The DMASS sample is a photometric sample from the DES Year 1 data set designed to replicate the properties of the CMASS sample from BOSS, in support of a joint analysis of DES and BOSS beyond the ...small overlapping area. In this paper, we present the measurement of galaxy-galaxy lensing using the DMASS sample as gravitational lenses in the DES Y1 imaging data. We test a number of potential systematics that can bias the galaxy-galaxy lensing signal, including those from shear estimation, photometric redshifts, and observing conditions. After careful systematic tests, we obtain a highly significant detection of the galaxy-galaxy lensing signal, with total \(S/N=25.7\). With the measured signal, we assess the feasibility of using DMASS as gravitational lenses equivalent to CMASS, by estimating the galaxy-matter cross-correlation coefficient \(r_{\rm cc}\). By jointly fitting the galaxy-galaxy lensing measurement with the galaxy clustering measurement from CMASS, we obtain \(r_{\rm cc}=1.09^{+0.12}_{-0.11}\) for the scale cut of \(4~h^{-1}{\rm Mpc}\) and \(r_{\rm cc}=1.06^{+0.13}_{-0.12}\) for \(12~h^{-1}{\rm Mpc}\) in fixed cosmology. By adding the angular galaxy clustering of DMASS, we obtain \(r_{\rm cc}=1.06\pm 0.10\) for the scale cut of \(4~h^{-1}{\rm Mpc}\) and \(r_{\rm cc}=1.03\pm 0.11\) for \(12~h^{-1}{\rm Mpc}\). The resulting values of \(r_{\rm cc}\) indicate that the lensing signal of DMASS is statistically consistent with the one that would have been measured if CMASS had populated the DES region within the given statistical uncertainty. The measurement of galaxy-galaxy lensing presented in this paper will serve as part of the data vector for the forthcoming cosmology analysis in preparation.
We report the identification of a low-mass AGN, DES J0218\(-\)0430, in a redshift \(z = 0.823\) galaxy in the Dark Energy Survey (DES) Supernova field. We select DES J0218\(-\)0430 as an AGN ...candidate by characterizing its long-term optical variability alone based on DES optical broad-band light curves spanning over 6 years. An archival optical spectrum from the fourth phase of the Sloan Digital Sky Survey shows both broad Mg II and broad H\(\beta\) lines, confirming its nature as a broad-line AGN. Archival XMM-Newton X-ray observations suggest an intrinsic hard X-ray luminosity of \(L_{{\rm 2-12\,keV}}\sim7.6\pm0.4\times10^{43}\) erg s\(^{-1}\), which exceeds those of the most X-ray luminous starburst galaxies, in support of an AGN driving the optical variability. Based on the broad H\(\beta\) from SDSS spectrum, we estimate a virial BH mass of \(M_{\bullet}\approx10^{6.43}\)-\(10^{6.72}M_{\odot}\) (with the error denoting 1\(\sigma\) statistical uncertainties only), consistent with the estimation from OzDES, making it the lowest mass AGN with redshift \(>\) 0.4 detected in optical. We estimate the host galaxy stellar mass to be \(M_{\ast}\sim10^{10.5\pm0.3}M_{\odot}\) based on modeling the multi-wavelength spectral energy distribution. DES J0218\(-\)0430 extends the \(M_{\bullet}\)-\(M_{\ast}\) relation observed in luminous AGNs at \(z\sim1\) to masses lower than being probed by previous work. Our work demonstrates the feasibility of using optical variability to identify low-mass AGNs at higher redshift in deeper synoptic surveys with direct implications for the upcoming Legacy Survey of Space and Time at Vera C. Rubin Observatory.
We report on small-amplitude optical variability and recent dissipation of the unusually persistent broad emission lines in the blue compact dwarf galaxy PHL 293B. The galaxy's unusual spectral ...features (P Cygni-like profiles with \(\sim\)800 km s\(^{-1}\) blueshifted absorption lines) have resulted in conflicting interpretations of the nature of this source in the literature. However, analysis of new Gemini spectroscopy reveals the broad emission has begun to fade after being persistent for over a decade prior. Precise difference imaging light curves constructed with the Sloan Digital Sky Survey and the Dark Energy Survey reveal small-amplitude optical variability of \(\sim\)0.1 mag in the g band offset by \(100\pm21\) pc from the brightest pixel of the host. The light curve is well-described by an active galactic nuclei (AGN)-like damped random walk process. However, we conclude that the origin of the optical variability and spectral features of PHL 293B is due to a long-lived stellar transient, likely a Type IIn supernova or non-terminal outburst, mimicking long-term AGN-like variability. This work highlights the challenges of discriminating between scenarios in such extreme environments, relevant to searches for AGNs in dwarf galaxies. This is the second long-lived transient discovered in a blue compact dwarf, after SDSS1133. Our result implies such long-lived stellar transients may be more common in metal-deficient galaxies. Systematic searches for low-level variability in dwarf galaxies will be possible with the upcoming Legacy Survey of Space and Time at Vera C. Rubin Observatory.
