J/AJ/169/103 Cosmic UV background spectra with New Horizons (Murthy+, 2025)
Excess ultraviolet emission at high galactic latitudes: A New Horizons view.
Murthy J., Shull J.M., Postman M., Parker J.W., Redfield S.,
Cunningham N., Gladstone G.R., Pineau J.P., Brandt P., Verbiscer A.J.,
Singer K.N., Weaver H.A., Henry R.C., Stern S.A.
<Astron. J., 169, 103 (2025)>
=2025AJ....169..103M 2025AJ....169..103M
ADC_Keywords: Spectra, ultraviolet
Keywords: Extrasolar radiation ; Ultraviolet astronomy ; Ultraviolet sources ;
Ultraviolet spectroscopy ; Cosmic background radiation ;
Diffuse radiation
Abstract:
We present new observations of the cosmic ultraviolet background
(CUVB) at high Galactic latitudes (|b|>40°), made using the Alice
UV spectrograph on board the New Horizons spacecraft. These
observations were taken at about 57AU from the Sun, outside much of
the foreground emission affecting previous missions, and allowed a new
determination of the spectrum of the CUVB between 912-1100Å and
1400-1800Å. We found a linear correlation between the CUVB and the
Planck E(B-V) with offsets at zero-reddening of 221±11 photon units
at 1000Å and 264±24 photon units at 1500Å (4.4±0.2nW/m2/sr
at 1000Å and 5.3±0.5nW/m/sr at 1500Å). The former is the first
firm detection of the offset in the range 912-1100Å while the
latter result confirms previous results from the Galaxy Evolution
Explorer, showing that there is little emission from the solar system
from 1400 to 1800Å. About half of the offset may be explained by
known sources (the integrated light of unresolved galaxies, unresolved
stars, emission from ionized gas, and two-photon emission from warm
hydrogen in the halo) with the source of the remaining emission as yet
unidentified. There is no detectable emission below the Lyman limit
with an upper limit of 3.2±3.0 photon units.
Description:
The data were taken in the far-ultraviolet (FUV) by the Alice
spectrograph onboard the New Horizons (NH) spacecraft at a distance of
57AU from the Sun.
The New Horizons Alice spectrograph is a Rowland Circle spectrograph
with spectral coverage from 520 to 1870Å, enabling direct
measurement of the FUV cosmic background over these wavelengths. The
main airglow channel (AGC) has an aperture comprising two connected
areas on the sky: a narrow "Stem" with a field of view (FOV) of
0.1°x4° and a square "Box" with an FOV of 2°x2°.
The Cosmic UV Background (CUVB) survey was designed in parallel with
the NH Cosmic Optical Background (COB) survey, described by
Postman+ (2024ApJ...972...95P 2024ApJ...972...95P). In brief, they observed several fields
with the New Horizons LORRI instrument to measure the COB intensity
and calibrate the observations. Of these, 16 fields were selected for
primary observation of the COB intensity; these were designated with
the "NCOB" prefix. Eight other fields, designated with the "DCAL"
prefix, were selected to develop a diffuse galactic light (DGL)
estimator when used in league with the NCOB fields. For the NCOB and
auxiliary fields, eight individual sky exposures of 3600s each were
obtained for a 28,800s total exposure, interlaced between nine 3600s
dark exposures. For the DCAL fields, four 3600s sky exposures, for a
total of 14,400s, were interlaced between five 3600s dark exposures.
Additionally, the NHTF01 field is the test of the NCOB field selection
and observational strategy published initially in
Lauer+ (2022ApJ...927L...8L 2022ApJ...927L...8L) and reanalyzed in
Postman+ (2024ApJ...972...95P 2024ApJ...972...95P).
Lastly, four fields unique to the CUVB program were defined to improve
the calibration of the Alice instrument or to augment the analysis of
the observations. Two fields, designated SHOCK1 and SHOCK2, were
defined to observe UV emission from highly shocked gas. H2_NE is a low
Galactic latitude field selected to observe possible molecular
hydrogen (H2) fluorescent emission. LYACAL is a reobservation of a
mosaic of a contiguous area of Box fields first observed with Alice in
2007.
A sequence of individual exposures, using the Alice Histogram Imaging
Mode (HIM), was taken at each field. The histogram data consists of
FITS images with the counts integrated over the entire observation.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 118 28 Survey field centers and observations
sp_csv/* . 56 Individual CSV file of the spectra (Data behind
Figure 11)
sp_fits/* . 56 Individual FITS file of the spectra (Data behind
Figure 11)
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See also:
J/A+A/411/447 : Interstellar NaI D-line (Lallement+, 2003)
J/ApJ/724/1389 : Mapping the diffuse UV sky with GALEX (Murthy+, 2010)
J/ApJ/818/187 : The cosmic TeV gamma-ray background spectrum (Inoue+, 2016)
J/A+A/557/L1 : WMAP cosmic microwave background data (Rassat+, 2013)
J/A+A/591/A50 : CMB intensity map from WMAP and Planck PR2 data (Bobin+, 2016)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 A6 --- ID Field identifier
8- 14 F7.3 deg RAdeg Right ascension (J2000)
16- 22 F7.3 deg DEdeg Declination (J2000)
24- 33 A10 "Y:M:D" Date Date of observation (UT)
35- 44 I10 s MET Mission elapsed time (from launch on 14:00
ET on 2006-01-19) of the first image in
each field sequence
46- 51 F6.2 deg lS Galactic longitude of the Stem (1)
53- 58 F6.2 deg bS Galactic latitude of the Stem (1)
60- 65 F6.2 deg lB Galactic longitude of the Box (1)
67- 72 F6.2 deg bB Galactic latitude of the Box (1)
74- 78 F5.3 --- E(B-V)S [0.01/0.75] Color excess mean values in the
Stem (2)
80- 84 F5.3 --- E(B-V)B [0.01/0.6] Color excess mean values in the
Box (2)
86- 87 A2 --- BoxSp Box spectrum with link to interactive plot
(column added by CDS)
89- 90 A2 --- StemSp Stem spectrum with link to interactive plot
(column added by CDS)
92-104 A13 --- BoxFITS Link to the spectrum file in FITS format
(column added by CDS) (3)
106-118 A13 --- StemFITS Link to the spectrum file in FITS format
(column added by CDS) (3)
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Note (1): A narrow "Stem" with a field of view (FOV) of 0.1°x4° and
a square "Box" with an FOV of 2°x2°.
Note (2): from Planck Collab+ (2016A&A...586A.132P 2016A&A...586A.132P) with a typical variation of
5mmag.
Note (3): The individual FITS files were created by CDS. The original FITS file
"publicdata.fits" was split into individual files for the table, but
it is available unaltered in the FTP.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Robin Leichtnam [CDS] 02-Dec-2025