We present an empirical assessment of the use of broadband optical colours as age indicators for unresolved extragalactic clusters and investigate stochastic sampling effects on integrated colours. We use the integrated properties of Galactic open clusters as models for unresolved extragalactic clusters. The population synthesis code Starburst99 (Leitherer et al. 1999) and four optical colours were used to estimate how well we can recover the ages of 62 well-studied Galactic open clusters with published ages. We provide a method for estimating the ages of unresolved clusters and for reliably determining the uncertainties in the age estimates. Our results support earlier conclusions based on comparisons to synthetic clusters, namely the (U-B) colour is critical to the estimation of the ages of star forming regions. We compare the observed optical colours with those obtained from Starburst99 using the published ages and get good agreement. The scatter in the (B-V)_observed-(B-V)_model is larger for lower luminosity clusters, perhaps due to stochastic effects.
For many years broadband colours have been used to obtain insight into the contents of galaxies, in particular to estimate stellar and dust content. Broadband colours are easy to obtain for large samples of objects, making them ideal for statistical studies. In this paper I use the radial distribution of the colours in galaxies, which gives more insight into the local processes driving the global colour differences than integrated colours. Almost all galaxies in my sample of 86 face-on galaxies become systematically bluer with increasing radius. The radial photometry is compared to new dust ex…
We present new K- and L'-band imaging of a representative sample of members of the young 3-5 Myr old sigma Orionis cluster. We identified objects with (K-L') excess by analysing colour-colour diagrams and comparing the observations with empirical main-sequence colours. The derived disk frequency depends on the method used: (54 +/- 15)% if measured directly from the JHKL' colour-colour diagram; or (46 +/-14)% if excesses are computed with respect to predicted photospheric colours (according to the objects spectral types, 2-sigma excess detections). We compare the (K-L') excess with other indica…
The I-Z colour has been recently shown to be a good temperature indicator for M dwarfs. We present the first IZ photometry of a small sample of L dwarfs ranging in spectral type from L0.5V to L6.0V. We find that the I-Z colour is not a good temperature indicator for objects between L1V and L5V, such objects having colours that overlap with mid M dwarfs. We attribute this to the reduction in the strength of the TiO and VO bands in the L dwarfs which are the dominant opacity source in the I band for late M dwarfs. Beyond L5V, I-Z appears to be a reasonable indicator. This has important implicati…
We present an improved calibration of photometric metallicity indicators, derived from the new metallicity scale for Globular Clusters presented by Carretta & Gratton (1997) and based on direct high resolution spectroscopy of 160 stars in 24 globular clusters. We have carefully recalibrated the traditional abundance indices based upon the red giant branch (RGB) morphology, both in the V,B-V and V,V-I planes, namely the dereddened colour at the luminosity level of the horizontal branch (HB), and the magnitude difference between the HB and the RBG at a given dereddened colour. Finally, we give n…
Context: The prediction of stellar angular diameters from broadband photometry plays an important role for different applications. In particular, long-baseline interferometry, gravitational microlensing, extrasolar planet transits, and many other observing techniques require accurate predictions of the angular size of stars. These predictions are based on the surface brightness-colour (SBC) relations. Aims: Our goal is to calibrate general-purpose SBC relations using visible colours, the most commonly available data for most stars. Methods: We compiled the existing long-baseline interferometri…
Broadband colours are often used as metallicity proxies in the study of extragalactic globular clusters. A common concern is the effect of variations in horizontal branch (HB) morphology--the second-parameter effect--on such colours. We have used UBVI, Washington, and DDO photometry for a compilation of over 80 Milky Way globular clusters to address this question. Our method is to fit linear relations between colour and [Fe/H], and study the correlations between the residuals about these fits and two quantitative measures of HB morphology. While there is a significant HB effect seen in U-B, fo…
A substantial fraction of the light emitted from young or star-forming galaxies at ultraviolet to near-infrared wavelengths comes from the ionized interstellar medium in the form of emission lines and a nebular continuum. At high redshifts, star formation rates are on average higher and stellar populations younger than in the local Universe. Both of these effects act to boost the impact of nebular emission on the overall spectrum of galaxies. Even so, the broadband fluxes and colours of high-redshift galaxies are routinely analyzed under the assumption that the light observed originates direct…
We present a study of flux-calibrated low-resolution optical spectroscopy of ten stars belonging to eight systems in the ~ 5 Myr-old Epsilon Chamaeleontis (Eps Cha) pre-main-sequence (PMS) star cluster. Using synthetic broadband colours, narrow-band continuum, atomic and molecular lines derived from the spectra, we compare the Eps Cha stars to a slightly older PMS cluster, the ~ 8 Myr-old Eta Cha cluster, and to main-sequence dwarfs. Using synthetic VRI colours and other indices, we find that the relationship between broadband colours and spectroscopic temperature indicators for Eps Cha cluste…
The main purpose of this study is to use the IUE spectra in the analysis of magnetic activity of main sequence F-K stars. Combining IUE observations of MgII and optical spectroscopy of Ca II, the registry of ctivity of stars can be extended in time. We retrieved all the high-resolution spectra of F, G, and K main sequence stars observed by IUE (i.e. 1623 spectra of 259 F to K dwarf stars). We obtained the continuum surface flux near the Mg II h+k lines near 2800 Åand the MgII line-core surface flux from the IUE spectra. We obtained a relation between the mean continuum flux near the MgII lines…
We perform an statistical analysis of galaxies in pairs in a Lambda-CDM scenario by using the chemical GADGET-2 of Scannapieco et al. (2005) in order to study the effects of galaxy interactions on colours and metallicities. We find that galaxy-galaxy interactions can produce a bimodal colour distribution with galaxies with significant recent star formation activity contributing mainly to blue colours. In the simulations, the colours and the fractions of recently formed stars of galaxies in pairs depend on environment more strongly than those of galaxies without a close companion, suggesting th…
We explore the origin of the colour-magnitude relation (CMR) of early type galaxies in the Virgo cluster using spectra of very high S/N ratio for six elliptical galaxies selected along the CMR. The data are analysed using a new evolutionary stellar population synthesis model to generate galaxy spectra at the resolution given by their velocity dispersions. In particular we use a new age indicator that is virtually free of the effects of metallicity. We find that the luminosity weighted mean ages of Virgo ellipticals are greater than ~8 Gyr, and show no clear trend with galaxy luminosity. We als…
The Edinburgh-Cape Blue Object Survey is an ongoing project to identify and analyse a large sample of hot stars selected initially on the basis of photographic colours (down to a magnitude limit B~18.0) over the entire high-Galactic-latitude southern sky, then studied with broadband UBV photometry and medium-resolution spectroscopy. Due to unavoidable errors in the initial candidate selection, stars that are likely metal-deficient dwarfs and giants of the halo and thick-disk populations are inadvertently included, yet are of interest in their own right. In this paper we discuss a total of 206 …
We present J and K-band near-infrared photometry of a sample of mid-infrared sources detected by the Infrared Space Observatory (ISO) as part of the European Large Area ISO-Survey (ELAIS) and study their classification and star-forming properties. We have used the Preliminary ELAIS Catalogue for the 6.7 micron (LW2) and 15 micron (LW3) fluxes. All of the high-reliability LW2 sources and 80 per cent of the LW3 sources are identified in the near-IR survey reaching K = 17.5 mag. The near- to mid-IR flux ratios can effectively be used to separate stars from galaxies in mid-IR surveys. The stars de…
We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyr using UCAC2 proper motion data and 2MASS photometry. For each cluster we consider the projected velocity distributions in the core and off-core regions separately. In the projected velocity distribution of all sample clusters we find a well-defined low-velocity peak, as well as an excess in the number of stars at larger velocities. The low-velocity peak is accounted for by the random motion of the single stars, while the high-velocity excess can be attributed to the large velocity changes produced by a significant fraction of …
We present the results of our analysis of the photometric data of globular clusters in the elliptical galaxy NGC 5128 (Cen A). We show that the integrated colour U-B can be an effective metallicity indicator for simple stellar populations. This is because it is sensitive to metallicity via the opacity effect but relatively insensitive to the effective main sequence turn-off temperature of the population (and thus to age) when Teff ~ 7000-12000K, that is, when the Balmer Jump is strong. This flat U-B vs Teff relation is a result of the fact that the blueing effect of the optical continuum with …
The M33 galaxy is a nearby, relatively metal-poor, late-type spiral. Its proximity and almost face-on inclination means that it projects over a large area on the sky, making it an ideal candidate for wide-field CCD mosaic imaging. Photometry was obtained for more than 10^6 stars covering a 74' x 56' field centered on M33. Main sequence (MS), supergiant branch (SGB), red giant branch (RGB) and asymptotic giant branch (AGB) populations are identified and classified based on broad-band V and I photometry. Narrow-band filters are used to measure spectral features allowing the AGB population to be …
We investigate the broadband optical emission of diffuse intergalactic stars in galaxy clusters at z~0.25 by means of an image stacking technique. The images of 654 clusters, selected with the max-BCG algorithm from a subsample of the SDSS-Data Release 1 (DR1), have been stacked after masking all the sources detected down to very low S/N. The resulting images in the g, r and i bands provide reliable photometric data at the level of >=30 mag arcsec^-2 (r band), out to >~ 600 kpc from the Brightest Cluster Galaxy (BCG). Our analysis shows that: i) the IntraCluster Light (ICL) is much more concen…
To investigate the distribution of dust in Sb and Sc galaxies we have analyzed near-infrared and optical surface photometry for an unbiased sample of 37 galaxies. Since light in the $K$-band is very little affected by extinction by dust, the $B-K$ colour is a good indicator of the amount of extinction, and using the colour-inclination relation we can statistically determine the extinction for an average Sb/Sc galaxy. We find in general a considerable amount of extinction in spiral galaxies in the central regions, all the way out to their effective radii. In the outer parts, at D$_{K,21}$, or a…
The main chromospheric activity indicator is the S index, which is esentially the ratio of the flux in the core of the Ca II H and K lines to the continuum nearby, and is well studied basically for stars from F to K. Another usual chromospheric proxy is the Hαline, which is beleived to be tightly correlated with the Ca II index. In this work we characterize both chromospheric activity indicators, one associated with the H and K Ca II lines and the other with Hα, for the whole range of late type stars, from F to M. We present periodical medium-resolution echelle observations covering the comple…
(abridged) Accurate reddenings for Globular Clusters could be obtained by comparing the colour-temperature obtained using temperatures from reddening-free indicator (Halpha), with that given by standard colour-temperature calibrations. From a single 1300 seconds exposure with FLAMES at VLT2 we obtained spectra centred on Halpha (R=6000, 5<S/N<50) for 120 stars near the turn-off of NGC6752. This set of spectra was used to derive effective temperatures from fittings of Halpha profiles with typical errors of about +/- 200 K and reddening estimates with individual errors of 0.05 mag. Averaging all…
We present results derived from VLT-FORS2 spectra of 17 globular clusters associated with the nearby lenticular galaxy NGC3115. Comparing line-strength indices to new stellar population models by Thomas et al. we determine ages, metallicities and element abundance ratios. Our data are also compared with the Lick/IDS observations of Milky Way and M31 globular clusters. Our best age estimates show that the observed clusters which sample the bimodal colour distribution of NGC3115 globular clusters are coeval within our observational errors (2-3 Gyr). Our best calibrated age/metallicity diagnostic…
Globular cluster age estimates based on the absolute magnitude of the main sequence turn-off (\mvto) are generally considered to be the most reliable from a theoretical viewpoint. However, the difficulty in determining \mvto\ in observed colour-magnitude diagrams leads to a large error in the derived age. In this paper, we advocate the use of the absolute magnitude of the point which is brighter than the turn-off and 0.05 mag redder (\mvb) as a precision age indicator. It is easy to measure this point on observed colour-magnitude diagrams, leading to small observational error bars. Furthermore…
We present results derived from VLT-FORS2 spectra of 24 different globular clusters associated with the lenticular galaxy NGC3115. A subsample of 17 globular clusters have sufficiently high signal-to-noise to allow precision measurements of absorption line-strengths. Comparing these indices to new stellar population models by Thomas et al. we determine ages, metallicities and element abundance ratios. Our data are also compared with the Lick/IDS observations of Milky Way and M31 globular clusters. Unpublished higher order Balmer lines (HgammaA,F and HdeltaA,F) from the Lick/IDS observations ar…
