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Register a new userSurface Brightness Profiles for a sample of LMC, SMC and Fornax galaxy Globular Clusters
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Authors
Eva Noyola
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We use Hubble Space Telescope archival images to measure central surface brightness profiles of globular clusters around satellite galaxies of the Milky Way. We report results for 21 clusters around the LMC, 5 around the SMC, and 4 around the Fornax dwarf galaxy. The profiles are obtained using a recently developed technique based on measuring integrated light, which is tested on an extensive simulated dataset. Our results show that for 70% of the sample, the central photometric points of our profiles are brighter than previous measurements using star counts with deviations as large as 2 mag/arcsec^2. About 40% of the objects have central profiles deviating from a flat central core, with central logarithmic slopes continuously distributed between -0.2 and -1.2. These results are compared with those found for a sample of Galactic clusters using the same method. We confirm the known correlation in which younger clusters tend to have smaller core radii, and we find that they also have brighter central surface brightness values. This seems to indicate that globular clusters might be born relatively concentrated, and that a profile with extended flat cores might not be the ideal choice for initial profiles in theoretical models.
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We present radial surface brightness profiles for all five globular clusters in the Fornax dwarf spheroidal galaxy, and for the four present members of the Sagittarius dwarf spheroidal galaxy. These profiles are derived from archival Hubble Space Telescope observations, and have been calculated using the same techniques with which we measured profiles in our previous studies of LMC and SMC clusters (astro-ph/0209031 and astro-ph/0209046 respectively), apart from some small modifications. From the surface brightness profiles, we have determined structural parameters for each cluster, including core radii and luminosity and mass estimates. We also provide a brief summary of literature measurements of other parameters for these clusters, including their ages, metallicities and distances. Our core radius measurements are mostly in good agreement with those from previous lower resolution studies, although for several clusters our new values are significantly different. The profile for Fornax cluster 5 does not appear to be well fit by a King-type model and we suggest that it is a post core-collapse candidate. We examine the distribution of cluster core radii in each of the two dwarf galaxy systems, and compare these with the distribution of core radii for old LMC clusters. The three distributions match within the limits of measurement errors and the small sample sizes. We discuss the implications of this in the context of the radius-age trend we have previously highlighted for the Magellanic Cloud clusters.
Hubble Space Telescope allows us to study the central surface brightness profiles for globular clusters at unprecedented detail. We have mined the HST archives to obtain 38 WFPC2 images of galactic globular clusters with adequate exposure times and filters, which we use to measure their central structure. We outline a reliable method to obtain surface brightness profiles from integrated light that we test on an extensive set of simulated images. Most clusters have central surface brightness about 0.5 mag brighter than previous measurements made from ground-based data, with the largest differences around 2 magnitudes. Including the uncertainties in the slope estimates, the surface brightness slope distribution is consistent with half of the sample having flat cores and the remaining half showing a gradual decline from 0 to -0.8 (dlog(Sigma)/dlogr). We deproject the surface brightness profiles in a non-parametric way to obtain luminosity density profiles. The distribution of luminosity density logarithmic slopes show similar features with half of the sample between -0.4 and -1.8. These results are in contrast to our theoretical bias that the central regions of globular clusters are either isothermal (i.e. flat central profiles) or very steep (i.e. luminosity density slope ~-1.6) for core-collapse clusters. With only 50% of our sample having central profiles consistent with isothermal cores, King models appear to poorly represent most globular clusters in their cores.
We present Chandra gas temperature profiles at large radii for a sample of 13 nearby, relaxed galaxy clusters and groups, which includes A133, A262, A383, A478, A907, A1413, A1795, A1991, A2029, A2390, MKW4, RXJ1159+5531, and USGC S152. The sample covers a range of average temperatures from 1 to 10 keV. The clusters are selected from the archive or observed by us to have sufficient exposures and off-center area coverage to enable accurate background subtraction and reach the temperature accuracy of better than 20-30% at least to r=0.4-0.5 r_180, and for the three best clusters, to 0.6-0.7 r_180. For all clusters, we find cool gas in the cores, outside of which the temperature reaches a peak at r =~ 0.15 r_180 and then declines to ~0.5 of its peak value at r =~ 0.5 r_180. When the profiles are scaled by the cluster average temperature (excluding cool cores) and the estimated virial radius, they show large scatter at small radii, but remarkable similarity at r>0.1-0.2 r_180 for all but one cluster (A2390). Our results are in good agreement with previous measurements from ASCA by Markevitch et al. and from Beppo-SAX by DeGrandi & Molendi. Four clusters have recent XMM-Newton temperature profiles, two of which agree with our results, and we discuss reasons for disagreement for the other two. The overall shape of temperature profiles at large radii is reproduced in recent cosmological simulations.
We present radial entropy profiles of the intracluster medium (ICM) for a collection of 239 clusters taken from the Chandra X-ray Observatorys Data Archive. Entropy is of great interest because it controls ICM global properties and records the thermal history of a cluster. Entropy is therefore a useful quantity for studying the effects of feedback on the cluster environment and investigating any breakdown of cluster self-similarity. We find that most ICM entropy profiles are well-fit by a model which is a power-law at large radii and approaches a constant value at small radii: K(r) = K0 + K100(r/100 kpc), where K0 quantifies the typical excess of core entropy above the best fitting power-law found at larger radii. We also show that the K0 distributions of both the full archival sample and the primary HIFLUGCS sample of Reiprich (2001) are bimodal with a distinct gap between K0 ~ 30 - 50 keV cm^2 and population peaks at K0 ~ 15 keV cm^2 and K0 ~ 150 keV cm^2. The effects of PSF smearing and angular resolution on best-fit K0 values are investigated using mock Chandra observations and degraded entropy profiles, respectively. We find that neither of these effects is sufficient to explain the entropy-profile flattening we measure at small radii. The influence of profile curvature and number of radial bins on best-fit K0 is also considered, and we find no indication K0 is significantly impacted by either. For completeness, we include previously unpublished optical spectroscopy of Halpha and [N II] emission lines discussed in Cavagnolo et al. (2008a). All data and results associated with this work are publicly available via the project web site.
The periphery of the Small Magellanic Cloud (SMC) can unlock important information regarding galaxy formation and evolution in interacting systems. Here, we present a detailed study of the extended stellar structure of the SMC using deep colour-magnitude diagrams (CMDs), obtained as part of the Survey of the MAgellanic Stellar History (SMASH). Special care was taken in the decontamination of our data from MW foreground stars, including from foreground globular clusters NGC 362 and 47 Tuc. We derived the SMC surface brightness using a ``conservative approach from which we calculated the general parameters of the SMC, finding a staggered surface brightness profile. We also traced the fainter outskirts by constructing a stellar density profile. This approach, based on stellar counts of the oldest main sequence turn-off (MSTO) stars, uncovered a tidally disrupted stellar feature that reaches as far out as 12 degrees from the SMC centre. We also serendipitously found a faint feature of unknown origin located at $sim 14$ degrees from the centre of the SMC and that we tentatively associated to a more distant structure. We compared our results to in-house simulations of a $1times10^{9} M_odot$ SMC, finding that its elliptical shape can be explained by its tidal disruption under the combined presence of the MW and the LMC. Finally, we found that the older stellar populations show a smooth profile while the younger component presents a jump in the density followed by a flat profile, confirming the heavily disturbed nature of the SMC.
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