Browsing by Author "Rhoads, James E."

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    Design for the First Narrowband Filter for the Dark Energy Camera: Optimizing the LAGER Survey for z ∼ 7 Galaxies
    (2019) Zheng, Zhen-Ya; Rhoads, James E.; Wang, Jun-Xian; Malhotra, Sangeeta; Walker, Alistair; Mooney, Thomas; Jiang, Chunyan; Hu, Weida; Hibon, Pascale; Jiang, Linhua; Infante, Leopoldo; Barrientos, L. Felipe; Galaz, Gaspar; Valdes, Francisco; Wester, William; Yang, Huan; Coughlin, Alicia; Harish, Santosh; Kang, Wenyong; Khostovan, Ali Ahmad; Kong, Xu; Perez, Lucia A.; Pharo, John; Wold, Isak; Zheng, XianZhong
    We present the design for the first narrowband filter, NB964, for the Dark Energy Camera (DECam), which is operated on the 4-m Blanco Telescope at the Cerro Tololo Inter-American Observatory. The NB964 filter profile is essentially defined by maximizing the power of searching for Lyman-alpha emitting galaxies (LAEs) in the epoch of reionization, with the consideration of the night sky background in the near-infrared and the DECam quantum efficiency. The NB964 filter was manufactured by Materion in 2015. It has a central wavelength of 964.2 nm and a full width at half maximum (FWHM) of 9.2 nm. An NB964 survey named Lyman Alpha Galaxies in the Epoch of Reionization (LAGER) has been ongoing since December 2015. Here, we report results of lab tests, on-site tests, and observations with the NB964 filter. The excellent performances of this filter ensure that the LAGER project is able to detect LAEs at z similar to 7 with a high efficiency.
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    Discovery of Local Analogs to JWST's Little Red Dots
    (2024) Lin, Ruqiu; Zheng, Zhen-Ya; Jiang, Chunyan; Yuan, Fang-Ting; Ho, Luis C.; Wang, Junxian; Jiang, Linhua; Rhoads, James E.; Malhotra, Sangeeta; Barrientos, Luis Felipe; Wold, Isak; Infante Lira, Leopoldo; Zhu, Shuairu; Ji, Xiang; Fu, Xiaodan
    Recently, the James Webb Space Telescope (JWST) has revealed a new class of high redshift (high- z , z > 4) compact galaxies which are red in the rest-frame optical and blue in the rest-frame UV as V-shaped spectral energy distributions (SEDs), referred to as “Little Red Dots” (LRDs). It is very likely that LRDs host obscured broad-line active galactic nuclei (AGNs). In the meanwhile, Green pea galaxies (GPs), which are compact dwarf galaxies at low redshift, share various similar properties with high redshift star-forming galaxies. Here we aim to find the connection between the LRDs and GPs hosting broad-line AGNs (BLGPs). With a sample of 19 BLGPs obtained from our previous work, we further identify 7 GPs with V-shaped rest-frame UV-to-optical SEDs that are likely local analogs to LRDs. These V-shaped BLGPs exhibit faint UV absolute magnitudes and sub-Eddington rates similar to those of LRDs. Three of them occupy a similar region as LRDs in the BPT diagram, suggesting they have similar ionization conditions and gas-phase metallicities to LRDs. These similarities suggest that V-shaped BLGPs can be taken as local analogs of high-redshift LRDs. In addition, most (16/19) BLGPs, including 6 V-shaped BLGPs, host over-massive black holes above the local M BH- M∗ relation, making it the first sample of galaxies hosting over-massive black holes at z < 0.4. These findings will help us learn more about the formation and co-evolution of early galaxies and black holes.
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    GREEN PEA GALAXIES REVEAL SECRETS OF Ly alpha ESCAPE
    (IOP PUBLISHING LTD, 2016) Yang, Huan; Malhotra, Sangeeta; Gronke, Max; Rhoads, James E.; Dijkstra, Mark; Jaskot, Anne; Zheng, Zhenya; Wang, Junxian
    We analyze archival Ly alpha spectra of 12 "Green Pea" galaxies observed with the Hubble Space Telescope, model their Ly alpha profiles with radiative transfer models, and explore the dependence of the Ly alpha escape fraction on various properties. Green Pea galaxies are nearby compact starburst galaxies with [O III] lambda 5007 equivalent widths (EWs) of hundreds of angstrom. All 12 Green Pea galaxies in our sample show Ly alpha lines in emission, with an Ly alpha EW distribution similar to high-redshift Ly alpha emitters. Combining the optical and UV spectra of Green Pea galaxies, we estimate their Ly alpha escape fractions and find correlations between Ly alpha escape fraction and kinematic features of Ly alpha profiles. The escape fraction of Ly alpha in these galaxies ranges from 1.4% to 67%. We also find that the Ly alpha escape fraction depends strongly on metallicity and moderately on dust extinction. We compare their high-quality Ly alpha profiles with single H I shell radiative transfer models and find that the Ly alpha escape fraction anticorrelates with the derived H I column densities. Single-shell models fit most Ly alpha profiles well, but not the ones with the. highest escape fractions of Ly alpha. Our results suggest that low H I column density and low metallicity are essential for Ly alpha escape. and make a galaxy an Ly alpha emitter.
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    Ly alpha EMITTER GALAXIES AT z similar to 2.8 IN THE EXTENDED CHANDRA DEEP FIELD SOUTH. I. TRACING THE LARGE-SCALE STRUCTURE VIA Ly alpha IMAGING
    (IOP PUBLISHING LTD, 2016) Zheng, Zhen Ya; Malhotra, Sangeeta; Rhoads, James E.; Finkelstein, Steven L.; Wang, Jun Xian; Jiang, Chun Yan; Cai, Zheng
    We present a narrowband survey with three adjacent filters for z - 2.8-2.9 Lyman alpha (Ly alpha) emitter (LAE) galaxies in the Extended Chandra Deep Field South (ECDFS), along with spectroscopic follow-up. With a complete sample of 96 LAE candidates in the narrowband NB466, we confirm the large-scale structure at z similar to 2.8 suggested by previous spectroscopic surveys. Compared to the blank field detected with the other two narrowband filters NB470 and NB475, the LAE-density excess in NB466 (900 arcmin(2)) is similar to 6.0 +/- 0.8 times the standard deviation expected at z similar to 2.8, assuming a linear bias of 2. The overdense large-scale structure in NB466 can be decomposed into four protoclusters, whose overdensities (each within an equivalent comoving volume 15(3) Mpc(3)) relative to the blank field (NB470+NB475) are in the range of 4.6-6.6. These four protoclusters are expected to evolve into a Coma-like cluster (M >= 10(15) M-circle dot) at z similar to 0. We also investigate the various properties of LAEs at z = 2.8-2.9 and their dependence on the environment. The average star formation rates derived from the Lya, rest-frame UV, and X-ray bands are similar to 4, 10, and < 16 M-circle dot yr(-1), respectively, implying a Ly alpha escape fraction of 25% less than or similar to f(ESC)(Ly alpha) less than or similar to 40% and a UV continuum escape fraction of f(ESC)(UV,cont) greater than or similar to 62% for LAEs at z similar to 2.8. The Ly alpha photon density calculated from the integrated Ly alpha luminosity function in the overdense field (NB466) is similar to 50% higher than that in the blank field (NB470+NB475), and more bright LAEs are found in the overdense field. The three brightest LAEs, including a quasar at z = 2.81, are all detected in the X-ray band and in NB466. These three LAE-active galactic nuclei contribute an extra 20%-30% Lya photon density compared to other LAE galaxies. Furthermore, we find that LAEs in overdense regions have larger equivalent width values, bluer U-B and V-R (similar to 2-3 sigma) colors compared with those in lower density regions, indicating that LAEs in overdense regions are younger and possible less dusty. We conclude that the structure at z similar to 2.8 in the ECDFS field is a very significant and rare density peak similar to the SSA22 protocluster, and narrowband imaging is an efficient method of detecting and studying such structures in the high-z universe.
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    New Spectroscopic Confirmations of Lyα Emitters at Z ∼ 7 from the LAGER Survey
    (2022) Harish, Santosh; Wold, Isak G. B.; Malhotra, Sangeeta; Rhoads, James E.; Hu, Weida; Wang, Junxian; Zheng, Zhen-ya; Barrientos, L. Felipe; Gonzalez-Lopez, Jorge; Perez, Lucia A.; Khostovan, Ali Ahmad; Infante, Leopoldo; Jiang, Chunyan; Moya-Sierralta, Cristobal; Pharo, John; Valdes, Francisco; Yang, Huan
    We report spectroscopic confirmations of 15 Ly alpha galaxies at z similar to 7, implying a spectroscopic confirmation rate of similar to 80% on candidates selected from the Ly alpha Galaxies in the Epoch of Reionization (LAGER), which is the largest (24 deg(2)) survey aimed at finding Ly alpha emitters (LAEs) at z similar to 7 and uses deep narrowband imaging from the Dark Energy Camera at CTIO. LAEs at high redshifts are sensitive probes of cosmic reionization, and narrowband imaging is a robust and effective method for selecting a large number of LAEs. In this work, we present results from the spectroscopic follow-up of LAE candidates in two LAGER fields, COSMOS and WIDE-12, using observations from Keck/LRIS. We report the successful detection of Ly alpha emission in 15 candidates. Three of these in COSMOS have matching confirmations from a previous spectroscopic follow-up and are part of the overdense region, LAGER-z7OD1. Two other candidates that were not detected with LRIS have prior spectroscopic confirmations from Magellan. Including these, we obtain a spectroscopic confirmation success rate of similar to 80% for LAGER LAE candidates. Thorough checks were performed to reject the possibility of these detections being foreground emission resulting with a probability of, at most, one contaminant. We do not detect any other UV nebular lines in our LRIS spectra, apart from Ly alpha. We estimate a 2 sigma upper limit for the ratio of N v/Ly alpha, f (NV)/f (Ly alpha ) less than or similar to 0.27. Including confirmations from this work, a total of 33 LAE sources from LAGER are now spectroscopically confirmed. LAGER has more than doubled the sample of spectroscopically confirmed LAE sources at z similar to 7.