Stellar Populations in the Outskirts of the Small Magellanic Cloud: No Outer Edge Yet

We report the detection of intermediate-age and old stars belonging to the SMC at 6.5 kpc from the SMC center in the southern direction. We show, from the analysis of three high quality 34\arcmin $\times$ 33\arcmin CMDs, that the age composition of the stellar population is similar at galactocentric distances of $\thicksim$4.7 kpc, $\thicksim$5.6 kpc, and $\thicksim$6.5 kpc. The surface brightness profile of the SMC follows an exponential law, with no evidence of truncation, all the way out to 6.5 kpc. These results, taken together, suggest that the SMC `disk' population is dominating over a possible old Milky Way-like stellar halo, and that the SMC may be significantly larger than previously thought.Comment: Accepted for publication in ApJ Letters. High resolution figures are available at ftp://ftp.iac.es/out/noe

The proper motion of the Magellanic Clouds, I: first results and description of the program

We present the first results of a ground-based program to determine the proper motion of the Magellanic Clouds (MCs) relative to background quasars (QSO), being carried out using the Iréneé du Pont 2.5 m telescope at Las Campanas Observatory, Chile. Eleven QSO fields have been targeted in the Small Magellanic Cloud (SMC) over a time base of six years, and with seven epochs of observation. One quasar field was targeted in the Large Magellanic Cloud (LMC), over a time base of five years, and with six epochs of observation. The shorter time base in the case of the LMC is compensated by the much larger amount of high-quality astrometry frames that could be secured for the LMC quasar field (124 frames), compared to the SMC fields (an average of roughly 45 frames). In this paper, we present final results for field Q0557-6713 in the LMC and field Q0036-7227 in the SMC. From field Q0557-6713, we have obtained a measured proper motion of μαcos δ = +1.95 ± 0.13 mas yr-1, μδ = +0.43 ± 0.18 mas yr-1 for the LMC. From field Q0036-7227, we have obtained a measured proper motion of μα cosδ = +0.95 ± 0.29 mas yr-1, μδ = -1.14 ± 0.18 mas yr-1 for the SMC. Although we went through the full procedure for another SMC field (QJ0036-7225), on account of unsolvable astrometric difficulties caused by blending of the QSO image, it was impossible to derive a reliable proper motion. Current model rotation curves for the plane of the LMC indicate that the rotational velocity (V rot) at the position of LMC field Q0557-6713 can be as low as 50 km s-1, or as high as 120 km s-1. A correction for perspective and rotation effects leads to a center of mass proper motion for the LMC of μα cosδ = +1.82 ± 0.13 mas yr-1, μδ = +0.39 ± 0.15 mas yr-1 (V rot = 50 km s-1), and to μα cosδ = +1.61 ± 0.13 mas yr-1, μδ = +0.60 ± 0.15 mas yr-1 (V rot = 120 km s-1). Assuming that the SMC has a disk-like central structure, but that it does not rotate, we obtain a center of mass proper motion for the SMC of μα cosδ = +1.03 ± 0.29 mas yr-1, μδ = -1.09 ± 0.18 mas yr-1. Our results are in reasonable agreement with most previous determinations of the proper motion of the MCs, including recent Hubble Space Telescope measurements. Complemented with published values of the radial velocity of the centers of the LMC and SMC, we have used our proper motions to derive the galactocentric (gc) velocity components of the MCs. For the LMC, we obtain V gc,t = +315 ± 20 km s-1, V gc,r = +86 ± 17 km s-1 (V rot = 50 km s-1), and V gc,t = +280 ± 24 km s-1, V gc,r = +94 ± 17 km s-1 (V rot = 120 km s-1). For the SMC, we obtain V gc,t = +258 ± 50 km s-1, V gc,r = +20 ± 44 km s-1. These velocities imply a relative velocity between the LMC and SMC of 84 ± 50 km s-1, for V rot,LMC = 50 km s-1, and 62 ± 63 km s-1 for V rot,LMC = 120 km s-1. Albeit our large errors, these values are not inconsistent with the standard assumption that the MCs are gravitationally bound to each other.Fil: Costa, Edgardo. Universidad de Chile; ChileFil: Méndez, René A.. Universidad de Chile; ChileFil: Pedreros, Mario H.. Universidad de Tarapaca; ChileFil: Moyano, Maximiliano. Universidad de Chile; ChileFil: Gallart, Carme. Instituto de Astrofísica de Canarias; EspañaFil: Noël, Noelia. Instituto de Astrofísica de Canarias; EspañaFil: Baume, Gustavo Luis. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; ArgentinaFil: Carraro, Giovanni. European Southern Observatory; Chil

The Chemical Enrichment History of the Small Magellanic Cloud and Its Gradients

We present stellar metallicities derived from Ca II triplet spectroscopy in over 350 red giant branch stars in 13 fields distributed in different positions in the SMC, ranging from $\sim$1\arcdeg\@ to $\sim$4\arcdeg\@ from its center. In the innermost fields the average metallicity is [Fe/H] $\sim -1$. This value decreases when we move away towards outermost regions. This is the first detection of a metallicity gradient in this galaxy. We show that the metallicity gradient is related to an age gradient, in the sense that more metal-rich stars, which are also younger, are concentrated in the central regions of the galaxy.Comment: 30 pages, 13 figures, accepted for publication in Astronomical Journa

The proper motion of the Magellanic clouds. I. First results and description of the program

We present the first results of a ground-based program to determine the proper motion of the Magellanic Clouds (MCs) relative to background quasars (QSO), being carried out using the Iréneé du Pont 2.5 m telescope at Las Campanas Observatory, Chile. Eleven QSO fields have been targeted in the Small Magellanic Cloud (SMC) over a time base of six years, and with seven epochs of observation. One quasar field was targeted in the Large Magellanic Cloud (LMC), over a time base of five years, and with six epochs of observation. The shorter time base in the case of the LMC is compensated by the much larger amount of high-quality astrometry frames that could be secured for the LMC quasar field (124 frames), compared to the SMC fields (an average of roughly 45 frames). In this paper, we present final results for field Q0557-6713 in the LMC and field Q0036-7227 in the SMC. From field Q0557-6713, we have obtained a measured proper motion of μαcos δ = +1.95 ± 0.13 mas yr-1, μδ = +0.43 ± 0.18 mas yr-1 for the LMC. From field Q0036-7227, we have obtained a measured proper motion of μα cosδ = +0.95 ± 0.29 mas yr-1, μδ = -1.14 ± 0.18 mas yr -1 for the SMC. Although we went through the full procedure for another SMC field (QJ0036-7225), on account of unsolvable astrometric difficulties caused by blending of the QSO image, it was impossible to derive a reliable proper motion. Current model rotation curves for the plane of the LMC indicate that the rotational velocity (Vrot) at the position of LMC field Q0557-6713 can be as low as 50 km s-1, or as high as 120 km s-1. A correction for perspective and rotation effects leads to a center of mass proper motion for the LMC of μα cosδ = +1.82 ± 0.13 mas yr-1, μδ = +0.39 ± 0.15 mas yr-1 (Vrot = 50 km s-1), and to μα cosδ = +1.61 ± 0.13 mas yr-1, μδ = +0.60 ± 0.15 mas yr-1 (V rot = 120 km s-1). Assuming that the SMC has a disk-like central structure, but that it does not rotate, we obtain a center of mass proper motion for the SMC of μα cosδ = +1.03 ± 0.29 mas yr-1, μδ = -1.09 ± 0.18 mas yr-1. Our results are in reasonable agreement with most previous determinations of the proper motion of the MCs, including recent Hubble Space Telescope measurements. Complemented with published values of the radial velocity of the centers of the LMC and SMC, we have used our proper motions to derive the galactocentric (gc) velocity components of the MCs. For the LMC, we obtain Vgc,t = +315 ± 20 km s-1, Vgc,r = +86 ± 17 km s-1 (Vrot = 50 km s-1), and Vgc,t = +280 ± 24 km s-1, Vgc,r = +94 ± 17 km s-1 (Vrot = 120 km s-1). For the SMC, we obtain Vgc,t = +258 ± 50 km s-1, V gc,r = +20 ± 44 km s-1. These velocities imply a relative velocity between the LMC and SMC of 84 ± 50 km s-1, for Vrot,LMC = 50 km s-1, and 62 ± 63 km s -1 for Vrot,LMC = 120 km s-1. Albeit our large errors, these values are not inconsistent with the standard assumption that the MCs are gravitationally bound to each other.Facultad de Ciencias Astronómicas y Geofísica

Old Main Sequence Turnoff Photometry in the Small Magellanic Cloud. II. Star Formation History and Its Spatial Gradients

We present a quantitative analysis of the SFH of 12 fields in the SMC. We find that there are four main periods of enhancement of star formation: a young one peaked at around 0.2-0.5 Gyr old, only present in the eastern and in the central-most fields; two at intermediate ages present in all fields (a conspicuous one peaked at 4-5 Gyr old, and a less significant one peaked at 1.5-2.5); and an old one, peaked at 10 Gyr in all fields but the western ones. In the western fields, this old enhancement splits into two, one peaked at around 8 Gyr old and another at around 12 Gyr old. This "two-enhancement" zone seems to be a robust feature since it is unaffected by our choice of stellar evolutionary library but more data covering other fields of the SMC are necessary in order to ascertain its significancy. Some correlation could exist with encounters taken from the orbit determination of Kallivayalil et al. (2006). But our results would be also fit in a first pericenter passage scenario like the one claimed by Besla et al. (2007). There is a strong dichotomy between East/Southeast and West in the current irregular shape of the SMC. We find that this dichotomy is produced by the youngest population and began about 1 Gyr ago or later. We do not find yet a region dominated by an old halo at 4.5 kpc from the SMC center, indicating either that this old stellar halo does not exist in the SMC or that its contribution to the stellar populations, at the galactocentric distances of our outermost field, is negligible. We derive the age-metallicity relation and find that the metallicity increased continuously from early epochs until now.Comment: Accepted for publication in AJ, 39 pages, 13 Postscript figures. High resolution available at: http://www.iac.es/galeria/noelia/PaperII_Figures/index.html or via email to: [email protected]

The proper motion of the Magellanic clouds. I. First results and description of the program

We present the first results of a ground-based program to determine the proper motion of the Magellanic Clouds (MCs) relative to background quasars (QSO), being carried out using the Iréneé du Pont 2.5 m telescope at Las Campanas Observatory, Chile. Eleven QSO fields have been targeted in the Small Magellanic Cloud (SMC) over a time base of six years, and with seven epochs of observation. One quasar field was targeted in the Large Magellanic Cloud (LMC), over a time base of five years, and with six epochs of observation. The shorter time base in the case of the LMC is compensated by the much larger amount of high-quality astrometry frames that could be secured for the LMC quasar field (124 frames), compared to the SMC fields (an average of roughly 45 frames). In this paper, we present final results for field Q0557-6713 in the LMC and field Q0036-7227 in the SMC. From field Q0557-6713, we have obtained a measured proper motion of μαcos δ = +1.95 ± 0.13 mas yr-1, μδ = +0.43 ± 0.18 mas yr-1 for the LMC. From field Q0036-7227, we have obtained a measured proper motion of μα cosδ = +0.95 ± 0.29 mas yr-1, μδ = -1.14 ± 0.18 mas yr -1 for the SMC. Although we went through the full procedure for another SMC field (QJ0036-7225), on account of unsolvable astrometric difficulties caused by blending of the QSO image, it was impossible to derive a reliable proper motion. Current model rotation curves for the plane of the LMC indicate that the rotational velocity (Vrot) at the position of LMC field Q0557-6713 can be as low as 50 km s-1, or as high as 120 km s-1. A correction for perspective and rotation effects leads to a center of mass proper motion for the LMC of μα cosδ = +1.82 ± 0.13 mas yr-1, μδ = +0.39 ± 0.15 mas yr-1 (Vrot = 50 km s-1), and to μα cosδ = +1.61 ± 0.13 mas yr-1, μδ = +0.60 ± 0.15 mas yr-1 (V rot = 120 km s-1). Assuming that the SMC has a disk-like central structure, but that it does not rotate, we obtain a center of mass proper motion for the SMC of μα cosδ = +1.03 ± 0.29 mas yr-1, μδ = -1.09 ± 0.18 mas yr-1. Our results are in reasonable agreement with most previous determinations of the proper motion of the MCs, including recent Hubble Space Telescope measurements. Complemented with published values of the radial velocity of the centers of the LMC and SMC, we have used our proper motions to derive the galactocentric (gc) velocity components of the MCs. For the LMC, we obtain Vgc,t = +315 ± 20 km s-1, Vgc,r = +86 ± 17 km s-1 (Vrot = 50 km s-1), and Vgc,t = +280 ± 24 km s-1, Vgc,r = +94 ± 17 km s-1 (Vrot = 120 km s-1). For the SMC, we obtain Vgc,t = +258 ± 50 km s-1, V gc,r = +20 ± 44 km s-1. These velocities imply a relative velocity between the LMC and SMC of 84 ± 50 km s-1, for Vrot,LMC = 50 km s-1, and 62 ± 63 km s -1 for Vrot,LMC = 120 km s-1. Albeit our large errors, these values are not inconsistent with the standard assumption that the MCs are gravitationally bound to each other.Facultad de Ciencias Astronómicas y Geofísica

EDGE: The direct link between mass growth history and the extended stellar haloes of the faintest dwarf galaxies

Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to the Milky Way and other massive spiral galaxies. As such, their projected stellar ellipticity and extended light distributions are often thought to owe to tidal forces. In this paper, we study the projected stellar ellipticities and faint stellar outskirts of tidally isolated ultra-faints drawn from the 'Engineering Dwarfs at Galaxy Formation's Edge' (EDGE) cosmological simulation suite. Despite their tidal isolation, our simulated dwarfs exhibit a wide range of projected ellipticities ($0.03 < \varepsilon < 0.85$), with many possessing anisotropic extended stellar haloes that mimic tidal tails, but owe instead to late-time accretion of lower mass companions. Furthermore, we find a strong causal relationship between ellipticity and formation time of an UFD, which is robust to a wide variation in the feedback model. We show that the distribution of projected ellipticities in our suite of simulated EDGE dwarfs matches well with that of 21 Local Group dwarf galaxies. Given the ellipticity in EDGE arises from an ex-situ accretion origin, the agreement in shape indicates the ellipticities of some observed dwarfs may also originate from a similar non-tidal scenario. The orbital parameters of these observed dwarfs further support that they are not currently tidally disrupting. If the baryonic content in these galaxies is still tidally intact, then the same may be true for their dark matter content, making these galaxies in our Local Group pristine laboratories for testing dark matter and galaxy formation models.Comment: 10 pages, 4 figures; submitted to MNRA

Slicing The Monoceros Overdensity with Suprime-Cam

We derive distance, density and metallicity distribution of the stellar Monoceros Overdensity (MO) in the outer Milky Way, based on deep imaging with the Subaru Telescope. We applied CMD fitting techniques in three stripes at galactic longitudes: l=130 deg, 150 deg, 170 deg; and galactic latitudes: +15 < b [deg] < +25 . The MO appears as a wall of stars at a heliocentric distance of ~ 10.1\pm0.5 kpc across the observed longitude range with no distance change. The MO stars are more metal rich ([Fe/H] ~ -1.0) than the nearby stars at the same latitude. These data are used to test three different models for the origin of the MO: a perturbed disc model, which predicts a significant drop in density adjacent to the MO that is not seen; a basic flared disc model, which can give a good match to the density profile but the MO metallicity implies the disc is too metal rich to source the MO stars; and a tidal stream model, which bracket the distances and densities we derive for the MO, suggesting that a model can be found that would fully fit the MO data. Further data and modeling will be required to confirm or rule out the MO feature as a stream or as a flaring of the disc.Comment: 15 pages, 12 figures, accepted for publication in Ap

EDGE: The direct link between mass growth history and the extended stellar haloes of the faintest dwarf galaxies

Ultra-faint dwarf galaxies (UFDs) are commonly found in close proximity to the Milky Way and other massive spiral galaxies. As such, their projected stellar ellipticity and extended light distributions are often thought to owe to tidal forces. In this paper, we study the projected stellar ellipticities and faint stellar outskirts of tidally isolated ultra-faints drawn from the ‘Engineering Dwarfs at Galaxy Formation’s Edge’ (EDGE) cosmological simulation suite. Despite their tidal isolation, our simulated dwarfs exhibit a wide range of projected ellipticities (0.03 &amp;lt; ε &amp;lt; 0.85), with many possessing anisotropic extended stellar haloes that mimic tidal tails, but owe instead to late-time accretion of lower mass companions. Furthermore, we find a strong causal relationship between ellipticity and formation time of a UFD, which is robust to a wide variation in the feedback model. We show that the distribution of projected ellipticities in our suite of simulated EDGE dwarfs matches well with a sample of 19 Local Group dwarf galaxies and a sample of 11 isolated dwarf galaxies. Given ellipticity in EDGE arises from an ex-situ accretion origin, the agreement in shape indicates the ellipticities of some observed dwarfs may also originate from a non-tidal scenario. The orbital parameters of these observed dwarfs further support that they are not currently tidally disrupting. If the baryonic content in these galaxies is still tidally intact, then the same may be true for their dark matter content, making these galaxies in our Local Group pristine laboratories for testing dark matter and galaxy formation models