X-rays across the galaxy population – I. Tracing the main sequence of star formation

We use deep $\textit{Chandra}$ imaging to measure the distribution of X-ray luminosities (LX) for samples of star-forming galaxies as a function of stellar mass and redshift, using a Bayesian method to push below the nominal X-ray detection limits. Our luminosity distributions all show narrow peaks at LX ≲ 10$^{42}$ erg s$^{-1}$ that we associate with star formation, as opposed to AGN that are traced by a broad tail to higher LX. Tracking the luminosity of these peaks as a function of stellar mass reveals an ‘X-ray main sequence’ with a constant slope ≈0.63 ± 0.03 over 8.5≲logM∗/M⊙≲11.5 and 0.1 ≲ z ≲ 4, with a normalization that increases with redshift as (1 + z)$^{3.79 ± 0.12}$. We also compare the peak X-ray luminosities with UV-to-IR tracers of star formation rates (SFRs) to calibrate the scaling between LX and SFR. We find that LX ∝ SFR$^{0.83}$ × (1 + z)$^{1.3}$, where the redshift evolution and non-linearity likely reflect changes in high-mass X-ray binary populations of star-forming galaxies. Using galaxies with a broader range of SFR, we also constrain a stellar-mass-dependent contribution to LX, likely related to low-mass X-ray binaries. Using this calibration, we convert our X-ray main sequence to SFRs and measure a star-forming main sequence with a constant slope ≈0.76 ± 0.06 and a normalization that evolves with redshift as (1 + z)$^{2.95 ± 0.33}$. Based on the X-ray emission, there is no evidence for a break in the main sequence at high stellar masses, although we cannot rule out a turnover given the uncertainties in the scaling of LX to SFR.JA acknowledges support from ERC Advanced Grant FEEDBACK 340442. ALC acknowledges support from NSF CAREER award AST-1055081. AG acknowledges the THALES project 383549 that is jointly funded by the European Union and the Greek Government in the framework of the programme “Education and lifelong learning”. This work is based in part on observations taken by the 3D-HST Treasury Program (GO 12177 and 12328) with the NASA/ESA HST, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Based in part on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile , under Large Program 185.A-0791, and made available by the VUDS team at the CESAM data center, Laboratoire d’Astrophysique de Marseille, France. The scientific results reported in this article are based to a significant degree on observations made by the Chandra X-ray Observatory

Human Immunodeficiency Virus and open fractures. Is wound or fracture healing affected in surgically stabilised open fractures? A prospective study

Background: 33 million people worldwide are infected with HIV, a complex disease that affects many of the processes involved in wound and fracture healing. There is little evidence available to guide acute management of open fractures in these patients and fears of acute and delayed sepsis often inhibit the use of surgical fixation, which may be the most effective way of achieving union. This study addresses the hypothesis that the presence of either HIV or advanced HIV (CD4 count 350), 7 refused CD4 count testing. This cohort was three times larger (number of HIV positive patients) than any similar previously published study. There was no randomised allocation; the treatment of these patients was based on locally developed protocols and was dependent on; fracture type, location and the grade of wound. Patients were followed up either till union had been achieved or for 6 months in tibia/femur fractures, and 3 months in other fractures. The primary outcome was acute wound infection, secondary outcomes tested were fracture union and pin site sepsis. The analysis of the binary nominal data was done using the Chi squared test. In cases where the expected value was less than 5, then the Fisher’s exact test was used. In the assessment of multiple potential risk factors, binary logistic regression was used. Results: Analysis of background characteristics showed that HIV positive and negative populations were broadly similar with regard to demographics, injury type/location and grade of wound. In the analysis of the primary outcome, the risk of wound infection was marginally higher in patients without HIV (22%) as compared to patients with HIV (15%). This difference was small and did not reach statistical significance (n=135, Risk Ratio 0.7, p value 0.40). However, as hypothesized, the infection risk was higher in patients with advanced HIV (26%), compared to patients with early HIV (5%). The numbers, however, were small and this did not reach statistical significance (n=33, Risk Ratio=4.8, P value= 0.12). Sub group analyses, conceived prior to the study, provided strong evidence that patients with Gustilo Anderson grade 1 injuries had a higher risk of wound infection in patients with advanced HIV than controls (HIV negative and early HIV) (n=46, Risk Ratio=6.3, P value =0.02). Of note, departmental guidelines meant that patients with grade 1 injury were not prioritised for theatre and had, on average, a delay of 3.5 days to surgery. The average delay was similar in both HIV positive and negative groups. Analysis of the secondary outcome, nonunion, provided strong evidence that the risk of nonunion was higher in HIV positive than HIV negative patients (n=115, Risk Ratio=4.1, P value=0.04). Interestingly, the patients with advanced HIV had a slightly lower nonunion risk (13%) than patients with early HIV (20%). However the numbers were small and the difference was not statistically significant (n=33, Risk Ratio=0.8, P value=1). The incidence of nonunion was not correlated with the presence of wound infection. The risk of mild pin site sepsis in fractures treated with external fixation was similar in both HIV positive (60%) and negative (67%) patients (n=31, Risk Ratio=0.9, P value=1). An increased risk of severe pin site sepsis was noted in patients with advanced HIV (50%), compared to controls (25%). Although the difference is large, the numbers are small and the difference was not statistically significant (n=28, Risk Ratio=2, P value= 0.31). It would require 160 patients to prove a difference of this size. Conclusions: Data from this study appears to dispute the conclusion of previous studies that suggest that all patients with HIV are at higher risk of wound infection, and therefore internal fixation should be considered with caution. In this study it was only the patients with advanced HIV that showed a small increase in the risk of wound infection. Based on this study the author suggests that early HIV should not be a contraindication to either internal or external fixation in open fractures, due to concerns of wound infection. However, advanced HIV should continue to be considered a relative contraindication to internal fixation, until further data becomes available. Since this finding applied equally to grade 1 (Gustilo Anderson) injuries, the data suggests that any theatre delays in patients with advanced HIV may be detrimental to outcomes. This is contrary to published data that suggests that grade 1 injuries do not need to be prioritised. The data provides strong evidence that HIV leads to an increased risk of non unions. Interestingly, the risk of non union is less in patients with advanced HIV. This may fit with recently published laboratory studies suggesting that the absence of lymphocytes is beneficial to bone healing. Based on this evidence the author suggests that in patients with HIV treatment strategies should be aimed at achieving union, rather than on potentially unfounded concerns of preventing infection. In patients treated with external fixation, the data provides weak evidence of an increased risk of severe pin site sepsis in advanced HIV. This observation may be due to an increased susceptibility to infection, or to problems with bone healing in these patients. Based on this evidence, and the evidence that patients with HIV may be at increased risk of non union, the author suggests that HIV positive patients being treated with external fixators, should be considered for treatment strategies that will prolong the life of the pin bone interface. These may include additional pins, wires and/or the use of hydroxyapatite coated half pins

Sustainable harvesting of Conomurex luhuanus and Rochia nilotica by Indigenous Australians on the Great Barrier Reef over the past 2000 years

Offshore island colonisation and use around the northern Australian coastline in the mid-to-late Holocene is associated with expanding Aboriginal and Torres Strait Islander populations and intensifying land-use activities. However, few explicit tests of the long-term effects of shellfish forager decision-making and associated impacts on intertidal ecosystems in these newly colonised island environments have been undertaken. We report morphometric analyses on two key reef flat Great Barrier Reef shellfish species, strawberry conch Conomurex luhuanus (n = 360) and top shell Rochia nilotica (n = 45), from two late Holocene archaeological shell midden assemblages on Lizard Island, northeast Queensland. Human foraging pressure was assessed through reconstructions of population age structure across time, highlighting the importance of determining size-at-age habitat preferences and species behaviour patterns when assessing long-term anthropogenic impacts on shellfish populations. Results show no evidence for resource depression across the late Holocene which is broadly in keeping with previous findings at other locales on the Great Barrier Reef, but contrary to expectations of resource intensification models. We conclude that the rich and abundant resources of reef flat environments were resilient to relatively low intensity and likely episodic Indigenous foraging. This sustainability contrasts with the scale and impacts of intensive industrialised harvesting in the historic period

Evidence for a mass-dependent AGN Eddington ratio distribution via the flat relationship between SFR and AGN luminosity

The lack of a strong correlation between AGN X-ray luminosity (L X ; a proxy for AGN power) and the star formation rate (SFR) of their host galaxies has recently been attributed to stochastic AGNvariability. Studies using population synthesis models have incorporated this by assuming a broad, universal (i.e. does not depend on the host galaxy properties) probability distribution for AGN specific X-ray luminosities (i.e. the ratio of L X to host stellar mass; a common proxy for Eddington ratio). However, recent studies have demonstrated that this universal Eddington ratio distribution fails to reproduce the observed X-ray luminosity functions beyond z ~ 1.2. Furthermore, empirical studies have recently shown that the Eddington ratio distribution may instead depend upon host galaxy properties, such as SFR and/or stellarmass. To investigate this further, we develop a population synthesis model in which the Eddington ratio distribution is different for star-forming and quiescent host galaxies. We showthat, although this model is able to reproduce the observed X-ray luminosity functions out to z ~ 2, it fails to simultaneously reproduce the observed flat relationship between SFR and X-ray luminosity. We can solve this, however, by incorporating a mass dependency in the AGN Eddington ratio distribution for starforming host galaxies. Overall, our models indicate that a relative suppression of low Eddington ratios (λ Edd < ~0.1) in lower mass galaxies (M * < ~10 10-11 M⊙) is required to reproduce both the observed X-ray luminosity functions and the observed flat SFR/X-ray relationship

The X-ray luminosity function of AGN at z~3

We combine Lyman-break colour selection with ultradeep (> 200 ks) Chandra X-ray imaging over a survey area of ~0.35 deg^2 to select high redshift AGN. Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at z~3. Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at z=1, we find no evidence that the faint slope of the XLF flattens at high z, but we do find significant (factor ~3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity L*. Our data therefore support models of luminosity-dependent density evolution between z=1 and z=3. A sharp upturn in the the XLF is seen at the very lowest luminosities (Lx < 10^42.5 erg s^-1), most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.Comment: 16 pages, 9 figures, accepted for publication in MNRA