Exploring the UV excess in star clusters of different mass

The Astrophysical Journal, 2008

A large fraction of stars form within young embedded clusters, and these environments produce a substantial ultraviolet (UV) background radiation field, which can provide feedback on the star formation process. To assess the possible effects of young stellar clusters on the formation of their constituent stars and planets, this paper constructs the expected radiation fields produced by these clusters. We include both the observed distribution of cluster sizes N in the solar neighborhood and an extended distribution that includes clusters with larger N. The paper presents distributions of the FUV and EUV luminosities for clusters with given stellar membership N, distributions of FUV and EUV luminosity convolved over the expected distribution of cluster sizes N, and the corresponding distributions of FUV and EUV fluxes. These flux distributions are calculated both with and without the effects of extinction. Finally, we consider the effects of variations in the stellar initial mass function on these radiation fields. Taken together, these results specify the distributions of radiation environments that forming solar systems are expected to experience.

Mon Notic Roy Astron Soc, 2005

Cooling in the first halos at high redshift is dominated by molecular hydrogen, H_2 - we call these Generation 1 objects. At lower redshift and higher virial temperature, T_vir > 10^4K, electron cooling dominates - we call these generation 2. Radiation fields act to photo-dissociate H_2, but also generate free electrons that can help to catalyse its production. At modest radiation levels, J_{21}/(1+z)^3 ~ 10^{-12}-10^{-7}, the nett effect is to enhance the formation of Generation 1 star-clusters. At higher fluxes the heating from photo-ionisation dominates and halts their production. With a realistic build-up of flux over time, the period of enhanced H_2 cooling is so fleeting as to be barely discernable and the nett effect is to move primordial star cluster formation towards Generation 2 objects at lower redshift. A similar effect is seen with local feedback. Provided that enough photons are produced to maintain ionization of their host halo, they will suppress the cooling in Generation 1 halos and boost the numbers of primordial star clusters in Generation 2 halos. Significant suppression of Generation~1 halos occurs for specific photon fluxes in excess of about 10^{43} ph s^{-1} Msun^{-1}.

To understand how galaxies form and evolve, we need to study their stellar populations. And Globular Clusters are among the best stellar populations to investigate. They are fossils of important events in the lives of galaxies, and they are intrinsically bright objects that can be observed at large distances so that they can sample very early phases of the formation of galaxies and test the age of the Universe. They are made up of a large population of stars, all located at virtually the same distance from us, and possibly of the same age and chemical composition. They are the best examples of simple stellar populations, and thus natural laboratories to study stellar evolution. Depending on its initial mass, every star goes through specific evolutionary stages dictated by its internal structure and how it produces energy. Each of these stages corresponds to a change in the temperature and luminosity of the star; therefore we can know a star’s internal structure and evolutionary stage simply by determining its position in the HR-Diagram. For globular clusters far away, individual stars cannot be measured, and the studies must rely on the summed light of all the stars: the integrated spectrum. When it comes to Globular Clusters it is necessary to use a discrete method to try and create an HR-diagram or an integrated spectrum similar to the real ones because we found out that for a small number of clusters we have a stochastic behavior. Many studies have been done to try and fit the integrated spectrum for a single stellar population and in this report we are going to introduce a new method based on a discrete mode of calculation. It is called Fast-Discrete that generates N number along the isochrones for a Single Stellar Population given under the assumption of Kroupa 2001 initial mass function for a given age, metallicity and for a result we will have a discrete HR-Diagram. The indices we focus on in this report are H_beta0 as age indicator, [MgFe]’as metal sensitive, NaD and TiO2 as IMF-sensitive indicator. The aim of this report is to study the effect of stochastic nature of finite stellar population on spectroscopic indices and to see if this dispersion neglected till now has an impact on the analysis that explores the indices H_beta0, [MgFe]’, NaD, TiO2. We find that stochastic fluctuations cannot be neglected for a small number of stars. When studying the stochastic behavior of the indices we saw that TiO2 so as NaD are both highly affected by fluctuations, but TiO2 is much more affected by the fluctuation than NaD. Which make us re-consider the use of TiO2 as a good IMF-indicator. Furthermore, we consider H_beta0 and [MgFe]’ respectively good age and metallicity indicators for not being highly affected by fluctuations.

The Astrophysical Journal, 1993

We present an analysis of the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is that the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. Such objects have Zero Age Horizontal Branch envelope masses M 0 env < 0:05M. Local examples of EHB stars exist in some globular clusters and in the Galactic disk field and serve both as a guide and constraint. We re-derive the broad-band UV colors 1500 V and 2500 V for globular clusters and elliptical galaxies from the available satellite data and investigate color-color and color-line strength correlations. There are several important distinctions between clusters and galaxies. They do not occupy a single Mg 2-color sequence. Clusters can be bluer than any galaxy in 15 V and 25 V , implying larger hot star populations, but galaxies are significantly bluer than clusters in 15 25 at a given 15 V. We attribute this primarily to the effect of metal abundance on the mid-UV (2500 A) light. It also implies that the UVX in galaxies is not produced by metal poor subpopulations similar to the clusters. We develop a simple spectral synthesis formulation for all phases of single star evolution from the ZAMS to the white dwarf cooling track that requires only one or two parameters for each choice of age and abundance. We provide the ingredients necessary for constructing models with arbitrary HB morphologies in the age range 2 < t < 20 Gyr and for 6 metallicities in the range 2:26 < [Fe=H] < 0:58; we also consider the effect of enhanced Y in metal rich models. UV properties of the models are predicted using the Kurucz (1991) atmospheres. The maximum lifetime UV output is produced by EHB stars with M 0 env 0:02M , and can be up to 30 times higher than for post-asymptotic-giant-branch (P-AGB) stars. The ultraviolet output of old populations is governed primarily by the distribution of M 0 env , P(M 0 env), on the ZAHB. The UV output is not very sensitive to [Fe=H] or to Y , but it can change very rapidly with M 0 env. Thus, it is extremely sensitive to the precise nature of giant branch mass loss. Because this process is not well understood physically, we choose to leave mass loss as an implicit free parameter. Our models use simple descriptions of P(M 0 env) to bracket the colors produced from any real distribution of stars. Our models accurately predict the range of UV colors observed for the globular clusters, given known constraints on their age, abundances, and HB morphologies. Clusters with "blue HB" morphologies do not require the hotter EHB stars to explain their UV colors, although a small EHB population is consistent with our models. The largest known population of these stars in a cluster, as a fraction of the total HB, is 20% in ! Cen. For [Fe=H] > 0:5, however, blue HB stars will be rare. As a consequence, we find that models with [Fe=H] > 0 which do not contain EHB stars cannot reproduce the colors of most of the galaxies. However, only small EHB fractions are required: < 5% for the bulk of the E galaxies and 20% for those with the strongest UVX. These results are independent of the assumed [Fe/H]. The EHB fraction required for most galaxies is comparable to the fraction of hot subdwarfs in the Galactic disk. Most of these are EHB stars, and their existence considerably strengthens the case for EHB populations as the source of elliptical galaxy UV light. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is 70% and 20% for moderate UVX and strong UVX systems, respectively. We find that 25 V , but not 15 V , is sensitive to the age and abundance, though these cannot always be cleanly distinguished. The galaxy colors place strong limits of h[Fe=H]i > 0:5 and < 15% on the contribution of globular cluster-type populations to the V light. Galaxy colors are consistent with solar-abundance models with ages in the range 6-14 Gyr. However, the 25 V colors of the galaxies other than the strong UVX systems are too blue to be consistent with [Fe=H] > 0:2 for any age. This may be additional evidence that [Mg/Fe] > 0 in elliptical galaxies. UV colors for M32 are consistent with the solar abundance, intermediate age (4-6 Gyr) population inferred from optical/IR observations.

Monthly Notices of the …, 2010

Monthly Notices of the Royal Astronomical Society, 2019

We explore the dependence of ultraviolet (UV) upturn colours in early-type cluster galaxies on the properties of their parent clusters (such as velocity dispersion and X-ray luminosity) and on the positions and kinematics of galaxies within them. We use a sample of 24 nearby clusters with highly complete spectroscopy and optical/infrared data to select a suitable sample of red-sequence galaxies, whose far-ultraviolet and NUV magnitudes we measure from archival GALEX data. Our results show that the UV upturn colour has no dependence on cluster properties and has the same range in all clusters. There is also no dependence on the projected position within clusters or on line-of-sight velocity. Therefore, our conclusion is that the UV upturn phenomenon is an intrinsic feature of cluster early-type galaxies, irrespective of their cluster environment.

Proceedings of the International Astronomical Union, 2012

This paper is part of a series devoted to the investigation of a large sample of brightest cluster galaxies (BCGs), their properties and the relationships between these and the properties of the host clusters. In this paper, we compare the stellar population properties derived from high signal-to-noise, optical long-slit spectra with the GALEX ultraviolet (UV) colour measurements for 36 nearby BCGs to understand the diversity in the most rapidly evolving feature in old stellar systems, the UV-upturn. We investigate: (1) the possible differences between the UV-upturn of BCGs and those of a control sample of ordinary ellipticals in the same mass range, as well as possible correlations between the UV-upturn and other general properties of the galaxies; (2) possible correlations between the UV-upturn and the properties of the host clusters;

2004

HST is very well tailored for observations of extragalactic star clusters. One obvious reason is HST's high spatial resolution, but equally important is the wavelength range offered by the instruments on board HST, in particular the blue and near-UV coverage which is essential for age-dating young clusters. HST observations have helped establish the ubiquity of young massive clusters (YMCs) in a wide variety of star-forming environments, from dwarf galaxies and spiral disks to nuclear starbursts and mergers. These YMCs have masses and sizes similar to those of old globular clusters (GCs), and the two may be closely related. A large fraction of all stars seem to be born in clusters, but most clusters disrupt rapidly and the stars disperse to become part of the field population. In most cases studied to date the luminosity functions of young cluster systems are well fit by power-laws dN(L)/dL ~ L^-2, and the luminosity of the brightest cluster can (with few exceptions) be predicted from simple sampling statistics. Mass functions have only been constrained in a few cases, but appear to be well approximated by similar power-laws. The absence of any characteristic mass scale for cluster formation suggests that star clusters of all masses form by the same basic process, without any need to invoke special mechanisms for the formation of YMCs. It is possible, however, that special conditions can lead to the formation of a few YMCs in some dwarfs where the mass function is discontinuous. Further studies of mass functions for star clusters of different ages may help test the theoretical prediction that the power-law mass distribution observed in young cluster systems can evolve towards the approximately log-normal distribution seen in old GC systems.

The Astrophysical Journal, 1993

As part of an observing program using the International UltravioletExplorer (IUE) satellite to investigate the ultraviolet properties of starsfound within the coresof galacticglobularclusters with blue horizontal branches (HBs), we obtained three spectraof the clusterNGC 1904 (iV[ 79). All three were long integration-time, short-wavelength(SWP) spectra obtained at the so-called "center-of-light" and allthree showed evidenceof multiplesourceswithin the IUE largeaperture (21_.4x 10"). In thispaper we shalldescribethe analysisof these spectra and present evidence that the UV sources representindividualhot starsin the post-HB stage of evolution_Describe results of UV star extraction.\Analyses of other clusters observed as part of thissame program } willbe discussedin a seriesof subsequent papers.

The Astrophysical …, 2010

We present UV broadband photometry and optical emission-line measurements for a sample of 32 Brightest Cluster Galaxies (BCGs) in clusters of the Representative XMM-Newton Cluster Structure Survey (REXCESS) with z = 0.06 − 0.18. The REXCESS clusters, chosen to study scaling relations in clusters of galaxies, have X-ray measurements of high quality. The trends of star formation and BCG colors with BCG and host properties can be investigated with this sample. The UV photometry comes from the XMM Optical Monitor, supplemented by existing archival GALEX photometry. We detected Hα and forbidden line emission in 7 (22%) of these BCGs, in optical spectra obtained using the SOAR Goodman Spectrograph. All of these emission-line BCGs occupy clusters classified as cool cores based on the central cooling time in the cluster core, for an emission-line incidence rate of 70% for BCGs in REXCESS cool core clusters. Significant correlations between the Hα equivalent widths, excess UV production in the BCG, and the presence of dense, X-ray bright intracluster gas with a short cooling time are seen, including the fact that all of the Hα emitters inhabit systems with short central cooling times and high central ICM densities. Estimates of the star formation rates based on Hα and UV excesses are consistent with each other in these 7 systems, ranging from 0.1 − 8 solar masses per year. The incidence of emission-line BCGs in the REXCESS sample is intermediate, somewhat lower than in other X-ray selected samples (∼ 35%), and somewhat higher than but statistically consistent with optically selected, slightly lower redshift BCG samples (∼ 10 − 15%). The UV-optical colors (UVW1-R ∼ 4.7 ± 0.3) of REXCESS BCGs without strong optical emission lines are consistent with those predicted from templates and observations of ellipticals dominated by old stellar populations. We see no trend in UV-optical colors with optical luminosity, R − K color, X-ray temperature, redshift, or offset between X-ray centroid and X-ray peak ( w ). The lack of such trends in these massive galaxies, particularly the ones lacking emission lines, suggests that the proportion of UV-emitting (200-300 nm) stars is insensitive to galaxy mass, cluster mass, cluster relaxation, and recent evolution, over the range of this sample.