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Register a new userThe UV-upturn in brightest cluster galaxies
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Added by
Susan Ilani Loubser
Publication date
2010
fields
Physics
and research's language is
English
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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; (3) recently proposed scenarios where helium-sedimentation in the cluster centre can produce an enhanced UV-upturn. We find systematic differences between the UV-colours of BCGs and ordinary ellipticals, but we do not find correlations between these colours and the properties of the host clusters. Furthermore, the observations do not support the predictions made by the helium-sedimentation model as an enhancer of the UV-upturn.
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We have measured the strength of the UV upturn for red sequence galaxies in the Abell~1689 cluster at $z=0.18$, reaching to or below the $L^*$ level and therefore probing the general evolution of the upturn phenomenon. We find that the range of UV upturn strengths in the population as a whole has not declined over the past 2.2 Gyrs. This is consistent with a model where hot horizontal branch stars, produced by a Helium-enriched population, provide the required UV flux. Based on local counterparts, this interpretation of the result implies Helium abundances of at least 1.5 times the primordial value for this HB population, along with high formation and assembly redshifts for the galaxies and at least a subset of their stellar populations.
We present the results of a survey of the brightest UV-selected galaxies in protoclusters. These proto-brightest cluster galaxy (proto-BCG) candidates are drawn from 179 overdense regions of $g$-dropout galaxies at $zsim4$ from the Hyper Suprime-Cam Subaru Strategic Program identified previously as good protocluster candidates. This study is the first to extend the systematic study of the progenitors of BCGs from $zsim2$ to $zsim4$. We carefully remove possible contaminants from foreground galaxies and, for each structure, we select the brightest galaxy that is at least 1 mag brighter than the fifth brightest galaxy. We select 63 proto-BCG candidates and compare their properties with those of galaxies in the field and those of other galaxies in overdense structures. The proto-BCG candidates and their surrounding galaxies have different rest-UV color $(i - z)$ distributions to field galaxies and other galaxies in protoclusters that do not host proto-BCGs. In addition, galaxies surrounding proto-BCGs are brighter than those in protoclusters without proto-BCGs. The image stacking analysis reveals that the average effective radius of proto-BCGs is $sim28%$ larger than that of field galaxies. The $i-z$ color differences suggest that proto-BCGs and their surrounding galaxies are dustier than other galaxies at $zsim4$. These results suggest that specific environmental effects or assembly biasses have already emerged in some protoclusters as early as $z sim 4$, and we suggest that proto-BCGs have different star formation histories than other galaxies in the same epoch.
Using GALEX, UVOT and optical photometry, we explore the prevalence and strength of the UV-upturn in the spectra of quiescent early type galaxies in several nearby clusters. Even for galaxies with completely passive optical colours, there is a large spread in vacuum UV colour consistent with almost all having some UV upturn component. Combining GALEX and UVOT data below 3000AA, we generate for the first time comparatively detailed UV SEDs for Coma cluster galaxies. Fitting the UV upturn component with a blackbody, twenty six of these show a range of characteristic temperatures (10000--21000K) for the UV upturn population. Assuming a single temperature to explain GALEX-optical colours could underestimate the fraction of galaxies with UV upturns and mis-classify some as systems with residual star formation. The UV upturn phenomenon is not an exclusive feature found only in giant galaxies; we identify galaxies with similar (or even bluer) $FUV-V$ colours to the giants with upturns over a range of fainter luminosities. The temperature and strength of the UV upturn are correlated with galaxy mass. Under the plausible hypothesis that the sources of the UV upturn are blue horizontal branch stars, the most likely mechanism for this is the presence of a substantial (between 4% and 20%) Helium rich ($Y > 0.3$) population of stars in these galaxies, potentially formed at $zsim 4$ and certainly at $z>2$; this plausibly sets a lower limit of $sim {rm 0.3 - 0.8} times 10^{10}$ $M_{odot}$ to the {it in situ} stellar mass of $sim L^*$ galaxies at this redshift.
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 Halpha and forbidden line emission in 7 (22%) of these BCGs, in optical spectra. All of the emission-line BCGs occupy clusters classified as cool cores, for an emission-line incidence rate of 70% for BCGs in cool core clusters. Significant correlations between the Halpha 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 Halpha emitters inhabit systems with short central cooling times and high central ICM densities. Estimates of the star formation rates based on Halpha 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-4.7pm0.3) 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>).
It is suspected that the ultraviolet (UV) upturn phenomenon in elliptical galaxies and extended horizontal-branch stars in globular clusters have a common origin. An extremely high abundance of helium (Y~0.4) allows for a working hypothesis, but its origin is unclear. Peng & Nagai (2009) proposed that primordial helium sedimentation in dark haloes over cosmic timescales may lead to extreme helium abundances in galaxy cluster centers. In this scenario UV upturn should be restricted to brightest cluster galaxies (BCGs) only. This is a clear and testable prediction. We present tests of this hypothesis using galaxy clusters from Yoon et al. (2008) that were detected by both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer Medium Imaging Survey. Using a new UV classification scheme based on far-UV, near-UV, and optical photometry we found only 5% of cluster elliptical galaxies show a UV upturn, while 27% and 68% are classified as recent star-formation and UV-weak ellipticals, respectively. The data reveal a modest positive dependence of the UV upturn fraction on galaxy velocity dispersion, which is in agreement with the earlier findings of Burstein et al. (1988) and possibly with the helium sedimentation theory. However, we do not see any dependency on rank or luminosity of galaxies. Besides, BCGs do not show any marked difference in UV upturn fraction or strength, which is inconsistent with the prediction. We conclude that the aforementioned helium sedimentation theory and its inferred environmental effects are not supported by the available data.
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