About "New supernova candidates from the SDSS-DR7 spectral survey" by Tu et al.(2009)

Zong-Wei Li

(Department of Astronomy, Beijing Normal University, 100875, Beijing, China Lizw625@sina.com)

Supernova studies have led to the most exciting astronomical discovery made in the last decade, the so-called “dark energy.” With a standardizable absolute luminosity and being visible at cosmological distances, type Ia supernovae (SNe Ia, i.e., thermonuclear explosions of white dwarfs) turn out to be powerful cosmological probes. Using SNe Ia as distance indicators, the High-z (Riess et al.1998) and SCP (Supernova Cosmology Project) (Perlmutter et al.1999) teams independently discovered the accelerating expansion of our universe, and hence the mysterious “dark energy” that drives it, which opened a new era for cosmology and fundamental physics.

Using SNe Ia as distance indicators will continue to be a very important tool for revealing the elusive nature of dark energy, together with new promising techniques like baryonic acoustic oscillation (Eisenstein et al.2005) and weak lensing (Albrecht et al.2009). Sky surveys dedicated to finding high-z SNe Ia, e.g., the latest SNLS (SuperNova Legacy Survey) (Astier et al.2006) and ESSENCE (Equation of State: SupErNovae trace Cosmic Expansion) (Wood-Vasey et al.2007), and the future space-borne JDEM (Joint Dark Energy Mission) (Gehrels2009), routinely search several small patches of sky taking images of each field with a certain cadence. Using the same strategy as in nearby SN surveys, images of consecutive observations of the same field are subtracted from each other to recognize new or variable objects, among which SNe candidates are then sent for spectroscopic measurements for identification and redshift determination (of the host galaxy). Surveys of SNe Ia at intermediate redshifts of z ~ 0.05 - 0.03 are also essential for precisely building the Hubble diagram. This redshift range was explored by the Supernova Survey of the SDSS-II (Sloan Digital Sky Survey II) (Frieman et al.2008). About 500 spectroscopically confirmed SNe, mostly of type Ia, were found over the course of a three 3-month campaign.

The LAMOST (Large sky-Area Multi-object fiber Spectroscopic Telescope) project, a forthcoming spectroscopic survey dedicated to galaxies and stars and a successor to SDSS, is certain to serendipitously catch a significant number of SNe within its scientific products of ~ 107 galaxy spectra (luo et al.PPT).  The 4-m LAMOST telescope, considerably larger than the 2.5-m Sloan telescope, is designed to take spectra of 4000 pre-selected targets simultaneously. However, the SN candidates have to be selected from the galaxy survey spectra directly, while the traditional image-subtraction method is completely of no relevance, as well as in a daily manner for the sake of follow-up observations. The selection method is crucial to the efficiency of SN discovery in the LAMOST galaxy survey.

In the current issue of Research in Astronomy and Astrophysics, (Tu et al.2009)demonstrated a novel SN selection method using Data Release 7 of the SDSS galaxy survey, following a preliminary study by Madgwick et al. (2003) on the SDSS Data Release 1. The algorithm, independently developed by Tu et al. (2009) and named Sample Decrease, successfully results in 25 SN candidates, among which 15 turn out to be newly discovered (14 of type Ia). Actually, a rather conservative selection criterion is used by the authors in the current paper. Since the quality and resolution of the LAMOST spectra will be similar to those from the SDSS, accordingly, about 2000 SNe of all types are estimated to exist in the galaxies observed by LAMOST. It is anticipated that the Sample Decrease algorithm, after further improvements, will routinely yield SNe by-products of the LAMOST galaxy survey to benefit SN and cosmology studies.

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