J/AJ/169/301 Masses & ages of MLW dwarf galaxy stars from HST (Archer+, 2025)
Stellar populations and molecular gas composition in the low-metallicity
environment of WLM.
Archer H.N., Hunter D.A., Elmegreen B.G., Hunt L.K., O'Brien R.,
Brinks E., Cigan P., Rubio M., Windhorst R.A., Jansen R.A., Mathews E.P.
<Astron. J., 169, 301 (2025)>
=2025AJ....169..301A 2025AJ....169..301A
ADC_Keywords: Photometry, HST; Star Forming Region; Stars, masses; Stars, ages;
Carbon monoxide; Galaxies, dwarf; Molecular clouds
Keywords: Local Group ; Dwarf irregular galaxies ; Star formation ;
Star forming regions ; Stellar populations
Abstract:
We investigate the stellar populations and molecular gas properties of
a star- forming region within the dwarf irregular (dIrr) galaxy
Wolf-Lundmark-Mellote (WLM). Low-metallicity dIrrs like WLM offer a
valuable window into star formation in environments that are unlike
those of larger, metal-rich galaxies such as the Milky Way. In these
conditions, carbon monoxide (CO), typically used to trace molecular
clouds, is more easily photodissociated by ultraviolet (UV) radiation,
leading to a larger fraction of CO-dark molecular gas, where H2 exists
without detectable CO emission, or CO-dark gas in the form of cold H
i. Understanding the molecular gas content and the stellar populations
in these star-forming regions provides important information about the
role of CO-bright and CO-dark gas in forming stars. Using Hubble Space
Telescope imaging across five Wide Field Camera 3 UVIS bands and CO
observations from the Atacama Large Millimeter Array, we examine
stellar populations within and outside CO cores and the
photodissociation region. Our findings indicate similar physical
characteristics such as age and mass across the different
environments. Assuming 2% of molecular gas is converted to stars, we
estimate the molecular gas content and determine that CO-dark gas
constitutes a large fraction of the molecular reservoir in WLM. These
results are consistent with molecular gas estimates using a previous
dust-derived CO-to-H2 conversion factor (αCO) for WLM. These
findings highlight the critical role of CO- dark gas in
low-metallicity star formation.
Description:
We obtained near-ultraviolet (NUV) and optical images covering most of
the star-forming area of WLM through the Hubble Space Telescope (HST)
GO program #17068. Focusing on the star-forming region constrained by
the [CII]-detected PDR, this project acquired Wide Field Camera 3
(WFC3) UVIS F275W (2709.7Å), F336W (3354.5Å), F438W
(4326.2Å), F555W (5308.4Å), and F625W (6242.6Å) images of the
region for detecting and analyzing the stellar population. The
exposure time in each filter was 2220s, 1230s, 1760s, 1125s, and 1050s
respectively. All HST data can be found in MAST: 10.17909/xyhn-3z68.
Crowded-field photometry was performed individually on all five HST
UVIS images using the Image Reduction and Analysis Facility (IRAF;
Tody 1986, 1986SPIE..627..733T 1986SPIE..627..733T) routine Daophot, derived from the
Stetson 1987 (1987PASP...99..191S 1987PASP...99..191S) version. Sources brighter than 24
were classified as stars if their sharpness is less than zero, while
sources fainter than 24 were classified as stars if both their
sharpness is less than zero and χ is less than 1. We first created
individual catalogs of stars detected in each of the five filters. To
construct a combined catalog of stars detected across all five
filters, we performed step-by-step matching.
In Cycle 1, Rubio+2015 (2015Natur.525..218R 2015Natur.525..218R) used ALMA to image two
star-forming regions in WLM, focusing on CO(1-0) emissions, and
detected 10 CO cores. The beam size for these observations was
0.9"x1.3". Of the 10 detected cores, six were located in the PDR,
referred to as Region B in Elmegreen+2013 (2013Natur.495..487E 2013Natur.495..487E),
the WLM-SE region in Rubio+2015, and Region 1 in
Archer+2022b (2022AJ....163..141A 2022AJ....163..141A), which is the primary focus of this
paper. An additional 35 CO cores were detected using CO(2-1)
observations at 1" resolution (4.8pc at WLM distance) with ALMA Cycle 6
(Rubio+2025, in preparation), all of which were detected outside the
PDR as the survey did not include it. Five of these 35 CO cores were
included when examining the environment surrounding the PDR to compare
stellar populations inside the CO cores and outside the PDR to stellar
populations inside the CO cores and inside the PDR.
Objects:
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RA (2000) DE Designation(s)
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00 01 57.90 -15 27 49.9 NAME Wolf-Lundmark-Melotte = Anon 2359-15
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table5.dat 69 443 Right ascension, declination, and Vega magnitudes
for the five HST filters for sources inside the
PDR and outside the CO cores
table7.dat 69 566 Right ascension, declination, and Vega magnitudes
for the five HST filters for sources outside the
PDR and outside the CO cores
table9.dat 58 144 Right ascension, declination, and Vega magnitudes
for the four HST filters for sources inside the
PDR and outside the CO cores not detected in the
F275W filter
table10.dat 58 78 Right ascension, declination, and Vega magnitudes
for the four HST filters for sources outside the
PDR and outside the CO cores not detected in the
F275W filter
table12.dat 29 443 Mass and age for stars inside the PDR and outside CO
cores.
table14.dat 29 566 Mass and age for stars outside the PDR and outside
CO cores
table16.dat 29 144 Mass and age for stars inside the PDR and outside CO
cores not detected in the F275W filter
table17.dat 29 78 Mass and age for stars outside the PDR and outside
CO cores not detected in the F275W filter.
table20.dat 54 443 The sharpness and chi parameters for the five HST
filters for sources inside the PDR and outside the
CO cores
table22.dat 54 566 The sharpness and chi parameters for the five HST
filters for sources outside the PDR and outside
the CO cores
table24.dat 43 144 The sharpness and chi parameters for the four HST
filters for sources inside the PDR and outside the
CO cores not detected in the F275W filter
table25.dat 43 78 The sharpness and chi parameters for the four HST
filters for sources outside the PDR and outside
the CO cores not detected in the F275W filter
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See also:
J/AJ/136/2782 : Star formation efficiency in nearby galaxies (Leroy+, 2008)
J/ApJ/703/517 : The Spitzer Local Volume Legacy: IR photometry (Dale+, 2009)
J/AJ/144/134 : LITTLE THINGS survey of nearby dwarf galaxies (Hunter+, 2012)
J/A+A/589/A28 : N66, N88 & N25+N26 emission line maps (Requena-Torres+, 2016)
J/A+A/643/A141 : Tracing total molecular gas in galaxies (Madden+, 2020)
J/ApJ/907/112 : The astrophysical dist. scale. III. The WLM gal. (Lee+, 2021)
J/ApJS/260/41 : Dust extinction law in nearby galaxies. II. M33 (Wang+, 2022)
J/A+A/672/A153 : CO(2-1) survey at 9pc resolution in SMC (Saldano+, 2023)
http://archive.stsci.edu/doi/resolve/resolve.html?doi=10.17909/xyhn-3z68 : MAST
data
Byte-by-byte Description of file: table5.dat table7.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 F6.4 deg RAdeg Right ascension (J2000)
8- 14 F7.3 deg DEdeg Declination (J2000)
16- 20 F5.2 mag F275W F275W apparent Vega magnitude
22- 25 F4.2 mag e_F275W Uncertainty in F275W
27- 31 F5.2 mag F336W F336W apparent Vega magnitude
33- 36 F4.2 mag e_F336W Uncertainty in F336W
38- 42 F5.2 mag F438W F438W apparent Vega magnitude
44- 47 F4.2 mag e_F438W Uncertainty in F438W
49- 53 F5.2 mag F555W F555W apparent Vega magnitude
55- 58 F4.2 mag e_F555W Uncertainty in F555W
60- 64 F5.2 mag F625W F625W apparent Vega magnitude
66- 69 F4.2 mag e_F625W Uncertainty in F625W
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Byte-by-byte Description of file: table9.dat table10.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 F6.4 deg RAdeg Right ascension (J2000)
8- 14 F7.3 deg DEdeg Declination (J2000)
16- 20 F5.2 mag F336W F336W apparent Vega magnitude
22- 25 F4.2 mag e_F336W Uncertainty in F336W
27- 31 F5.2 mag F438W F438W apparent Vega magnitude
33- 36 F4.2 mag e_F438W Uncertainty in F438W
38- 42 F5.2 mag F555W F555W apparent Vega magnitude
44- 47 F4.2 mag e_F555W Uncertainty in F555W
49- 53 F5.2 mag F625W F625W apparent Vega magnitude
55- 58 F4.2 mag e_F625W Uncertainty in F625W
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Byte-by-byte Description of file: table1[2467].dat
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Bytes Format Units Label Explanations
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1- 4 F4.2 [Msun] log(M) Log mass of star
6- 9 F4.2 [Msun] e_log(M) Uncertainty in log(Mass)
11- 14 F4.2 [yr] log(Age) Log age of star
16- 19 F4.2 [yr] e_log(Age) Uncertainty in log(Age)
21- 24 F4.2 mag Av AV of star
26- 29 F4.2 mag e_Av Uncertainty in Dust-Atten
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Byte-by-byte Description of file: table20.dat table22.dat
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Bytes Format Units Label Explanations
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1- 5 F5.2 --- shpF275W F275W sharpness
7- 10 F4.2 --- chiF275W F275W chi
12- 16 F5.2 --- shpF336W F336W sharpness
18- 21 F4.2 --- chiF336W F336W chi
23- 27 F5.2 --- shpF438W F438W sharpness
29- 32 F4.2 --- chiF438W F438W chi
34- 38 F5.2 --- shpF555W F555W sharpness
40- 43 F4.2 --- chiF555W F555W chi
45- 49 F5.2 --- shpF625W F625W sharpness
51- 54 F4.2 --- chiF625W F625W chi
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Byte-by-byte Description of file: table24.dat table25.dat
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Bytes Format Units Label Explanations
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1- 5 F5.2 --- shpF336W F336W sharpness
7- 10 F4.2 --- chiF336W F336W chi
12- 16 F5.2 --- shpF438W F438W sharpness
18- 21 F4.2 --- chiF438W F438W chi
23- 27 F5.2 --- shpF555W F555W sharpness
29- 32 F4.2 --- chiF555W F555W chi
34- 38 F5.2 --- shpF625W F625W sharpness
40- 43 F4.2 --- chiF625W F625W chi
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History:
From electronic version of the journal
(End) Prepared by [AAS], Robin Leichtnam [CDS] 02-Mar-2026