The field of NGC 6397 as a test for Galactic models

2003

We present the results of a Galactic model able to reproduce star counts and synthetic color-magnitude diagrams of field stars, including the white dwarf (WD) population in an evolutionary consistent way. The results on the expected distribution of the Galactic WD population appear in rather good agreement with recent estimates of the local WD luminosity function. Uncertainties still present in

Monthly Notices of the Royal Astronomical Society, 2013

We present multichromatic isochrone fits to the colour magnitude data of the globular cluster NGC 6366, based on HST ACS/WFC and SOAR photometric data. We corrected the photometric data for differential reddening and calculated the mean ridge line of the colour magnitude diagrams. We compared the isochrones of Dartmouth Stellar Evolution Database and PAdova and TRieste Stellar Evolution Code both with microscopic diffusion starting on the main sequence. Bracketing all previous determinations of this cluster we tested metallicities from [Fe/H]=-1.00 to [Fe/H]=-0.50, and ages from 9 to 13 Gyrs. After determining the total to selective extinction ratio only from stars belonging to this cluster, R V = 3.06±0.14, we found the parameters for this cluster to be E(B − V ) = 0.69 ± 0.02(int) ±0.04(ext), (m − M ) V = 15.02 ± 0.07(int) ±0.13(ext), Age = 11 ± 1.15Gyr. Evolutionary models fail to reproduce the low-T eff sequence in multi-band colour magnitude diagrams, indicating that they still have an incomplete physics. We found that the Dartmouth Stellar Evolution Database isochrones fit better the sub giant branch and low main sequence than the PAdova and TRieste Stellar Evolution Code.

Astronomy and Astrophysics, 2001

We present deep V and I photometry for the open cluster NGC 4815 and four surrounding Galactic fields down to a limiting magnitude V ∼ 25. These data are used to study cluster spatial extension by means of star counts, and to derive the luminosity (LF) and mass function (MF). The radius turns out to be 3.6 ± 0.3arcmin at V=19.0 level, whereas the mass amounts at 880 ± 230m ⊙ down to V=20.8. From the color-magnitude diagram, we obtain the LFs in the V and I bands, using both the standard histogram and an adaptive kernel. After correction for incompleteness and field star contamination, the LFs have been transformed into the present day mass functions (PDMF). The PDMFs from the V and I photometry can be represented as a power-law with a slope α = 3.1±0.3 and α = 2.9±0.3 (the (?) MF in this notation has a slope α = 2.35) respectively, in the mass range 2.5 ≤ m m⊙ ≤ 0.8. Below this mass, the MF cannot be considered as representative of the cluster IMF, as it is the result of the combined effect of strong irregularities in the stellar background, probable internal dynamical evolution of the cluster and/or interaction of the cluster with the dense Galactic field. Unresolved binaries and mass segregation can only flatten the apparent derived IMF, so we expect that the real IMF must be steeper than the quoted slope by an unknown amount.

Monthly Notices of the Royal Astronomical Society, 2002

We present line-strengths and kinematics from the central regions of 32 galaxies with Hubble types ranging from E to Sbc. Spectral indices, based on the Lick system, are measured in the optical and near infra-red (NIR). The 24 indices measured, in conjunction with models of the effects of varying abundance ratios, permit the breaking of age/metallicity degeneracy and allow estimation of enhancements in specific light elements (particularly C and Mg). The large range of Hubble types observed allows direct comparison of line-strengths in the centres of early-type galaxies (E and S0) with those in spiral bulges, free from systematic differences that have plagued comparisons of results from different studies. Our sample includes field and Virgo cluster galaxies. For early-type galaxies our data are consistent with previously reported trends of Mg 2 and Mgb with velocity dispersion. In spiral bulges we find trends in all indices with velocity dispersion. We estimate luminosity-weighted ages, metallicities and heavy element abundance ratios (enhancements) from optical indices. These show that bulges are less enhanced in light (α-capture) elements and have lower average age than earlytype galaxies. Trends involving age and metallicity also differ sharply between early and late types. An anti-correlation exists between age and metallicity in early types, while, in bulges, metallicity is correlated with velocity dispersion. We consider the implications of these findings for models of the formation of these galaxies. We find that primordial collapse models of galaxy formation are ruled out by our observations, while several predictions of hierarchical clustering (merger) models are confirmed.

Monthly Notices of the Royal Astronomical Society, 2004

We estimated the Galactic model parameters by means of a new approach based on the comparison of the observed space density functions per absolute magnitude interval with a unique density law for each population individually, and via the procedure in situ for the field SA 114 (α = 22 h 40 m 00 s , δ = 00 • 00 00 ; l = 68. • 15, b = -48. • 38; 4.239 deg 2 ; epoch 2000). The separation of stars into different populations has been carried out by their spatial distribution. The new approach reveals that model parameters are absolute-magnitude-dependent. The scaleheight for a thin disc decreases monotonically from absolutely bright [M(g ) = 5] to absolutely faint [M(g ) = 13] stars in the range 265-495 pc, but there is a discontunity at the absolute magnitude M(g ) = 10 where the sech 2 density law replaces the exponential one. The range of the scaleheight for a thick disc, dominant in the absolute magnitude interval 5 < M(g ) 9, is less: 805-970 pc. The local space density for a thick disc relative to a thin disc decreases from 9.5 to 5.2 per cent when one goes from absolutely bright to faint magnitudes. The halo is dominant in three absolute magnitude intervals, namely 5 < M(g ) 6, 6 < M(g ) 7, and 7 < M(g ) 8, and the axial ratio for this component is almost the same for these intervals where c/a ∼ 0.7. The same holds for the local space density relative to the space density of the thin disc with range (0.02-0.15) per cent. The model parameters estimated by comparison of the observed space density functions combined for three populations per absolute magnitude interval with the combined density laws agree with the cited values in the literature. Also, each parameter is equal to at least one of the corresponding parameters estimated for different absolute magnitude intervals by the new approach. We argue that the most appropriate Galactic model parameters are those that are magnitude dependent.

2003

Using the population synthesis code StarTrack we construct the first synthetic X-ray binary populations for direct comparison with the X-ray luminosity function (XLF) of NGC 1569 observed with Chandra. Our main goal is to examine whether it is possible to reproduce the XLF shape with our models, given the current knowledge for the star-formation history of this starburst galaxy. We thus produce hybrid models meant to represent the two stellar populations: one old, metal-poor with continuous star-formation for ∼ 1.5 Gyr and another recent and metal-rich population. To examine the validity of the models we compare XLFs calculated for varying ages of the populations and varying relative weights for the star-formation rates in the two populations. We find that, for typical binary evolution parameters, it is indeed possible to quite closely match the observed XLF shape. The robust match is achieved for an age of the young population and a ratio of star formation rates in the two populations that are within factors of 1.5 and 2, respectively, of those inferred from HST observations of NGC 1569. In view of this encouraging first step, we discuss the implications of our X-ray binary models and their potential as tools to study binary populations in galaxies.

The Astrophysical Journal, 2013

NGC 6388 and NGC 6441 are two massive Galactic bulge globular clusters which share many properties, including the presence of an extended horizontal branch (HB), quite unexpected because of their high metal content. In this paper we use HST's WFPC2, ACS, and WFC3 images and present a broad multicolor study of their stellar content, covering all main evolutionary branches. The color-magnitude diagrams (CMDs) give compelling evidence that both clusters host at least two stellar populations, which manifest themselves in different ways. NGC 6388 has a broadened main sequence (MS), a split sub-giant branch (SGB), and a split red giant branch (RGB) that becomes evident above the HB in our data set; its red HB is also split into two branches. NGC 6441 has a split MS, but only an indication of two SGB populations, while the RGB clearly splits in two from the SGB level upward, and no red HB structure. The multicolor analysis of the CMDs confirms that the He difference between the two main stellar populations in the two clusters must be similar. This is observationally supported by the HB morphology, but also confirmed by the color distribution of the stars in the MS optical band CMDs. However, a MS split becomes evident in NGC 6441 using UV colors, but not in NGC 6388, indicating that the chemical patterns of the different populations are different in the two clusters, with C, N, O abundance differences likely playing a major role. We also analyze the radial distribution of the two populations.

The Astronomical Journal, 2008

We present measurements of the luminosity and mass functions of low-mass stars constructed from a catalog of matched Sloan Digital Sky Survey (SDSS) and 2 Micron All Sky Survey (2MASS) detections. This photometric catalog contains more than 25,000 matched SDSS and 2MASS point sources spanning ∼30 square degrees on the sky. We have obtained follow-up spectroscopy, complete to J=16, of more than 500 low mass dwarf candidates within a 1 square degree sub-sample, and thousands of additional dwarf candidates in the remaining 29 square degrees. This spectroscopic sample verifies that the photometric sample is complete, uncontaminated, and unbiased at the 99% level globally, and at the 95% level in each color range. We use this sample to derive the luminosity and mass functions of lowmass stars over nearly a decade in mass (0.7 M ⊙ > M * > 0.1 M ⊙ ). The luminosity function of the Galactic disk is statistically consistent with that measured from volume complete samples in the solar neighborhood. We find that the logarithmically binned mass function is best fit with an M c =0.29 lognormal distribution, with a 90% confidence interval of M c =0.20-0.50. These 90% confidence intervals correspond to linearly binned mass functions peaking between 0.27 M ⊙ and 0.12 M ⊙ , where the best fit MF turns over at 0.17 M ⊙ . A power law fit to the entire mass range sampled here, however, returns a best fit of α=1.1 (where the Salpeter slope is α = 2.35); a broken power law returns α=2.04 at masses greater than log M = -0.5 (M=0.32 M ⊙ ), and α=0.2 at lower masses. These results agree well with most previous investigations, though differences in the analytic formalisms adopted to describe those mass functions, as well as the range over which the data are fit, can give the false impression of disagreement. Given the richness of modern-day astronomical datasets, we are entering the regime whereby stronger conclusions can be drawn by comparing the actual datapoints measured in different mass functions, rather than the results of analytic analyses that impose structure on the data a priori. Having validated this method to generate a low-mass luminosity function from matched SDSS/2MASS datasets, future studies will extend this technique to the entirety of the SDSS footprint.

The Large Scale Structure of the Universe, 1978

Publications of the Astronomical Society of Australia, 2004

With the advent of large scale surveys (i.e., Legacy Surveys) it is now possible to start looking beyond the galaxy luminosity function (LF) to more detailed statistical representations of the galaxy population, i.e., multivariate distributions. In this review I first summarise the current state-of-play of the B-band global and cluster LFs and then briefly present two promising bivariate distributions: the luminosity-surface brightness plane (LSP); and the colour-luminosity plane (CLP). In both planes galaxy bulges and galaxy disks form marginally overlapping but distinct distributions, indicating two key formation/evolutionary processes (presumably merger and accretion). Forward progress in this subject now requires the routine application of reliable bulge-disk decomposition codes to allow independent investigation of these two key components.

Astrophysical Journal - ASTROPHYS J, 1997

In this work we investigate the stellar content of three circumnuclear giant H II regions in the starburst galaxy NGC 7714. We model the stellar population that best reproduces the observational constraints given by the Ha image and the optical spectroscopy from 3710 to 9700 Ó.

The Astrophysical Journal, 1996

In this paper, we discuss a method to conduct a quantitative study of the star formation history (SFH) of Local Group (LG) galaxies using Hubble Space Telescope (HST) data. This method has proven to be successful in the analysis of the SFH of the same kind of galaxies using ground-based observations. It is based on the comparison of observed CMDs with a set of model CMDs. The latter are computed assuming different evolutionary scenarios, and include a detailed simulation of observational effects.

2007

The main goal of this work is to find the best-fitting galaxy model for the Milky Way by comparing 10 different models with observed star counts and colour distributions. The observational data are taken from the Basel High-Latitude (Basel)and the Sloan Digital Sky (SDSS) photometric field star Surveys, respectively. We develop a code to derive synthetic spectra, absolute and apparent magnitudes and colours for stars given by a galactic evolutionary model (3D chemodynamical code for the Milky Way by Samland & Gerhard (2003)). The model galaxies provide stellar masses, ages, chemical abundances, velocities and positions at an evolutionary time of 13.5 Gyr. By means of the spectrophotometric data (given in the libraries of stellar evolutionary tracks (Padova 1994) and synthetic stellar spectra (BaSeL 3.2)), we build synthetic colour-magnitude diagrams, star counts, and ageand metallicity distributions for a number of viewing directions and field sizes. Our intention is to first compar...

Monthly Notices of the Royal Astronomical Society, 2013

We present Hubble Space Telescope (HST) Wide-Field Camera 3 (WFC3) images of the merger remnant NGC 7252. In particular, we focus on the surface brightness profiles and effective radii R eff of 36 young massive clusters (YMCs) within the galaxy. All the clusters have masses exceeding 10 5 M ⊙ and are, despite the 64 Mpc distance to the galaxy, (partly) resolved on the HST images. Effective radii can be measured down to ∼ 2.5 pc, and the largest clusters have R eff approaching 20 pc. The median R eff of our sample clusters is ∼ 6 − 7 pc, which is larger than typical radii of YMCs (∼ 2.5 pc). This could be due to our sample selection (only selecting resolved sources) or to an intrinsic mass-radius relation within the cluster population. We find at least three clusters that have power-law profiles of the Elson, Fall, & Freeman (1987, "EFF") type extending out to 150 pc. Among them are the two most massive clusters, W3 and W30, which have profiles that extend to at least 500 and 250 pc, respectively. Despite their extended profiles, the effective radii of the three clusters are 17.2, 12.6 and 9.1 pc for W3, W26 and W30, respectively. We compare these extended profiles with those of YMCs in the LMC (R136 in 30 Dor), the Antennae galaxies (Knot S) and in the nearby spiral galaxy NGC 6946. Extended profiles seem to be a somewhat common feature, even though many nearby YMCs show distinct truncations. A continuous distribution between these two extremes, i.e. truncated or extremely extended, is the most likely interpretation. We suggest that the presence or absence of an extended envelope in very young clusters may be due to the gas distribution of the proto-cluster giant molecular cloud, in particular if the proto-cluster core becomes distinct from the surrounding gas before star formation begins.

Galaxy Evolution in Groups and Clusters, 2003

We review selected measurements of the galaxy luminosity function including the global field, the local group, the local sphere, nearby clusters (Virgo, Coma and Fornax) and clusters in general. We conclude that the overall cluster luminosity function is consistent with the global luminosity function over the magnitude range in common (−22 ≤ MB − 5logh0.68 ≤ −17). We find that only in the core regions of clusters (r ≤ 300 kpc) does the overall form of the luminosity function show significant variation. However when the luminosity function is subdivided by spectral type some further variations are seen. We argue that these results imply: substantial late infall, efficient star-formation suppression, and the confinement of mass-changing evolutionary processes to the core regions only.