Matias Gomez

Cepheid period-radius and period-luminosity relations and the distance to the large magellanic cloud

We have used the infrared Barnes-Evans surface brightness technique to derive the radii and distances of 34 Galactic Cepheid variables. Radius and distance results obtained from both versions of the technique are in excellent agreement. The radii of 28 variables are used to determine the period-radius (PR) relation. This relation is found to have a smaller dispersion than in previous studies, and is identical to the PR relation found by Laney & Stobie from a completely independent method, a fact which provides persuasive evidence that the Cepheid PR relation is now determined at a very high confidence level. We use the accurate infrared distances to determine period-luminosity (PL) relations in the V, I, J, H, and K passbands from the Galactic sample of Cepheids. We derive improved slopes of these relations from updated LMC Cepheid samples and adopt these slopes to obtain accurate absolute calibrations of the PL relation. By comparing these relations to the ones defined by the LMC Cepheids, we derive strikingly consistent and precise values for the LMC distance modulus in each of the passbands that yield a mean value of μ0(LMC) = 18.46 ± 0.02. By analyzing the observed dispersions of the PL relations defined by the LMC and Galactic samples of Cepheids, we disentangle the contributions due to uncertainties in the reddenings, in distance measurement, and due to metallicity effects, and we estimate the intrinsic dispersion of the PL relation with the Wesenheit function. Assuming that the Galactic Cepheid distances are typically accurate to ±3% (as shown in a previous paper), and assuming an intrinsic spread in [Fe/H] of ~0.4 dex among the Cepheids of our sample as obtained by Fry & Carney, the observed dispersion of the Galactic Cepheid PL relation suggests a metallicity dependence of Δμ/Δ[Fe/H] ≈ 0.2, about half the value suggested by Sasselov et al. from EROS data. Since this estimate of the metallicity dependence of the PL (V) relation is rather uncertain, however, we prefer to retain μ0(LMC) = 18.46 as our best value, but with an increased uncertainty of ±0.06, most of which is due to the uncertainty in the appropriate metallicity correction. Our results show that the infrared Barnes-Evans technique is very insensitive to both Cepheid metallicity and adopted reddening, and therefore is a very powerful tool to derive accurate distances to nearby galaxies by a direct application of the technique to their Cepheid variables, rather than by comparing PL relations of different galaxies, which introduces much more sensitivity to metallicity and absorption corrections that are usually difficult to determine.

The ages, metallicities, and alpha element enhancements of globular clusters in the elliptical NGC 5128: A homogeneous spectroscopic study with gemini/gemini multi-object spectrograph

We present new integrated light spectroscopy of globular clusters (GCs) in NGC 5128, a nearby giant elliptical galaxy less than 4 Mpc away, in order to measure radial velocities and derive ages, metallicities, and alpha-element abundance ratios. Using the Gemini South 8 meter telescope with the instrument Gemini Multi-Object Spectrograph, we obtained spectroscopy in the range of ~3400-5700 ? for 72 GCs with a signal-to-noise ratio greater than 30 ??1; and we have also discovered 35 new GCs within NGC 5128 from our radial velocity measurements. We measured and compared the Lick indices from H? A through Fe5406 with the single stellar population models of Thomas et?al. in order to derive age, metallicity, and [?/Fe] values. We also measure Lick indices for 41 Milky Way GCs from Puzia et?al. and Schiavon et?al. with the same methodology for direct comparison. Our results show that 68% of the NGC 5128 GCs have old ages (>8 Gyr), 14% have intermediate ages (5-8 Gyr), and 18% have young ages ( 8 Gyr, indicating that the majority of both metallicity subpopulations of GCs formed earlier, with a significant population of young and metal-rich GCs forming later. Our metallicity distribution function generated directly from spectroscopic Lick indices is clearly bimodal, as is the color distribution of the same set of GCs. Thus, the metallicity bimodality is real and not an artifact of the color to metallicity conversion. However, the metallicity distribution function obtained from comparison with the single stellar population models is consistent with a unimodal, bimodal, or multimodal shape. The [?/Fe] values are supersolar with a mean value of 0.14 ? 0.04, indicating a fast formation timescale. However, the GCs in NGC 5128 are not as [?/Fe] enhanced as the Milky Way GCs also examined in this study. Our measured indices also indicate that the GCs in NGC 5128 may have a slight overabundance in nitrogen and a wider range of calcium strength compared to the Milky Way GCs. Our results support a rapid, early formation of the GC system in NGC 5128, with subsequent major accretion and/or GC and star-forming events in more recent times.

Very Accurate Distances and Radii of Open Cluster Cepheids from a Near-Infrared Surface Brightness Technique

We have obtained the radii and distances of 16 galactic Cepheids supposed to be members in open clusters or associations using a new optical and two near-infrared calibrations of the surface brightness (Barnes-Evans) method. We find excellent agreement of the radii and distances produced by both infrared techniques, which use the V, V - K (K on the Carter system) and the K, J - K magnitude-color combinations, respectively, with typical random errors that are as little as ~2%. We discuss possible systematic errors in our infrared solutions in detail and conclude that the typical total uncertainty of the infrared distance and radius of a Cepheid is about 3% in both infrared solutions, provided that the data are of excellent quality and that the amplitude of the color curve used in the solution is larger than ~0.3 mag. The optical V, V - R distance and radius of a given Cepheid can deviate by as much as ~30% from the infrared value because of large systematic and random errors caused by microturbulence and gravity variations: these affect the optical but not the infrared colors. We find excellent agreement of our infrared radii with the infrared radii derived previously for these variables by Laney & Stobie from an application of the maximum likelihood technique, which further increases our confidence that the total errors in our infrared solutions are not larger than ~3%. In an Appendix we discuss the relative advantages and disadvantages of our infrared surface brightness technique and the maximum likelihood technique. We compare the adopted infrared distances of the Cepheid variables to the zero-age main-sequence-fitting (ZAMS-fitting) distances of their supposed host clusters and associations (assuming a Pleiades distances modulus of 5.57) and find an unweighted mean value of the distance ratio of 1.02 ± 0.04. A detailed discussion of the individual Cepheids shows that the uncertainty of the ZAMS-fitting distances varies considerably from cluster to cluster. We find clear evidence that four Cepheids are not cluster members (SZ Tau, T Mon, U Car, and SV Vul), while we confirm cluster membership for V Cen and BB Sgr, for which former evidence for cluster membership was only weak. After rejection of nonmembers, we find a weighted mean distance ratio of 0.969 ± 0.014, with a standard deviation of 0.05, which demonstrates that both distance indicators are accurate to better than 5%, including systematic errors, and that there is excellent agreement between both distance scales.

BV(RI)C Photometry of Cepheids in the Magellanic Clouds

ABSTRACT We present V(RI)C data for 13 Cepheids in the Large Magellanic Cloud and eight in the Small Magellanic Cloud. The total number of new measures is 55 in each wavelength band. The median uncertainty in the photometry is ±0.03 mag. These results supplement a larger photometric program presented in the second paper in this series which contained 1000 measures (±0.01 mag) in each wavelength band on 22 variables with periods in the range 8–133 days.

Structural parameters from ground-based observations of newly discovered globular clusters in NGC 5128

We have investigated a number of globular cluster candidates from a recent wide-field study by Harris et al. (2004a, AJ, 128, 712) of the giant elliptical galaxy NGC 5128. We used the Magellan I telescope + MagIC camera under excellent seeing conditions (

STRUCTURAL PARAMETERS OF THE MESSIER 87 GLOBULAR CLUSTERS

0.3\arcsec{-}0.6\arcsec

The globular cluster system of NGC 4374

) and obtained very high resolution images for a sample of 44 candidates. Of these, 15 appear to be bonafide globular clusters in NGC 5128 while the rest are either foreground stars or background galaxies. We also serendipitously discovered 18 new cluster candidates in the same fields. Our images allow us to study the light profiles of the likely clusters, all of which are well resolved. This is the first ground-based study of structural parameters for globular clusters outside the Local Group. We compare the psf-deconvolved profiles with King models and derive structural parameters, ellipticities and surface brightnesses. We compare the derived structural properties with those of other well-studied globular cluster systems. In general, our clusters are similar in size, ellipticity, core radius and central surface brightness to their counterparts in other galaxies, in particular those in NGC 5128 observed with HST by Harris et al. (2002, AJ, 124, 1435). However, our clusters extend to higher ellipticities and larger half-light radii than their Galactic counterparts, as do the Harris et al. sample. Combining our results with those of Harris et al. fills in the gaps previously existing in

OBSERVATIONAL EVIDENCE FOR A DARK SIDE TO NGC 5128’S GLOBULAR CLUSTER SYSTEM*

r_{\rm h} - M_V

The Globular Cluster System of NGC 5128: Ages, Metallicities, Kinematics and Structural Parameters

parameter space and indicates that any substantial difference between presumed distinct cluster types in this diagram, including for example the Faint Fuzzies of Larsen & Brodie (2000, AJ, 120, 2938) and the “extended, luminous” M 31 clusters of Huxor et al. (2005, MNRAS, 360, 1007) is now removed and that clusters form a continuum in this diagram. Indeed, this continuum now extends to the realm of the Ultra Compact Dwarfs. The metal-rich clusters in our sample have half-light radii that are almost twice as large in the mean as their metal-poor counterparts, at odds with the generally accepted trend. The possibility exists that this result could be due in part to contamination by background galaxies. We have carried out additional analysis to quantify this contamination. This shows that, although galaxies cannot be easily told apart from clusters in some of the structural diagrams, the combination of excellent image quality and Washington photometry should limit the contamination to roughly 10% of the population of cluster candidates. Finally, our discovery of a substantial number of new cluster candidates in the relatively distant regions of the NGC 5128 halo suggests that current values of the total number of globular clusters may be underestimates.

A Direct Distance to the Large Magellanic Cloud Cepheid HV 12198 from the Infrared Surface Brightness Technique

We derive structural parameters for ~2000 globular clusters in the giant Virgo elliptical Messier 87 (M87) using extremely deep Hubble Space Telescope images in F606W (V) and F814W (I) taken with the ACS/WFC. The cluster scale sizes (half-light radii rh ) and ellipticities are determined from point-spread-function -convolved King-model profile fitting. We find that the rh distribution closely resembles the inner Milky Way clusters, peaking at rh 2.5 pc and with virtually no clusters more compact than rh 1 pc. The metal-poor clusters have on average an rh 24% larger than the metal-rich ones. The cluster scale size shows a gradual and noticeable increase with galactocentric distance. Clusters are very slightly larger in the bluer waveband V, a possible hint that we may be beginning to see the effects of mass segregation within the clusters. We also derived a color magnitude diagram for the M87 globular cluster system which shows a striking bimodal distribution.