J/MNRAS/439/788 Stromgren and Washington photometry in Boo I (Hughes+, 2014)
Photometric metallicities in Bootes I.
Hughes J., Wallerstein G., Dotter A., Geisler D.
<Mon. Not. R. Astron. Soc., 439, 788-809 (2014)>
=2014MNRAS.439..788H 2014MNRAS.439..788H
ADC_Keywords: Galaxies, nearby ; Photometry, CMT1T2V ; Photometry, ugriz
Keywords: galaxies: abundances - galaxies: dwarf - Local Group
Abstract:
We present new Stromgren and Washington data sets for the Bootes I
dwarf galaxy, and combine them with the available Sloan Digital Sky
Survey photometry. The goal of this project is to refine a
ground-based, practical, accurate method to determine age and
metallicity for individual stars in Bootes I that can be selected in
an unbiased imaging survey, without having to take spectra. With few
bright upper red giant branch stars and distances of about 35-250kpc,
the ultra-faint dwarf galaxies (UDFs) present observational challenges
in characterizing their stellar population. Other recent studies have
produced spectra and proper motions, making Bootes I an ideal test
case for our photometric methods. We produce photometric metallicities
from Stromgren and Washington photometry, for stellar systems with a
range of -1.0>[Fe/H]>-3.5. Needing to avoid the collapse of the
metallicity sensitivity of the Stromgren m1-index on the lower red
giant branch, we replace the Stromgren v filter with the broader
Washington C filter to minimize observing time. We construct two
indices: m*=(C-T1)0-(T1-T2)0 and m**=(C-b)0-(b-y)0. We find that CT1by
is the most successful filter combination, for individual stars with
[Fe/H] < -2.0, to maintain ∼0.2dex [Fe/H]-resolution over the whole
red giant branch. The m**-index would be the best choice for
space-based observations because the (C-y) colour is not sufficient to
fix metallicity alone in an understudied system. Our photometric
metallicites of stars in the central regions of Bootes I confirm that
there is a metallicity spread of at least -1.9>[Fe/H]>-3.7. The
best-fitting Dartmouth isochrones give a mean age, for all the Bootes
I stars in our data set, of 11.5±0.4Gyr. From ground-based
telescopes, we show that the optimal filter combination is CT1by,
avoiding the v filter entirely. We demonstrate that we can break the
isochrones' age-metallicity degeneracy with the CT1by filters, using
stars with logg=2.5-3.0, which have less than a 2percent change in
their (C-T1) colour due to age, over a range of 10-14Gyr.
Description:
As in HWB (Hughes et al., 2008AJ....136.2321H 2008AJ....136.2321H, Cat. J/AJ/136/2321), we
observed the same central field in Bootes I (RA=14:00:06, DE=14.5
J2000) with the Apache Point Observatory's 3.5-m telescope, using the
direct imaging SPIcam system. The detector is a backside-illuminated
SITe TK2048E 2048x2048 pixel CCD with 24 micron pixels, which we
binned (2x2), giving a plate scale of 0.28arcsec/pix, and a field of
view (FOV) of 4.78x4.78arcmin2. The HWB data set for Bootes I was
taken on 2007 March 19 (with a comparison field in M92 taken on 2007
May 24). We took 21 frames in Washington C, and Cousins R and I
filters, with exposure time ranging from 1 to 1000s.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table3.dat 125 120 Objects in Bootes I with Washington and
Stroemgren photometry
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See also:
J/AJ/136/2321 : Washington photometry of Bootes I stars (Hughes+, 2008)
Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1 I1 --- Code [1/3] Code (1)
3- 6 A4 --- --- [HWB-]
7- 9 I3 --- HWB [2/163]? Hughes et al. name (see J/AJ/136/2321)
<[HWB2008] NNN> in Simbad
10 A1 --- --- [/]
11- 14 A4 --- --- [Boo-]
15- 18 I4 --- BooI ? Other name, <[NWG2010] BooI NNNN> in Simbad
20- 25 F6.2 pix Xpos ?=- X position from the Fig. 1
27- 32 F6.2 pix Ypos ?=- Y position from the Fig. 1
34- 35 I2 h RAh Right ascension (J2000)
37- 38 I2 min RAm Right ascension (J2000)
40- 44 F5.2 s RAs Right ascension (J2000)
46 A1 --- DE- Declination sign (J2000)
47- 48 I2 deg DEd Declination (J2000)
50- 51 I2 arcmin DEm Declination (J2000)
53- 56 F4.1 arcsec DEs Declination (J2000)
58 A1 --- Mm [A-F] HWB's photometric membership class (2)
60- 64 F5.2 mag T1mag [14/24] Washington T1 magnitude
66- 69 F4.2 mag e_T1mag [0.01/0.12] rms uncertainty on T1mag
71- 75 F5.2 mag C-T1 Washington C-T1 colour index
77- 80 F4.2 mag e_C-T1 rms uncertainty on C-T1
82- 86 F5.2 mag T1-T2 Washington T1-T2 colour index
88- 91 F4.2 mag e_T1-T2 rms uncertainty on T1-T2
93- 97 F5.2 mag Vmag [14/24.7] Stroemgren V magnitude
99-103 F5.3 mag e_Vmag rms uncertainty on Vmag
105-109 F5.2 mag b-y Stroemgren b-y colour index
111-114 F4.2 mag e_b-y rms uncertainty on b-y
116-120 F5.2 mag m1 ?=- Stroemgren m1 index
122-125 F4.2 mag e_m1 ?=- rms uncertainty on m1
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Note (1): Code as follows:
1 = stars with radial velocity-confirmed membership
2 = stars with measurements on more than one night
3 = stars with measurements on one night but photometry taken from
median-filtered images
Note (2): HWB's object classes as follows:
A = If sources passed the statistical cleaning process, had the correct
colours and had photometry in all filters with uncertainties <0.05
B = Objects passed the cleaning program, but had uncertainties in all
filters ≥0.05
C = Passed the statistical cleaning process, had the correct colours and
A-type good photometry, but failed the comparison with the randomly
generated probability
D = Objects failed the statistical cleaning process (also had the right
colours but poor photometry)
E = Passed statistical cleaning but failed colour selection (according to HWB)
F = These objects failed both statistical cleaning and colour selection, and
tended to be well outside the CMD area of a metal-poor dwarf.
Usually bright foreground stars.
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History:
From electronic version of the journal
(End) Patricia Vannier [CDS] 15-Dec-2014