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Optical broad-band photometry and reference image for APMUKS(BJ) B215839.70-615403.9 / ASASSN-15lh from the Dark Energy Survey

ATel #7843; Peter Melchior (CCAPP/The Ohio State University), Alexander Drlica-Wagner (Fermi National Accelerator Laboratory), Keith Bechtol (WIPAC/University of Wisconsin), Eli Rykoff (SLAC National Accelerator Laboratory), William Hartley (ETH Zurich) for the Dark Energy Survey (http://www. darkenergysurvey. org/)
on 26 Jul 2015; 04:18 UT
Credential Certification: Peter Melchior (melchior.12@osu.edu)

Subjects: Optical, Supernovae

We report optical broad-band photometry of the host galaxy APMUKS(BJ) B215839.70-615403.9 of SLSN ASASSN-15lh (ATel #7642; Dong et al., arXiv:1507.03010). The images were obtained using the DECam imager on the Blanco 4-m telescope at NOAOâs Cerro Tololo Inter-American Observatory during Year-2 observations of the Dark Energy Survey (DES). A preliminary reduction of the images was performed by the DES Data Management pipeline (Mohr et al. 2012, SPIE Conference Series, 84510D; Desai et al. 2012, ApJ, 757, 83). The photometry was measured using SExtractor with additional calibration via stellar locus regression to provide magnitude zero points with 2-3% calibration uncertainty for point sources (relative to 2MASS) and mildly increased uncertainties for extended sources.

 
DATE       TIME     BAND FLUX_AUTO FLUXERR_AUTO MAG_ZERO MAG_AUTO 
2014-10-21 00:21:06 g    11246.50  188.82       29.9136  19.7861 
2014-10-22 01:36:15 g    10822.80  159.36       29.9470  19.8612 
2014-09-30 02:00:34 r    48037.40  328.52       30.1500  18.4460 
2014-10-21 00:19:08 r    48670.40  332.94       30.1573  18.4391 
2014-10-22 01:38:13 r    48431.70  320.94       30.1733  18.4604 
2014-09-03 04:07:50 i    53504.31  607.53       29.8298  18.0089 
2014-10-21 01:05:57 i    71295.50  481.46       30.1054  17.9728 
2014-10-23 01:59:59 i    70043.20  482.51       30.0893  17.9759 
2014-09-30 02:02:32 z    67086.50  546.36       29.7606  17.6940 
2014-10-21 00:23:06 z    71313.20  639.76       29.7784  17.6455 
2014-09-30 02:06:28 Y    14626.40  338.19       27.9374  17.5245 
2014-10-02 01:24:43 Y    14308.70  320.13       27.9395  17.5505

We fit the DES g-r, r-i, and i-z colors to a red-sequence model from redMaPPer (Rykoff et al. 2014, ApJ, 785, 104) and obtain a redshift 0.25±0.02, consistent with the spectroscopic redshift of z = 0.2326 (ATel #7774). Fixed to that redshift, the host photometry is fully consistent with a red-sequence galaxy. We combine photometry from DES grizY with VHS NIR (ATel #7776) and WISE IR and fit a linear combination of single stellar populations with a variety of ages. We find that the fit is dominated by an old (5 Gyr) component, consistent with those of elliptical galaxies. We further compare with dusty SEDs taken from the latest set of templates in EAZY (Brammer, van Dokkum & Coppi, 2008, ApJ, 686, 1503). The combined host photometry is not well described by any of the dusty templates.

We also release a multi-color cutout around the host, utilizing all DES exposures listed above, and the FITS image of a single DECam CCD from the r-band exposure taken on 2014-10-22 for further studies. The relative astrometric error is expected to be approximately 55 mas.