Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Stellar Halo in the Large Magellanic Cloud: Mass, Luminosity, and Microlensing Predictions
54
0
0.0
(
0
)
Authors
David R. Alves
Ask ChatGPT about the research
No Arabic abstract
Recently obtained kinematic data has shown that the Large Magellanic Cloud (LMC) possesses an old stellar halo. In order to further characterize the properties of this halo, parametric King models are fit to the surface density of RR Lyrae stars. Using data from both the MACHO and OGLE II microlensing surveys, the model fits yield the center of their distribution at RA = 05:21.1+-0.8, Dec = -69:45+-6 (J2000) and a core radius of 1.42+-0.12 kpc. As a check the halo model is compared with RR Lyrae star counts in fields near the LMCs periphery previously surveyed with photographic plates. These data, however, require a cautious interpretation. Several topics regarding the LMC stellar halo are discussed. First, the properties of the halo imply a global mass-to-light ratio of M/L_V = 5.3+-2.1 and a total mass of 1.6+-0.6 10^10 M_sun for the LMC in good agreement with estimates based on the rotation curve. Second, although the LMCs disk and halo are kinematically distinct, the shape of the surface density profile of the halo is remarkably similar to that of the young disk. For example, the best-fit exponential scale length for the RR Lyrae stars is 1.47+-0.08 kpc, which compares to 1.46 kpc for the LMCs blue light. In the Galaxy, the halo and disk do not resemble each other like this. Finally, a local maximum in the LMCs microlensing optical depth due to halo-on-disk stellar self-lensing is predicted. For the parameters of the stellar halo obtained, this maximum is located near MACHO events LMC-4 and LMC-23, and is large enough to possibly account for these two events, but not for all of the observed microlensing.
rate research
Read More
(Abridged) Photometry of archival Spitzer observations of the Large Magellanic Cloud (LMC) are used to search for young stellar objects (YSOs). Simple mid-infrared selection criteria were used to exclude most normal and evolved stars and background galaxies. We identify a sample of 2,910 sources in the LMC that could potentially be YSOs. We then simultaneously considered images and photometry from the optical through mid-IR wavelengths to assess the source morphology, spectral energy distribution (SED), and the surrounding interstellar environment to determine the most likely nature of each source. From this examination of the initial sample, we suggest 1,172 sources are most likely YSOs and 1,075 probable background galaxies, consistent with expectations based on SWIRE survey data. Spitzer IRS observations of 269 of the brightest YSOs from our sample have confirmed that ~>95% are indeed YSOs. A comprehensive search for YSOs in the LMC has also been carried out by the SAGE team. There are three major differences between these two searches. (1) In the common region of color-magnitude space, ~850 of our 1,172 probable YSOs are missed in the SAGE YSO catalog because their conservative point source identification criteria have excluded YSOs superposed on complex diffuse emission. (2) About 20-30% of the YSOs identified by the SAGE team are sources we classify as background galaxies. (3) the SAGE YSO catalog identifies YSO in parts of color-magnitude space that we excluded and thus contains more evolved or fainter YSOs missed by our analysis. Finally, the mid-IR luminosity functions of our most likely YSO candidates in the LMC can be well described by N(L) propto L^-1, which is consistent with the Salpeter initial mass function if a mass-luminosity relation of L propto M^2.4 is adopted.
We present a morphological analysis of the feature-rich 2MASS LMC color-magnitude diagram, identifying Galactic and LMC populations and estimating the density of LMC populations alone. We also present the projected spatial distributions of various stellar populations. Major populations are identified based on matching morphological features of the CMD with expected positions of known populations, isochrone fits, and analysis of the projected spatial distributions. The LMC populations along the first-ascent RGB and AGB are quantified. We find the RGB tip at $K_s=12.3pm0.1$. Preliminary isochrone analysis is done for giant populations in the bar and the outer regions of the Cloud. We find no significant differences in metallicities and ages between the fields. The observed LMC giant branch is well-fit by published tracks in the CIT/CTIO system with a distance modulus of $mu=18.5pm0.1$, reddening $E_{B-V}=0.15-0.20$, metallicity $Z=0.004^{+0.002}_{-0.001}$ and age 3-13 Gyr. Analysis of deep 2MASS engineering data with six times the standard exposure produces similar estimates.
We have undertaken a detailed analysis of HST/WFPC2 and STIS imaging observations, and of supplementary wide-field ground-based observations obtained with the NTT of two young ~10-25 Myr) compact star clusters in the LMC, NGC 1805 and NGC 1818. The ultimate goal of our work is to improve our understanding of the degree of primordial mass segregation in star clusters. This is crucial for the interpretation of observational luminosity functions (LFs) in terms of the initial mass function (IMF), and for constraining the universality of the IMF. We present evidence for strong luminosity segregation in both clusters. The LF slopes steepen with cluster radius; in both NGC 1805 and NGC 1818 the LF slopes reach a stable level well beyond the clusters core or half-light radii. In addition, the brightest cluster stars are strongly concentrated within the inner ~4 R_hl. The global cluster LF, although strongly nonlinear, is fairly well approximated by the core or half-light LF; the (annular) LFs at these radii are dominated by the segregated high-luminosity stars, however. We present tentative evidence for the presence of an excess number of bright stars surrounding NGC 1818, for which we argue that they are most likely massive stars that have been collisionally ejected from the cluster core. We therefore suggest that the cores of massive young stars clusters undergo significant dynamical evolution, even on time-scales as short as ~25 Myr.
We present Hubble Space Telescope (HST) WFPC2 photometry of 13 microlensed source stars from the 5.7 year Large Magellanic Cloud (LMC) survey conducted by the MACHO Project. The microlensing source stars are identified by deriving accurate centroids in the ground-based MACHO images using difference image analysis (DIA) and then transforming the DIA coordinates to the HST frame. None of these sources is coincident with a background galaxy, which rules out the possibility that the MACHO LMC microlensing sample is contaminated with misidentified supernovae or AGN in galaxies behind the LMC. This supports the conclusion that the MACHO LMC microlensing sample has only a small amount of contamination due to non-microlensing forms of variability. We compare the WFPC2 source star magnitudes with the lensed flux predictions derived from microlensing fits to the light curve data. In most cases the source star brightness is accurately predicted. Finally, we develop a statistic which constrains the location of the Large Magellanic Cloud (LMC) microlensing source stars with respect to the distributions of stars and dust in the LMC and compare this to the predictions of various models of LMC microlensing. This test excludes at > 90% confidence level models where more than 80% of the source stars lie behind the LMC. Exotic models that attempt to explain the excess LMC microlensing optical depth seen by MACHO with a population of background sources are disfavored or excluded by this test. Models in which most of the lenses reside in a halo or spheroid distribution associated with either the Milky Way or the LMC are consistent which these data, but LMC halo or spheroid models are favored by the combined MACHO and EROS microlensing results.
Stellar streams are excellent probes of the underlying gravitational potential in which they evolve. In this work, we fit dynamical models to five streams in the Southern Galactic hemisphere, combining observations from the Southern Stellar Stream Spectroscopic Survey (${S}^5$), Gaia EDR3, and the Dark Energy Survey (DES), to measure the mass of the Large Magellanic Cloud (LMC). With an ensemble of streams, we find a mass of the LMC ranging from 14 to $19 times 10^{10} mathrm{M}_{odot}$, probed over a range of closest approach times and distances. With the most constraining stream (Orphan-Chenab), we measure an LMC mass of $18.8^{+ 3.5}_{- 4.0} times 10^{10} mathrm{M}_{odot}$, probed at a closest approach time of 310 Myr and a closest approach distance of 25.4 kpc. This mass is compatible with previous measurements, showing that a consistent picture is emerging of the LMCs influence on structures in the Milky Way. Using this sample of streams, we find that the LMCs effect depends on the relative orientation of the stream and LMC at their point of closest approach. To better understand this, we present a simple model based on the impulse approximation and we show that the LMCs effect depends both on the magnitude of the velocity kick imparted to the stream and the direction of this kick.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
