Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userMilky Way Red Dwarfs in the BoRG Survey; Galactic scale-height and the distribution of dwarfs stars in WFC3 imaging
103
0
0.0
(
0
)
Authors
B. W.Holwerda
Ask ChatGPT about the research
No Arabic abstract
We present a tally of Milky Way late-type dwarf stars in 68 WFC3 pure-parallel fields (227 arcmin^2) from the Brightest of Reionizing Galaxies (BoRG) survey for high-redshift galaxies. Using spectroscopically identified M-dwarfs in two public surveys, the CANDELS and the ERS mosaics, we identify a morphological selection criterion using the half-light radius (r50), a near-infrared J-H, G-J color region where M-dwarfs are found, and a V-J relation with M-dwarf subtype. We apply this morphological selection of stellar objects, color-color selection of M-dwarfs and optical-near-infrared color subtyping to compile a catalog of 274 M-dwarfs belonging to the disk of the Milky Way with a limiting magnitude of m_F125W < 24. Based on the M-dwarfs statistics, we conclude that (a) the previously identified North/South discrepancy in M-dwarf numbers persists in our sample; there are more M-dwarfs in the Northern fields on average than in Southern ones, (b) the Milky Ways single disk scale-height for M-dwarfs is 0.3-4 kpc, depending on sub-type, (c) {bf ERRATUM:} we present corrected coordinates (AstroPy) and distances and find a constant $z_0$=600 pc for all types. (d) a second component is visible in the vertical distribution, with a different, much higher scale-height. We report the M-dwarf component of the Sagittarius stream in one of our fields with 11 confirmed M-dwarfs, 7 of which are at the streams distance. The dwarf scale-height and the relative low incidence in our fields of L- and T-dwarfs in these fields makes it unlikely that these stars will be interlopers in great numbers in color-selected samples of high-redshift galaxies. The relative ubiquity of M-dwarfs however will make them ideal tracers of Galactic Halo substructure with EUCLID and reference stars for JWST observations.
rate research
Read More
We present optical spectroscopy for 18 halo white dwarfs identified using photometry from the Canada-France Imaging Survey and Pan-STARRS1 DR1 3$pi$ survey combined with astrometry from Gaia DR2. The sample contains 13 DA, 1 DZ, 2 DC, and two potentially exotic types of white dwarf. We fit both the spectrum and the spectral energy distribution in order to obtain the temperature and surface gravity, which we then convert into a mass, and then an age, using stellar isochrones and the initial-to-final mass relation. We find a large spread in ages that is not consistent with expected formation scenarios for the Galactic halo. We find a mean age of 9.03$^{+2.13}_{-2.03}$ Gyr and a dispersion of 4.21$^{+2.33}_{-1.58}$ Gyr for the inner halo using a maximum likelihood method. This result suggests an extended star formation history within the local halo population.
The James Webb Space Telescope (JWST) will be an exquisite new near-infrared observatory with imaging and multi-object spectroscopy through ESAs NIRspec instrument with its unique Micro-Shutter Array (MSA), allowing for slits to be positioned on astronomical targets by opening specific 0.002-wide micro shutter doors. To ensure proper target acquisition, the on-sky position of the MSA needs to be verified before spectroscopic observations start. An onboard centroiding program registers the position of pre-identified guide stars in a Target Acquisition (TA) image, a short pre-spectroscopy exposure without dispersion (image mode) through the MSA with all shutters open. The outstanding issue is the availability of Galactic stars in the right luminosity range for TA relative to typical high redshift targets. We explore this here using the stars and $zsim8$ candidate galaxies identified in the source extractor catalogs of Brightest of Reionizing Galaxies survey (BoRG[z8]), a pure-parallel program with Hubble Space Telescope Wide-Field Camera 3. We find that (a) a single WFC3 field contains enough Galactic stars to satisfy the NIRspec astrometry requirement (20 milli-arcseconds), provided its and the NIRspec TAs are $m_{lim}>24.5$ AB in WFC3 F125W, (b) a single WFC3 image can therefore serve as the pre-image if need be, (c) a WFC3 mosaic and accompanying TA image satisfy the astrometry requirement at $sim23$ AB mag in WFC3 F125W, (d) no specific Galactic latitude requires deeper TA imaging due to a lack of Galactic stars, and (e) a depth of $sim24$ AB mag in WFC3 F125W is needed if a guide star in the same MSA quadrant as a target is required. We take the example of a BoRG identified $zsim8$ candidate galaxy and require a Galactic star within 20 of it. In this case, a depth of 25.5 AB in F125W is required (with $sim$97% confidence).
Near the minor axis of the Galactic bulge, at latitudes b < -5 degrees, the red giant clump stars are split into two components along the line of sight. We investigate this split using the three fields from the ARGOS survey that lie on the minor axis at (l,b) = (0,-5), (0,-7.5), (0,-10) degrees. The separation is evident for stars with [Fe/H] > -0.5 in the two higher-latitude fields, but not in the field at b = -5 degrees. Stars with [Fe/H] < -0.5 do not show the split. We compare the spatial distribution and kinematics of the clump stars with predictions from an evolutionary N-body model of a bulge that grew from a disk via bar-related instabilities. The density distribution of the peanut-shaped model is depressed near its minor axis. This produces a bimodal distribution of stars along the line of sight through the bulge near its minor axis, very much as seen in our observations. The observed and modelled kinematics of the two groups of stars are also similar. We conclude that the split red clump of the bulge is probably a generic feature of boxy/peanut bulges that grew from disks, and that the disk from which the bulge grew had relatively few stars with [Fe/H] < -0.5
We have assembled a spectroscopic sample of low-mass dwarfs observed as part of the Sloan Digital Sky Survey along one Galactic sightline, designed to investigate the observable properties of the thin and thick disks. This sample of ~7400 K and M stars also has measured ugriz photometry, proper motions, and radial velocities. We have computed UVW space motion distributions, and investigate their structure with respect to vertical distance from the Galactic Plane. We place constraints on the velocity dispersions of the thin and thick disks, using two-component Gaussian fits. We also compare these kinematic distributions to a leading Galactic model. Finally, we investigate other possible observable differences between the thin and thick disks, such as color, active fraction and metallicity.
If the Galaxy is axisymmetric and in dynamical equilibrium, we expect negligible fluctuations in the residual line-of-sight velocity field. Recent results using the apg{} survey find significant fluctuations in velocity for stars in the midplane ($|z|<$0.25 kpc) out to 5 kpc, suggesting that the dynamical influence of non-axisymmetric features i.e., the Milky Ways bar, spiral arms and merger events extends out to the Solar neighborhood. Their measured power spectrum has a characteristic amplitude of 11 kms{} on a scale of 2.5 kpc. The existence of such large-scale streaming motions has important implications for determining the Suns motion about the Galactic Centre. Using Red Clump stars from glh{} and apg{}, we map the line-of-sight velocities around the Sun (d$<$5 kpc), and $|z|<$1.25 kpc from the midplane. By subtracting a smooth axisymmetric model for the velocity field, we study the residual fluctuations and compare our findings with mock survey generated by glx{}. We find negligible large-scale fluctuations away from the plane. In the mid-plane, we reproduce the earlier apg{} power spectrum but with 20% smaller amplitude (9.3 kms{}) after taking into account a few systematics (e.g., volume completeness). Using a flexible axisymmetric model the power-amplitude is further reduced to 6.3 kms{}. Additionally, our simulations show that, in the plane, distances are underestimated for high-mass Red Clump stars which can lead to spurious power-amplitude of about 5.2 kms{}. Taking this into account, we estimate the amplitude of real fluctuations to be $<$4.6 kms{}, about a factor of three less than the apg{} result.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
