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Register a new userThe outer disc in shambles: blind detection of Monoceros and ACS with Gaias astrometric sample
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The astrometric sample of Gaia allows us to study the outermost Galactic disc, the halo and their interface. It is precisely at the very edge of the disc where the effects of external perturbations are expected to be the most noticeable. Our goal is to detect the kinematic substructure present in the halo and at the edge of the Milky Way (MW) disc, and provide observational constraints on their phase-space distribution. We download, one HEALpix at a time, the proper motion histogram of distant stars, to which we apply a Wavelet Transformation to reveal the significant overdensities. We then analyse the large coherent structures that appear in the sky. We reveal a sharp yet complex anticentre dominated by Monoceros (MNC) and the Anticentre Stream (ACS) in the north, which we find with an intensity comparable to the Magellanic clouds and the Sagittarius stream, and by MNC south and TriAnd at negative latitudes. Our method allows us to perform a morphological analysis of MNC and ACS, both spanning more than 100$^circ$ in longitude, and to provide a high purity sample of giants with which we track MNC down to latitudes as low as $sim$5$^circ$. Their colour-magnitude diagram is consistent with extended structures at a distance of $sim$10-11 kpc originated in the disc, with a very low ratio of RR Lyrae over M giants, and kinematics compatible with the rotation curve at those distances or only slightly slower. We present a precise characterisation of MNC and ACS, two previously known structures that our method reveals naturally, allowing us to detect them without limiting ourselves to a particular stellar type and, for the first time, using only kinematics. Our results allow future studies to model their chemo-dynamics and evolution, thus constraining some of the most influential processes that shaped the MW.
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We use detailed simulations of the Gaia observations of synthetic planetary systems and develop and utilize independent software codes in double-blind mode to analyze the data, including statistical tools for planet detection and different algorithms for single and multiple Keplerian orbit fitting that use no a priori knowledge of the true orbital parameters of the systems. 1) Planets with astrometric signatures $alphasimeq 3$ times the single-measurement error $sigma_psi$ and period $Pleq 5$ yr can be detected reliably, with a very small number of false positives. 2) At twice the detection limit, uncertainties in orbital parameters and masses are typically $15%-20%$. 3) Over 70% of two-planet systems with well-separated periods in the range $0.2leq Pleq 9$ yr, $2leqalpha/sigma_psileq 50$, and eccentricity $eleq 0.6$ are correctly identified. 4) Favorable orbital configurations have orbital elements measured to better than 10% accuracy $> 90%$ of the time, and the value of the mutual inclination angle determined with uncertainties $leq 10^{degr}$. 5) Finally, uncertainties obtained from the fitting procedures are a good estimate of the actual errors. Extrapolating from the present-day statistical properties of the exoplanet sample, the results imply that a Gaia with $sigma_psi$ = 8 $mu$as, in its unbiased and complete magnitude-limited census of planetary systems, will measure several thousand giant planets out to 3-4 AUs from stars within 200 pc, and will characterize hundreds of multiple-planet systems, including meaningful coplanarity tests. Finally, we put Gaia into context, identifying several areas of planetary-system science in which Gaia can be expected to have a relevant impact, when combined with data coming from other ongoing and future planet search programs.
The outer regions of disc galaxies are becoming increasingly recognized as key testing sites for models of disc assembly and evolution. Important issues are the epoch at which the bulk of the stars in these regions formed and how discs grow radially over time. To address these issues, we use Hubble Space Telescope Advanced Camera for Surveys imaging to study the star formation history (SFH) of two fields at 9.1 and 11.6 kpc along M33s northern major axis. These fields lie at ~ 4 and 5 V-band disc scale-lengths and straddle the break in M33s surface brightness profile. The colour-magnitude diagrams (CMDs) reach the ancient main sequence turnoff with a signal-to-noise ratio of ~ 5. From detailed modelling of the CMDs, we find that the majority of stars in both fields combined formed at z < 1. The mean age in the inner field, S1, is ~ 3 +/- 1 Gyr and the mean metallicity is [M/H] ~ -0.5 +/- 0.2 dex. The star formation history of S1 unambiguously reveals how the inside-out growth previously measured for M33s inner disc out to ~ 6 kpc extends out to the disc edge at ~ 9 kpc. In comparison, the outer field, S2, is older (mean age ~ 7 +/- 2 Gyr), more metal-poor (mean [M/H] ~ -0.8 +/- 0.3 dex), and contains ~ 30 times less stellar mass. These results provide the most compelling evidence yet that M33s age gradient reverses at large radii near the disc break and that this reversal is accompanied by a break in stellar mass surface density. We discuss several possible interpretations of this behaviour including radial stellar mixing, warping of the gaseous disc, a change in star formation efficiency, and a transition to another structural component. These results offer one of the most detailed views yet of the peripheral regions of any disc galaxy and provide a much-needed observational constraint on the last major epoch of star formation in the outer disc.
We present a detailed analysis of two fields located 26 kpc (~5 scalelengths) from the centre of M31. One field samples the major axis populations--the Outer Disc field--while the other is offset by ~18 and samples the Warp in the stellar disc. The CMDs based on HST/ACS imaging reach old main-sequence turn-offs (~12.5 Gyr). We apply the CMD-fitting technique to the Warp field to reconstruct the star formation history (SFH). We find that after undergoing roughly constant SF until about 4.5 Gyr ago, there was a rapid decline in activity and then a ~1.5 Gyr lull, followed by a strong burst lasting 1.5 Gyr and responsible for 25% of the total stellar mass in this field. This burst appears to be accompanied by a decline in metallicity which could be a signature of the inflow of metal-poor gas. The onset of the burst (~3 Gyr ago) corresponds to the last close passage of M31 and M33 as predicted by detailed N-body modelling, and may have been triggered by this event. We reprocess the deep M33 outer disc field data of Barker et al. (2011) in order to compare consistently-derived SFHs. This reveals a similar duration burst that is exactly coeval with that seen in the M31 Warp field, lending further support to the interaction hypothesis. The complex SFHs and the smoothly-varying age-metallicity relations suggest that the stellar populations observed in the far outer discs of both galaxies have largely formed in situ rather than migrated from smaller galactocentric radii. The strong differential reddening affecting the CMD of the Outer Disc field prevents derivation of the SFH. Instead, we quantify this reddening and find that the fine-scale distribution of dust precisely follows that of the HI gas. This indicates that the outer HI disc of M31 contains a substantial amount of dust and therefore suggests significant metal enrichment in these parts, consistent with inferences from our CMD analysis.
We present the peculiar in-plane velocities derived from the LAMOST red clump stars, which are purified and separated by a novel approach into two groups with different ages. The samples are mostly contributed around the Galactic anti-centre direction such that we are able to map the radial profiles of the radial and azimuthal velocities in the outer disc. From the variations of the in-plane velocities with the Galactocentric radius for the younger and older populations, we find that both radial and azimuthal velocities are not axisymmetric at $8<R<14,kpc$. The two red clump populations show that the mean radial velocity is negative within $Rsim9,kpc$ and positive beyond. This is likely because of the perturbation induced by the rotating bar. The cross-zero radius, $Rsim9$, kpc, essentially indicates the rough location of the outer Lindblad resonance (OLR) radius. Given the circular speed of 238,km$rm s^{-1}$, then the pattern speed of the bar can be approximated as $45$,km$rm s^{-1}rm kpc^{-1}$. The young red clump stars show larger mean radial velocity than the old population by about 3$,kmrm s^{-1}$ between $Rsim9$ and 12,kpc. This is possibly because the younger population is more sensitive to the perturbation than the older one. The radial profiles of the mean azimuthal velocity for the two populations show an interesting U-shape, i.e. at $R<10.5,kpc$, the azimuthal velocity declines with $R$ by about 10$,kmrm s^{-1}$, while at $R>10.5$ it increases with $R$ to 240-245$,kmrm s^{-1}$. It is not clear why the mean azimuthal velocity shows the U-shape along the Galactic anti-centre direction. Meanwhile, the azimuthal velocity for the younger population is slightly larger than the older one and the difference moderately declines with $R$. Beyond $Rsim12,kpc$, the azimuthal velocities for the two populations are indistinguishable.
Using Hubble Space Telescope (HST) ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations in the outskirts of NGC 2915, a blue compact dwarf with an extended HI disc. These observations reveal an elliptical distribution of red giant branch stars, and a clumpy distribution of main-sequence stars that correlate with the HI gas distribution. We constrain the upper-end initial mass function (IMF) and determine the star formation law (SFL) in this field, using the observed main-sequence stars and an assumed constant star formation rate. Previously published H{alpha} observations of the field, which show one faint HII region, are used to provide further constraints on the IMF. We find that the main-sequence luminosity function analysis alone results in a best-fitting IMF with a power-law slope {alpha}=-2.85 and upper-mass limit M$_rm{u}$ = 60 M$_odot$. However, if we assume that all H{alpha} emission is confined to HII regions then the upper-mass limit is restricted to M$_rm{u}$ $le$20 M$_odot$. For the luminosity function fit to be correct we have to discount the H{alpha} observations implying significant diffuse ionized gas or escaping ionizing photons. Combining the HST photometry with HI imaging we find the SFL has a power law index $N=1.53 pm 0.21$. Applying these results to the entire outer HI disc indicates that it contributes 11--28% of the total recent star formation in NGC 2915, depending on whether the IMF is constant within the disc or varies from the centre to the outer region.
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