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

Abstract

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.

Supernovae and cosmology

“…This could finally be shown convincingly with the first distant SN Ia, SN 1995K, by Leibundgut et al (1996) and was further confirmed on a large sample by Goldhaber et al (1997Goldhaber et al ( , 2001). In the meantime this test has been performed following the detailed spectral evolution (Riess et al 1997, Foley et al 2005, Hook et al 2005, Blondin et al 2006. The predictions of a universal expansion have been confirmed in all cases ruling out alternative theories of "tired light.…”

“…Observational trends appear to emerge in the way the velocities within the supernova ejecta evolve , but the interpretation of these correlations are not clear yet. It is noteworthy that the distant objects appear to follow the general spectral evolution of their nearby counterparts and there is no obvious sign of differences in the spectral appearance of SNe Ia (Blondin et al 2006, Garavini et al 2007b. The interpretation of the spectra has now also been expanded to reconstruct the element distribution in the ejecta through the spectral evolution (Fisher et al 1999, Stehle et al 2005, which gives a direct input to the explosion models.…”

Determining the Type, Redshift, and Phase of a Supernova Spectrum

“…This is due to the SN Ia template phase distribution in our database: many input spectra at post-maximum phases (t +10 days) are attracted to higher phases ( +10 days), where the position of SN spectral features has shifted redward in wavelength due to the expansion of the supernova envelope. The template needs to be shifted less in ln λ space to match the redshift of the input spectrum, which leads to an under-estimation of the redshift (by ∼ 0.01) corresponding to a combination of the typical velocity shift in SN Ia absorption features from maximum to ∼ 10 days past maximum, and the spread of these velocities at a given phase for different supernovae (∼ 3000 km s −1 ; see [14,15]). …”

The Rise Time of Type Ia Supernovae from the Supernova Legacy Survey

“…An evolutionary effect of 0.2 mag to z ¼ 0:5 would nullify the SN Ia evidence that the universe is accelerating, and measuring w to 10% requires that any effect be smaller than 0.04 mag. There are other routes to studying evolution with redshift: by comparing SNe Ia in different host galaxy environments (Hamuy et al 2000;Sullivan et al 2003;Gallagher 2005) and via detailed spectroscopic studies (Hook et al 2005;Blondin et al 2006;J. Bronder et al 2006, in preparation).…”