, central absorption line strengths, and central surface brightnesses. However, BCGs are more luminous due to their much larger radial extent. These properties result in a flattening of the Faber–Jackson and Mgb–luminosity relations above 1010.6 . We use this effect to estimate an amount of 60%–80% of accreted and merged light in BCGs relative to LNEs, which agrees with results from cosmological simulations. We determine the contribution of this excess light (EL) at each radius from the difference between the surface flux profiles of BCGs and LNEs. It is small in the center but increases steeply to 50% at ∼3 kpc radius. The shape of these profiles suggests that BCGs could be formed from LNEs in three major merger processes. This is also consistent with the mild increase of the Sérsic indices from n ≈ 4 to n ≈ 6, as confirmed in merger simulations. We note that minor mergers cannot be the dominant origin of the BCG’s EL because they deposit too few stars at intermediate radii r ≲ 20 kpc. The shape of the EL profile also explains a detected offset of 0.14 dex of the fundamental planes for BCGs and LNEs relative to each other.

The radio emission in radio-quiet quasars: the VLBA perspective

ABSTRACT The origin of the radio emission in radio-quiet quasars (RQQ) is not established yet. We present new VLBA observations at 1.6 and 4.9 GHz of 10 RQQ (9 detected), which together with published earlier observations of 8 RQQ (5 detected), forms a representative sample of 18 RQQ drawn from the Palomar–Green sample of low z (< 0.5) AGN. The spectral slope of the integrated emission extends from very steep (α < −1.98) to strongly inverted (α = +2.18), and the slopes of 9 of the 14 objects are flat (α > −0.5). Most objects have an unresolved flat-spectrum core, which coincides with the optical Gaia position. The extended emission is generally steep-spectrum, has a low brightness temperature (< 107 K), and is displaced from the optical core (the Gaia position) by ∼ 5–100 pc. The VLBA core flux is tightly correlated with the X-ray flux, and follows a radio to X-ray luminosity relation of log LR/LX ≃ −6, for all objects with a black hole mass log MBH/M⊙ < 8.5. The flatness of the core emission implies a compact source size (≲ 0.1 pc), which likely originates from the accretion disc corona. The mas-scale extended emission is optically thin and of clumpy structure, and is likely produced by an outflow from the center. Radio observations at higher frequencies can further test the accretion disc coronal emission interpretation for the core emission in RQQ.