arXiv subjects

M. Romaniello

Publications and source records attributed to M. Romaniello.

At least 73 records · Page 4Linked to original sources

arXiv2007

Cepheids as tracers of the metallicity gradient across the Galactic disk

We present iron abundance measurements, based on high resolution spectroscopy, and accurate distance determinations, based on near infrared photometry, for 34 Galactic Cepheids. The new data are used to constrain the Galactic iron abundance gradient in the outer disk, namely from 10 to 14 kpc. We confirm the flattening of the gradient toward the outer disk. In this region we also found an increase in the metallicity dispersion. Current data do not support the occurrence of a jump in the metallicity gradient for Galactocentric distances of the order of 10-12 kpc.

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arXiv2007

Stroemgren metallicity calibration: the m1, b-y relation

We performed a new calibration of the Stroemgren metallicity index m1 based on the b-y color of cluster red giant stars. The current Metallicity-Index-Color relation is not linear in the color range 0.40 < b-y < 1.0, but provides iron abundances of cluster and field red giants with an accuracy of ~ 0.25 dex.

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arXiv2007

The Peculiar Horizontal Branch Morphology of the Galactic Globular Clusters NGC 6388 and NGC 6441: new insights from UV observations

In this paper we present multiband optical and UV Hubble Space Telescope photometry of the two Galactic globular clusters NGC 6388 and NGC 6441. We have obtained the first UV color-magnitude diagrams for NGC 6388 and NGC 6441. These diagrams confirm previous results, obtained in optical bands, about the presence of a sizeable stellar population of extremely hot horizontal branch stars. At least in NGC 6388, we find a clear indication that at the hot end of the horizontal branch the distribution of stars forms a hook-like feature, closely resembling those observed in NGC 2808 and Omega Centauri. We briefly review the theoretical scenarios that have been suggested for interpreting this observational feature. We also investigate the tilted horizontal branch morphology and provide further evidence that supports early suggestions that this feature cannot be interpreted as an effect of differential reddening. We show that a possible solution of the puzzle is to assume that a small fraction - ranging between 10-20% - of the stellar population in the two clusters is strongly helium enriched (Y=0.40 in NGC 6388 and Y=0.35 in NGC 6441). The occurrence of a spread in the He abundance between the canonical value (Y=0.26) and the quoted upper limits can significantly help in explaining the whole morphology of the horizontal branch and the pulsational properties of the variable stars in the target clusters.

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arXiv2007

Stroemgren photometry of Galactic Globular Clusters. I. New Calibrations of the metallicity index

We present a new calibration of the Stroemgren metallicity index m1 using red giant (RG) stars in 4 globular clusters (GCs:M92,M13,NGC1851,47Tuc) with metallicity ranging from [Fe/H]=-2.2 to -0.7, marginally affected by reddening (E(B-V)<0.04) and with accurate u,v,b,y photometry.The main difference between the new metallicity-index-color (MIC) relations and similar relations available in the literature is that we adopted the u-y/v-y colors instead of the b-y.These colors present a stronger sensitivity to effective temperature, and the MIC relations show a linear slope. The difference between photometric estimates and spectroscopic measurements for RGs in M71,NGC288,NGC362,NGC6397, and NGC6752 is 0.04+/-0.03dex (sigma=0.11dex). We also apply the MIC relations to 85 field RGs with metallicity raning from [Fe/H]=-2.4 to -0.5 and accurate reddening estimates. We find that the difference between photometric estimates and spectroscopic measurements is-0.14+/-0.01dex (sig=0.17dex). We also provide two sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semi-empirical or theoretical color-temperature relations. We apply the semi-empirical relations to the 9 GCs and find that the difference between photometric and spectroscopic metallicities is 0.04+/-0.03dex (sig=0.10dex).A similar agreement is found for the sample of field RGs, with a difference of -0.09+/-0.03dex (sig=0.19dex).The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11+/-0.03dex (sig=0.14dex) for the 9 GGCs and -0.24+/-0.03dex (sig=0.15dex) for the field RGs. Current evidence indicates that new MIC relations provide metallicities with an intrinsic accuracy better than 0.2dex.

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arXiv2007

A Hubble View of Star Forming Regions in the Magellanic Clouds

The Magellanic Clouds (MCs) offer an outstanding variety of young stellar associations, in which large samples of low-mass stars (with masses less than 1 solar mass) currently in the act of formation can be resolved and explored sufficiently with the Hubble Space Telescope. These pre-main sequence (PMS) stars provide a unique snapshot of the star formation process, as it is being recorded for the last 20 Myr, and they give important information on the low-mass Initial Mass Function (IMF) of their host environments. We present the latest results from observations with the Advanced Camera for Surveys (ACS) of such star-forming regions in the MCs, and discuss the importance of Hubble}for a comprehensive collection of substantial information on the most recent low-mass star formation and the low-mass IMF in the MCs.

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arXiv2007

Detailed chemical composition of Galactic Cepheids. A determination of the Galactic abundance gradient in the 8-12 kpc region

The recent introduction of high resolution/large spectral range spectrographs provided the opportunity to investigate in detail the chemical composition of classical Cepheids. This paper is focussed on new abundance determinations for iron and 6 light metals (O, Na, Mg, Al, Si, Ca) in 30 Galactic Cepheids. We also give a new estimate of the Galactic radial abundance gradient. The stellar effective temperatures have been determined using the method of line depth ratios, while the surface gravity and the microturbulent velocity v$_{t}$ by imposing the ionization balance between Fe I and Fe II and the help of the curves of growth. Abundances were calculated with classical LTE atmosphere models. Abundances are obtained with RMS accuracies of the order of 0.05-0.10 dex for Fe, and 0.05-0.20 dex for the other elements. Cepheids in our sample have solar-like abundances and current measurements agree quite-well with previous determinations. We computed ``single zone'' Galactic radial abundance gradients for the 8-12 kpc region and found a slope for iron of -0.061 dex kpc$^{-1}$.

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arXiv2006

Photometry with FORS at the ESO VLT

ESO's two FOcal Reducer and low dispersion Spectrographs (FORS) are the primary imaging cameras for the VLT. Since they are not direct-imaging cameras, the accuracy of photometry which can routinely be obtained is limited by significant sky concentration and other effects. Photometric standard observations are routinely obtained by ESO, and nightly zero points are computed mainly for the purpose of monitoring the instrument performance. The accuracy of these zero points is about 10%. Recently, we have started a program to investigate, if and how percent-level absolute photometric accuracy with FORS can be achieved. The main results of this project are presented in this paper. We first discuss the quality of the flatfields and how it can be improved. We then use data with improved flat-fielding to investigate the usefulness of Stetson standard fields for FORS calibration and the accuracy which can be achieved. The main findings of the FORS Absolute Photometry Project program are as follows. There are significant differences between the sky flats and the true photometric response of the instrument which partially depend on the rotator angle. A second order correction to the sky flat significantly improves the relative photometry within the field. Percent level photometric accuracy can be achieved with FORS1. To achieve this accuracy, observers need to invest some of the assigned science time for imaging of photometric standard fields in addition to the routine nightly photometric calibration.

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arXiv2006

RASS-SDSS Galaxy Cluster Survey. VI. The dependence of the cluster SFR on the cluster global properties

Using a subsample of 79 nearby clusters from the RASS-SDSS galaxy cluster catalogue of Popesso et al. (2005a), we perform a regression analysis between the cluster integrated star formation rate (Sigma_SFR) the cluster total stellar mass (M_star), the fractions of star forming (f_SF) and blue (f_b) galaxies and other cluster global properties, namely its richness (N_gal, i.e. the total number of cluster members within the cluster virial radius), velocity dispersion (sigma_v), virial mass (M_200), and X-ray luminosity (L_X). All cluster global quantities are corrected for projection effects before the analysis. Galaxy SFRs and stellar masses are taken from the catalog of Brinchmann et al. (2004), which is based on SDSS spectra. We only consider galaxies with M_r <= -20.25 in our analysis, and exclude AGNs. We find that both Sigma_SFR and M_star are correlated with all the cluster global quantities. A partial correlation analysis show that all the correlations are induced by the fundamental one between Sigma_SFR and N_gal, hence there is no evidence that the cluster properties affect the mean SFR or M_star per galaxy. The relations between Sigma_SFR and M_star, on one side, and both N_gal and M_200, on the other side, are linear, i.e. we see no evidence that different clusters have different SFR or different M_star per galaxy and per unit mass. The fraction f_SF does not depend on any cluster property considered, while f_b does depend on L_X. We note that a significant fraction of star-forming cluster galaxies are red (~25% of the whole cluster galaxy population). We conclude that the global cluster properties are unable to affect the SF properties of cluster galaxies, but the presence of the X-ray luminous intra-cluster medium can affect their colors, perhaps through the ram-pressure stripping mechanism.

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arXiv2006

RASS-SDSS Galaxy Cluster Survey. VII. On the Cluster Mass to Light ratio and the Halo Occupation Distribution

We explore the mass-to-light ratio in galaxy clusters and its relation to the cluster mass. We study the relations among the optical luminosity ($L_{op}$), the cluster mass ($M_{200}$) and the number of cluster galaxies within $r_{200}$ ($N_{gal}$) in a sample of 217 galaxy clusters with confirmed 3D overdensity. We correct for projection effects, by determining the galaxy surface number density profile in our cluster sample. This is best fitted by a cored King profile in low and intermediate mass systems. The core radius decreases with cluster mass, and, for the highest mass clusters, the profile is better represented by a generalized King profile or a cuspy Navarro, Frenk & White profile. We find a very tight proportionality between $L_{op}$ and $N_{gal}$, which, in turn, links the cluster mass-to-light ratio to the Halo Occupation Distribution $N_{gal}$ vs. $M_{200}$. After correcting for projection effects, the slope of the $L_{op}-M_{200}$ and $N_{gal}-M_{200}$ relations is found to be $0.92\pm0.03$, close, but still significantly less than unity. We show that the non-linearity of these relations cannot be explained by variations of the galaxy luminosity distributions and of the galaxy M/L with the cluster mass. We suggest that the nonlinear relation between number of galaxies and cluster mass reflects an underlying nonlinear relation between number of subhaloes and halo mass.

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arXiv2006

RASS-SDSS Galaxy Cluster Survey. V. The X-ray-Underluminous Abell Clusters

In this paper we consider a large sample of optically selected clusters, in order to elucidate the physical reasons for the existence of X-ray underluminous clusters. For this purpose we analyze the correlations of the X-ray and optical properties of a sample of 137 spectroscopically confirmed Abell clusters in the SDSS database. We search for the X-ray counterpart of each cluster in the ROSAT All Sky Survey. We find that 40% of our clusters have a marginal X-ray detection or remain undetected in X-rays. These clusters appear too X-ray faint on average for their velocity dispersion determined mass, i.e. they do not follow the scaling relation between X-ray luminosity and virial mass traced by the other clusters. On the other hand, they do follow the general scaling relation between optical luminosity and virial mass. We refer to these clusters as the X-ray-Underluminous Abell clusters (AXU clusters, for short) and designate as 'normal' the X-ray detected Abell systems. We examine the distributions and properties of the galaxy populations of the normal and the AXU clusters, separately. The AXU clusters are characterized by leptokurtic (more centrally concentrated than a Gaussian) velocity distribution of their member galaxies in the outskirts ($1.5 < r/r_{200} \leq 3.5$), as expected for the systems in accretion. In addition, the AXU clusters have a higher fraction of blue galaxies in the external region and show a marginally significant paucity of galaxies at the center. Our results seem to support the interpretation that the AXU clusters are systems in formation undergoing a phase of mass accretion. Their low X-ray luminosity should be due to the still accreting Intracluster gas or to an ongoing merging process.

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arXiv2005

The Chemical Composition of Cepheids in the Milky Way and the Magellanic Clouds

We have measured the elemental abundances of 68 Galactic and Magellanic Cepheids from FEROS and UVES high-resolution and high signal-to-noise spectra in order to establish the influence of the chemical composition on the properties of these stars (see Romaniello et al. 2005). Here we describe the robust analytical procedure we have developed to accurately determine them. The resulting iron abundances span a range between ~ -0.80 dex for stars in the Small Magellanic Cloud and ~ +0.20 dex for the most metal-rich ones in the Galaxy. While the average values for each galaxy are in good agreement with non-pulsating stars of similar age, Cepheids display a significant spread. Thus it is fundamental to measure the metallicity of individual stars.

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arXiv2005

The Metallicity Dependence of the Cepheid Period-Luminosity Relation

We have assessed the influence of the stellar iron content on the Cepheid Period-Luminosity (PL) relation by relating the V band residuals from the Freedman et al (2001) PL relation to [Fe/H] for 68 Galactic and Magellanic Cloud Cepheids. The iron abundances were measured from FEROS and UVES high-resolution and high signal-to-noise optical spectra. Our data indicate that the stars become fainter as metallicity increases, until a plateau or turnover point is reached at about solar metallicity. This behavior appears at odds both with the PL relation being independent from iron abundance and with Cepheids becoming monotonically brighter as metallicity increases (e.g. Kennicutt et al 1998, Sakai et al 2004).

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arXiv2005

RASS-SDSS Galaxy Cluster Survey. IV. A ubiquitous dwarf galaxy population in clusters

We analyze the Luminosity Functions (LFs) of a subsample of 69 clusters from the RASS-SDSS galaxy cluster catalog. When calculated within the cluster physical sizes, given by r200 or r500, all the cluster LFs appear to have the same shape, well fitted by a composite of two Schechter functions with a marked upturn and a steepening at the faint-end. Previously reported cluster-to-cluster variations of the LF faint-end slope are due to the use of a metric cluster aperture for computing the LF of clusters of different masses. We determine the composite LF for early- and late-type galaxies, where the typing is based on the galaxy u-r colors. The late-type LF is well fitted by a single Schechter function with a steep slope (alpha=-2.0 in the r band, within r200}). The early-type LF instead cannot be fitted by a single Schechter function, and a composite of two Schechter functions is needed. The faint-end upturn of the global cluster LF is due to the early-type cluster galaxies. The shape of the bright-end tail of the early-type LF does not seem to depend upon the local galaxy density or the distance from the cluster center. The late-type LF shows a significant variation only very near the cluster center. On the other hand, the faint-end tail of the early-type LF shows a significant and continuous variation with the environment. We provide evidence that the process responsible for creating the excess population of dwarf early type galaxies in clusters is a threshold process that occurs when the density exceeds ~500 times the critical density of the Universe. We interpret our results in the context of the 'harassment' scenario, where faint early-type cluster galaxies are predicted to be the descendants of tidally-stripped late-type galaxies.

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arXiv2004

The influence of chemical composition on the properties of Cepheid stars I - Period-Luminosity relation vs iron abundance

We have assessed the influence of the stellar iron content on the Cepheid Period-Luminosity (PL) relation by relating the V band residuals from the Freedman et al (2001) PL relation to [Fe/H] for 37 Galactic and Magellanic Clouds Cepheids. The iron abundances were measured from FEROS and UVES high-resolution and high-signal to noise optical spectra. Our data indicate that the stars become fainter as metallicity increases, until a plateau or turnover point is reached at about solar metallicity. Our data are incompatible with both no dependence of the PL relation on iron abundance, and with the linearly decreasing behavior often found in the literature (e.g. Kennicutt et al 1998, Sakai et al 2004). On the other hand, non-linear theoretical models of Fiorentino et al (2002) provide a fairly good description of the data.

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arXiv2004

The RASS-SDSS Galaxy Cluster Survey. III. Scaling relations of galaxy clusters

We use the RASS-SDSS galaxy cluster sample to compare the quality of optical and X-ray luminosities as predictors of other cluster properties such as their masses, temperatures, and velocity dispersions. We use the SDSS spectroscopic data to estimate the velocity dispersions and the virial masses of a subsample of 69 clusters within r_{500} and r_{200}. The ASCA temperature of the intra-cluster medium, T_X, is retrieved from the literature for a subsample of 49 clusters. For this subsample we estimate the cluster masses also by using the mass-temperature relation. We show that the optical luminosity, L_{op}, correlates with the cluster mass much better than the X-ray luminosity, L_X. L_{op} can be used to estimate the cluster mass with an accuracy of 40% while L_X can predict the mass only with a 55% accuracy. We show that correcting $L_X$ for the effect of a cool core at the center of a cluster, lowers the scatter of the $L_X-M$ relation only by 3%. We find that the scatter observed in the L_{op}-L_X relation is determined by the scatter of the L_X-M relation. The mass-to-light ratio in the SDSS i band clearly increases with the cluster mass with a slope 0.2\pm0.08. The optical and X-ray luminosities correlate in excellent way with both T_X and σ_V with an orthogonal scatter of 20% in both relations. Moreover, L_{op} and L_X can predict with the same accuracy both variables. We conclude that the cluster optical luminosity is a key cluster parameter since it can give important information about fundamental cluster properties such as the mass, the velocity dispersion, and the temperature of the intra-cluster medium.

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arXiv2004

The Chandra Deep Field South: Optical Spectroscopy I

We present the results of our spectroscopic follow-up program of the X-ray sources detected in the 942 ks exposure of the Chandra Deep Field South (CDFS). 288 possible counterparts were observed at the VLT with the FORS1/FORS2 spectrographs for 251 of the 349 Chandra sources (including three additional faint X-ray sources). Spectra and R-band images are shown for all the observed sources and R-K colours are given for most of them. Spectroscopic redshifts were obtained for 168 X-ray sources, of which 137 have both reliable optical identification and redshift estimate (including 16 external identifications). The R<24 observed sample comprises 161 X-ray objects (181 optical counterparts) and 126 of them have unambiguous spectroscopic identification. We introduce a new classification based solely on X-ray properties, HR and X-ray luminosity, consistent with the unified AGN model. This X-ray classification uncovers a large fraction of optically obscured, X-ray luminous AGNs missed by the classical optical classification.

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arXiv2004

The metallicity dependence of the Cepheid PL-relation

A sample of 37 Galactic, 10 LMC and 6 SMC cepheids is compiled for which individual metallicity estimates exist and BVIK photometry in almost all cases. The Galactic cepheids all have an individual distance estimate available. For the MC objects different sources of photometry are combined to obtain improved periods and mean magnitudes. A multi-parameter Period-Luminosity relation is fitted to the data which also solves for the distance to the LMC and SMC. When all three galaxies are considered, without metallicity effect, a significant quadratic term in log P is found, as previously observed and also predicted in some theoretical calculations. For the present sample it is empirically determined that for log P < 1.65 linear PL-relations may be adopted, but this restricts the sample to only 4 LMC and 1 SMC cepheid. Considering the Galactic sample a metallicity effect is found in the zero point in the VIWK PL-relation (-0.6 \pm 0.4 or -0.8 \pm 0.3 mag/dex depending on the in- or exclusion of one object), in the sense that metal-rich cepheids are brighter. The small significance is mostly due to the fact that the Galactic sample spans a narrow metallicity range. The error is to a significant part due to the error in the metallicity determinations and not to the error in the fit. Including the 5 MC cepheids broadens the observed metallicity range and a metallity effect of about -0.27 \pm 0.08 mag/dex in the zero point is found in VIWK, in agreement with some previous empirical estimates, but now derived using direct metallicity determinations for the cepheids themselves.

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arXiv2003

Determining the Metallicity of Cepheid Stars in the SMC, LMC and the Galaxy

The Cepheid Period-Luminosity relation is unquestionably one of the most powerful tools at our disposal for determining the extragalactic distance scale. While significant progress has been made in the past few years towards its understanding and characterisation, both on the observational (e.g. the HST Key Project) and theoretical (e.g. non-linear pulsation models, non-LTE atmospheres etc.) sides, the debate on the influence that chemical composition may have on the Period-Luminosity relation is still unsettled. Current estimates lead to differences in the distance as large as 15%, effectively limiting the accuracy of Cepheids as distance indicators. To further tackle this problem, we have obtained high resolution spectra of a large sample of Cepheids in our Galaxy and the Magellanic Clouds. The superb quality of the data allow us to probe the detailed effects of chemical composition (alpha, iron-group, and heavy elements) over more than a factor of ten in metallicity. Here, we present the first preliminary results of the analysis of iron abundances in a sub-sample of Cepheids.

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