Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userOn the Nature of Ultra-faint Dwarf Galaxy Candidates. III. Horologium I, Pictor I, Grus I, and Phoenix II
125
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We use deep Gemini/GMOS-S $g,r$ photometry to study the stellar populations of the recently discovered Milky Way satellite candidates Horologium I, Pictor I, Grus I, and Phoenix II. Horologium I is most likely an ultra-faint dwarf galaxy at $D_odot = 68pm3$ kpc, with $r_h = 23^{+4}_{-3}$pc and $langle $[Fe/H]$ rangle = -2.40^{+0.10}_{-0.35}$,dex. Its color-magnitude diagram shows evidence of a split sub-giant branch similar to that seen in some globular clusters. Additionally, Gaia DR2 data suggests it is, or was, a member of the Magellanic Cloud group. Pictor I with its compact size ($r_h = 12.9^{+0.3}_{-0.2}$pc) and metal-poor stellar population ($langle $[Fe/H]$ rangle = -2.28^{+0.30}_{-0.25}$) closely resembles confirmed star clusters. Grus I lacks a well-defined centre, but has two stellar concentrations within the reported half-light radius ($r_h = 1.77^{+0.85}_{-0.39}$ arcmin) and has a mean metallicity of $langle $[Fe/H]$ rangle = -2.5pm0.3$. Phoenix II has a half-light radius of $r_h = 12.6pm2.5$pc and an $langle $[Fe/H]$ rangle = -2.10^{+0.25}_{-0.20}$ and exhibits S-shaped tidal arms extending from its compact core. Great circles through each of these substructures intersect at the Large Magellanic Cloud (LMC). This suggests that these objects are, or once were, satellites of the LMC.
rate research
Read More
This work presents the first search for RR Lyrae stars (RRLs) in four of the ultra-faint systems imaged by the Dark Energy Survey (DES) using SOAR/Goodman and Blanco/DECam imagers. We have detected two RRLs in the field of Grus I, none in Kim 2, one in Phoenix II, and four in Grus II. With the detection of these stars, we accurately determine the distance moduli for these ultra-faint dwarf satellite galaxies; $mu_0$=20.51$pm$0.10 mag (D$_{odot}$=127$pm$6 kpc) for Grus I and $mu_0$=20.01$pm$0.10 mag (D$_{odot}$=100$pm$5 kpc) for Phoenix II. These measurements are larger than previous estimations by Koposov et al. 2015 and Bechtol et al. 2015, implying larger physical sizes; 5% for Grus I and 33% for Phoenix II. For Grus II, out of the four RRLs detected, one is consistent with being a member of the galactic halo (D$_odot$=24$pm$1 kpc, $mu_0$=16.86$pm$0.10 mag), another is at D$_odot$=55$pm$2 kpc ($mu_0$=18.71$pm$0.10 mag), which we associate with Grus II, and the two remaining at D$_odot$=43$pm$2 kpc ($mu_0$=18.17$pm$0.10 mag). Moreover, the appearance of a subtle red horizontal branch in the color-magnitude diagram of Grus II at the same brightness level of the latter two RRLs, which are at the same distance and in the same region, suggests that a more metal-rich system may be located in front of Grus II. The most plausible scenario is the association of these stars with the Chenab/Orphan Stream. Finally, we performed a comprehensive and updated analysis of the number of RRLs in dwarf galaxies. This allows us to predict that the method of finding new ultra-faint dwarf galaxies by using two or more clumped RRLs will work only for systems brighter than M$_Vsim-6$ mag.
We present deep $g$- and $r$-band Magellan/Megacam photometry of two dwarf galaxy candidates discovered in the Dark Energy Survey (DES), Grus I and Indus II (DES J2038-4609). For the case of Grus I, we resolved the main sequence turn-off (MSTO) and $sim 2$ mags below it. The MSTO can be seen at $g_0sim 24$ with a photometric uncertainty of $0.03$ mag. We show Grus I to be consistent with an old, metal-poor ($sim 13.3$ Gyr, [Fe/H]$sim-1.9$) dwarf galaxy. We derive updated distance and structural parameters for Grus I using this deep, uniform, wide-field data set. We find an azimuthally averaged half-light radius more than two times larger ($sim 151^{+21}_{-31}$ pc; $sim 4.^{prime} 16^{+0.54}_{-0.74}$) and an absolute $V$-band magnitude $sim-4.1$ that is $sim 1$ magnitude brighter than previous studies. We obtain updated distance, ellipticity, and centroid parameters which are in agreement with other studies within uncertainties. Although our photometry of Indus II is $sim 2-3$ magnitudes deeper than the DES Y1 Public release, we find no coherent stellar population at its reported location. The original detection was located in an incomplete region of sky in the DES Y2Q1 data set and was flagged due to potential blue horizontal branch member stars. The best fit isochrone parameters are physically inconsistent with both dwarf galaxies and globular clusters. We conclude that Indus II is likely a false-positive, flagged due to a chance alignment of stars along the line of sight.
We present chemical abundance measurements of two metal-poor red giant stars in the ultra-faint dwarf galaxy Bootes I, based on Magellan/MIKE high-resolution spectra. For Boo I-980, with [Fe/H]=-3.1, we present the first elemental abundance measurements while Boo I-127, with [Fe/H]=-2.0, shows abundances in good agreement with previous measurements. Light and iron-peak element abundance ratios in the two Bootes I stars, as well as those of most other Boootes I members, collected from the literature, closely resemble those of regular metal-poor halo stars. Neutron-capture element abundances Sr and Ba are systematically lower than the main halo trend, and also show a significant abundance spread. Overall, this is similar to what has been found for other ultra-faint dwarf galaxies. We apply corrections to the carbon abundances (commensurate with stellar evolutionary status) of the entire sample and find 21% of stars to be carbon-enhanced metal-poor (CEMP) stars, compared to 13% without using the carbon correction. We reassess the metallicity distribution functions (MDF) for the CEMP stars and non-CEMP stars, and confirm earlier claims that CEMP stars might belong to a different, earlier population. Applying a set of abundance criteria to test to what extent Bootes I could be a surviving first galaxy suggests that it is one of the earliest assembled systems that perhaps received gas from accretion from other clouds in the system, or from swallowing a first galaxy or building block type object. This resulted in the two stellar populations observable today.
We present chemical abundance measurements of three stars in the ultra-faint dwarf galaxy Horologium I, a Milky Way satellite discovered by the Dark Energy Survey. Using high resolution spectroscopic observations we measure the metallicity of the three stars as well as abundance ratios of several $alpha$-elements, iron-peak elements, and neutron-capture elements. The abundance pattern is relatively consistent among all three stars, which have a low average metallicity of [Fe/H] $sim -2.6$ and are not $alpha$-enhanced ([$alpha$/Fe] $sim 0.0$). This result is unexpected when compared to other low-metallicity stars in the Galactic halo and other ultra-faint dwarfs and hints at an entirely different mechanism for the enrichment of Hor I compared to other satellites. We discuss possible scenarios that could lead to this observed nucleosynthetic signature including extended star formation, a Population III supernova, and a possible association with the Large Magellanic Cloud.
We present a study of the ultra-faint Milky Way dwarf satellite galaxy Tucana II using deep photometry from the 1.3m SkyMapper telescope at Siding Spring Observatory, Australia. The SkyMapper filter-set contains a metallicity-sensitive intermediate-band $v$ filter covering the prominent Ca II K feature at 3933.7A. When combined with photometry from the SkyMapper $u, g$, and $i$ filters, we demonstrate that $v$ band photometry can be used to obtain stellar metallicities with a precision of $sim0.20$dex when [Fe/H] $> -2.5$, and $sim0.34$dex when [Fe/H] $< -2.5$. Since the $u$ and $v$ filters bracket the Balmer Jump at 3646A, we also find that the filter-set can be used to derive surface gravities. We thus derive photometric metallicities and surface gravities for all stars down to a magnitude of $gsim20$ within $sim$75 arcminutes of Tucana II. Photometric metallicity and surface gravity cuts remove nearly all foreground contamination. By incorporating Gaia proper motions, we derive quantitative membership probabilities which recover all known members on the red giant branch of Tucana II. Additionally, we identify multiple likely new members in the center of the system and candidate members several half-light radii from the center of the system. Finally, we present a metallicity distribution function derived from the photometric metallicities of likely Tucana II members. This result demonstrates the utility of wide-field imaging with the SkyMapper filter-set in studying UFDs, and in general, low surface brightness populations of metal-poor stars. Upcoming work will clarify the membership status of several distant stars identified as candidate members of Tucana II.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
