The distribution of stars in our IMACS photometry and corresponding color-magnitude diagram are presented in Figure 5.1, reaching ∼3 mag fainter than our previous DECam photometry at the same S/N levels. The stars within an elliptical radius of 2.055, equivalent to 3 half-light radii, of the center of Peg III are highlighted with black large dots. The stars outside the 3 half-light radii but within a circular radius of 2.055 are also highlighted with smaller black dots to take into account the large uncertainty of ellipticity derived in Section 4. The large red and blue dots in Figure 5.1 represent kinematically confirmed member and non-member stars respectively (see Section 5 for more details).
We constrain the heliocentric distance of Peg III using the luminosity of its HB and the fiducial HB track of a globular cluster. Since the absolute total luminosity of Peg III was estimated to be −4.1±0.5 in our previous work, the mean metallicity of the system is expected to be as low as [Fe/H]∼ −2.5 according to the mass-metallicity relation by Kirby et al. (2013). We note that the recent metallicity measurements of the MW satellite dwarf galaxies in the same luminosity range as Peg III, for example Psc II (h[Fe/H]i=−2.45±0.07; Kirby et al., 2015a) and Reticulum II (h[Fe/H]i=−2.65±0.07; Simon et al., 2015) are consis- tent with the mass-metallicity relation. Accordingly, we adopted the fiducial HB sequence of M 15, one of the most metal-poor globular clusters ([Fe/H]=−2.42), from Bernard et al. (2014). We converted the fiducial into the SDSS photometric system with the help of transformation equations and coefficients provided by Tonry et al. (2012). We took liter- ature values of E(B−V)=0.11 and (m−M)0= 15.25 (Kraft & Ivans, 2003) to obtain the
reddening-corrected fiducial HB sequence. We then expressed this fiducial HB sequence as a function of color (g−r)0by means of a fifth order polynomial regression, fit this function
to the six BHB candidate stars in the blue polygon in the lower left panel of Figure 5.1 with the least-squares method, and derived a distance modulus of (m−M)0 = 21.66±0.12. For
the uncertainty in the final estimate of the distance modulus, we combined in quadrature the uncertainties associated with our calibration to our DECam photometry (<0.01 mag), the adopted distance modulus of M 15 (∼0.1 mag; Kraft & Ivans, 2003), our fiducial HB sequence fit (0.03 mag, determined by jackknife resampling), and galactic reddening in the r band (< 0.01 mag). In addition, we took into account the systematic uncertainty associated with the metallicity-luminosity relation, for which our estimate is ∼0.05 mag. In the middle panel, a Dartmouth isochrone (Dotter et al., 2008) for age 13.5 Gyr, [Fe/H]=−2.5, and [α/Fe]=+0.4 (solid curve), an isochrone from the same set but for age 12 Gyr, [Fe/H]=−1.1, [α/Fe]=+0.2 (dashed curve), and the M 15 fiducial HB sequence are plotted on the CMD at a distance modulus of (m−M)0 = 21.66, or a heliocentric
distance of 215 kpc. The three kinematic member stars in the red polygon appear sys- tematically brighter than the blue horizontal branch (BHB) and bluer than the red giant
Chapter 5 A PHOTOMETRIC AND SPECTROSCOPIC STUDY OF THE
ULTRA-FAINT MILKY WAY SATELLITE PEGASUS III 65
Figure 5.3 Convolved density contour map of Peg III candidate stars that pass the photo- metric filter illustrated in the lower left panel of Figure 5.1. The density map was binned with a pixel size of 10.000 and smoothed with a Gaussian kernel with FWHM of 23.006. The contours mark the levels of star density in units of the standard deviation above the back- ground (median value). The white dotted ellipse represents 3 half-light radii of the center of Peg III. The left and right arrows point to the nearby outer halo satellite Psc II and the Galactic Center, respectively.
branch (RGB). An excess of such stars relative to the RGB has been noticed in the CMD of the Hercules dwarf galaxy (e.g., Figure 2 in Sand et al., 2009) and the majority of them has been identified as its asymptotic giant branch (AGB) or red horizontal branch (RHB) population by photometric and spectroscopic studies (e.g., Ad´en et al., 2009; Fabrizio et al., 2014). Most likely the three Peg III member stars in the red polygon are AGB/RHB stars, too. Three of the other four kinematic members of Peg III are consistent with the red giant branch (RGB) while the last one, star#2, is almost 0.1 mag redder. That color difference cannot be explained by photometric uncertainties alone. There are different factors that can cause a color spread in the RGB, including dispersions in metallicity and carbon abundances. The metallicity of stars in MW satellite dwarf galaxies with similar total luminosities to Peg III ranges largely from [Fe/H]=−3.5 up to [Fe/H]=−1.0 dex (e.g., Ursa Major II; Vargas et al., 2013). The red star #2 of Peg III can be indeed fitted with an isochrone for a higher metallicity of [Fe/H]=−1.1 (dashed curve in the middle panel) at the same distance modulus. Carbon stars ([Ca/Fe]+1.0) in dwarf galaxies also tend to be redder than carbon-normal RGB stars due to the Bond-Neff effect (Bond & Neff, 1969), as shown for instance in Figure 7 of Kirby et al. (2015c). The possibility of a metallicity and carbon spread among the Peg III stars can be tested once the information on the
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Chapter 5 A PHOTOMETRIC AND SPECTROSCOPIC STUDY OF THE ULTRA-FAINT MILKY WAY SATELLITE PEGASUS III
chemical abundances of the individual stars becomes available. The low signal-to-noise of our spectra does not permit a detailed analysis for the chemical abundances of the individual targets.
We present a background-subtracted Hess diagram in Figure 5.2, which allows us to qual- itatively assess the stellar population of Peg III by means of model isochrone fitting for different properties. The Hess diagram was constructed based on the CMD of all stars within the radial distance of 2.055 and subtracting a control CMD of all stars in an equal area outside 4.00. We overplot Dartmouth isochrones with different ages (8,10,12,13.5 Gyr) and metallicity [F/H] values (-2.5, -2.0, -1.5 dex). The [α/Fe] values for the isochrones are determined based on the [Fe/H]-[α/Fe] relation from Vargas et al. (2013). The dis- tance modulus is fixed at 21.66 mag. The most notable difference among the isochrone fits is found in the main-sequence turnoff region, where the isochrones for metal poor ([Fe/H].−2.0) and old (&12 Gyr) stellar populations appear to be most consistent. This suggests that Peg III shares similar properties, i.e. low metallicities and old ages, of stellar populations with previously known ultra-faint MW satellite dwarf galaxies (e.g., Brown et al., 2014, and references therein).