We present the results of spectroscopic observations of targets discovered during the first 2 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 P z P 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 on the knowledge that the objects at high redshift have the same properties 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, five corecollapse SNe, 12 active galactic nuclei, 19 galaxies, four possibly variable stars, and 16 objects with uncertain identifications.
Using archival data of low-redshift (z < 0.01; CfA and SUSPECT databases) 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 (v abs ) and peak emission (v peak ). Our measurement technique makes it easier to compare low and high signal-tonoise ratio observations. We also quantify the associated sources of error, assessing the effect of line blending with assistance from the parametrized code SYNOW . We find that the evolution of v abs and v peak for our sample lines (Ca ii l3945, Si ii l6355, and S ii l l5454, 5640) is similar for both the low-and high-redshift samples. We find that v abs for the weak S ii l l5454, 5640 lines, and v peak for S ii l5454, can be used to identify fast-declining [∆m 15 (B) > 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 doubleabsorption feature in Ca ii l3945, an event also observed, though infrequently, in low-redshift SN Ia spectra (6/22 SN Ia in our local sample). Moreover, echoing the recent studies of Dessart & Hillier (2005a,b) in the context of Type II supernovae (SN II), we see similar P-Cygni line profiles in our large sample of SN Ia spectra. First, the magnitude of the velocity location at maximum profile absorption may underestimate that at the continuum photosphere, as observed for example in the optically thinner line S ii l5640. Second, 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 −1 . All the high-z SN Ia analyzed in this paper were discovered and followed up by the ESSENCE collaboration, and are now publicly available.
We present the results of spectroscopic observations from the ESSENCE high-redshift supernova (SN) survey during its first four years of operation. This sample includes spectra of all SNe Ia whose light curves were presented by Miknaitis et al. (2007) and used in the cosmological analyses of Davis et al. (2007) and Wood-Vasey et al. (2007). The sample represents 273 hours of spectroscopic observations with 6.5-10-m-class telescopes of objects detected and selected for spectroscopy by the ESSENCE team. We present 174 spectra of 156 objects. Combining this sample with that of Matheson et al. (2005), we have a total sample of 329 spectra of 274 objects. From this, we are able to spectroscopically classify 118 Type Ia SNe. As the survey has matured, the efficiency of classifying SNe Ia has remained constant while we have observed both higher-redshift SNe Ia and SNe Ia farther from maximum brightness. Examining the subsample of SNe Ia with host-galaxy redshifts shows that redshifts derived from only the SN Ia spectra are consistent with redshifts found from host-galaxy spectra. Moreover, the phases derived from only the SN Ia spectra are consistent with those derived from light-curve fits. By comparing our spectra to local templates, we find that the rate of objects similar to the overluminous SN 1991T and the underluminous SN 1991bg in our sample are consistent with that of the local sample. We do note, however, that we detect no object spectroscopically or photometrically similar to SN 1991bg. Although systematic effects could reduce the high-redshift rate we expect based on the low-redshift surveys, it is possible that SN 1991bg-like SNe Ia are less prevalent at high redshift.
We present broadband light curves of nine supernovae ranging in redshift from 0.5 to 0.8. The supernovae were discovered as part of the ESSENCE project, and the light curves are a combination of Cerro Tololo 4 m and Hubble Space Telescope (HST ) photometry. On the basis of spectra and/or light-curve fitting, eight of these objects are definitely Type Ia supernovae, while the classification of one is problematic. The ESSENCE project is a 5 yr endeavor to discover about 200 high-redshift Type Ia supernovae, with the goal of tightly constraining the time average of the equation-ofstate parameter [w ¼ p/(c 2 )] of the ''dark energy.'' To help minimize our systematic errors, all of our ground-based photometry is obtained with the same telescope and instrument. In 2003 the highest redshift subset of ESSENCE supernovae was selected for detailed study with HST. Here we present the first photometric results of the survey. We find that all but one of the ESSENCE supernovae have slowly declining light curves and that the sample is not representative of the low-redshift set of ESSENCE Type Ia supernovae. This is unlikely to be a sign of evolution in the population. We attribute the decline-rate distribution of HSTevents to a selection bias at the high-redshift edge of our sample and find that such a bias will infect other magnitude-limited Type Ia supernova searches unless appropriate precautions are taken.
In many decision making contexts, people often persist with their previous selections. This predisposition to choose to maintain a current (or previous) choice is referred to as the status quo bias (SQB). In this work, we propose that increased attention towards the status quo option – enabled by its visual salience – is a previously underappreciated driver of SQB. We base this hypothesis on three propositions: 1) the status-quo bias option is often more visually salient relative to the non-status quo options on offer, 2) greater visual salience of an option biases attention towards that option, and 3) increased attention towards an option leads to that option being selected at greater rates. We examined the attention hypothesis among 6,854 participants in four studies. Study 1 and Study 2 showed that increasing the visual salience of a non-status quo option (i.e., the type of visual salience often garnered by the status quo option) increased the selection rate of that option. Study 3 directly tested the hypothesis by lessening the visual salience of the status quo option. Doing so eliminated SQB. Study 4 replicated and extended the findings of Study 3 in a real-world decision context. Collectively, these studies suggest that the selection of the status quo may often be related to its saliency relative to other available options.
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