Thin Disk Scale Research Articles

Vertical Structure of the Milky Way Disk with Gaia DR3

Using a complete sample of about 330,000 dwarf stars, well measured by Gaia DR3, limited to the galactic north and south solid angles |b|<75° and up to a vertical distance of 2 kpc, we analyze the vertical structure of the Milky Way stellar disks, based on projected tangential velocities. From selected subsamples dominated by their corresponding population, we obtain the thin and thick disk scale heights as hZ=279.76±12.49 pc and HZ=797.23±12.34 pc, respectively. Then from the simultaneous fitting of the sum of two populations over the whole sample, assuming these scale heights, we estimate the thick-to-thin disk number density ratio at the galactic plane to be ρT/ρt=0.750±0.049, which is consistent with a previous result by the authors: in the galactic plane there is a significant number of thick disk stars, possibly as many as thin disk ones, which also points to the existence of more thick disk stars than generally thought. The overall fit does not closely follow the data for |Z|>700 pc and points to the presence of more stars beyond the thin disk that cannot be accounted for by the two-disk model.

Weighing the Galactic disk using phase-space spirals

We have applied our method to weigh the Galactic disk using phase-space spirals to the proper motion sample of Gaia’s early third release (EDR3). For stars in distant regions of the Galactic disk, the latitudinal proper motion has a close projection with vertical velocity, such that the phase-space spiral in the plane of vertical position and vertical velocity can be observed without requiring that all stars have available radial velocity information. We divided the Galactic plane into 360 separate data samples, each corresponding to an area cell in the Galactic plane in the distance range of 1.4–3.4 kpc, with an approximate cell length of 200–400 pc. Roughly half of our data samples were disqualified altogether due to severe selection effects, especially in the direction of the Galactic centre. In the remainder, we were able to infer the vertical gravitational potential by fitting an analytic model of the phase-space spiral to the data. This work is the first of its kind, in the sense that we are weighing distant regions of the Galactic disk with a high spatial resolution, without relying on the strong assumptions of axisymmetry. Post-inference, we fitted a thin disk scale length of 2.2 ± 0.1 kpc, although this value is sensitive to the considered spatial region. We see surface density variations as a function of azimuth of the order of 10–20%, which is roughly the size of our estimated sum of potential systematic biases. With this work, we have demonstrated that our method can be used to weigh distant regions of the Galactic disk despite strong selection effects. We expect to reach even greater distances and improve our accuracy with future Gaia data releases and further improvements to our method.

The photo-astrometric vertical tracer density of the Milky Way – II. Results from Gaia

ABSTRACT We use Gaia photometry and astrometry to estimate the vertical spatial structure of the Milky Way at the Solar radius, formally accounting for sample incompleteness (the selection function) and parallax measurement uncertainty. Our results show impressive precision demonstrating the power of the Gaia data. However, systematic errors dominate the parameter value uncertainties. We thoroughly test and quantify the impacts of all systematic uncertainties. The vertical tracer density is modelled as a sum of two exponential profiles for the thin and thick discs, together with a spherically symmetric power law for the stellar halo. We constrain the thin disc scale height as ${h_\mathrm{Tn}=260 \pm 3\, (\mathrm{stat}) \pm 26\, \mathrm{pc}\, (\mathrm{sys})}$ and thick disc ${h_\mathrm{Tk}=693 \pm 7 \, (\mathrm{stat}) \pm 121\, \mathrm{pc}\, (\mathrm{sys})}$. For the halo, we obtain a power-law profile with $n_\mathrm{H}=3.543\pm 0.023 \, (\mathrm{stat}) \pm 0.259\, (\mathrm{sys})$. We infer a local stellar mass density for non-compact object stars of ${\rho _\mathrm{local}^{*} = 3.66\pm 0.03\, (\mathrm{stat})\pm 0.52 \times 10^{-2}\, \mathrm{M}_\odot \, \mathrm{pc}^{-3}\, (\mathrm{sys})}$ and surface density of ${\Sigma _\mathrm{local}^{*} = 23.17\pm 0.08\, (\mathrm{stat})\pm 2.43\, \mathrm{M}_\odot \, \mathrm{pc}^{-2}\, (\mathrm{sys})}$. We find asymmetries above and below the disc with longer disc scale heights in the north but a flatter halo in the south at the ≲ 10 per cent level.

A Bayesian Approach to the Vertical Structure of the Disk of the Milky Way

This work investigates the vertical profile of the stars in the disk of the Milky Way. The models investigated are of the form <em>sech^2/n(nz/(2H))</em> where, setting <em>α = 2/n</em>, the three functions of the sequence <em>α= 0, 1, 2</em> correspond to exponential, <em>sech</em>, and <em>sech²</em> functions respectively. We consider symmetric models and asymmetric models, above and below the plane. The study uses the large sample of K and M stars of Ferguson et al. (2017) and applies the methods of Bayesian model comparison to discriminate between the 6 models. Two inconsistencies in Ferguson et al. (2017), concerning the vertical height cut and the model continuity across the plane, are noted and addressed. We find that (1) in the Milky Way the symmetric disc models are decisively ruled out, with northern thin disc scale heights about 25% larger than southern, (2) there is moderate evidence for the exponential and sech models over the <em>sech²</em> model, though a sample extending further into the Galactic mid-plane is needed to strengthen this result, (3) the photometric distances used by Ferguson et al. underestimate the GAIA distances by a factor of roughly 1.16, and (4) the increase of scale height with Galactic latitude observed by Ferguson et al. is due to incorrect cuts to the data. <em>Open J. Astrophys. 3(2020) 5</em>

Brown dwarf census with the Dark Energy Survey year 3 data and the thin disc scale height of early L types

ABSTRACT In this paper we present a catalogue of 11 745 brown dwarfs with spectral types ranging from L0 to T9, photometrically classified using data from the Dark Energy Survey (DES) year 3 release matched to the Vista Hemisphere Survey (VHS) DR3 and Wide-field Infrared Survey Explorer (WISE) data, covering ≈2400 deg2 up to iAB = 22. The classification method follows the same phototype method previously applied to SDSS-UKIDSS-WISE data. The most significant difference comes from the use of DES data instead of SDSS, which allow us to classify almost an order of magnitude more brown dwarfs than any previous search and reaching distances beyond 400 pc for the earliest types. Next, we also present and validate the GalmodBD simulation, which produces brown dwarf number counts as a function of structural parameters with realistic photometric properties of a given survey. We use this simulation to estimate the completeness and purity of our photometric LT catalogue down to iAB = 22, as well as to compare to the observed number of LT types. We put constraints on the thin disc scale height for the early L (L0–L3) population to be around 450 pc, in agreement with previous findings. For completeness, we also publish in a separate table a catalogue of 20 863 M dwarfs that passed our colour cut with spectral types greater than M6. Both the LT and the late M catalogues are found at DES release page https://des.ncsa.illinois.edu/releases/other/y3-mlt.

Evaluation of the Vertical Scale Height of L Dwarfs in the Galactic Thin Disk

Abstract Using data release 1 of the Hyper Suprime-Cam Subaru Strategic Program covering about 130 square degrees at high galactic latitudes, we have obtained L dwarf counts based on the selection criteria on colors, limiting magnitude, and PSF morphology using the i, z, and y bands. Using these criteria, we have detected 3665 L dwarfs brighter than z = 24. The surface number counts obtained differentially in z magnitude are compared with predictions of an exponential disk model to estimate the thin-disk scale height in the vicinity of the Sun. In the exponential disk model, we first fix the local luminosity function (LLF) to the mean LLF of Cruz et al. and derive the best-fit scale height of 260 pc. However, this fit appears to be poor. We then allow the LLF to vary along with the scale height. We use the LLF of Cruz et al. as a starting point. The best-fit model is found for the vertical scale height of 380 pc. However, the χ 2 minimum is rather broad and the 90% confidence interval is between 320 and 520 pc. We investigate another model by varying the scale height and the density of the brightest magnitude bin, while other magnitude bins are fixed to the mean LLF of Cruz et al. We find an equally good fit with the two free parameters and the best-fit scale height is again 380 pc, but the 90% confidence interval is between 340 and 420 pc.

The vertical force in the solar neighbourhood using red clump stars in TGAS and RAVE

Aims. We investigate the kinematics of red clump (RC) stars in the solar neighbourhood by combining data from Tycho-Gaia Astrometric Solution (TGAS) and Radial Velocity Experiment (RAVE) to constrain the local dark matter density. Methods. After calibrating the absolute magnitude of RC stars, we characterized their velocity distribution over a radial distance range of 6−10 kpc and up to 1.5 kpc away from the Galactic plane. We then applied the axisymmetric Jeans equations on subsets representing the thin and thick disks to determine the (local) distribution of mass near the disk of our Galaxy. Results. Our kinematic maps are well behaved, permitting a straightforward local determination of the vertical force, which we find to be Kzthin = − 2454 ± 619 (km s−1)2 kpc−1 and Kzthick = − 2141 ± 774 (km s−1)2 kpc−1 at 1.5 kpc away from the Galactic plane for the thin and thick disk samples and for thin and thick disk scale heights of 0.28 kpc and 1.12 kpc, respectively. These measurements can be translated into a local dark matter density ρDM ~ 0.018 ± 0.002 M⊙ pc−3. The systematic error on this estimate is much larger than the quoted statistical error, since even a 10% difference in the scale height of the thin disk leads to a 30% change in the value of ρDM and a nearly equally good fit to the data.

Evolution over time of the Milky Way’s disc shape

Context. Galactic structure studies can be used as a path to constrain the scenario of formation and evolution of our Galaxy. The dependence with the age of stellar population parameters would be linked with the history of star formation and dynamical evolution. Aims. We aim to investigate the structures of the outer Galaxy, such as the scale length, disc truncation, warp and flare of the thin disc and study their dependence with age by using 2MASS data and a population synthesis model (the so-called Besançon Galaxy Model). Methods. We have used a genetic algorithm to adjust the parameters on the observed colour–magnitude diagrams at longitudes 80° ≤ ℓ ≤ 280° for | b | ≤ 5.5°. We explored parameter degeneracies and uncertainties. Results. We identify a clear dependence of the thin disc scale length, warp and flare shapes with age. The scale length is found to vary between 3.8 kpc for the youngest to about 2 kpc for the oldest. The warp shows a complex structure, clearly asymmetrical with a node angle changing with age from approximately 165° for old stars to 195° for young stars. The outer disc is also flaring with a scale height that varies by a factor of two between the solar neighbourhood and a Galactocentric distance of 12 kpc. Conclusions. We conclude that the thin disc scale length is in good agreement with the inside-out formation scenario and that the outer disc is not in dynamical equilibrium. The warp deformation with time may provide some clues to its origin.

A Milky Way with a massive, centrally concentrated thick disc: new Galactic mass models for orbit computations

In this work, two new axisymmetric models for the Galactic mass distribution are presented. Motivated by recent results, these two models include the contribution of a stellar thin disc and of a thick disc, as massive as the thin counterpart but with a shorter scale-length. Both models satisfy a number of observational constraints: stellar densities at the solar vicinity, thin and thick disc scale lengths and heights, rotation curve(s), and the absolute value of the perpendicular force Kz as a function of distance to the Galactic centre. We numerically integrate into these new models the motion of all Galactic globular clusters for which distances, proper motions, and radial velocities are available, and the orbits of about one thousand stars in the solar vicinity. The retrieved orbital characteristics are compared to those obtained by integrating the clusters and stellar orbits in pure thin disc models. We find that, due to the possible presence of a thick disc, the computed orbital parameters of disc stars can vary by as much as 30–40%. We also show that the systematic uncertainties that affect the rotation curve still plague computed orbital parameters of globular clusters by similar amounts.

Constraining the Galactic structure parameters with the XSTPS-GAC and SDSS photometric surveys

Photometric data from the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anticentre (XSTPS-GAC) and the Sloan Digital Sky Survey (SDSS) are used to derive the global structure parameters of the smooth components of the Milky Way. The data, which cover nearly 11,000 deg$^2$ sky area and the full range of Galactic latitude, allow us to construct a globally representative Galactic model. The number density distribution of Galactic halo stars is fitted with an oblate spheroid that decays by power law. The best-fit yields an axis ratio and a power law index $\kappa=0.65$ and $p=2.79$, respectively. The $r$-band differential star counts of three dwarf samples are then fitted with a Galactic model. The best-fit model yielded by a Markov Chain Monte Carlo analysis has thin and thick disk scale heights and lengths of $H_{1}=$ 322\,pc and $L_{1}=$2343\,pc, $H_{2}=$794\,pc and $L_{2}=$3638\,pc, a local thick-to-thin disk density ratio of $f_2=$11\,per\,cent, and a local density ratio of the oblate halo to the thin disk of $f_h=$0.16\,per\,cent. The measured star count distribution, which is in good agreement with the above model for most of the sky area, shows a number of statistically significant large scale overdensities, including some of the previously known substructures, such as the Virgo overdensity and the so-called "north near structure", and a new feature between 150\degr $< l < $ 240\degr~and $-1$5\degr $< b < $ $-$5\degr, at an estimated distance between 1.0 and 1.5\,kpc. The Galactic North-South asymmetry in the anticentre is even stronger than previously thought.

Clues on the Milky Way disc formation from population synthesis simulations

AbstractIn recent years the stellar populations of the Milky Way have been investigated from large scale surveys in different ways, from pure star count analysis to detailed studies based on spectroscopic surveys. While in the former case the data can constrain the scale height and scale length thanks to completeness, they suffer from high correlation between these two values. On the other hand, spectroscopic surveys suffer from complex selection functions which hardly allow to derive accurate density distributions. The scale length in particular has been difficult to be constrained, resulting in discrepant values in the literature. Here, we investigate the thick disc characteristics by comparing model simulations with large scale data sets. The simulations are done from the population synthesis model of Besançon. We explore the parameters of the thick disc (shape, local density, age, metallicity) using a Monte Carlo Markov Chain method to constrain the model free parameters (Robin et al. 2014). Correlations between parameters are limited due to the vast spatial coverage of the used surveys (SDSS + 2MASS). We show that the thick disc was created during a long phase of formation, starting about 12 Gyr ago and finishing about 10 Gyr ago, during which gravitational contraction occurred, both vertically and radially. Moreover, in its early phase the thick disc was flaring in the outskirts. We conclude that the thick disc has been created prior to the thin disc during a gravitational collapse phase, slowed down by turbulence related to a high star formation rate, as explained for example in Bournaud et al. (2009) or Lehnert et al. (2009). Our result does not favor a formation from an initial thin disc thickened later by merger events or by secular evolution of the thin disc. We then study the in‐plane distribution of stars in the thin disc from 2MASS and show that the thin disc scale length varies as a function of age, indicating an inside out formation. Moreover, we investigate the warp and flare and demonstrate that the warp amplitude is changing with time and the node angle is slightly precessing. Finally, we show comparisons between the new model and spectroscopic surveys. The new model allows to correctly simulate the kinematics, the metallicity, and α‐abundance distributions in the solar neighbourhood as well as in the bulge region.

Flare in the Galactic stellar outer disc detected in SDSS-SEGUE data

Aims. We explore the outer Galactic disc up to a Galactocentric distance of 30 kpc to derive its parameters and measure the magnitude of its flare. Methods. We obtained the 3D density of stars of type F8V-G5V with a colour selection from extinction-corrected photometric data of the Sloan Digital Sky Survey - Sloan Extension for Galactic Understanding and Exploration (SDSS-SEGUE) over 1,400 deg^2 in off-plane low Galactic latitude regions and fitted it to a model of flared thin+thick disc. Results. The best-fit parameters are a thin-disc scale length of 2.0 kpc, a thin-disc scale height at solar Galactocentric distance of 0.24 kpc, a thick-disc scale length of 2.5 kpc, and a thick-disc scale height at solar Galactocentric distance of 0.71 kpc. We derive a flaring in both discs that causes the scale height of the average disc to be multiplied with respect to the solar neighbourhood value by a factor of 3.3^{+2.2}_{-1.6} at R=15 kpc and by a factor of 12^{+20}_{-7} at R=25 kpc. Conclusions. The flare is quite prominent at large R and its presence explains the apparent depletion of in-plane stars that are often confused with a cut-off at R>15 kpc. Indeed, our Galactic disc does not present a truncation or abrupt fall-off there, but the stars are spread in off-plane regions, even at z of several kpc for R>20 kpc. Moreover, the smoothness of the observed stellar distribution also suggests that there is a continuous structure and not a combination of a Galactic disc plus some other substructure or extragalactic component: the hypothesis to interpret the Monoceros ring in terms of a tidal stream of a putative accreted dwarf galaxy is not only unnecessary because the observed flare explains the overdensity in the Monoceros ring observed in SDSS fields, but it appears to be inappropriate.

TRIGONOMETRIC PARALLAXES OF HIGH MASS STAR FORMING REGIONS: THE STRUCTURE AND KINEMATICS OF THE MILKY WAY

Over 100 trigonometric parallaxes and proper motions for masers associated with young, high-mass stars have been measured with the BeSSeL Survey, a VLBA key science project, the EVN, and the Japanese VERA project. These measurements provide strong evidence for the existence of spiral arms in the Milky Way, accurately locating many arm segments and yielding spiral pitch angles ranging from 7 to 20 degrees. The widths of spiral arms increase with distance from the Galactic center. Fitting axially symmetric models of the Milky Way with the 3-D position and velocity information and conservative priors for the solar and average source peculiar motions, we estimate the distance to the Galactic center, Ro, to be 8.34 +/- 0.16 kpc, a circular rotation speed at the Sun, To, to be 240 +/- 8 km/s, and a rotation curve that is nearly flat (a slope of -0.2 +/- 0.4 km/s/kpc) between Galactocentric radii of 5 and 16 kpc. Assuming a "universal" spiral galaxy form for the rotation curve, we estimate the thin disk scale length to be 2.44 +/- 0.16 kpc. The parameters Ro and To are not highly correlated and are relatively insensitive to different forms of the rotation curve. Adopting a theoretically motivated prior that high-mass star forming regions are in nearly circular Galactic orbits, we estimate a global solar motion component in the direction of Galactic rotation, Vsun = 14.6 +/- 5.0 km/s. While To and Vsun are significantly correlated, the sum of these parameters is well constrained, To + Vsun = 255.2 +/- 5.1 km/s, as is the angular speed of the Sun in its orbit about the Galactic center, (To + Vsun)/Ro = 30.57 +/- 0.43 km/s/kpc. These parameters improve the accuracy of estimates of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their Galactic orbits, significantly reducing the uncertainty in tests of gravitational radiation predicted by general relativity.

The effects of a hot gaseous halo on disc thickening in galaxy minor mergers

We employ hydrodynamical simulations to study the effects of dissipational gas physics on the vertical heating and thickening of disc galaxies during minor mergers. For the first time we present a suite of simulations that includes a diffuse, rotating, cooling, hot gaseous halo, as predicted by cosmological hydrodynamical simulations as well as models of galaxy formation. We study the effect of this new gaseous component on the vertical structure of a Milky Way-like stellar disc during 1:10 and 1:5 mergers. For 1:10 mergers, we find no increased final thin disc scale height compared to the isolated simulation, leading to the conclusion that thin discs can be present even after a 1:10 merger if a reasonable amount of hot gas is present. The reason for this is the accretion of new cold gas, leading to the formation of a massive new thin stellar disc that dominates the surface brightness profile. In a previous study, in which we included only cold gas in the disc, we showed that the presence of cold gas decreased the thickening by a minor merger relative to the no-gas case. Here, we show that the evolution of the scale height in the presence of a cooling hot halo is dominated by the formation of the new stellar disc. In this scenario, the thick disc is the old stellar disc that has been thickened in a minor merger at z ≳ 1, while the thin disc is the new stellar disc that reforms after this merger. When galactic winds are also considered, the final scale height is larger due to two effects. First, the winds reduce the star formation rate, leading to a less massive new stellar disc, such that the thickened old disc still dominates. Secondly, the winds exert a pressure force on the gas in the disc, leading to a shallower gas profile and thus to a thicker new stellar disc. In addition, we study the evolution of the scale height during a 1:5 merger and find that a thin disc can be present even after this merger, provided enough hot gas is available. The final scale height in our simulations depends on the mass of the hot gaseous halo, the efficiency of the winds and the merger mass ratio. We find post-merger values in the range 0.5 ≲ z0 ≲ 1.0 kpc in good agreement with observational constraints by local galaxies.

Characteristics of thick disks formed through minor mergers: stellar excesses and scale lengths

By means of N-body/SPH simulations we investigate the morphological properties of thick disks formed through minor mergers. We show that the vertical surface density profile of the post-merger thick disk follows a sech function and has an excess in the regions far from the disk mid-plane (z>2kpc). This stellar excess also follows a sech function with a larger scale height than the main thick disk component, and it is usually dominated by stars from the primary galaxy. Stars in the excess have a rotational velocity lower than that of stars in the thick disk, and they may thus be confused with stars in the inner galactic halo. The thick disk scale height increases with radius and the rate of its increase is smaller for more gas rich primary galaxies. On the contrary, the scale height of the stellar excess is independent of both radius and gas fraction. We also find that the post-merger thick disk has a radial scale length which is 10-50% larger than that of the thin disk. Two consecutive mergers have basically the same effect on heating the stellar disk as a single merger of the same total mass. To investigate how thick disks produced through secular processes may differ from those produced by minor mergers, we also simulated gravitationally unstable gas-rich disks. These disks do not produce either a stellar excess or a ratio of thick to thin disk scale lengths greater than one. Our simulation results are consistent with observations of the ratio of thick to thin disk scale lengths of the Milky Way and nearby galaxies, and with the Toomre diagram of the Milky Way. We conclude that minor mergers are a viable mechanism for the creation of galactic thick disks and investigating stars at several kpc above the mid-plane of the Milky Way and other galaxies may provide a quantitative method for studying the minor merger history of galaxies.

THE THREE-DIMENSIONAL VELOCITY STRUCTURE OF THE THICK DISK FROM SPM4 AND RAVE DR2

We analyze the 3D kinematics of a sample of $\sim 4400$ red clump stars ranging between 5 and 10 kpc from the Galactic center and up to 3 kpc from the Galactic plane. This sample is representative for the metal-rich ([Fe/H] = -0.6 to 0.5) thick disk. Absolute proper motions are from the fourth release of the Southern Proper Motion Program, and radial velocities from the second release of the Radial Velocity Experiment. The derived kinematical properties of the thick disk include: the rotational velocity gradient $\partial V_{\theta} / \partial z = -25.2 \pm 2.1$ km s$^{-1}$ kpc$^{-1}$, velocity dispersions $(\sigma_{V_R}, \sigma_{V_{\theta}}, \sigma_{V_z})|_{z=1} = (70.4, 48.0, 36.2) \pm(4.1,8.3,4.0)$ km s$^{-1}$, and velocity-ellipsoid tilt angle $\alpha_{Rz} = 8.6\arcdeg \pm 1.8 \arcdeg$. Our dynamical estimate of the thin-disk scale length is $R_{thin} = 2.0 \pm 0.4$ kpc and the thick-disk scale height is $z_{thick} = 0.7 \pm 0.1$ kpc. The observed orbital eccentricity distribution compared with those from four different models of the formation of the thick disk from Sales et al. favor the gas-rich merger model and the minor merger heating model. Interestingly, when referred to the currently accepted value of the LSR, stars more distant than 0.7 kpc from the Sun show a net average radial velocity of $13 \pm3 $ km s$^{-1}$. This result is seen in previous kinematical studiesusing other tracers at distances larger than $\sim 1$ kpc. We suggest this motion reflects an inward perturbation of the locally-defined LSR induced by the spiral density wave.

SPECTROPHOTOMETRICALLY IDENTIFIED STARS IN THE PEARS-N AND PEARS-S FIELDS

Deep ACS slitless grism observations and identification of stellar sources are presented within the Great Observatories Origins Deep Survey (GOODS) North and South fields which were obtained in the Probing Evolution And Reionization Spectroscopically (PEARS) program. It is demonstrated that even low resolution spectra can be a very powerful means to identify stars in the field, especially low mass stars with stellar types M0 and later. The PEARS fields lay within the larger GOODS fields, and we used new, deeper images to further refine the selection of stars in the PEARS field, down to a magnitude of mz = 25 using a newly developed stellarity parameter. The total number of stars with reliable spectroscopic and morphological identification was 95 and 108 in the north and south fields respectively. The sample of spectroscopically identified stars allows constraints to be set on the thickness of the Galactic thin disk as well as contributions from a thick disk and a halo component. We derive a thin disk scale height, as traced by the population of M4 to M9 dwarfs along two independent lines of sight, of h_thin = 370 +60/-65 pc. When including the more massive M0 to M4 dwarf population, we derive h_thin = 300 +/- 70pc. In both cases, we observe that we must include a combination of thick and halo components in our models in order to account for the observed numbers of faint dwarfs. The required thick disk scale height is typically h_thick=1000 pc and the acceptable relative stellar densities of the thin disk to thick disk and the thin disk to halo components are in the range of 0.00025<f_halo<0.0005 and 0.05<f_thick<0.08 and are somewhat dependent on whether the more massive M0 to M4 dwarfs are included in our sample.

Disentangling the Galaxy at low Galactic latitudes

We have used the field stars from the open cluster survey BOCCE, to study three low-latitude fields imaged with the Canada-France-Hawaii telescope (CFHT), with the aim of better understanding the Galactic structure in those directions. Due to the deep and accurate photometry in these fields, they provide a powerful discriminant among Galactic structure models. In the present paper we discuss if a canonical star count model, expressed in terms of thin and thick disc radial scales, thick disc normalization and reddening distribution, can explain the observed CMDs. Disc and thick disc are described with double exponentials, the spheroid is represented with a De Vaucouleurs density law. In order to assess the fit quality of a particular set of parameters, the colour distribution and luminosity function of synthetic photometry is compared to that of target stars selected from the blue sequence of the observed colour-magnitude diagrams. Through a Kolmogorov-Smirnov test we find that the classical decomposition halo-thin/thick disc is sufficient to reproduce the observations--no additional population is strictly necessary. In terms of solutions common to all three fields, we have found a thick disc scale length that is equal to (or slightly longer than) the thin disc scale.

Constraining the Distribution of L and T Dwarfs in the Galaxy

We estimate the thin-disk scale height of the Galactic population of L and T dwarfs based on star counts from 15 deep parallel fields from the Hubble Space Telescope. From these observations, we have identified 28 candidate L and T dwarfs based on their i'-z' color and morphology. By comparing these star counts with a simple Galactic model, we estimate the scale height to be 350 ± 50 pc, which is consistent with the increase in vertical scale with decreasing stellar mass and is independent of reddening, color-magnitude limits, and other Galactic parameters. With this refined measure, we predict that less than 109 M☉ of the Milky Way can be in the form of L and T dwarfs and confirm that high-latitude, z 6 galaxy surveys that use the i'-band dropout technique are 97%-100% free of L and T dwarf interlopers.

A Proper‐Motion Survey for White Dwarfs with the Wide Field Planetary Camera 2

We have performed a search for halo white dwarfs as high proper motion objects in a second-epoch Wide Field Planetary Camera 2 image of the Groth-Westphal strip. The survey covers 74.8 arcmin2 and is complete to V ~ 26.5. We identify 24 high proper motion objects with μ > 0014 yr-1. Five of these high proper motion objects are identified as strong white dwarf candidates on the basis of their position in a reduced proper motion diagram. We also identify two marginal candidates whose photometric errors place them within ~1 σ of the white dwarf region of the reduced proper motion diagram. We create a model of the Milky Way thin disk, thick disk, and stellar halo and find that this sample of white dwarfs is clearly an excess above the ≤2 detections expected from these known stellar populations. The origin of the excess signal is less clear. Possibly, the excess cannot be explained without invoking a fourth galactic component: a white dwarf dark halo. Previous work of this nature has separated white dwarf samples into various galactic components based on kinematics; distances, and thus velocities, are unavailable for a sample this faint. Therefore, we present a statistical separation of our sample into the four components and estimate the corresponding local white dwarf densities using only the directly observable variables, V, (V-I), and μ. For all Galactic models explored, our five white dwarf sample separates into about three disk white dwarfs and two halo white dwarfs. However, the further subdivision into the thin and thick disk and the stellar and dark halo, and the subsequent calculation of the local densities, are sensitive to the input parameters of our model for each Galactic component. Using the lowest mean mass model for the dark halo and the five white dwarf sample, we find n0, thin disk = 2.4 × 10-2 pc-3, n0, thick disk = 0.0+7.6 × 10-4 pc-3, n0, stellar halo = 0.0+7.7 × 10-5 pc-3, and n0, dark halo = 1.0 × 10-3 pc-3. This implies a 7% white dwarf halo and 6 times the canonical value for the thin disk white dwarf density (at marginal statistical significance), but possible systematic errors due to uncertainty in the model parameters likely dominate these statistical error bars. The white dwarf halo can be reduced to ~1.5% of the halo dark matter by changing the initial mass function slightly. The local thin disk white dwarf density in our solution can be made consistent with the canonical value by assuming a larger thin disk scale height of 500 pc.