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We cross-match and compare characteristics of galaxy clusters identified in observations from two sky surveys using two completely different techniques. One sample is optically selected from the analysis of 3 years of Dark Energy Survey observations using the redMaPPer cluster detection algorithm. The second is X-ray selected from XMM observations analysed by the XMM Cluster Survey. The samples comprise a total area of 57.4 deg2, bounded by the area of four contiguous XMM survey regions that overlap the DES footprint. We find that the X-ray-selected sample is fully matched with entries in the redMaPPer catalogue, above λ > 20 and within 0.1 <$z$ <0.9. Conversely, only 38 per cent of the redMaPPer catalogue is matched to an X-ray extended source. Next, using 120 optically clusters and 184 X-ray-selected clusters, we investigate the form of the X-ray luminosity–temperature (LX –TX ), luminosity–richness (LX –λ), and temperature–richness (TX –λ) scaling relations. We find that the fitted forms of the LX –TX relations are consistent between the two selection methods and also with other studies in the literature. However, we find tentative evidence for a steepening of the slope of the relation for low richness systems in the X-ray-selected sample. When considering the scaling of richness with X-ray properties, we again find consistency in the relations (i.e. LX –λ and TX –λ) between the optical and X-ray-selected samples. This is contrary to previous similar works that find a significant increase in the scatter of the luminosity scaling relation for X-ray-selected samples compared to optically selected samples.

 

Ecosystem engineers such as the Antarctic scallop (Adamussium colbecki) shape marine communities. Thus, changes to their lifespan and growth could have far-reaching effects on other organisms. Sea ice is critical to polar marine ecosystem function, attenuating light and thereby affecting nutrient availability. Sea ice could therefore impact longevity and growth in polar bivalves unless temperature is the overriding factor. Here, we compare the longevity and growth of A. colbecki from two Antarctic sites: Explorers Cove and Bay of Sails, which differ by sea-ice cover, but share similar seawater temperatures, the coldest on Earth (-1.97°C). We hypothesize that scallops from the multiannual sea-ice site will have slower growth and greater longevity. We found maximum ages to be similar at both sites (18–19 years). Growth was slower, with higher inter-individual variability, under multiannual sea ice than under annual sea ice, which we attribute to patchier nutrient availability under multiannual sea ice. Contrary to expectations, A. colbecki growth, but not longevity, is affected by sea-ice duration when temperatures are comparable. Recent dramatic reductions in Antarctic sea ice and predicted temperature increases may irrevocably alter the life histories of this ecosystem engineer and other polar organisms. 

The Atlantic surfclam, Spisula solidissima, is distinguished by a well-documented shift in range that accelerated in the 2000s as the northwest Atlantic warmed. Here the extension of the Atlantic surfclam into heretofore Acadian Province waters off the island of Nantucket is documented and compared to the distribution of surfclam shell as an indicator of recent colonization, to the timing of range expansion, and to the physiological implications of a range extension into deeper water. The primary demographic difference observed is the dichotomous distribution of sizes. Smaller surfclams averaged higher in abundance at the deeper offshore sites, whereat the number of large animals was distinctly fewer; thus, the size-frequency distributions at deeper sites were shifted towards the smaller sizes, a finding consistent with the expectation of recruitment into deeper water during a period of range expansion. In confirmation, deeper-water stations where surfclams were aged yielded surfclams no older than 13 yr, whereas shallow-water stations had a mature age frequency with some surfclams exceeding 20 yr. Further support for the more recent occupation of deeper-water sites comes from the distribution of surfclam shell, that was found in limited quantities at stations where recent colonization is inferred and in greater quantities in shallower water where longer-term occupation is surmised. For the shallower-water sites with a mature demographic, growth rates were comparable or higher than observed elsewhere in the stock and surfclam maximum sizes were larger than elsewhere in the geographic range. In contrast, surfclams colonizing deeper water post-2000 grew at a slower rate likely due to a lower average temperature near the deep-water range boundary. The penalty for colonization pushing the range boundary into deeper, cooler water lasted no more than 4–5 years, however, after which growth rates increased to rates typical of surfclams in shallower water. Thus, surfclams responded quickly to a period of rapid climate change in contrast to expectation from their known longevity. 

Context. Protoplanetary disks in dense, massive star-forming regions are strongly affected by their environment. How this environmental impact changes over time is an important constraint on disk evolution and external photoevaporation models. Aims. We characterize the dust emission from 179 disks in the core of the young (0.5 Myr) NGC 2024 cluster. By studying how the disk mass varies within the cluster, and comparing these disks to those in other regions, we aim to determine how external photoevaporation influences disk properties over time. Methods. Using the Atacama Large Millimeter/submillimeter Array, a 2.9′× 2.9′ mosaic centered on NGC 2024 FIR 3 was observed at 225 GHz with a resolution of 0.25″, or ~100 AU. The imaged region contains 179 disks identified at IR wavelengths, seven new disk candidates, and several protostars. Results. The overall detection rate of disks is 32 ± 4%. Few of the disks are resolved, with the exception of a giant ( R = 300 AU) transition disk. Serendipitously, we observe a millimeter flare from an X-ray bright young stellar object (YSO), and resolve continuum emission from a Class 0 YSO in the FIR 3 core. Two distinct disk populations are present: a more massive one in the east, along the dense molecular ridge hosting the FIR 1-5 YSOs, with a detection rate of 45 ± 7%. In the western population, towards IRS 1, only 15 ± 4% of disks are detected. Conclusions. NGC 2024 hosts two distinct disk populations. Disks along the dense molecular ridge are young (0.2–0.5 Myr) and partly shielded from the far ultraviolet radiation of IRS 2b; their masses are similar to isolated 1–3 Myr old SFRs. The western population is older and at lower extinctions, and may be affected by external photoevaporation from both IRS 1 and IRS 2b. However, it is possible these disks had lower masses to begin with. 

Context. The ESA Gaia mission provides a unique time-domain survey for more than 1.6 billion sources with G ≲ 21 mag. Aims. We showcase stellar variability in the Galactic colour-absolute magnitude diagram (CaMD). We focus on pulsating, eruptive, and cataclysmic variables, as well as on stars that exhibit variability that is due to rotation and eclipses. Methods. We describe the locations of variable star classes, variable object fractions, and typical variability amplitudes throughout the CaMD and show how variability-related changes in colour and brightness induce “motions”. To do this, we use 22 months of calibrated photometric, spectro-photometric, and astrometric Gaia data of stars with a significant parallax. To ensure that a large variety of variable star classes populate the CaMD, we crossmatched Gaia sources with known variable stars. We also used the statistics and variability detection modules of the Gaia variability pipeline. Corrections for interstellar extinction are not implemented in this article. Results. Gaia enables the first investigation of Galactic variable star populations in the CaMD on a similar, if not larger, scale as was previously done in the Magellanic Clouds. Although the observed colours are not corrected for reddening, distinct regions are visible in which variable stars occur. We determine variable star fractions to within the current detection thresholds of Gaia . Finally, we report the most complete description of variability-induced motion within the CaMD to date. Conclusions. Gaia enables novel insights into variability phenomena for an unprecedented number of stars, which will benefit the understanding of stellar astrophysics. The CaMD of Galactic variable stars provides crucial information on physical origins of variability in a way that has previously only been accessible for Galactic star clusters or external galaxies. Future Gaia data releases will enable significant improvements over this preview by providing longer time series, more accurate astrometry, and additional data types (time series BP and RP spectra, RVS spectra, and radial velocities), all for much larger samples of stars.