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STScI Newsletter
2022 / Volume 39 / Issue 01

About this Article

B. James (bjames[at]stsci.edu) and D. Berg (daberg[at]austin.utexas.edu)

Introduction

Rest-frame far-ultraviolet (FUV) 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 James Webb Space Telescope (JWST) and extremely large telescopes (ELTs) will soon usher in a new era, pushing the FUV spectroscopic frontier to higher redshifts than ever before (z > 10). The success of these future endeavors hinges on a comprehensive understanding of the massive star populations and interstellar medium (ISM) gas conditions that power the observed FUV spectral features. The Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY (CLASSY) Treasury is a powerful new solution.

The CLASSY treasury builds upon the legacy of HST/COS's archive to produce the first high-signal-to-noise (S/N), high-resolution full-FUV spectral atlas of nearby star-forming galaxies (Berg et al. 2022; James et al. 2022). CLASSY covers a local (0.002 < z < 0.182) sample of 45 star-forming galaxies with a large range of masses (log(M*) ~ 6–10 M, metallicities (12 + log(O/H) ~ 7–9), star-formation rates (–2 < log(SFR/M yr-1) < 2), ionization (0.5 < O32 < 38.0), and electron densities (10 < ne < 1000 cm−3).  With a wavelength range of roughly 1200–2000 Å (e.g., Fig. 1), the spectra contain a suite of emission and absorption lines that characterize the massive stellar populations that populate metal-poor galaxies (e.g., metallicity, age), the physical properties of large-scale outflows that regulate star formation (e.g., velocities, column densities, ionization phases), and the physical conditions of the multiphase interstellar gas (e.g., metallicity, temperature, ionization) within the same galaxies. CLASSY will improve the diagnostic power of the UV spectra for future JWST/ELT surveys, providing a long-lasting legacy to the astronomical community for decades to come.

In order to achieve nearly-panchromatic FUV spectral coverage with the highest spectral resolution possible, CLASSY efficiently augments 177 orbits of existing archival data with 135 orbits of new HST/COS observations, uniting the high-resolution G130M, G160M, and G185M gratings for a sample of galaxies spanning broad parameter space. As a result, CLASSY provides a well-controlled, local FUV sample with the requisite sensitivity and spectral resolution to enable synergistic studies of stars and gas within the same galaxies.

The CLASSY atlas was designed to provide a benchmark training set for star-forming galaxies across cosmic time, including the earliest galaxies that will be observed with JWST and the next generation of ELTs. To this end, CLASSY will provide a number of state-of-the-art data products to the astronomical community via MAST to ensure their enduring value and utility. The primary data product, available now, is the atlas of high-resolution (R ~ 15,000), high-S/N (S/N1500 > 5/resel) coadded multi-grating FUV+NUV spectral templates of the CLASSY sample of 45 star-forming galaxies. Data products are publicly available via the MAST high-level science products (HLSP) CLASSY webpage.  Importantly, this website provides simple tarball downloads of individual CLASSY HLSPs, as well as the entire CLASSY HLSP data collection.

In addition to full data access, the CLASSY HLSP home page provides interactive spectra that allow quick-look examination of the coadded spectral features, user-friendly jupyter notebook guides to accessing and using the CLASSY HLSPs, and important links such as to the NASA Astrophysics Database System (ADS) library of CLASSY publications. The second platform for accessing the HLPSs is the CLASSY MAST portal, which provides a unique search-and-selection method to access the CLASSY HLSPs, where users can select a subsample of the data products based on HLSP type, filename, or target properties such as coordinates or stellar mass.

Additional planned CLASSY data products include stellar continuum fits, nebular abundance measurements, improved UV emission-line diagnostics, feedback properties, radiative transfer models, and more, which will be released to the community as they become available. With these HLSPs, the CLASSY Treasury will complete the vital picture that is needed to diagnose the suite of star-forming galaxy properties that will be offered to us in the next decade. Given the uncertain future of observed-frame UV capabilities, the CLASSY templates and models embody an indispensable toolset to the astronomical community.

A benefit of the CLASSY sample is the availability of ample ancillary data, including excellent optical spectra for the entire sample. Optical spectra have long been the gold standard source for calculating important nebular properties, including the oxygen abundance, density, and reddening (see Berg et al. 2022, Table 1). The majority of the CLASSY sample was also observed as part of the Sloan Digital Sky Survey, which provides full optical spectra using a similar aperture (3" diameter) to that of the COS UV aperture (2.5" diameter). With this unique combination of UV+optical data in Berg et al. (2022) we show that the CLASSY sample is consistent with the well-known mass-metallicity relationship of nearby galaxies (Fig. 2), but is offset to higher star-formation rates by roughly 2 dex, similar to distant galaxies at z ~ 2. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.

The scientific return of CLASSY has already begun, with several publications either published, in review, or in preparation. PaperⅠ(Berg et al. 2022) provides a comprehensive overview of the CLASSY Treasury Atlas, including exciting insights into each of the scientific areas that CLASSY will probe. PaperⅡ(James et al. 2022) provides a detailed discussion of the data reduction and coaddtion process, along with best practices for COS data of extended sources. PaperⅢ(Xu et al. 2022) explores the properties of the outflowing gas in the CLASSY sample and provides interesting constraints on, and new insights into, models and simulations of galactic winds in dwarf galaxies. A full list of the future CLASSY science products are listed in Berg et al. (2022) and James et al. (2022), as well as the CLASSY MAST homepage.

This animated figure consists of two parts. On the right, changing jagged lines depict the high signal-to-noise, high-resolution, coadded FUV spectra of the CLASSY atlas, which are highlighted for all 45 galaxies in the sample. The spectra show a wide range of nebular emission and stellar and ISM absorption features that vary with galaxy property. On the left, a box shows colored dots depicting the different locations for the galaxies on the mass-metallicity relationship. The spectra of more massive, metal-rich galaxies are dominated by absorption lines, but lack significant emission. Less massive, more metal-poor galaxies are dominated by emission lines, but have minimal absorption.
Figure 1: The high-S/N, high-resolution coadded FUV spectra of the CLASSY atlas are highlighted for all 45 galaxies in the sample (right). The spectra show a wide range of nebular emission and stellar and ISM absorption features that vary with galaxy property. As the sample steps through different locations on the mass-metallicity relationship (left), the spectra of more massive, metal-rich galaxies are dominated by absorption lines, but lack significant emission, while less massive, more metal-poor galaxies are dominated by emission lines, but have minimal absorption. Cementing the legacy of Hubble, the CLASSY atlas is our most comprehensive FUV atlas to date and the benchmark toolset to interpret early universe spectra from JWST.

 

This figure features two plots. The plot on the left shows property trends for the total star-formation rate versus total stellar mass. The plot on the right shows the gas-phase oxygen abundance versus total stellar mass, or the mass-metallicity relationship. The plots show that the CLASSY mass-metallicity relationship is typical of local low-mass star-forming galaxies, but the sample is offset to higher star-formation rates that are more typical of galaxies at cosmic noon.
Figure 2: The CLASSY sample covers broad ranges in typical galaxy parameters, as measured from the optical spectra and UV+optical photometry. Property trends are plotted for the total star-formation rate versus total stellar mass (SFMR; left) and gas-phase oxygen abundance versus total stellar mass, or the mass-metallicity relationship (MZR; right). Clearly the CLASSY MZR is typical of local low-mass star-forming galaxies, but the sample is offset to higher SFRs that are more typical of galaxies at cosmic noon. Full details can be found in Berg et al. (2022).

References

Berg, D., et al. 2022, ApJS accepted, arXiv:2203.07357

James, B., et al. 2022, ApJS submitted

Xu, X., et al. 2022, ApJ submitted

 

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