J/ApJ/902/106 HST obs. & Gaia data of Hercules dwarf gal. (Mutlu-Pakdil+, 2020)
The elusive distance gradient in the ultrafaint dwarf galaxy Hercules: a
combined Hubble Space Telescope and Gaia view.
Mutlu-Pakdil B., Sand D.J., Crnojevic D., Olszewski E.W.,
Zaritsky D., Strader J., Collins M.L., Seth A.C., Willman B.
<Astrophys. J., 902, 106 (2020)>
=2020ApJ...902..106M 2020ApJ...902..106M
ADC_Keywords: Galaxies, photometry; Photometry, HST; Proper motions; Extinction;
Photometry, RI
Keywords: Dwarf galaxies ; Galaxy interactions ; Galaxy kinematics ;
Galaxy dynamics ; Local Group ; HST photometry ; Proper motions
Abstract:
The ultrafaint dwarf galaxy Hercules has an extremely elongated
morphology with both photometric overdensities and kinematic members
at large radii, suggesting that it may be tidally disrupting due to a
previous close encounter with the Milky Way. To explain its
observational peculiarities, we present a deep Hubble Space Telescope
(HST) imaging study of Hercules and its surrounding regions and
investigate its tidal history through a careful search for a distance
gradient along its stretched body. Our off-center HST data clearly
resolve a main sequence, showing that the stellar extension seen along
the major axis of Hercules is genuine, not a clump of background
galaxies. Utilizing Gaia DR2 data, we clean the region around the
Hercules of field contamination, and find four new plausible member
stars, all of which are located on the outskirts of the dwarf galaxy.
We update the distance to Hercules, and find 130.6R±6.1kpc
(m-M=20.58±0.10) for the main body, which is consistent with earlier
estimates in the literature. While we find no conclusive evidence for
a distance gradient, our work demonstrates that constraining a
distance gradient in such a faint system is not trivial, and the
possible thickness of the dwarf along the line of sight and field
contamination make it harder to make decisive conclusions even with
these high-precision data. Future studies coupled with tailored
theoretical models are needed to understand the true nature of
Hercules and of tidal distortion observables in ultrafaint galaxies in
general.
Description:
Deep optical observations along the major axis of Hercules were
performed using the F606W and F814W filters on the Advanced Camera for
Surveys (ACS) (HST-GO-15182; Cycle 25, PI: D. Sand). The new HST
observations were obtained from 2018 March through 2020 February with
10 orbits per field.
We also included archived HST/ACS and Wide Field Camera 3 (WFC3)
imaging in our analysis (HST-GO-12549; Cycle 19, PI: T. Brown).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 72 22833 Photometry of Hercules fields
table4.dat 76 37 A clean members catalog for Hercules within
our HST footprint
table5.dat 73 4 List of plausible Hercules members, requiring
further follow-up
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See also:
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
J/AJ/129/2714 : JHK photometry of M92 variable stars (Del Principe+, 2005)
J/MNRAS/397/1748 : Spectroscopic study of Segue 2 (Belokurov+, 2009)
J/ApJ/704/898 : Photometry of the Hercules dwarf galaxy (Sand+, 2009)
J/AJ/142/22 : Heavy-element dispersion in M92 (Roederer+, 2011)
J/ApJ/756/121 : Variable stars in Her dwarf galaxy (Musella+, 2012)
J/ApJ/756/79 : Optical phot. in 3 Local Group dwarf galaxies (Sand+, 2012)
J/ApJ/770/16 : Spectroscopic members of Segue 2 galaxy (Kirby+, 2013)
J/A+A/570/A61 : PM-selected stars of Hercules galaxy (Fabrizio+, 2014)
J/ApJ/866/22 : Spectroscopic analysis of Tuc III stream (Li+, 2018)
J/ApJ/863/25 : Deep Magellan obs. of 4 MW satellites (Mutlu-Pakdil+, 2018)
J/ApJ/863/89 : Gaia DR2 PMs of stars in MW satellites (Simon, 2018)
J/ApJ/883/11 : Magellan/IMACS spectra of Crater II & Hercules (Fu+, 2019)
J/ApJ/885/53 : Spectrosc. & HST phot. of galaxy Leo V (Mutlu-Pakdil+, 2019)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 8 A8 --- Field Field Name
10- 13 A4 --- Cam Camera used ("ACS" or "WFC3")
15- 18 I4 --- Seq [0/6435] Sequential identifier in
Field/Camera subset
20- 28 F9.5 deg RAdeg [247.5/248] Right Ascension (J2000)
30- 38 F9.6 deg DEdeg [12.6/12.9] Declination (J2000)
40- 44 F5.2 mag F606W [19.22/29.75] HST F606W band magnitude
in Camera
46- 50 F5.3 mag e_F606W [0.001/0.3] Uncertainty (DOLPHOT) in F606W
52- 55 F4.2 mag AF606W [0.13/0.16] Galactic extinction correction
in F606W
57- 61 F5.2 mag F814W [18.07/28.93] F814W band magnitude in Camera
63- 67 F5.3 mag e_F814W [0.001/0.3] Uncertainty (DOLPHOT) in F814W
69- 72 F4.2 mag AF814W [0.08/0.1] Galactic extinction correction
in F814W
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- Seq [1/37] Assigned number for each star
4- 12 F9.5 deg RAdeg [247.5/248] Right Ascension (J2000)
14- 22 F9.6 deg DEdeg [12.7/12.9] Declination (J2000)
24- 28 F5.2 mag F606W [19.17/22.11] HST F606W band magnitude
in Camera/Filter catalog
30- 34 F5.2 mag F814W [18.3/22.1] HST F814W band magnitude
in Camera/Filter catalog
36- 41 F6.3 mas/yr pmRA [-1.6/0.6]? Gaia DR2 proper motion in RA
43- 47 F5.3 mas/yr e_pmRA [0.3/1.8]? Uncertainty in pmRA
49- 54 F6.3 mas/yr pmDE [-2.2/1]? Gaia DR2 proper motion in DEC
56- 60 F5.3 mas/yr e_pmDE [0.27/1.6]? Uncertainty in pmDE
62- 70 A9 --- Field Field Name
72- 76 A5 --- Ref References (1)
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Note (1): References as follows:
1 = Simon & Geha (2007ApJ...670..313S 2007ApJ...670..313S);
2 = Aden et al. (2009ApJ...706L.150A 2009ApJ...706L.150A);
3 = Deason et al. (2012MNRAS.425L.101D 2012MNRAS.425L.101D);
4 = Musella et al. (2012, J/ApJ/756/121);
5 = Garling et al. (2018ApJ...852...44G 2018ApJ...852...44G);
6 = Gregory et al. (2020MNRAS.496.1092G 2020MNRAS.496.1092G)
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Byte-by-byte Description of file: table5.dat
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Bytes Format Units Label Explanations
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1- 11 A11 --- Field Field Name
13- 21 F9.5 deg RAdeg [247.6/247.9] Right Ascension (J2000)
23- 31 F9.6 deg DEdeg [12.6/12.9] Declination (J2000)
33- 37 F5.2 mag F606W [19.8/20.71] HST/ACS F606W band magnitude
in Camera/Filter catalog
39- 43 F5.2 mag F814W [19/20.3] HST/ACS F814W band magnitude
in Camera/Filter catalog
45- 50 F6.3 mas/yr pmRA [-1.3/-0.2] Gaia DR2 proper motion in RA
52- 56 F5.3 mas/yr e_pmRA [0.6/2.4] Uncertainty in pmRA
58- 63 F6.3 mas/yr pmDE [-1.7/-0.27] Gaia DR2 proper motion in DEC
65- 69 F5.3 mas/yr e_pmDE [0.5/2] Uncertainty in pmDE
71- 73 A3 --- Type Position on CMD (1)
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Note (1): Our proper motion members located in the West and West-WFC3 fields
are well within the red giant branch (RGB) locus while the remaining
two, located in Center-WFC3, are good RR Lyrae candidates.
See Section 4.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 20-Apr-2022