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Register a new userDynamics and Stellar Content of the Giant Southern Stream in M31. I. Keck Spectroscopy of Red Giant Stars
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Puragra GuhaThakurta
Publication date
2004
fields
Physics
and research's language is
English
Authors
Puragra Guhathakurta
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We present results from a large spectroscopic survey of M31 red giants using the Keck telescope/DEIMOS. Photometric pre-screening, based on the 100A-wide DDO51 band centered on the Mgb/MgH feature, was used to select spectroscopic targets. Red giant candidates were targeted in a small field on M31s giant southern tidal stream at a projected distance of 31kpc from the galaxy center. We isolate a clean sample of 68 giants by removing contaminants (foreground Galactic dwarfs and background galaxies) using spectroscopic, imaging, and photometric methods. About 65% of the M31 stars are found to be members of the stream, while the rest appear to be members of the general halo population. The mean (heliocentric) radial velocity of the stream in our field is -458 km/s, or -158 km/s relative to M31s systemic velocity, in good agreement with recent measurements at other stream locations. The intrinsic velocity dispersion of the stream is constrained to be 15_{-15}^{+8} km/s (90% confidence limits). The companion paper by Font et al. (2004, astro-ph/0406146) discusses possible orbits, implications of the coldness of the stream, and progenitor satellite properties. The kinematics (and perhaps [Fe/H] distribution) of our halo sample indicate that it is different from other M31 halo samples; this may be an indication of substructure in the halo. The stream seems to have a higher mean [Fe/H] than the halo, -0.51 vs -0.74 dex, and a smaller [Fe/H] spread. The streams high metallicity implies that its progenitor must have been a luminous dwarf galaxy. The CaII triplet strengths of the M31 giants are generally consistent with photometric estimates of their metallicity (derived by fitting RGB fiducials in the color-magnitude diagram). There is indirect evidence of intermediate-age stars in the stream.
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We examine the nature of the progenitor of the giant stellar stream in M31 using as constraints new radial velocity measurements of stream red giant stars (presented in the companion paper by Guhathakurta et al. 2005, astro-ph/0406145) along with other M31 data sets available in the literature. We find that the observations are best fit by orbits that are highly eccentric and close to edge-on, with apo- to peri-center ratios of order 25 - 30, and with apocenters at or only slightly beyond the southern edge of the current data. Among these orbits, we are able to find a few that plausibly connect the stream with the northern spur or with the low-surface-brightness feature of similar high metallicity as the stream (originally reported by Ferguson et al. 2002) to the east of M31s center. In the latter case, if the connection is real, then the eastern debris should lie well in front of M31 near the apocenter of the orbit. Both the width of the debris and velocity dispersion measurements imply a rough lower limit on the mass of the progenitor of 10^8 M_sun. We use this limit and our orbits to discuss which of M31s satellites could be plausibly associated with the stream. In addition, we predict that the width of the stream should increase beyond the southern edge of the current data around the apocenter of the orbit and that the line-of-sight velocity dispersion should exhibit significant variations along the stream.
We are using the DEIMOS multi-object spectrograph on the Keck II 10m telescope to conduct a spectroscopic survey of red giant branch stars in the outskirts of M31. To date, velocities have been obtained for most of the major substructures in the halo as well as at several positions in the far outer disk and inner halo. First results concerning the giant stellar stream and major axis substructures are presented here.
We analyze existing measurements of [Fe/H] and [$alpha$/Fe] for individual red giant branch (RGB) stars in the Giant Stellar Stream (GSS) of M31 to determine whether spatial abundance gradients are present. These measurements were obtained from low- ($R sim 3000$) and moderate- ($R sim 6000$) resolution Keck/DEIMOS spectroscopy using spectral synthesis techniques as part of the Elemental Abundances in M31 survey. From a sample of 62 RGB stars spanning the GSS at 17, 22, and 33 projected kpc, we measure a [Fe/H] gradient of $-$0.018 $pm$ 0.003 dex kpc$^{-1}$ and negligible [$alpha$/Fe] gradient with M31-centric radius. We investigate GSS abundance patterns in the outer halo using additional [Fe/H] and [$alpha$/Fe] measurements for 6 RGB stars located along the stream at 45 and 58 projected kpc. These abundances provide tentative evidence that the trends in [Fe/H] and [$alpha$/Fe] beyond 40 kpc in the GSS are consistent with those within 33 kpc. We also compare the GSS abundances to 65 RGB stars located along the possibly related Southeast (SE) shelf substructure at 12 and 18 projected kpc. The abundances of the GSS and SE shelf are consistent, supporting a common origin hypothesis, although this interpretation may be complicated by the presence of [Fe/H] gradients in the GSS. We discuss the abundance patterns in the context of photometric studies from the literature and explore implications for the properties of the GSS progenitor, suggesting that the high $langle$[$alpha$/Fe]$rangle$ of the GSS (+0.40 $pm$ 0.05 dex) favors a major merger scenario for its formation.
In the fourth paper of this series, we present the metallicity-dependent Sloan Digital Sky Survey (SDSS) stellar color loci of red giant stars, using a spectroscopic sample of red giants in the SDSS Stripe 82 region. The stars span a range of 0.55 -- 1.2 mag in color g-i, -0.3 -- -2.5 in metallicity [Fe/H], and have values of surface gravity log g smaller than 3.5 dex. As in the case of main-sequence (MS) stars, the intrinsic widths of loci of red giants are also found to be quite narrow, a few mmag at maximum. There are however systematic differences between the metallicity-dependent stellar loci of red giants and MS stars. The colors of red giants are less sensitive to metallicity than those of MS stars. With good photometry, photometric metallicities of red giants can be reliably determined by fitting the u-g, g-r, r-i, and i-z colors simultaneously to an accuracy of 0.2 -- 0.25 dex, comparable to the precision achievable with low-resolution spectroscopy for a signal-to-noise ratio of 10. By comparing fitting results to the stellar loci of red giants and MS stars, we propose a new technique to discriminate between red giants and MS stars based on the SDSS photometry. The technique achieves completeness of ~ 70 per cent and efficiency of ~ 80 per cent in selecting metal-poor red giant stars of [Fe/H] $le$ -1.2. It thus provides an important tool to probe the structure and assemblage history of the Galactic halo using red giant stars.
We present spectroscopic observations of red giant branch (RGB) stars in the Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck II 10-m telescope. The three fields targeted in this study are in the M31 spheroid, outer disk, and giant southern stream. In this paper, we focus on the kinematics and chemical composition of RGB stars in the stream field located at a projected distance of R = 20 kpc from M31s center. A mix of stellar populations is found in this field. M31 RGB stars are isolated from Milky Way dwarf star contaminants using a variety of spectral and photometric diagnostics. The radial velocity distribution of RGB stars displays a clear bimodality -- a primary peak centered at v = -513 km/s and a secondary one at v = -417 km/s -- along with an underlying broad component that is presumably representative of the smooth spheroid of M31. Both peaks are found to be dynamically cold with intrinsic velocity dispersions of sigma(v) = 16 km/s. The mean metallicity and metallicity dispersion of stars in the two peaks is also found to be similar: [Fe/H] = -0.45 and sigma([Fe/H]) = 0.2. The observed velocity of the primary peak is consistent with that predicted by dynamical models for the stream, but there is no obvious explanation for the secondary peak. The nature of the secondary cold population is unclear: it may represent: (1) tidal debris from a satellite merger event that is superimposed on, but unrelated to, the giant southern stream; (2) a wrapped around component of the giant southern stream; (3) a warp or overdensity in M31s disk at R > 50 kpc (this component is well above the outward extrapolation of the smooth exponential disk brightness profile).
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