ORCID Profile
0000-0002-2644-3518
Current Organisation
The University of Edinburgh
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Publisher: American Astronomical Society
Date: 20-09-2022
Abstract: The COS Legacy Archive Spectroscopic SurveY (CLASSY) is designed to provide the community with a spectral atlas of 45 nearby star-forming galaxies that were chosen to cover similar properties to those seen at high z ( z 6). The prime high-level science product of CLASSY is accurately coadded UV spectra, ranging from ∼1000 to 2000 Å, derived from a combination of archival and new data obtained with HST’s Cosmic Origins Spectrograph (COS). This paper details the multistage technical processes of creating this prime data product and the methodologies involved in extracting, reducing, aligning, and coadding far-ultraviolet and near-ultraviolet (NUV) spectra. We provide guidelines on how to successfully utilize COS observations of extended sources, despite COS being optimized for point sources, and best-practice recommendations for the coaddition of UV spectra in general. Moreover, we discuss the effects of our reduction and coaddition techniques in the scientific application of the CLASSY data. In particular, we find that accurately accounting for flux calibration offsets can affect the derived properties of the stellar populations, while customized extractions of NUV spectra for extended sources are essential for correctly diagnosing the metallicity of galaxies via C iii ] nebular emission. Despite changes in spectral resolution of up to ∼25% between in idual data sets (due to changes in the COS line-spread function), no adverse affects were observed on the difference in velocity width and outflow velocities of isolated absorption lines when measured in the final combined data products, owing in part to our signal-to-noise regime of S/N 20.
Publisher: American Astronomical Society
Date: 11-2022
Abstract: The Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY (CLASSY) provides the first high-resolution spectral catalog of 45 local high- z analogs in the ultraviolet (UV 1200–2000 Å) to investigate their stellar and gas properties. Here we present a toolkit of UV interstellar medium (ISM) diagnostics, analyzing the main emission lines of CLASSY spectra (N iv ] λ λ 1483,87, C iv λλ 1548,51, He ii λ 1640, O iii ] λ λ 1661,6, Si iii ] λλ 1883,92, C iii ] λ 1907,9). Specifically, our aim is to provide accurate diagnostics for the reddening E ( B − V ), electron density n e , electron temperature T e , metallicity 12+log(O/H), and ionization parameter log( U ), taking the different ISM ionization zones into account. We calibrate our UV toolkit using well-known optical diagnostics, analyzing archival optical spectra for all CLASSY targets. We find that UV density diagnostics estimate n e values that are ∼1–2 dex higher (e.g., n e (C iii ] λ λ 1907,9) ∼ 10 4 cm −3 ) than those inferred from their optical counterparts (e.g., n e ([S ii ] λ λ 6717,31) ∼ 10 2 cm −3 n e ([Ar iv ] λ λ 4714,41) ∼ 10 3 cm −3 ). T e derived from the hybrid ratio [O iii ] λ 1666/ λ 5007 proves to be reliable, implying differences in determining 12+log(O/H) compared to the optical counterpart O iii ] λ 4363/[O iii ] λ 5007 within ∼ ±0.3 dex. We also investigate the relation between the stellar and gas E ( B − V ), finding consistent values at high specific star formation rates (sSFRs log ( sSFR ) ≳ − 8 yr −1 ), while at low sSFRs we confirmed an excess of dust attenuation in the gas. Finally, we investigate UV line ratios and equivalent widths to provide correlations with 12+log(O/H) and log( U ), but note that there are degeneracies between the two. With this suite of UV-based diagnostics, we illustrate the pivotal role CLASSY plays in understanding the chemical and physical properties of high-z systems that JWST can observe in the rest-frame UV.
Publisher: American Astronomical Society
Date: 27-07-2022
Abstract: Far-ultraviolet (FUV ∼1200–2000 Å) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of the James Webb Space Telescope will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of le wavelength coverage, signal-to-noise, spectral-resolution, and s le ersity that has not yet been achieved by any FUV spectral database. We present the Cosmic Origins Spectrograph Legacy Spectroscopic Survey (CLASSY) treasury and its first high-level science product, the CLASSY atlas. CLASSY builds on the Hubble Space Telescope (HST) archive to construct the first high-quality (S/N 1500 Å ≳ 5/resel), high-resolution ( R ∼ 15,000) FUV spectral database of 45 nearby (0.002 z 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY s le covers a broad range of properties including stellar mass (6.2 log M ⋆ ( M ⊙ ) 10.1), star formation rate (−2.0 log SFR ( M ⊙ yr −1 ) +1.6), direct gas-phase metallicity (7.0 12+log(O/H) 8.8), ionization (0.5 O 32 38.0), reddening (0.02 E ( B − V ) 0.67), and nebular density (10 n e (cm −3 ) 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a s le that is consistent with the z ∼ 0 mass–metallicity relationship, but is offset to higher star formation rates by roughly 2 dex, similar to z ≳ 2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.
Publisher: American Astronomical Society
Date: 08-2022
Abstract: Strong nebular emission lines are an important diagnostic tool for tracing the evolution of star-forming galaxies across cosmic time. However, different observational setups can affect these lines, and the derivation of the physical nebular properties. We analyze 12 local star-forming galaxies from the COS Legacy Spectroscopy SurveY (CLASSY) to assess the impact of using different aperture combinations on the determination of the physical conditions and gas-phase metallicity. We compare optical spectra observed with the Sloan Digital Sky Survey Data Release aperture, which has a 3″ diameter similar to COS, IFU, and long-slit spectra, including new LBT/MODS observations of five CLASSY galaxies. We calculate the reddening, electron densities and temperatures, metallicities, star formation rates, and equivalent widths (EWs). We find that measurements of the electron densities and temperatures, and metallicity remained roughly constant with aperture size, indicating that the gas conditions are relatively uniform for this s le. However, using IFU observations of three galaxies, we find that the E ( B − V ) values derived from the Balmer ratios decrease (by up to 53%) with increasing aperture size. The values change most significantly in the center of the galaxies, and level out near the COS aperture diameter of 2.″5. We examine the relative contributions from the gas and stars using the H α and [O iii ] λ 5007 EWs as a function of aperture light fraction, but find little to no variations within a given galaxy. These results imply that the optical spectra provide nebular properties appropriate for the far-UV CLASSY spectra, even when narrow 1.″0 long-slit observations are used.
Location: United Kingdom of Great Britain and Northern Ireland
Location: Mexico
No related grants have been discovered for Karla Ziboney Arellano-Córdova.