ORCID Profile
0000-0002-0302-2577
Current Organisation
The University of Texas at Austin
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Publisher: American Astronomical Society
Date: 25-01-2018
Publisher: Elsevier BV
Date: 05-2019
Publisher: American Astronomical Society
Date: 30-08-2021
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: Oxford University Press (OUP)
Date: 25-02-2022
Abstract: We study star formation-driven outflows in a z ∼ 0.02 starbursting disc galaxy, IRAS08339+6517, using spatially resolved measurements from the Keck Cosmic Web Imager (KCWI). We develop a new method incorporating a multistep process to determine whether an outflow should be fit in each spaxel, and then subsequently decompose the emission line into multiple components. We detect outflows ranging in velocity, vout, from 100 to 600 km s−1 across a range of star formation rate surface densities, ΣSFR, from ∼0.01 to 10 M⊙ yr−1 kpc−2 in resolution elements of a few hundred parsec. Outflows are detected in ∼100 per cent of all spaxels within the half-light radius, and ∼70 per cent within r90, suggestive of a high covering fraction for this starbursting disc galaxy. Around 2/3 of the total outflowing mass originates from the star forming ring, which corresponds to ${\\lt}10{{\\ \\rm per\\ cent}}$ of the total area of the galaxy. We find that the relationship between vout and the ΣSFR, as well as between the mass loading factor, η, and the ΣSFR, are consistent with trends expected from energy-driven feedback models. We study the resolution effects on this relationship and find stronger correlations above a re-binned size-scale of ∼500 pc. Conversely, we do not find statistically significant consistency with the prediction from momentum-driven winds.
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.
Publisher: American Astronomical Society
Date: 12-2022
Abstract: We compare 500 pc scale, resolved observations of ionized and molecular gas for the z ∼ 0.02 starbursting disk galaxy IRAS08339+6517, using measurements from KCWI and NOEMA. We explore the relationship of the star-formation-driven ionized gas outflows with colocated galaxy properties. We find a roughly linear relationship between the outflow mass flux ( Σ ̇ out ) and star formation rate surface density (Σ SFR ), Σ ̇ out ∝ Σ SFR 1.06 ± 0.10 , and a strong correlation between Σ ̇ out and the gas depletion time, such that Σ ̇ out ∝ t dep − 1.1 ± 0.06 . Moreover, we find these outflows are so-called breakout outflows, according to the relationship between the gas fraction and disk kinematics. Assuming that ionized outflow mass scales with total outflow mass, our observations suggest that the regions of highest Σ SFR in IRAS08 are removing more gas via the outflow than through the conversion of gas into stars. Our results are consistent with a picture in which the outflow limits the ability of a region of a disk to maintain short depletion times. Our results underline the need for resolved observations of outflows in more galaxies.
Publisher: American Astronomical Society
Date: 21-10-2022
Abstract: The CLusters in the Uv as EngineS (CLUES) survey is a Cosmic Origins Spectrograph (COS) c aign aimed at acquiring the 1130–1770 Å rest-frame spectroscopy of very young ( Myr) and massive ( 4 M ⊙ ) star clusters in galaxies that are part of the Hubble treasury program Legacy ExtraGalactic UV Survey. In this first paper of a series, we describe the CLUES s le consisting of 20 young star clusters and report their physical properties as derived by both multiwavelength photometry and far-UV (FUV) spectroscopy with Hubble Space Telescope. Thanks to the synergy of the two different data sets, we build a coherent picture of the erse stellar populations found in each region (with sizes of 40–160 pc). We associate the FUV-brightest stellar population to the central targeted star cluster and the other modeled population to the diffuse stars that are included in the COS aperture. We observe better agreement between photometric and spectroscopic ages for star clusters younger than 5 Myr. For clusters older than 5 Myr, photometry and spectroscopy measurements deviate, with the latter producing older ages, due to the degeneracy of photometric models. FUV spectroscopy enables us to better constrain the stellar metallicities, a parameter that optical colors are insensitive to. Finally, the derived E ( B − V ) are quite similar, with a tendency for FUV spectroscopy to favor solutions with higher extinctions. The recovered masses are in agreement within a factor of 2 for all of the clusters.
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: EDP Sciences
Date: 28-10-2019
DOI: 10.1051/0004-6361/201935896
Abstract: We compare the molecular and ionized gas kinematics of two strongly lensed galaxies at z ∼ 1 that lie on the main sequence at this redshift. The observations were made with ALMA and MUSE, respectively. We derive the CO and [O II ] rotation curves and dispersion profiles of these two galaxies. We find a difference between the observed molecular and ionized gas rotation curves for one of the two galaxies, the Cosmic Snake, for which we obtain a spatial resolution of a few hundred parsec along the major axis. The rotation curve of the molecular gas is steeper than the rotation curve of the ionized gas. In the second galaxy, A521, the molecular and ionized gas rotation curves are consistent, but the spatial resolution is only a few kiloparsec on the major axis. Using simulations, we investigate the effect of the thickness of the gas disk and effective radius on the observed rotation curves and find that a more extended and thicker disk smoothens the curve. We also find that the presence of a strongly inclined ( 70°) thick disk ( 1 kpc) can smoothen the rotation curve because it degrades the spatial resolution along the line of sight. By building a model using a stellar disk and two gas disks, we reproduce the rotation curves of the Cosmic Snake with a molecular gas disk that is more massive and more radially and vertically concentrated than the ionized gas disk. Finally, we also obtain an intrinsic velocity dispersion in the Cosmic Snake of 18.5 ± 7 km s −1 and 19.5 ± 6 km s −1 for the molecular and ionized gas, respectively, which is consistent with a molecular disk with a smaller and thinner disk. For A521, the intrinsic velocity dispersion values are 11 ± 8 km s −1 and 54 ± 11 km s −1 , with a higher value for the ionized gas. This could indicate that the ionized gas disk is thicker and more turbulent in this galaxy. These results highlight the ersity of the kinematics of galaxies at z ∼ 1 and the different spatial distribution of the molecular and ionized gas disks. It suggests the presence of thick ionized gas disks at this epoch and that the formation of the molecular gas is limited to the midplane and center of the galaxy in some objects.
Publisher: Oxford University Press (OUP)
Date: 06-09-2023
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: Oxford University Press (OUP)
Date: 16-08-2023
Abstract: We present the JWST cycle 1 53.8 h medium program FRESCO, short for ‘First Reionization Epoch Spectroscopically Complete Observations’. FRESCO covers 62 arcmin2 in each of the two GOODS/CANDELS fields for a total area of 124 arcmin2 exploiting JWST’s powerful new grism spectroscopic capabilities at near-infrared wavelengths. By obtaining ∼2 h deep NIRCam/grism observations with the F444W filter, FRESCO yields unprecedented spectra at R ∼ 1600 covering 3.8–5.0 µm for most galaxies in the NIRCam field of view. This setup enables emission line measurements over most of cosmic history, from strong PAH lines at z ∼ 0.2–0.5, to Pa α and Pa β at z ∼ 1–3, He i and [S iii] at z ∼ 2.5–4.5, H α and [N ii] at z ∼ 5–6.5, up to [O iii] and H β for z ∼ 7–9 galaxies. FRESCO’s grism observations provide total line fluxes for accurately estimating galaxy stellar masses and calibrating slit-loss corrections of NIRSpec/MSA spectra in the same field. Additionally, FRESCO results in a mosaic of F182M, F210M, and F444W imaging in the same fields to a depth of ∼28.2 mag (5σ in 0${_{.}^{\\prime\\prime}}$32 diameter apertures). Here, we describe the overall survey design and the key science goals that can be addressed with FRESCO. We also highlight several, early science results, including: spectroscopic redshifts of Lyman break galaxies that were identified almost 20 yr ago, the discovery of broad-line active galactic nuclei at z & 4, and resolved Pa α maps of galaxies at z ∼ 1.4. These results demonstrate the enormous power for serendipitous discovery of NIRCam/grism observations.
No related grants have been discovered for John Chisholm.