Infrared studies of the structure of the galaxy

A simple three-component model of the Galaxy is presented. The Galaxy is represented by (1) a disk whose stellar density decreases exponentially with distance from the galactic centre as well as perpendicularly as a function of height above or below the galactic plane; plus a (2) ring with a radius of R₀sin25 kpc considered to be an enhancement of the disk and finally by (3) an oblate de Vaucouleurs spheroid meant to represent the galactic bulge. The absorbing material is also assumed to be distributed as an exponential layer. The model is used to fit the 2.4 μm integrated flux observations of the Galaxy, which it does remarkably well, and to determine some of the model constants such as the radial length scale for the disk and the central densities for the bulge and ring components. Based on this model and the derived values of the constants, a series of stellar count predictions for the Km agnitude (2.2 μm) were made. These predictions were com pared to those made by the Bahcall and Soneira model of the Galaxy. The model predicts an almost complete dominance of the bright K-magnitude (K ≤ 14-15) realm by the disk population. At fainter magnitudes the bulge population dominates. A transition region at which disk and bulge contributions are similar occurs at different K -m agnitudes and depends on galactic longitude (from K ~ +9.5 at l ~ 0 to K ~ +20 at l ~ 180). Stellar counts along the galactic plane in the range 220 ≤ l ≤ 60 have been obtained, and have been fitted to the galactic model described above. The model constants utilised in fitting the stellar counts are those derived from the model fit to the 2.4 μm observed integrated flux. Considering the general nature of the model (same constants for all regions), the agreement between the model predictions and the observations is remarkable. The slight discrepancies which have arisen between theory and observations have been interpreted, in most cases, as inhomogeneities in the absorbing material. In the longitude range 0 ≤ l ≤ 60, there appears to be a slight theoretical excess over the observed counts at bright magnitudes. A carefully chosen alteration of the absolute K-magnitude of all the stars in the Luminosity Function (LF) may be necessary to produce a perfect fit between predicted and observed Cumulative Counts Functions (CCF’s). A low absorption window in the direction l — 0, 6 ~ -4, known as Baade’s Window (BW) has been observed photometrically as well as spectroscopically. A K-scan of this region provided an observational CCF which, with the aid of the model discussed above, was decomposed into possible disk plus bulge contributions. The following results were obtained: • At bright K-magnitudes the slope of the disk CCF is different from that of the bulge CCF. This suggests the presence of two radically different stellar populations within a radius of ± 3 kpc from the galactic centre. • Possible similarities between the real BW bulge population and globular clusters stars is suggested from the similar slope of both stellar populations’ CCF’s. • A significant spread in the (J-H) vs. (H-K), as well as in the CO vs. (J-K) diagram suggests an intrinsic spread in the colour of the sources, which may indicate a possible spread in the metallicity of the objects observed in BW. • What may be identified as the brightest infrared (IR) magnitude locus for stars in BW, is composed of young (2 x 10⁹ years) Asymptotic Giant Branch (AGB) stars with a luminosity of the order ~ 10⁴Lʘ. • In BW ‘true’ bulge members appear to be CO-poor as opposed to ‘true’ disk members which seem to have higher CO-index values. This suggests there is a difference in metallicity of the stars in the disk and the bulge. ‘True’ bulge members appear to be more metal poor than 47-Tuc. There is a hint of kinematical differences between CO-weak and CO-strong stars which is consistent with our picture of ‘true’ bulge and ‘true’ disk populations respectively. Further photometric, spectroscopic and kinematical studies of IR sources in low absorption regions will help to confirm the points put forward in this thesis

CCD Photometry of the Globular Cluster NGC 5897

We report CCD photometric observations of the globular cluster NGC 5897, in the Johnson system filters B, V , R, and I. With the values for these magnitudes we obtain various colour indices and produce several colour-magnitude diagrams. We present eight colour-magnitude diagrams: V vs B-V , B vs B-V , V vs V-I, I vs V-I, R vs R-I, I vs R-I, V vs V-R, and R vs V-R. In all of these diagrams we can clearly see the Giant Branch, the Horizontal Branch and the beginning of the Main Sequence. To the left of the Main Sequence turn-off point we detect a somewhat large number of Blue Straggler stars. We determine the mean value of the visual magnitude of the HB as $16.60 \pm 0.46$. This value is fainter than the value found by other authors.Comment: 28 pages, 7 figures, accepted for publication RevMexAA, vol. 60-1, April 2024. arXiv admin note: text overlap with arXiv:1606.0452

How much dark matter is there inside early-type galaxies?

We study the luminous mass as a function of the dynamical mass inside the effective radius (r_e) of early-type galaxies (ETGs) to search for differences between these masses. We assume Newtonian dynamics and that any difference between these masses is due to the presence of dark matter. We use several samples of ETGs -ranging from 19 000 to 98 000 objects- from the ninth data release of the Sloan Digital Sky Survey. We perform Monte Carlo (MC) simulations of galaxy samples and compare them with real samples. The main results are: i) MC simulations show that the distribution of the dynamical vs. luminous mass depends on the mass range where the ETGs are distributed (geometric effect). This dependence is caused by selection effects and intrinsic properties of the ETGs. ii) The amount of dark matter inside r_e is approximately 7% +- 22%. iii) This amount of dark matter is lower than the minimum estimate (10%) found in the literature and four times lower than the average (30%) of literature estimates. However, if we consider the associated error, our estimate is of the order of the literature average.Comment: 24 pages, 12 figures. MNRAS accepte

Sub-arcsecond radio and optical observations of the likely counterpart to the gamma-ray source 2FGL J2056.7+4939

We have searched and reviewed all multi- wavelength data available for the region towards the gamma-ray source 2FGL J2056.7+4939 in order to con- strain its possible counterpart at lower energies. As a result, only a point-like optical/infrared source with flat-spectrum radio emission is found to be consistent with all X-ray and gamma-ray error circles. Its struc- ture is marginally resolved at radio wavelengths at the sub-arcsecond level. An extragalactic scenario appears to be the most likely interpretation for this object.Comment: 5 pages, 3 figures, 1 tabl

Virial masses of late-type galaxies from the SDSS DR16

Motivated by the challenges of calculating the dynamical masses of late-type galaxies (LTGs) and the enormous amount of data from the Sloan Digital Sky Survey (SDSS), we calculate virial masses of a sample of approximately 126 000 LTGs from the sixteenth data release of the SDSS. The virial mass estimations were made considering Newtonian mechanics, virial equilibrium and velocity dispersion from stars and gas. The procedure gave as a result seven mass estimations for each galaxy. The calculated masses were calibrated using a sample of spiral galaxies with velocity rotation curves. Considering the results from the calibration, we find that the correlation between virial and dynamical (rotation curve) masses is stronger for high inclination values. Therefore, the calibration relies more on the available data for higher inclination angle galaxies. We also show that if we have a heterogeneous sample of galaxies one must take into consideration the size and colour of these galaxies by using the following variables: Sersic index n, concentration index, and colour of the stars. For relatively smaller and bluer LTGs, the gas velocity dispersion provides a more consistent mass calculation, while for LTGs that are relatively larger and redder the stellar velocity dispersion provides a better correlated mass calculation.Fil: Nigoche Netro, A.. Universidad de Guadalajara; México. Instituto de Astronomía y Meteorologia de la Universidad de Guadalajara; MéxicoFil: De La Fuente, E.. Universidad de Guadalajara; México. University of Tokyo; JapónFil: Diaz, Ruben Joaquin. United States Gemini Office; Estados Unidos. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; ArgentinaFil: Agüero, Maria Paz. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; ArgentinaFil: Kemp, S. N.. Instituto de Astronomía y Meteorologia de la Universidad de Guadalajara; MéxicoFil: Marquez Lugo, R. A.. Instituto de Astronomía y Meteorologia de la Universidad de Guadalajara; MéxicoFil: Lagos, P.. Centro de Astrofísica Da Universidade Do Porto; PortugalFil: Ruelas Mayorga, A.. Universidad Nacional Autónoma de México; MéxicoFil: López Contreras, N. L.. Instituto de Astronomía y Meteorologia de la Universidad de Guadalajara; Méxic

A major star formation region in the receding tip of the stellar Galactic bar

We present an analysis of the optical spectroscopy of 58 stars in the Galactic plane at $l=27$\arcdeg, where a prominent excess in the flux distribution and star counts have been observed in several spectral regions, in particular in the Two Micron Galactic Survey (TMGS) catalog. The sources were selected from the TMGS, to have a $K$ magnitude brighter than +5 mag and be within 2 degrees of the Galactic plane. More than 60% of the spectra correspond to stars of luminosity class I, and a significant proportion of the remainder are very late giants which would also be fast evolving. This very high concentration of young sources points to the existence of a major star formation region in the Galactic plane, located just inside the assumed origin of the Scutum spiral arm. Such regions can form due to the concentrations of shocked gas where a galactic bar meets a spiral arm, as is observed at the ends of the bars of face-on external galaxies. Thus, the presence of a massive star formation region is very strong supporting evidence for the presence of a bar in our Galaxy.Comment: 13 pages (latex) + 4 figures (eps), accepted in ApJ Let