Bayesian single-epoch photometric classification of supernovae

Ongoing supernova (SN) surveys find hundreds of candidates that require confirmation for their various uses. Traditional classification based on follow- up spectroscopy of all candidates is virtually impossible for these large samples. The use of Type Ia SNe as standard candles is at an evolved stage that requires pure, uncontaminated samples. However, other SN survey applications, such as measuring cosmic SN rates, could benefit froma classification of SNe on a statistical basis, rather than case by case. With this objective in mind, we have developed the SN-ABC, an automatic Bayesian classifying algorithm for supernovae. We rely solely on single- epoch multiband photometry and host-galaxy (photometric) redshift information to sort SN candidates into the two major types, Ia and core-collapse supernovae. We test the SN-ABC performance on published samples of SNe from the Supernova Legacy Survey (SNLS) and GOODS projects that have both broadband photometry and spectroscopic classification (so the true type is known). The SN- ABC correctly classifies up to 97% (85%) of the Type Ia (II-P) SNe in SNLS, and similar fractions of the GOODS SNe, depending on photometric redshift quality. Using simulations with large artificial samples, we find similarly high success fractions for Types Ia and II-P, and reasonable (~75%) success rates in classifying Type Ibc SNe as core-collapse. Type IIn SNe, however, are often misclassified as Type Ia. In deep surveys, SNe Ia are best classified at redshifts z ≳ 0.6 or when near maximum. Core-collapse SNe (other than Type IIn) are best recognized several weeks after maximum, or at z ≾ 0.6. Assuming the SNe are young, as would be the case for rolling surveys, the success fractions improve by a degree dependent on the type and redshift. The fractional contamination of a single-epoch photometrically selected sample of SNe la by core-collapse SNe varies between less than 10% and as much as 30%, depending on the intrinsic fraction and redshift distribution of the core-collapse SNe in a given survey. The SN-ABC also allows the rejection of SN "impostors" such as active galactic nuclei (AGNs), with half of the AGNs we simulate rejected by the algorithm. Our algorithm also supplies a good measure of the quality of the classification, which is valuable for error estimation

Spectroscopic identification of a redshift 1.55 supernova host galaxy from the Subaru Deep Field Supernova Survey

Context: The Subaru Deep Field (SDF) Supernova Survey discovered 10 Type Ia supernovae (SNe Ia) in the redshift range 1.5<z<2.0, as determined solely from photometric redshifts of the host galaxies. However, photometric redshifts might be biased, and the SN sample could be contaminated by active galactic nuclei (AGNs). Aims: We aim to obtain the first robust redshift measurement and classification of a z > 1.5 SDF SN Ia host galaxy candidate Methods: We use the X-shooter (U-to-K-band) spectrograph on the Very Large Telescope to allow the detection of different emission lines in a wide spectral range. Results: We measure a spectroscopic redshift of 1.54563 +/- 0.00027 of hSDF0705.25, consistent with its photometric redshift of 1.552 +/- 0.018. From the strong emission-line spectrum we rule out AGN activity, thereby confirming the optical transient as a SN. The host galaxy follows the fundamental metallicity relation defined in Mannucci et al. (2010, 2011) showing that the properties of this high-redshift SN Ia host galaxy is similar to other field galaxies. Conclusions: Spectroscopic confirmation of additional SDF SN hosts would be required to confirm the cosmic SN rate evolution measured in the SDF.Comment: 14 pages, 2 figures, Accepted A&A Upload of the Journal versio

Dusting off the diffuse interstellar bands: DIBs and dust in extragalactic SDSS spectra

Using over a million and a half extragalactic spectra we study the properties of the mysterious Diffuse Interstellar Bands (DIBs) in the Milky Way. These data provide us with an unprecedented sampling of the skies at high Galactic-latitude and low dust-column-density. We present our method, study the correlation of the equivalent width of 8 DIBs with dust extinction and with a few atomic species, and the distribution of four DIBs - 5780.6A, 5797.1A, 6204.3A, and 6613.6A - over nearly 15000 squared degrees. As previously found, DIBs strengths correlate with extinction and therefore inevitably with each other. However, we show that DIBs can exist even in dust free areas. Furthermore, we find that the DIBs correlation with dust varies significantly over the sky. DIB under- or over-densities, relative to the expectation from dust, are often spread over hundreds of square degrees. These patches are different for the four DIBs, showing that they are unlikely to originate from the same carrier, as previously suggested.Comment: MNRAS accepte