J/ApJ/958/167 Metallicity measurements of ∼500 stars across 13 UFDs (Fu+, 2023)
Metallicity distribution functions of 13 ultra-faint dwarf galaxy candidates
from Hubble Space Telescope narrowband imaging.
Fu S.W., Weisz D.R., Starkenburg E., Martin N., Savino A.,
Boylan-Kolchin M., Cote P., Dolphin A.E., Ji A.P., Longeard N.,
Mateo M.L., Patel E., Sandford N.R.
<Astrophys. J., 958, 167 (2023)>
=2023ApJ...958..167F 2023ApJ...958..167F
ADC_Keywords: Galaxies, dwarf; Photometry, HST; Abundances, [Fe/H]; Colors;
Optical
Keywords: Dwarf galaxies ; HST photometry ; Stellar abundances ; Local Group
Abstract:
We present uniformly measured stellar metallicities of 463 stars in
13 Milky Way (MW) ultra-faint dwarf galaxies (UFDs; MV=-7.1 to -0.8)
using narrowband CaHK (F395N) imaging taken with the Hubble Space
Telescope. This represents the largest homogeneous set of stellar
metallicities in UFDs, increasing the number of metallicities in these
13 galaxies by a factor of 5 and doubling the number of metallicities
in all known MW UFDs. We provide the first well-populated MDFs for all
galaxies in this sample, with <[Fe/H]> ranging from -3.0 to -2.0dex,
and σ[Fe/H] ranging from 0.3-0.7dex. We find a nearly constant
[Fe/H]~-2.6 over 3 decades in luminosity (∼102-105L☉),
suggesting that the mass-metallicity relationship does not hold for
such faint systems. We find a larger fraction (24%) of extremely
metal-poor ([Fe/H]<-3) stars across our sample compared to the
literature (14%), but note that uncertainties in our most metal-poor
measurements make this an upper limit. We find 19% of stars in our UFD
sample to be metal-rich ([Fe/H]>-2), consistent with the sum of
literature spectroscopic studies. MW UFDs are known to be
predominantly >13Gyr old, meaning that all stars in our sample are
truly ancient, unlike metal-poor stars in the MW, which have a range
of possible ages. Our UFD metallicities are not well matched to known
streams in the MW, providing further evidence that known MW
substructures are not related to UFDs. We include a catalog of our
stars to encourage community follow-up studies, including priority
targets for ELT-era observations.
Description:
To select our sample, we started from all known ultra-faint dwarf
galaxies (UFDs) within ∼300kpc of the Milky Way (MW) as listed in
Simon (2019ARA&A..57..375S 2019ARA&A..57..375S).
For this work, we present metallicity distribution function (MDF)
measurements focusing on 13 UFDs observed by our HST program
(GO-15901; PI: Weisz). Table 1 lists the observational properties of
our UFD sample.
We acquired new HST/UVIS F395N and F475W observations for our systems
between 2020 January and October. The F395N and F475W filters are
equivalent to the Pristine CaHK narrowband and g-band filters used for
measuring stellar metallicities. We required that the UVIS fields
spatially overlap archival ACS F814W and F606W imaging. All of the HST
data analyzed in this program can be found in MAST at this link:
10.17909/cfn9-gt96
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 93 13 Observational properties of ultra-faint dwarf
galaxies (UFDs)
table2.dat 62 13 Summary of metallicity distribution function (MDF)
measurements
table4.dat 156 462 Metallicity measurements of all stars
table5.dat 156 111 *EMP star candidates
table6.dat 141 86 *Metal-rich ([Fe/H]>-2.0) stars
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Note on table5.dat: Extremely metal-poor (EMP; [Fe/H]<-3.0) star candidates
identified in our work.
Note on table6.dat: MR ([Fe/H]>-2.0) stars identified in our work.
--------------------------------------------------------------------------------
See also:
B/hst : HST Archived Exposures Catalog (STScI, 2007)
J/AJ/131/2114 : Radial velocities in Fornax dSph (Walker+, 2006)
J/ApJ/692/1464 : Spectroscopy of Segue 1 (Geha+, 2009)
J/ApJ/733/46 : Velocity measurements in Segue 1 (Simon+, 2011)
J/ApJ/756/79 : Optical phot. in 3 Local Group dwarf galaxies (Sand+, 2012)
J/MNRAS/434/1681 : Extremely metal-poor stars CaII triplet (Carrera+, 2013)
J/ApJ/779/102 : Metallicities of RGB stars in dwarf galaxies (Kirby+, 2013)
J/ApJ/767/134 : Abund. of red giant stars in UFD galaxies (Vargas+, 2013)
J/ApJ/786/74 : EW measurements of 6 Segue 1 red giants (Frebel+, 2014)
J/ApJ/790/73 : Abundances of RGBs in M31 satellites (Vargas+, 2014)
J/ApJ/789/147 : Star formation histories of LG dwarf galaxies (Weisz+, 2014)
J/AJ/149/153 : Abund. of red giants in 10 GCs (Meszaros+, 2015)
J/ApJ/808/95 : Spectroscopy of Reticulum II (Simon+, 2015)
J/ApJ/808/108 : M2FS stellar spectroscopy of Reticulum 2 (Walker+, 2015)
J/AJ/151/82 : The 4 brightest red giants in Ret 2 (Roederer+, 2016)
J/ApJ/819/53 : Magellan/M2FS sp. of Tucana 2 and Grus 1 (Walker+, 2016)
J/MNRAS/472/2963 : Metallicities of Pristine stars (Youakim+, 2017)
J/ApJ/866/22 : Spectroscopic analysis of Tuc III stream (Li+, 2018)
J/ApJ/863/25 : Magellan obs. of 4 MW satellites (Mutlu-Pakdil+, 2018)
J/ApJ/852/99 : Abund. of 3 metal-poor stars in Hor I (Nagasawa+, 2018)
J/MNRAS/490/2241 : Spectroscopy of Pristine EMP star candidates (Aguado+, 2019)
J/A+A/623/A129 : New satellites of the LMC search (Fritz+, 2019)
J/ApJ/870/83 : Abund. in the UFD galaxies GruI & TriII (Ji+, 2019)
J/ApJS/247/7 : Narrowband Ca phot. for dSph galaxies (Han+, 2020)
J/ApJ/889/27 : Abundances of 11 stars in Carina II and III (Ji+, 2020)
J/A+A/640/A25 : Metal-poor stars limb-darkening coeff. (Karovicova+, 2020)
J/MNRAS/491/356 : Sagittarius II study with Pristine (Longeard+, 2020)
J/MNRAS/497/1236 : Chemo-dynamics of outer halo dwarf stars (Monty+, 2020)
J/ApJ/892/137 : Spectroscopy of Grus II, Tuc IV and Tuc V (Simon+, 2020)
J/MNRAS/492/3241 : The Pristine survey. IX. (Venn+, 2020)
J/ApJ/906/96 : Dwarfs gal. in DC Justice League Simu. (Applebaum+, 2021)
J/ApJ/916/81 : Grus I and Indus II deep DES photometry (Cantu+, 2021)
J/ApJ/909/192 : HST/ACS photometry of Eridanus II (Gallart+, 2021)
J/ApJ/914/123 : Gaia DR2 and EDR3 stars with sp. follow-up (Ibata+, 2021)
J/ApJ/920/92 : VLT Sp. of Ultra-faint Dwarf Galaxies (Jenkins+, 2021)
J/ApJ/908/18 : HST stellar photometry of Eridanus II (Simon+, 2021)
J/MNRAS/516/2348 : Sp. of metal-poor stars in Bootes I (Longeard+, 2022)
J/AJ/165/100 : Stellar param. and abundances in Reticulum II (Ji+, 2023)
J/ApJS/267/18 : Abundance measurements in M15 RGB stars (Sandford+, 2023)
J/ApJ/944/43 : HST/ACS stellar photometry for Reticulum II (Simon+, 2023)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 17 A17 --- Name Galaxy name
19- 22 F4.1 mag VMag [-7.1/-0.8] Absolute V-band magnitude
24- 28 F5.3 arcmin rh [0.315/6.3] Half-light radius
30- 33 F4.2 arcmin e_rh [0.03/1]? rh uncertainty
35 A1 --- r_rh Flag on rh (1)
37- 41 F5.2 mag m-M [16.8/22.8] Distance modulus
43- 46 F4.2 mag e_m-M [0.04/0.4] m-M uncertainty
48 A1 --- r_m-M Flag on m-M (1)
50- 53 F4.2 --- FoV [0.41/8.25] On-sky size in relation to the
HST/WFC3 FoV
55- 58 I4 s Exp1 [3120/9481] Exposure time of the F395N image
60- 63 I4 s Exp2 [730/7644] Exposure time of the F475Wimage
65- 69 I5 s Exp3 [4605/28580] Exposure time of the F606W image (2)
71- 75 I5 s Exp4 [4605/20860] Exposure time of the F814W image (2)
77- 80 F4.1 mag F395N [25/26] HST/WFC3 F395N magnitude
where F395N S/N=5
82- 85 F4.1 mag F475W [25/26] HST/WFC3 F475W magnitude
where F395N S/N=5
87- 93 A7 --- UFD Abbreviated galaxy name as in tables 4-6;
column added by CDS
--------------------------------------------------------------------------------
Note (1): Reference code as follows:
a = Crnojevic et al. (2016ApJ...824L..14C 2016ApJ...824L..14C)
b = Sand et al. (2012, J/ApJ/756/79)
c = Greco et al. (2008ApJ...675L..73G 2008ApJ...675L..73G)
d = Martin et al. (2015ApJ...804L...5M 2015ApJ...804L...5M)
e = Vivas et al. (2016AJ....151..118V 2016AJ....151..118V)
f = Mutlu-Pakdil et al. (2018ApJ...863...25M 2018ApJ...863...25M)
g = Jerjen et al. (2018arXiv180902259J 2018arXiv180902259J)
h = Koposov et al. (2015ApJ...805..130K 2015ApJ...805..130K)
i = Drlica-Wagner et al. (2015ApJ...813..109D 2015ApJ...813..109D)
j = Munoz et al. (2018ApJ...860...66M 2018ApJ...860...66M)
k = Willman et al. (2006astro.ph..3486W 2006astro.ph..3486W)
l = Conn et al. (2018ApJ...852...68C 2018ApJ...852...68C)
m = Belokurov et al. (2007ApJ...654..897B 2007ApJ...654..897B)
n = Laevens et al. (2015ApJ...813...44L 2015ApJ...813...44L)
Note (2): In all cases except Eri II and CVn II, the archival F606W and F814W
imaging are from GO-14734 (PI: Kallivayalil). For CVn II, the F606W
and F814W imaging are from GO-12549 (PI: Brown). For Eri II, all of
the broadband imaging is from programs GO-14224 (PI: Gallart) and
GO-14234 (PI: Simon). All of the data can be found in MAST at the
following doi:10.17909/cfn9-gt96
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 17 A17 --- Name Galaxy name
19- 22 F4.1 mag VMag [-7.1/-0.8] Absolute V magnitude
24- 28 F5.2 [-] [Fe/H] [-3.05/-2.03] Abundance [Fe/H]
30- 33 F4.2 [-] e_[Fe/H] [0.06/0.34] Lower uncertainty on [Fe/H]
35- 38 F4.2 [-] E_[Fe/H] [0.06/0.26] Upper uncertainty on [Fe/H]
40- 43 F4.2 --- Sig [0.26/0.72] σ[Fe/H] measurement
45- 48 F4.2 --- e_Sig [0.08/0.28] Lower uncertainty on Sig
50- 53 F4.2 --- E_Sig [0.08/0.44] Upper uncertainty on Sig
55- 56 I2 --- Ns [6/76] Number of stars used to make
the measurement
58- 59 I2 --- Nemp [0/17] Number of stars with [Fe/H]<-3
61- 62 I2 --- Nmr [1/21] Number of stars with [Fe/H]>-2
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- UFD Galaxy
9- 10 I2 --- StarID [0/82] Star identifier within UFD
12- 21 F10.6 deg RAdeg [35.6/355.1] Right ascension (J2000)
23- 32 F10.6 deg DEdeg [-63.3/64.6] Declination (J2000)
34- 39 F6.3 mag F814W [18/24.1] Apparent magnitude in HST/ACS
F814W filter
41- 45 F5.3 mag e_F814W [0/0.02] Uncertainty on F814W
47- 52 F6.3 mag F606W [18.6/24.5] Apparent magnitude in HST/ACS
F606W filter
54- 58 F5.3 mag e_F606W [0.001/0.02] Uncertainty on F606W
60- 65 F6.3 mag F475W [19.3/25] Apparent magnitude in HST/WFC3
F475W filter
67- 71 F5.3 mag e_F475W [0.002/0.07] Uncertainty on F475W
73- 78 F6.3 mag F395N [19.9/25.3] Apparent magnitude in HST/WFC3
F395N filter
80- 84 F5.3 mag e_F395N [0.006/0.2] Uncertainty on F395N
86- 90 F5.3 mag V-I [0.8/1.94] F475W-F814W color index
92- 96 F5.3 mag e_V-I [0.002/0.07] Uncertainty on V-I
98- 103 F6.3 mag CaHK [-1.8/-0.68] CaHK color index, defined as
F395N-F475W-1.5(F475W-F814W)
105- 109 F5.3 mag e_CaHK [0.007/0.2] Uncertainty on CaHK
111- 115 F5.2 [-] [Fe/H] [-4/-0.95] Metallicity in units of dex
117- 120 F4.2 [-] E_[Fe/H] [0.04/0.81]? Upper uncertainty on [Fe/H] (1)
122- 125 F4.2 [-] e_[Fe/H] [0.03/1.1]? Lower uncertainty on [Fe/H], all
negative values (1)
127- 129 F3.1 [-] [Fe/H]err [0.2/1] Error floor adopted for star in
units of dex (2)
131- 156 A26 --- Notes Additional measurement notes
(i.e. constrained vs unconstrained
measurements, EMP star candidates)
--------------------------------------------------------------------------------
Note (1): Statistical uncertainties on the metallicity measurements originate
from photometric uncertainties.
Note (2): Systematic uncertainties on the metallicity measurements are assigned
following the investigation in Appendix B.5.
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Byte-by-byte Description of file: table5.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- UFD Galaxy
9- 10 I2 --- StarID [0/81] Star Identifier within UFD
12- 21 F10.6 deg RAdeg [43.8/355.1] Right ascension (J2000)
23- 32 F10.6 deg DEdeg [-63.3/64.6] Declination (J2000)
34- 39 F6.3 mag F814W [18.5/24.1] Apparent magnitude in HST/ACS
F814W filter
41- 45 F5.3 mag e_F814W [0.001/0.02] Uncertainty on F814W
47- 52 F6.3 mag F606W [19.2/24.4] Apparent magnitude in HST/ACS
F606W filter
54- 58 F5.3 mag e_F606W [0.001/0.02] Uncertainty on F606W
60- 65 F6.3 mag F475W [19.8/24.9] Apparent magnitude in HST/WFC3
F475W filter
67- 71 F5.3 mag e_F475W [0.002/0.05] Uncertainty on F475W
73- 78 F6.3 mag F395N [20.3/25] Apparent magnitude in HST/WFC3
F395N filter
80- 84 F5.3 mag e_F395N [0.006/0.2] Uncertainty on F395N
86- 90 F5.3 mag V-I [0.8/1.8] F475W-F814W color index
92- 96 F5.3 mag e_V-I [0.002/0.05] Uncertainty on V-I
98- 103 F6.3 mag CaHK [-1.76/-0.89] CaHK color index, defined as
F395N-F475W-1.5(F475W-F814W)
105- 109 F5.3 mag e_CaHK [0.007/0.2] Uncertainty on CaHK
111- 115 F5.2 [-] [Fe/H] [-4/-3] Metallicity in units of dex
117- 120 F4.2 [-] E_[Fe/H] [0.09/0.8]? Upper uncertainty on [Fe/H]
122- 125 F4.2 [-] e_[Fe/H] [0.1/0.63]? Lower uncertainty on [Fe/H], all
negative values
127- 129 F3.1 [-] [Fe/H]err [0.2/1] Error floor adopted for star in
units of dex
131- 156 A26 --- Notes Additional measurement notes
(i.e. constrained vs unconstrained
measurements, EMP star candidates)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 7 A7 --- UFD Galaxy
9- 10 I2 --- StarID [0/70] Star identifier within UFD
12- 21 F10.6 deg RAdeg [35.6/355] Right ascension (J2000)
23- 32 F10.6 deg DEdeg [-63.3/64.6] Declination (J2000)
34- 39 F6.3 mag F814W [18/23.7] Apparent magnitude in HST/ACS
F814W filter
41- 45 F5.3 mag e_F814W [0.001/0.02] Uncertainty on F814W
47- 52 F6.3 mag F606W [18.6/24.2] Apparent magnitude in HST/ACS
F606W filter
54- 58 F5.3 mag e_F606W [0.001/0.02] Uncertainty on F606W
60- 65 F6.3 mag F475W [19.3/24.7] Apparent magnitude in HST/WFC3
F475W filter
67- 71 F5.3 mag e_F475W [0.002/0.05] Uncertainty on F475W
73- 78 F6.3 mag F395N [20/25.15] Apparent magnitude in HST/WFC3
F395N filter
80- 84 F5.3 mag e_F395N [0.006/0.11] Uncertainty on F395N
86- 90 F5.3 mag V-I [0.8/1.9] F475W-F814W color index
92- 96 F5.3 mag e_V-I [0.003/0.05] Uncertainty on V-I
98- 103 F6.3 mag CaHK [-1.23/-0.68] CaHK color index, defined as
F395N-F475W-1.5(F475W-F814W)
105- 109 F5.3 mag e_CaHK [0.01/0.12] Uncertainty on CaHK
111- 115 F5.2 [-] [Fe/H] [-1.99/-0.95] Metallicity in units of dex
117- 120 F4.2 [-] E_[Fe/H] [0.05/0.5]? Upper uncertainty on [Fe/H]
122- 125 F4.2 [-] e_[Fe/H] [0.03/1.1]? Lower uncertainty on [Fe/H], all
negative values
127- 129 F3.1 [-] [Fe/H]err [0.2/0.3] Error floor adopted for star in
units of dex
131- 141 A11 --- Notes Additional measurement notes
(i.e. constrained vs unconstrained
measurements, EMP star candidates)
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 23-Jan-2026