7 research outputs found
SKYSURF-4: Panchromatic HST All-Sky Surface-Brightness Measurement Methods and Results
The diffuse, unresolved sky provides most of the photons that the Hubble
Space Telescope (HST) receives, yet remains poorly understood. HST Archival
Legacy program SKYSURF aims to measure the 0.2-1.6 m sky surface
brightness (sky-SB) from over 140,000 HST images. We describe a sky-SB
measurement algorithm designed for SKYSURF that is able to recover the input
sky-SB from simulated images to within 1% uncertainty. We present our sky-SB
measurements estimated using this algorithm on the entire SKYSURF database.
Comparing our sky-SB spectral energy distribution (SED) to measurements from
the literature shows general agreements. Our SKYSURF SED also reveals a
possible dependence on Sun angle, indicating either non-isotropic scattering of
solar photons off interplanetary dust or an additional component to Zodiacal
Light. Finally, we update Diffuse Light limits in the near-IR based on the
methods from Carleton et al. (2022), with values of 0.009 MJy sr (22 nW
m sr) at 1.25 m, 0.015 MJy sr (32 nW m
sr) at 1.4 m, and 0.013 MJy sr (25 nW m sr) at
1.6 m. These estimates provide the most stringent all-sky constraints to
date in this wavelength range. SKYSURF sky-SB measurements are made public on
the official SKYSURF website and will be used to constrain Diffuse Light in
future papers.Comment: Revised based on helpful comments from the reviewer, and accepted to
AJ on April 12th, 2023. Main paper: 18 pages, 9 figures, 4 tables.
Appendices: 16 pages, 10 figures, 1 table. Main results shown in Figure 7 and
Table
JWST NIRCam Photometry: A Study of Globular Clusters Surrounding Bright Elliptical Galaxy VV 191a at z=0.0513
James Webb Space Telescope NIRCam images have revealed 443 globular cluster
(GC) candidates around the elliptical galaxy VV 191a. NIRCam
broadband observations are made at 0.9-4.5 m using filters F090W, F150W,
F356W, and F444W. Using photometry, the data is analyzed to present
color-magnitude diagrams (CMDs) that suggest a fairly uniform population of
GCs. Color histograms show a unimodal color distribution that is well fit by a
single Gaussian, using color to primarily trace the metallicity. The findings
show the sample's globular cluster luminosity function (GCLF) does not reach
the turnover value and is, therefore, more luminous than what is typically
expected, with an absolute AB magnitude, mag, reaching
within nearly one magnitude of the classical turnover value. We attribute this
to the completeness in the sample. Models show that the mass estimate of the
GCs detected tends to be more massive, reaching upward of . However, the results show that current GC models do not quite align
with the data. We find that the models appear to be bluer than the JWST data in
the reddest (F356W-F444W) filters and redder than the data in the bluest
(F090W-F150W) filters and may need to be revised to improve the modeling of
near-IR colors of old, metal-poor stellar populations.Comment: 11 pages, 7 figure
JWST's PEARLS: 119 multiply imaged galaxies behind MACS0416, lensing properties of caustic crossing galaxies, and the relation between halo mass and number of globular clusters at
We present a new lens model for the galaxy cluster MACS
J0416.12403 based on a previously known set of 77 spectroscopically
confirmed, multiply imaged galaxies plus an additional set of 42 candidate
multiply imaged galaxies from past HST and new JWST data. The new galaxies lack
spectroscopic redshifts but have geometric and/or photometric redshift
estimates that are presented here. The new model predicts magnifications and
time delays for all multiple images. The full set of constraints totals 343,
constituting the largest sample of multiple images lensed by a single cluster
to date. Caustic-crossing galaxies lensed by this cluster are especially
interesting. Some of these galaxies show transient events, most of which are
interpreted as micro-lensing of stars at cosmological distances. These
caustic-crossing arcs are expected to show similar events in future, deeper
JWST observations. We provide time delay and magnification models for all these
arcs. The time delays and the magnifications for different arcs are generally
anti-correlated, as expected from -body simulations.
In the major sub-halos of the cluster, the dark-matter mass from our lens
model correlates well with the observed number of globular clusters. This
confirms earlier results, derived at lower redshifts, which suggest that
globular clusters can be used as powerful mass proxies for the halo masses when
lensing constraints are scarce or not available.Comment: 21 pages and 11 figure
SKYSURF: Constraints on Zodiacal Light and Extragalactic Background Light through Panchromatic HST All-Sky Surface-Brightness Measurements: I. Survey Overview and Methods
We give an overview and describe the rationale, methods, and testing of the
Hubble Space Telescope (HST) Archival Legacy project "SKYSURF." SKYSURF uses
HST's unique capability as an absolute photometer to measure the ~0.2-1.7
m sky surface brightness (SB) from 249,861 WFPC2, ACS, and WFC3 exposures
in ~1400 independent HST fields. SKYSURF's panchromatic dataset is designed to
constrain the discrete and diffuse UV to near-IR sky components: Zodiacal Light
(ZL; inner Solar System), Kuiper Belt Objects (KBOs; outer Solar System),
Diffuse Galactic Light (DGL), and the discrete plus diffuse Extragalactic
Background Light (EBL). We outline SKYSURF's methods to: (1) measure sky-SB
levels between its detected objects; (2) measure the integrated discrete EBL,
most of which comes from AB17-22 mag galaxies; and (3) estimate how
much diffuse light may exist in addition to the extrapolated discrete galaxy
counts. Simulations of HST WFC3/IR images with known sky-values and gradients,
realistic cosmic ray (CR) distributions, and star plus galaxy counts were
processed with nine different algorithms to measure the "Lowest Estimated
Sky-SB" (LES) in each image between the discrete objects. The best algorithms
recover the inserted LES values within 0.2% when there are no image gradients,
and within 0.2-0.4% when there are 5-10% gradients. SKYSURF requires
non-standard re-processing of these HST images that includes restoring the
lowest sky-level from each visit into each drizzled image. We provide a proof
of concept of our methods from the WFC3/IR F125W images, where any residual
diffuse light that HST sees in excess of the Kelsall et al. (1998) Zodiacal
model prediction does not depend on the total object flux that each image
contains. This enables us to present our first SKYSURF results on diffuse light
in Carleton et al. (2022).Comment: Accepted to AJ; see accompanying paper Carleton et al. 2022:
arXiv:2205.06347. Comments welcome
SKYSURF-4: Panchromatic Hubble Space Telescope All-Sky Surface-brightness Measurement Methods and Results
The diffuse, unresolved sky provides most of the photons that the Hubble Space Telescope (HST) receives, yet remains poorly understood. The HST Archival Legacy program SKYSURF aims to measure the 0.2–1.6 μ m sky surface brightness (sky-SB) from over 140,000 HST images. We describe a sky-SB measurement algorithm designed for SKYSURF that is able to recover the input sky-SB from simulated images to within 1% uncertainty. We present our sky-SB measurements estimated using this algorithm on the entire SKYSURF database. Comparing our sky-SB spectral energy distribution (SED) to measurements from the literature shows general agreements. Our SKYSURF SED also reveals a possible dependence on the Sun angle, indicating either nonisotropic scattering of solar photons off interplanetary dust or an additional component to zodiacal light. Finally, we update the diffuse light limits in the near-IR based on the methods from Carleton et al., with values of 0.009 MJy sr ^−1 (22 nW m ^−2 sr ^−1 ) at 1.25 μ m, 0.015 MJy sr ^−1 (32 nW m ^−2 sr ^−1 ) at 1.4 μ m, and 0.013 MJy sr ^−1 (25 nW m ^−2 sr ^−1 ) at 1.6 μ m. These estimates provide the most stringent all-sky constraints to date in this wavelength range. SKYSURF sky-SB measurements are made public on the official SKYSURF website and will be used to constrain diffuse light in future papers
SKYSURF: Constraints on Zodiacal Light and Extragalactic Background Light through Panchromatic HST All-Sky Surface-Brightness Measurements: II. First Limits on Diffuse Light at 1.25, 1.4, and 1.6 microns
We present the first results from the HST Archival Legacy project "SKYSURF."
As described in Windhorst et al. 2022, SKYSURF utilizes the large HST archive
to study the diffuse UV, optical, and near-IR backgrounds and foregrounds in
detail. Here we utilize SKYSURF's first sky-surface brightness measurements to
constrain the level of near-IR diffuse Extragalactic Background Light (EBL).
Our sky-surface brightness measurements have been verified to an accuracy of
better than 1%, which when combined with systematic errors associated with HST,
results in sky brightness uncertainties of 2-4% 0.005 MJy/sr in
each image. We put limits on the amount of diffuse EBL in three near-IR filters
(F125W, F140W, and F160W) by comparing our preliminary sky measurements of images to Zodiacal light models, carefully selecting the darkest images
to avoid contamination from stray light. In addition, we investigate the impact
that instrumental thermal emission has on our measurements, finding that it has
a limited impact on F125W and F140W measurements, whereas uncertainties in the
exact thermal state of HST results in significant uncertainties in the level of
astrophysical diffuse light in F160W images. When compared to the Kelsall et
al. (1998) Zodiacal model, an isotropic diffuse background of nW m
sr remains, whereas using the Wright (1998) Zodiacal model results in no
discernible diffuse background. Based primarily on uncertainties in the
foreground model subtraction, we present limits on the amount of diffuse EBL of
29 nW m sr, 40 nW m sr, and 29 nW m
sr for F125W, F140W, and F160W respectively. While this light is
generally isotropic, our modeling at this point does not distinguish between a
cosmological origin or a Solar System origin (such as a dim, diffuse, spherical
cloud of cometary dust).Comment: To be submitted with Windhorst et al. 2022 to AJ. Main figures are
Fig. 10 and 11. Comments welcome
JWST NIRCam Photometry: A Study of Globular Clusters Surrounding Bright Elliptical Galaxy VV 191a at z = 0.0513
James Webb Space Telescope NIRCam images have revealed 154 reliable globular cluster (GC) candidates around the z = 0.0513 elliptical galaxy VV 191a after subtracting 34 likely interlopers from background galaxies inside our search area. NIRCam broadband observations are made at 0.9–4.5 μ m using the F090W, F150W, F356W, and F444W filters. Using point-spread-function-matched photometry, the data are analyzed to present color–magnitude diagrams and color distributions that suggest a relatively uniform population of GCs, except for small fractions of reddest (5%–8%) and bluest (2%–4%) outliers. GC models in the F090W versus (F090W–F150W) diagram fit the NIRCam data well and show that the majority of GCs detected have a mass of ∼10 ^6.5 M _⊙ , with metallicities [Fe/H] spanning the typical range expected for GCs (−2.5 ≲ [Fe/H]≲ 0.5). However, the models predict ∼0.3–0.4 mag bluer (F356W–F444W) colors than the NIRCam data for a reasonable range of GC ages, metallicities, and reddening. Although our data do not quite reach the luminosity function turnover, the measured luminosity function is consistent with previous measurements, suggesting an estimated peak at m _AB ∼ −9.4 ± 0.2 mag in the F090W filter