J/ApJ/908/18 HST stellar photometry of Eridanus II (Simon+, 2021)
Eridanus II: A Fossil from Reionization with an Off-center Star Cluster.
Simon J.D., Brown T.M., Drlica-Wagner A., Li T.S., Avila R.J., Bechtol K.,
Clementini G., Crnojevic D., Garofalo A., Geha M., Sand D.J., Strader J.,
Willman B.
<Astrophys. J., 908, 18 (2021)>
=2021ApJ...908...18S 2021ApJ...908...18S
ADC_Keywords: Galaxies; Clusters, globular; Photometry, HST; Optical
Keywords: Dwarf galaxies ; Local Group ; Globular star clusters ;
Stellar populations ; Galaxy ages ; Dark matter ; Galaxy
structure
Abstract:
We present deep Hubble Space Telescope (HST) photometry of the
ultra-faint dwarf galaxy EridanusII (EriII). EriII, which has an
absolute magnitude of MV=-7.1, is located at a distance of 339kpc,
just beyond the virial radius of the Milky Way. We determine the star
formation history of EriII and measure the structure of the galaxy and
its star cluster. We find that a star formation history consisting of
two bursts, constrained to match the spectroscopic metallicity
distribution of the galaxy, accurately describes the EriII stellar
population. The best-fit model implies a rapid truncation of star
formation at early times, with >80% of the stellar mass in place
before z∼6. A small fraction of the stars could be as young as 8Gyr,
but this population is not statistically significant; Monte Carlo
simulations recover a component younger than 9Gyr only 15% of the
time, where they represent an average of 7±4% of the population.
These results are consistent with theoretical expectations for
quenching by reionization. The HST depth and angular resolution enable
us to show that EriII's cluster is offset from the center of the
galaxy by a projected distance of 23±3pc. This offset could be an
indication of a small (∼50-75pc) dark matter core in EriII. Moreover,
we demonstrate that the cluster has a high ellipticity of
0.31-0.06+0.05 and is aligned with the orientation of EriII within
3°±6°, likely due to tides. The stellar population of the
cluster is indistinguishable from that of EriII itself.
Description:
We observed EriII with the Wide Field Channel of the Advanced Camera
for Surveys (ACS) on Hubble Space Telescope (HST) through program
GO-14234 (PI:Simon). The observations were scheduled over seven visits
between 2016 January 16 and 2016 February 8. We devoted four visits (8
orbits) to imaging in the F814W filter, totaling 20680s. The remaining
three visits (two visits of two orbits each and one single-orbit
visit) were used to image EriII in the F606W filter, totaling 12830s.
Objects:
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RA (2000) DE Designation(s)
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03 44 21.1 -43 32 00 Eri II = NAME Eridanus II
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 57 27299 Eri II stellar photometry
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See also:
I/337 : Gaia DR1 (Gaia Collaboration, 2016)
II/357 : The Dark Energy Survey (DES) Data Release 1 (Abbott+, 2018)
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
VII/195 : Globular Clusters in the Milky Way (Harris, 1996)
VII/202 : Globular Clusters in the Milky Way (Harris, 1997)
J/AJ/130/1693 : HST-ACS observations of 5 globular clusters (Brown+, 2005)
J/AJ/129/2714 : JHK photometry of M92 variable stars (Del Principe+, 2005)
J/MNRAS/411/1013 : CaII triplet in Sextans dSph galaxy (Battaglia+, 2011)
J/ApJ/756/108 : HST VI photometry of variable stars in LeoT (Clementini+, 2012)
J/AJ/148/83 : Spectroscopy in NGC 2360, NGC 3680, and NGC 5822 (Sales+, 2014)
J/ApJ/789/147 : Star formation histories of LG dwarf galaxies (Weisz+, 2014)
J/ApJ/808/16 : The Cannon, new approach to determine abundances (Ness+, 2015)
J/ApJ/829/26 : Variable stars in field of Andromeda XXV. III. (Cusano+, 2016)
J/A+A/612/A26 : Deep HI observations of Leo T with WSRT (Adams+, 2018)
J/ApJ/863/25 : Deep Magellan/Megacam obs. 4 MWsatellites (Mutlu-Pakdil+, 2018)
J/ApJ/885/53 : Spectroscopy & HST photometry galaxy LeoV (Mutlu-Pakdil+, 2019)
J/ApJ/875/77 : Proper motions of MW satellites with Gaia & DES (Pace+, 2019)
J/ApJ/875/L13 : DECam r and i magnitudes in the Fornax 6 cluster (Wang+, 2019)
J/ApJ/893/47 : Milky Way satellite census. I. DES & PS1 (Drlica-Wagner+, 2020)
J/A+A/635/A107 : Spectroscopy of the Eri 2 cluster (Zoutendijk+, 2020)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 5 I5 --- ID [1/27299] Internal index number
7- 15 F9.6 deg RAdeg [56.04/56.14] Right Ascension (J2000)
17- 26 F10.6 deg DEdeg [-43.57/-43.4] Declination (J2000)
28- 33 F6.3 mag F606mag [18.8/31.2] HST/ACS F606W filter magnitude; STMAG
35- 39 F5.3 mag e_F606mag [0.004/0.6] Uncertainty in F606mag
41- 46 F6.3 mag F814mag [18.6/31.6] HST/ACS F814W filter magnitude; STMAG
48- 52 F5.3 mag e_F814mag [0.004/0.6] Uncertainty in F814mag
54- 57 I4 --- Flag [0/2047] Photometry flag (1)
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Note (1): Flags as follows:
0 = no flag;
1 = fails χ criterion in F606W;
2 = fails χ criterion in F814W;
4 = fails sharp criterion in F606W;
8 = fails sharp criterion in F814W;
16 = bright neighbor within 4 pixels;
32 = bright neighbor within 8 pixels;
64 = bright neighbor within 12 pixels and 2.5 mag;
128 = bright neighbor within 16 pixels and 2.5 mag;
256 = bright neighbor within 20 pixels and 1.5 mag;
512 = bright neighbor within 24 pixels and 1.5 mag;
1024 = fails photometric uncertainty criterion in both bands.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Coralie Fix [CDS], 23-Jun-2022