The NN2 Flux Difference Method for Constructing Variable Object Light Curves

We present a new method for optimally extracting point-source time variability information from a series of images. Differential photometry is generally best accomplished by subtracting two images separated in time, since this removes all constant objects in the field. By removing background sources such as the host galaxies of supernovae, such subtractions make possible the measurement of the proper flux of point-source objects superimposed on extended sources. In traditional difference photometry, a single image is designated as the ``template'' image and subtracted from all other observations. This procedure does not take all the available information into account and for sub-optimal template images may produce poor results. Given N total observations of an object, we show how to obtain an estimate of the vector of fluxes from the individual images using the antisymmetric matrix of flux differences formed from the N(N-1)/2 distinct possible subtractions and provide a prescription for estimating the associated uncertainties. We then demonstrate how this method improves results over the standard procedure of designating one image as a ``template'' and differencing against only that image.Comment: Accepted to AJ. To be published in November 2005 issue. 16 page, 2 figures, 2 tables. Source code available at http://www.ctio.noao.edu/essence/nn2

Redshift-Independent Distances to Type Ia Supernovae

We describe a procedure for accurately determining luminosity distances to Type Ia supernovae (SNe Ia) without knowledge of redshift. This procedure, which may be used as an extension of any of the various distance determination methods currently in use, is based on marginalizing over redshift, removing the requirement of knowing $z$ a priori. We demonstrate that the Hubble diagram scatter of distances measured with this technique is approximately equal to that of distances derived from conventional redshift-specific methods for a set of 60 nearby SNe Ia. This indicates that accurate distances for cosmological SNe Ia may be determined without the requirement of spectroscopic redshifts, which are typically the limiting factor for the number of SNe that modern surveys can collect. Removing this limitation would greatly increase the number of SNe for which current and future SN surveys will be able to accurately measure distance. The method may also be able to be used for high-$z$ SNe Ia to determine cosmological density parameters without redshift information.Comment: 12 pages, 3 figures, accepted for publication in Astrophysical Journal Letter

The Rate of Type Ia Supernovae at High Redshift

We derive the rates of Type Ia supernovae (SNIa) over a wide range of redshifts using a complete sample from the IfA Deep Survey. This sample of more than 100 SNIa is the largest set ever collected from a single survey, and therefore uniquely powerful for a detailed supernova rate (SNR) calculation. Measurements of the SNR as a function of cosmological time offer a glimpse into the relationship between the star formation rate (SFR) and Type Ia SNR, and may provide evidence for the progenitor pathway. We observe a progressively increasing Type Ia SNR between redshifts z~0.3-0.8. The Type Ia SNR measurements are consistent with a short time delay (t~1 Gyr) with respect to the SFR, indicating a fairly prompt evolution of SNIa progenitor systems. We derive a best-fit value of SFR/SNR 580 h_70^(-2) M_solar/SNIa for the conversion factor between star formation and SNIa rates, as determined for a delay time of t~1 Gyr between the SFR and the Type Ia SNR. More complete measurements of the Type Ia SNR at z>1 are necessary to conclusively determine the SFR--SNR relationship and constrain SNIa evolutionary pathways.Comment: 37 pages, 9 figures, accepted for publication in Astrophysical Journal. Figures 7-9 correcte

Discovery of a Methane Dwarf from the IfA-Deep Survey

We present the discovery of a distant methane dwarf, the first from the Institute for Astronomy (IfA) Deep Survey. The object ("IfA 0230-Z1") was identified from deep optical I and z'-band imaging, being conducted as an IfA-wide collaboration using the prime-focus imager Suprime-Cam on the Subaru 8.2-m Telescope. IfA 0230-Z1 is extremely red in the Iz'J (0.8--1.2 micron) bands but relatively blue in J-H; such colors are uniquely characteristic of T dwarfs. A near-IR spectrum taken with the Keck Telescope shows strong H2O absorption and a continuum break indicative of CH4, confirming the object has a very cool atmosphere. Comparison with nearby T dwarfs gives a spectral type of T3-T4 and a distance of ~45 pc. Simple estimates based on previous T dwarf discoveries suggest that the IfA survey will find a comparable number of T dwarfs as the 2MASS survey, albeit at a much larger average distance. We also discuss the survey's ability to probe the galactic scale height of ultracool (L and T) dwarfs.Comment: Astrophysical Journal Letters, in pres

Imaging and Demography of the Host Galaxies of High-Redshift Type Ia Supernovae

We present the results of a study of the host galaxies of high redshift Type Ia supernovae (SNe Ia). We provide a catalog of 18 hosts of SNe Ia observed with the Hubble Space Telescope (HST) by the High-z Supernova Search Team (HZT), including images, scale-lengths, measurements of integrated (Hubble equivalent) BVRIZ photometry in bands where the galaxies are brighter than m ~ 25 mag, and galactocentric distances of the supernovae. We compare the residuals of SN Ia distance measurements from cosmological fits to measurable properties of the supernova host galaxies that might be expected to correlate with variable properties of the progenitor population, such as host galaxy color and position of the supernova. We find mostly null results; the current data are generally consistent with no correlations of the distance residuals with host galaxy properties in the redshift range 0.42 < z < 1.06. Although a subsample of SN hosts shows a formally significant (3-sigma) correlation between apparent V-R host color and distance residuals, the correlation is not consistent with the null results from other host colors probed by our largest samples. There is also evidence for the same correlations between SN Ia properties and host type at low redshift and high redshift. These similarities support the current practice of extrapolating properties of the nearby population to high redshifts pending more robust detections of any correlations between distance residuals from cosmological fits and host properties.Comment: 35 pages, 12 figures, 4 tables, accepted for publication in A

23 High Redshift Supernovae from the IfA Deep Survey: Doubling the SN Sample at z>0.7

We present photometric and spectroscopic observations of 23 high redshift supernovae spanning a range of z=0.34-1.03, 9 of which are unambiguously classified as Type Ia. These supernovae were discovered during the IfA Deep Survey, which began in September 2001 and observed a total of 2.5 square degrees to a depth of approximately m=25-26 in RIZ over 9-17 visits, typically every 1-3 weeks for nearly 5 months, with additional observations continuing until April 2002. We give a brief description of the survey motivations, observational strategy, and reduction process. This sample of 23 high-redshift supernovae includes 15 at z>0.7, doubling the published number of objects at these redshifts, and indicates that the evidence for acceleration of the universe is not due to a systematic effect proportional to redshift. In combination with the recent compilation of Tonry et al. (2003), we calculate cosmological parameter density contours which are consistent with the flat universe indicated by the CMB (Spergel et al. 2003). Adopting the constraint that Omega_total = 1.0, we obtain best-fit values of (Omega_m, Omega_Lambda)=(0.33, 0.67) using 22 SNe from this survey augmented by the literature compilation. We show that using the empty-beam model for gravitational lensing does not eliminate the need for Omega_Lambda > 0. Experience from this survey indicates great potential for similar large-scale surveys while also revealing the limitations of performing surveys for z>1 SNe from the ground.Comment: 67 pages, 12 figures, 12 tables, accepted for publication in the Astrophysical Journa

Spectroscopy of High-Redshift Supernovae from the ESSENCE Project: The First Two Years

We present the results of spectroscopic observations of targets discovered during the first two years of the ESSENCE project. The goal of ESSENCE is to use a sample of ~200 Type Ia supernovae (SNe Ia) at moderate redshifts (0.2 < z < 0.8) to place constraints on the equation of state of the Universe. Spectroscopy not only provides the redshifts of the objects, but also confirms that some of the discoveries are indeed SNe Ia. This confirmation is critical to the project, as techniques developed to determine luminosity distances to SNe Ia depend upon the knowledge that the objects at high redshift are the same as the ones at low redshift. We describe the methods of target selection and prioritization, the telescopes and detectors, and the software used to identify objects. The redshifts deduced from spectral matching of high-redshift SNe Ia with low-redshift SNe Ia are consistent with those determined from host-galaxy spectra. We show that the high-redshift SNe Ia match well with low-redshift templates. We include all spectra obtained by the ESSENCE project, including 52 SNe Ia, 5 core-collapse SNe, 12 active galactic nuclei, 19 galaxies, 4 possibly variable stars, and 16 objects with uncertain identifications.Comment: 38 pages, 9 figures (many with multiple parts), submitted to A

Using Line Profiles to Test the Fraternity of Type Ia Supernovae at High and Low Redshifts

Using archival data of low-redshift (z < 0.01) Type Ia supernovae (SN Ia) and recent observations of high-redshift (0.16 < z <0.64; Matheson et al. 2005) SN Ia, we study the "uniformity'' of the spectroscopic properties of nearby and distant SN Ia. We find no difference in the measures we describe here. In this paper, we base our analysis solely on line-profile morphology, focusing on measurements of the velocity location of maximum absorption (vabs) and peak emission (vpeak). We find that the evolution of vabs and vpeak for our sample lines (Ca II 3945, Si II 6355, and S II 5454, 5640) is similar for both the low- and high-redshift samples. We find that vabs for the weak S II 5454, 5640 lines, and vpeak for S II 5454, can be used to identify fast-declining [dm15 > 1.7] SN Ia, which are also subluminous. In addition, we give the first direct evidence in two high-z SN Ia spectra of a double-absorption feature in Ca II 3945, an event also observed, though infrequently, in low-redshift SN Ia spectra (6/22 SN Ia in our local sample). We report for the first time the unambiguous and systematic intrinsic blueshift of peak emission of optical P-Cygni line profiles in Type Ia spectra, by as much as 8000 km/s. All the high-z SN Ia analyzed in this paper were discovered and followed up by the ESSENCE collaboration, and are now publicly available.Comment: 28 pages (emulateapj), 15 figures; accepted for publication in A

Berkeley Supernova Ia Program I: Observations, Data Reduction, and Spectroscopic Sample of 582 Low-Redshift Type Ia Supernovae

In this first paper in a series we present 1298 low-redshift (z\leq0.2) optical spectra of 582 Type Ia supernovae (SNe Ia) observed from 1989 through 2008 as part of the Berkeley SN Ia Program (BSNIP). 584 spectra of 199 SNe Ia have well-calibrated light curves with measured distance moduli, and many of the spectra have been corrected for host-galaxy contamination. Most of the data were obtained using the Kast double spectrograph mounted on the Shane 3 m telescope at Lick Observatory and have a typical wavelength range of 3300-10,400 Ang., roughly twice as wide as spectra from most previously published datasets. We present our observing and reduction procedures, and we describe the resulting SN Database (SNDB), which will be an online, public, searchable database containing all of our fully reduced spectra and companion photometry. In addition, we discuss our spectral classification scheme (using the SuperNova IDentification code, SNID; Blondin & Tonry 2007), utilising our newly constructed set of SNID spectral templates. These templates allow us to accurately classify our entire dataset, and by doing so we are able to reclassify a handful of objects as bona fide SNe Ia and a few other objects as members of some of the peculiar SN Ia subtypes. In fact, our dataset includes spectra of nearly 90 spectroscopically peculiar SNe Ia. We also present spectroscopic host-galaxy redshifts of some SNe Ia where these values were previously unknown. [Abridged]Comment: 34 pages, 11 figures, 11 tables, revised version, re-submitted to MNRAS. Spectra will be released in January 2013. The SN Database homepage (http://hercules.berkeley.edu/database/index_public.html) contains the full tables, plots of all spectra, and our new SNID template