The projected structures and integrated properties of the Andromeda I, II, III, V, VI, VII and Cetus dwarf spheroidal galaxies are analysed based upon resolved counts of red giant branch stars. The ...observations were taken as part of the Isaac Newton Telescope Wide Field Survey of M31 and its environs. For each object, we have derived isopleth maps, surface brightness profiles, intensity-weighted centres, position angles, ellipticities, tidal radii, core radii, concentration parameters, exponential scalelengths, Plummer scalelengths, half-light radii, absolute magnitudes and central surface brightnesses. Our analysis probes into larger radius and fainter surface brightnesses than most previous studies, and as a result we find that the galaxies are generally larger and brighter than has previously been recognized. In particular, the luminosity of Andromeda V is found to be consistent with the higher metallicity value which has been derived for it. We find that exponential and Plummer profiles provide adequate fits to the surface brightness profiles, although the more general King models provide the best formal fits. Andromeda I shows strong evidence of tidal disruption and S-shaped tidal tails are clearly visible. On the other hand, Cetus does not show any evidence of tidal truncation, let alone disruption, which is perhaps unsurprising given its isolated location. Andromeda II shows compelling evidence of a large excess of stars at small radius and suggests that this galaxy consists of a secondary core component, in analogy with recent results for Sculptor and Sextans. Comparing the M31 dwarf spheroidal population with the Galactic population, we find that the scaleradii of the M31 population are larger than those for the Galactic population by at least a factor of 2, for all absolute magnitudes. This difference is either due to environmental factors or due to orbital properties, suggesting that the ensemble average tidal field experienced by the M31 dwarf spheroidals is weaker than that experienced by the Galactic dwarf spheroidals. We find that the two populations are offset from one another in the central surface brightness — luminosity relation, which is probably related to this difference in their scale sizes. Finally, we find that the M31 dwarf spheroidals show the same correlation with distance from host as shown by the Galactic population, such that dwarf spheroidals with a higher central surface brightness are found further from their host. This again suggests that environment plays a significant role in dwarf galaxy evolution, and requires detailed modelling to explain the origin of this result.
We have obtained Johnson V and Gunn i photometry for a large number of Local Group galaxies using the Isaac Newton Telescope Wide Field Camera (INT WFC). The majority of these galaxies are members of ...the M31 subgroup and the observations are deep enough to study the top few magnitudes of the red giant branch in each system. We previously measured the location of the tip of the red giant branch (TRGB) for Andromeda I, Andromeda II and M33 to within systematic uncertainties of typically <0.05 mag. As the TRGB acts as a standard candle in old, metal-poor stellar populations, we were able to derive distances to each of these galaxies. Here we derive TRGB distances to the giant spiral galaxy M31 and 13 additional dwarf galaxies – NGC 205, 185, 147, Pegasus, WLM, LGS3, Cetus, Aquarius, And III, V, VI, VII and the newly discovered dwarf spheroidal And IX. The observations for each of the dwarf galaxies were intentionally taken in photometric conditions. In addition to the distances, we also self-consistently derive the median metallicity of each system from the colour of their red giant branches. This allows us to take into account the small metallicity variation of the absolute I magnitude of the TRGB. The homogeneous nature of our data and the identical analysis applied to each of the 17 Local Group galaxies ensures that these estimates form a reliable set of distance and metallicity determinations that are ideal for comparative studies of Local Group galaxy properties.
We present a deep photometric survey of the Andromeda galaxy, conducted with the wide-field cameras of CFHT and INT, that covers the inner 50 kpc of the galaxy and the southern quadrant out to ...similar to 150 kpc and includes an extension to M33 at >200 kpc. This is the first systematic panoramic study of this very outermost region of galaxies. We detect a multitude of large-scale structures of low surface brightness, including several streams, and two new relatively luminous (M sub(v) similar to -9) dwarf galaxies: And XV and And XVI. Significant variations in stellar populations due to intervening stream-like structures are detected in the inner halo, which is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Underlying the many substructures lies a faint, smooth, and extremely extended halo component, reaching out to 150 kpc, whose stellar populations are predominantly metal-poor. We find that the smooth halo component in M31 has a radially decreasing profile that can be fitted with a Hernquist model of immense scale radius similar to 55 kpc, almost 4 times larger than theoretical predictions. Alternatively a power law with capital sigma sub(v) alpha R super(-1.01 plus or minus 0.11) can be fitted to the projected profile, similar to the density profile in the Milky Way. If it is symmetric, the total luminosity of this structure is similar to 10 super(9) L unk, again similar to the stellar halo of the Milky Way. This vast, smooth, underlying halo is reminiscent of a classical "monolithic" model and completely unexpected from modern galaxy formation models. M33 is also found to have an extended metal-poor halo component, which can be fitted with a Hernquist model also of scale radius similar to 55 kpc. These extended slowly decreasing halos will provide a challenge and strong constraints for further modeling.
We undertake an investigation into the spatial structure of the M31 satellite system utilizing the distance distributions presented in a previous publication. These distances make use of the unique ...combination of depth and spatial coverage of the Pan-Andromeda Archaeological Survey to provide a large, homogeneous sample consisting of 27 of M31's satellites, as well as M31 itself. We find that the satellite distribution, when viewed as a whole, is no more planar than one would expect from a random distribution of equal size. A disk consisting of 15 of the satellites is however found to be highly significant, and strikingly thin, with an rms thickness of just (ProQuest: Formulae and/or non-USASCII text omitted) kpc. This disk is oriented approximately edge-on with respect to the Milky Way and almost perpendicular to the Milky Way disk. It is also roughly orthogonal to the disk-like structure regularly reported for the Milky Way satellite system and in close alignment with M31's Giant Stellar Stream. A similar analysis of the asymmetry of the M31 satellite distribution finds that it is also significantly larger than one would expect from a random distribution. In particular, it is remarkable that 20 of the 27 satellites most likely lie on the Milky Way side of the galaxy, with the asymmetry being most pronounced within the satellite subset forming the aforementioned disk. This lopsidedness is all the more intriguing in light of the apparent orthogonality observed between the satellite disk structures of the Milky Way and M31.
In "A Bayesian Approach to Locating the Red Giant Branch Tip Magnitude (Part I)," a new technique was introduced for obtaining distances using the tip of the red giant branch (TRGB) standard candle. ...Here we describe a useful complement to the technique with the potential to further reduce the uncertainty in our distance measurements by incorporating a matched-filter weighting scheme into the model likelihood calculations. In this scheme, stars are weighted according to their probability of being true object members. We then re-test our modified algorithm using random-realization artificial data to verify the validity of the generated posterior probability distributions (PPDs) and proceed to apply the algorithm to the satellite system of M31, culminating in a three-dimensional view of the system. Finally, a preliminary investigation into the satellite density distribution within the halo is made using the obtained distance distributions. For simplicity, this investigation assumes a single power law for the density as a function of radius, with the slope of this power law examined for several subsets of the entire satellite sample.
ABSTRACT
We compare and contrast the stellar structures of isolated Local Group dwarf galaxies, as traced by their oldest stellar populations, with the satellite dwarf galaxies of the Milky Way and M ...31. All Local Group dwarfs with Mv ≤ −6 and μo < 26.5 mag arcsec−2 are considered, taking advantage of measurements from surveys that use similar observations and analysis techniques. For the isolated dwarfs, we use the results from Solitary Local (Solo) Dwarf Galaxy Survey. We begin by confirming that the structural and dynamical properties of the two satellite populations are not obviously statistically different from each other, but we note that there many more satellites around M 31 than around the Milky Way down to equivalent magnitude and surface brightness limits. We find that dwarfs in close proximity to a massive galaxy generally show more scatter in their Kormendy relations than those in isolation. Specifically, isolated Local Group dwarf galaxies show a tighter trend of half-light radius versus magnitude than the satellite populations, and similar effects are also seen for related parameters. There appears to be a transition in the structural and dynamical properties of the dwarf galaxy population around ∼400 kpc from the Milky Way and M 31, such that the smallest, faintest, and most circular dwarf galaxies are found closer than this separation. We discuss the impact of selection effects on our analysis, and we argue that our results point to the significance of tidal interactions on the population of systems within approximately 400 kpc from the Milky Way and M 31.
Carbon-enhanced metal-poor (CEMP) stars comprise a high percentage of stars at the lowest metallicities. The stars in the CEMP-no subcategory do not show any s-process enhancement and therefore ...cannot easily be explained by transfer of carbon and s-process elements from a binary AGB companion. We have performed radial velocity monitoring of a sample of 22 CEMP-no stars to further study the role that binarity plays in this type of CEMP star. We find four new binary CEMP-no stars based on their radial velocity variations; this significantly enlarges the population of known binaries to a total of 11. One of the new stars found to be in a binary system is HE 0107–5240, which is one of the most iron-poor stars known. This supports the binary transfer model for the origin of the abundance pattern of this star. We find a difference in binary fraction in our sample that depends on the absolute carbon abundance, with a binary fraction of 47 +15−14% 47 − 14 + 15 % $ 47^{+15}_{-14}\% $ for stars with a higher absolute carbon abundance and 18 +14−9% 18 − 9 + 14 % $ 18^{+14}_{-9}\% $ for stars with a lower absolute carbon abundance. This might imply a relation between a high carbon abundance and the binarity of a metal-poor star. Although binarity does not equate to mass transfer, there is a possibility that a CEMP-no star in a binary system has been polluted, and care has to be taken in the interpretation of their abundance patterns. We furthermore demonstrate the potential of Gaia of discovering additional binary candidates.
ABSTRACT
We present an analysis of the proper motion of the Andromeda galaxy (M31), based on the Early Third Data Release of the Gaia mission. We use the Gaia photometry to select young blue ...main-sequence stars and apply several quality cuts to obtain clean samples of these tracers. After correcting the proper motion measurements for the internal rotation of the M31 disc motion, we derive an apparent motion of $52.5 \pm 5.8{\rm \, \mu as\,yr^{-1}}$ with respect to the Gaia reference frame, or $61.9 \pm 9.7{\rm \, \mu as\,yr^{-1}}$ after applying a zero-point correction determined from quasars within 20° from M31 and a correction from systemic biases. Accounting for the Solar reflex motion, we deduce a relative velocity between Andromeda and the Milky way (in a non-rotating frame at the current location of the Sun) of $42.2 \pm 39.3 {\rm \, km\, s^{-1}}$ along right ascension ($40.0 \pm 39.3 {\rm \, km\, s^{-1}}$ along galactic longitude) and $-59.4 \pm 30.3 {\rm \, km\, s^{-1}}$ along declination ($-60.9 \pm 30.3 {\rm \, km\, s^{-1}}$ along galactic latitude), with a total transverse velocity of $V_{\rm trans} = 82.4 \pm 31.2 {\rm \, km\, s^{-1}}$. These values are consistent with (but more accurate than) earlier Hubble Space Telescope measurements that predict a future merger between the two galaxies. We also note a surprisingly large difference in the derived proper motion between the blue stars in M31 and samples of red stars that appear to lie in that galaxy. We propose several hypotheses to explain the discrepancy but found no clear evidence with the current data to privilege any one of them.
We present a renewed look at M31's giant stellar stream along with the nearby structures streams C and D, exploiting a new algorithm capable of fitting to the red giant branch (RGB) of a structure in ...both colour and magnitude space. Using this algorithm, we are able to generate probability distributions in distance, metallicity and RGB width for a series of subfields spanning these structures. Specifically, we confirm a distance gradient of approximately 20 kpc per degree along a 6 deg extension of the giant stellar stream, with the farthest subfields from M31 lying ∼120 kpc more distant than the innermost subfields. Further, we find a metallicity that steadily increases from
$-0.7^{+0.1}_{-0.1}$
to
$-0.2^{+0.2}_{-0.1}$
dex along the inner half of the stream before steadily dropping to a value of
$-1.0^{+0.2}_{-0.2}$
dex at the farthest reaches of our coverage. The RGB width is found to increase rapidly from
$0.4^{+0.1}_{-0.1}$
to
$1.1^{+0.2}_{-0.1}$
dex in the inner portion of the stream before plateauing and decreasing marginally in the outer subfields of the stream. In addition, we estimate stream C to lie at a distance between 794 and 862 kpc and stream D between 758 and 868 kpc. We estimate the median metallicity of stream C to lie in the range −0.7 to −1.6 dex and a metallicity of
$-1.1^{+0.3}_{-0.2}$
dex for stream D. RGB widths for the two structures are estimated to lie in the range 0.4–1.2 dex and 0.3–0.7 dex, respectively. In total, measurements are obtained for 19 subfields along the giant stellar stream, four along stream C, five along stream D and three general M31 spheroid fields for comparison. We thus provide a higher resolution coverage of the structures in these parameters than has previously been available in the literature.
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