[en] We consider a novel mechanism to account for the observed distance-redshift relation. This is done by presenting a toy model for the large-scale matter distribution in a static Universe. Our model mainly concerns particles with masses far below those in the Standard Model of Particle Physics. The model is founded on three main assumptions: (1) a mass spectrum $\text{d}{N}_i$/$\text{d}{m}_i=\mathrm{\beta <!--\; ??\; -->}{m}_i^{-<!--\; ???\; -->\mathrm{\alpha <!--\; ??\; -->}}$ (where α and β are both positive constants) for low-mass particles with $m_i\ll <!--\; ???\; -->{10}^{-<!--\; ???\; -->22}$ eV $\ll <!--\; ???\; -->$ M _P, where M _P is the Planck mass; (2) a particle mass-wavelength relation of the form ${\mathrm{\lambda <!--\; ??\; -->}}_i=\mathrm{\hslash <!--\; ???\; -->}$/${\mathrm{\delta <!--\; ??\; -->}}_i{m}_i$c, where ${\mathrm{\delta <!--\; ??\; -->}}_i=\mathrm{\eta <!--\; ??\; -->}{m}_i^{\mathrm{\gamma <!--\; ??\; -->}}$ and η and γ are both constants; and (3) for such low-mass particles, locality can only be defined on large spatial scales, comparable to or exceeding the particle wavelengths.

We use our model to derive the cosmological redshift characteristic of the Standard Model of Cosmology, which becomes a gravitational redshift in our model. We compare the results of our model to empirical data and show that, in order to reproduce the sub-linear form of the observed distance-redshift relation, our model requires $\mathrm{\alpha <!--\; ??\; -->}+\mathrm{\gamma <!--\; ??\; -->}<0$. We further place our toy model in the context of the Friedmann Universe via a superposition of Einstein Universes, each with its own scale factor a _i. Given the overwhelming evidence supporting an expanding Universe, we then address possible modifications to our base model that would be required to account for the available empirical constraints, including the addition of some initial expansion. Finally, we consider potentially observable distinctions between the cosmological redshift and our proposed mechanism to account for the observed distance-redshift relation. (paper)

[en] We present the first systematic investigation of spectral properties of 17 Type Ic Supernovae (SNe Ic), 10 broad-lined SNe Ic (SNe Ic-bl) without observed gamma-ray bursts (GRBs), and 11 SNe Ic-bl with GRBs (SN-GRBs) as a function of time in order to probe their explosion conditions and progenitors. Using a number of novel methods, we analyze a total of 407 spectra, which were drawn from published spectra of individual SNe as well as from the densely time-sampled spectra of Modjaz et al (2014). In order to quantify the diversity of the SN spectra as a function of SN subtype, we construct average spectra of SNe Ic, SNe Ic-bl without GRBs, and SNe Ic-bl with GRBs. We find that SN 1994I is not a typical SN Ic, contrasting the general view, while the spectra of SN 1998bw/GRB 980425 are representative of mean spectra of SNe Ic-bl. We measure the ejecta absorption and width velocities using a new method described here and find that SNe Ic-bl with GRBs, on average, have quantifiably higher absorption velocities, as well as broader line widths than SNe without observed GRBs. In addition, we search for correlations between SN-GRB spectral properties and the energies of their accompanying GRBs. Finally, we show that the absence of clear He lines in optical spectra of SNe Ic-bl, and in particular of SN-GRBs, is not due to them being too smeared-out due to the high velocities present in the ejecta. This implies that the progenitor stars of SN-GRBs are probably free of the He-layer, in addition to being H-free, which puts strong constraints on the stellar evolutionary paths needed to produce such SN-GRB progenitors at the observed low metallicities.

[en] Using the largest spectroscopic data set of stripped-envelope core-collapse supernovae (stripped SNe), we present a systematic investigation of spectral properties of Type IIb SNe (SNe IIb), Type Ib SNe (SNe Ib), and Type Ic SNe (SNe Ic). Prior studies have been based on individual objects or small samples. Here, we analyze 242 spectra of 14 SNe IIb, 262 spectra of 21 SNe Ib, and 207 spectra of 17 SNe Ic based on the stripped SN data set of Modjaz et al. and other published spectra of individual SNe. Each SN in our sample has a secure spectroscopic ID, a date of V -band maximum light, and most have multiple spectra at different phases. We analyze these spectra as a function of subtype and phase in order to improve the SN identification scheme and constrain the progenitors of different kinds of stripped SNe. By comparing spectra of SNe IIb with those of SNe Ib, we find that the strength of H α can be used to quantitatively differentiate between these two subtypes at all epochs. Moreover, we find a continuum in observational properties between SNe IIb and Ib. We address the question of hidden He in SNe Ic by comparing our observations with predictions from various models that either include hidden He or in which He has been burnt. Our results favor the He-free progenitor models for SNe Ic. Finally, we construct continuum-divided average spectra as a function of subtype and phase to quantify the spectral diversity of the different types of stripped SNe.

[en] Recent versions of the observed cosmic star formation history (SFH) have resolved an inconsistency with the stellar mass density history. We show that the revised SFH also scales up the delay-time distribution (DTD) of Type Ia supernovae (SNe Ia), as determined from the observed volumetric SN Ia rate history, aligning it with other field-galaxy SN Ia DTD measurements. The revised-SFH-based DTD has a $t^{-<!--\; ???\; -->1.1\pm <!--\; ??\; -->0.1}$ form and a Hubble-time-integrated production efficiency of $N/M_{\star <!--\; ???\; -->}=1.3\pm <!--\; ??\; -->0.1$ SNe Ia per $1000M_{\odot <!--\; ???\; -->}$ of formed stellar mass. Using these revised histories and updated empirical iron yields of the various SN types, we re-derive the cosmic iron accumulation history. Core-collapse SNe and SNe Ia have contributed about equally to the total mass of iron in the universe today. We find the track of the average cosmic gas element in the [α/Fe] versus [Fe/H] abundance-ratio plane. The track is broadly similar to the observed main locus of Galactic stars in this plane, indicating a Milky Way (MW) SFH similar in form to the cosmic one. We easily find a simple MW SFH that makes the track closely match this stellar locus. Galaxy clusters appear to have a higher-normalization DTD. This cluster DTD, combined with a short-burst MW SFH peaked at z = 3, produces a track that matches remarkably well the observed “high-α” locus of MW stars, suggesting the halo/thick-disk population has had a galaxy-cluster-like formation mode. Thus, a simple two-component SFH, combined with empirical DTDs and SN iron yields, suffices to closely reproduce the MW’s stellar abundance patterns.

[en] We present analytical reconstructions of SN Ia delay time distributions (DTDs) by way of two independent methods: by a Markov Chain Monte Carlo best-fit technique comparing the volumetric SN Ia rate history to today’s compendium cosmic star formation history, and second through a maximum likelihood analysis of the star formation rate histories of individual galaxies in the GOODS/CANDELS field, in comparison to their resultant SN Ia yields. We adopt a flexible skew-normal DTD model, which could match a wide range of physically motivated DTD forms. We find a family of solutions that are essentially exponential DTDs, similar in shape to the β ≈ −1 power-law DTDs, but with more delayed events (>1 Gyr in age) than prompt events (<1 Gyr). Comparing these solutions to delay time measures separately derived from field galaxies and galaxy clusters, we find the skew-normal solutions can accommodate both without requiring a different DTD form in different environments. These model fits are generally inconsistent with results from single-degenerate binary population synthesis models, and are seemingly supportive of double-degenerate progenitors for most SN Ia events.

[en] In Paper I of this series, we showed that the ratio between stripped-envelope (SE) supernova (SN) and Type II SN rates reveals a significant SE SN deficiency in galaxies with stellar masses $\lesssim <!--\; ???\; -->{10}^{10}{M}_{\odot <!--\; ???\; -->}$. Here, we test this result by splitting the volume-limited subsample of the Lick Observatory Supernova Search (LOSS) SN sample into low- and high-mass galaxies and comparing the relative rates of various SN types found in them. The LOSS volume-limited sample contains 180 SNe and SN impostors and is complete for SNe Ia out to 80 Mpc and core-collapse SNe out to 60 Mpc. All of these transients were recently reclassified by us in Shivvers et al. We find that the relative rates of some types of SNe differ between low- and high-mass galaxies: SNe Ib and Ic are underrepresented by a factor of ∼3 in low-mass galaxies. These galaxies also contain the only examples of SN 1987A-like SNe in the sample and host about nine times as many SN impostors. Normal SNe Ia seem to be ∼30% more common in low-mass galaxies, making these galaxies better sources for homogeneous SN Ia cosmology samples. The relative rates of SNe IIb are consistent in both low- and high-mass galaxies. The same is true for broad-line SNe Ic, although our sample includes only two such objects. The results presented here are in tension with a similar analysis from the Palomar Transient Factory, especially as regards SNe IIb.

[en] Most types of supernovae (SNe) have yet to be connected with their progenitor stellar systems. Here, we reanalyze the 10-year SN sample collected during 1998–2008 by the Lick Observatory Supernova Search (LOSS) in order to constrain the progenitors of SNe Ia and stripped-envelope SNe (SE SNe, i.e., SNe IIb, Ib, Ic, and broad-lined Ic). We matched the LOSS galaxy sample with spectroscopy from the Sloan Digital Sky Survey and measured SN rates as a function of galaxy stellar mass, specific star formation rate, and oxygen abundance (metallicity). We find significant correlations between the SN rates and all three galaxy properties. The SN Ia correlations are consistent with other measurements, as well as with our previous explanation of these measurements in the form of a combination of the SN Ia delay-time distribution and the correlation between galaxy mass and age. The ratio between the SE SN and SN II rates declines significantly in low-mass galaxies. This rules out single stars as SE SN progenitors, and is consistent with predictions from binary-system progenitor models. Using well-known galaxy scaling relations, any correlation between the rates and one of the galaxy properties examined here can be expressed as a correlation with the other two. These redundant correlations preclude us from establishing causality—that is, from ascertaining which of the galaxy properties (or their combination) is the physical driver for the difference between the SE SN and SN II rates. We outline several methods that have the potential to overcome this problem in future works.

[en] The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and Cluster Lensing And Supernova survey with Hubble multi-cycle treasury programs with the Hubble Space Telescope (HST) have provided new opportunities to probe the rate of core-collapse supernovae (CCSNe) at high redshift, now extending to $z\approx <!--\; ???\; -->\mathrm{2.5.}$ Here we use a sample of approximately 44 CCSNe to determine volumetric rates, R_CC, in six redshift bins in the range $0.1<<!--\; <\; -->z<<!--\; <\; -->\mathrm{2.5.}$ Together with rates from our previous HST program, and rates from the literature, we trace a more complete history of $R_{\mathrm{C}\mathrm{C}}(z),$ with $R_{\mathrm{C}\mathrm{C}}=0.72\pm <!--\; ??\; -->0.06$ yr⁻¹ Mpc⁻³ 10⁻⁴$h_{70}^3$ at $z<<!--\; <\; -->0.08,$ and increasing to ${3.7}_{-<!--\; ???\; -->1.6}^{+3.1}$ yr⁻¹ Mpc⁻³ 10⁻⁴$h_{70}^3$ to $z\approx <!--\; ???\; -->\mathrm{2.0.}$ The statistical precision in each bin is several factors better than than the systematic error, with significant contributions from host extinction, and average peak absolute magnitudes of the assumed luminosity functions for CCSN types. Assuming negligible time delays from stellar formation to explosion, we find these composite CCSN rates to be in excellent agreement with cosmic star formation rate density (SFRs) derived largely from dust-corrected rest-frame UV emission, with a scaling factor of $k=0.0091\pm <!--\; ??\; -->0.0017M_{\odot <!--\; ???\; -->}^{-<!--\; ???\; -->1},$ and inconsistent (to $><!--\; >\; -->95\mathrm{\%<!--\; \%\; -->}$ confidence) with SFRs from IR luminous galaxies, or with SFR models that include simple evolution in the initial mass function over time. This scaling factor is expected if the fraction of the IMF contributing to CCSN progenitors is in the 8–50 M_⊙ range. It is not supportive, however, of an upper mass limit for progenitors at $<<!--\; <\; -->20M_{\odot <!--\; ???\; -->}.$

[en] Seitenzahl et al. have predicted that roughly three years after its explosion, the light we receive from a Type Ia supernova (SN Ia) will come mostly from reprocessing of electrons and X-rays emitted by the radioactive decay chain ⁵⁷Co → ⁵⁷Fe, instead of positrons from the decay chain ⁵⁶Co → ⁵⁶Fe that dominates the SN light at earlier times. Using the Hubble Space Telescope, we followed the light curve of the SN Ia SN 2012cg out to 1055 days after maximum light. Our measurements are consistent with the light curves predicted by the contribution of energy from the reprocessing of electrons and X-rays emitted by the decay of ⁵⁷Co, offering evidence that ⁵⁷Co is produced in SN Ia explosions. However, the data are also consistent with a light echo ∼14 mag fainter than SN 2012cg at peak. Assuming no light-echo contamination, the mass ratio of ⁵⁷Ni and ⁵⁶Ni produced by the explosion, a strong constraint on any SN Ia explosion models, is ${0.043}_{-<!--\; ???\; -->0.011}^{+0.012}$, roughly twice Solar. In the context of current explosion models, this value favors a progenitor white dwarf with a mass near the Chandrasekhar limit.

[en] We use time-domain optical spectroscopy to distinguish between broad emission lines powered by accreting black holes (BHs) and stellar processes (i.e., supernovae) for 16 galaxies identified as active galactic nucleus (AGN) candidates by Reines et al (2013). Our study is primarily focused on those objects with narrow emission line ratios dominated by star formation, for which the origin of the broad Hα emission was unclear. Based on follow-up spectroscopy, we find that the broad Hα emission has faded or was ambiguous for all of the star-forming objects (14/16), over baselines ranging from 5–14 years, suggesting a transient stellar process was responsible for the broad emission in previous Sloan Digital Sky Survey observations. For the two objects in our follow-up sample with narrow-line AGN signatures (RGG 9 and RGG 119), we find persistent broad Hα emission consistent with an AGN origin. Additionally, we use high spectral resolution observations to measure stellar velocity dispersions for 15 objects in the Reines et al. (2013) sample, all with narrow-line ratios indicating the presence of an AGN. Stellar masses range from $\sim <!--\; ???\; -->5\times <!--\; ??\; -->{10}^8$ to $3\times <!--\; ??\; -->{10}^9$ M _⊙, and we measure ${\mathrm{\sigma <!--\; ??\; -->}}_{\ast <!--\; ???\; -->}$ in the range of $28\text{--}71\mathrm{k}\mathrm{m}{\mathrm{s}}^{-<!--\; ???\; -->1}$. These ${\mathrm{\sigma <!--\; ??\; -->}}_{\ast <!--\; ???\; -->}$ correspond to some of the lowest-mass galaxies with optical signatures of AGN activity. We show that RGG 119, the one object that has both a measured ${\mathrm{\sigma <!--\; ??\; -->}}_{\ast <!--\; ???\; -->}$ and persistent broad Hα emission, falls near the extrapolation of the $M_{\mathrm{B}\mathrm{H}}-<!--\; ???\; -->{\mathrm{\sigma <!--\; ??\; -->}}_{\star <!--\; ???\; -->}$ relation to the low-mass end.