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Red Supergiants As Cosmic Abundance Probes

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Red Supergiants As Cosmic Abundance Probes Astronomical Science Red Supergiants as Cosmic Abundance Probes Ben Davies1 to the shallower potential well, prevent- infrared (NIR). At these wavelengths, the Rolf-Peter Kudritzki2,3 ing them from reaching high metallicities, gain from adaptive optics will be the most Maria Bergemann4 and gives rise to the functional form of profound, as the increase in sensitivity Chris Evans5 the mass–metallicity [Z] relation we see scales with the fourth power of the mirror Zach Gazak2 today. State­of­the­art cosmo logical sim- diameter. Carmela Lardo1 ulations (e.g., Shaye et al., 2015) make Lee Patrick6 specific predictions about the quantitative Bertrand Plez7 nature of the mass–metallicity relation, Red supergiants: A novel, NIR­based Nate Bastian1 which makes it possible to test the current technique for extragalactic metallicity theories of galaxy evolution under the studies dark energy, cold dark matter framework. 1 Astrophysics Research Institute, To this end, in recent years we have been Liverpool John Moores University, Such tests, however, require reliable and developing a technique to obtain extra- United Kingdom robust measurements of a galaxy’s galactic abundances using NIR spectros- 2 Institute for Astronomy, University of chemical abundances. Historically this copy of red supergiants (RSGs). These 5 Hawaii, USA has been done using emission lines from stars are extremely bright (> 10 LA), 3 Munich University Observatory, Germany extragalactic H II regions. The “direct” young (< 30 Myr), and have the advan- 4 Max Planck Institute for Astronomy, method requires measurements of auro- tage that their fluxes peak at NIR wave- Garching, Germany ral lines, such as [O III] 436.3 nm, which lengths. A challenge for abundance work 5 UK Astronomy Technology Centre allows the observer to uniquely determine using these stars is that they are cool, (UKATC), Edinburgh, Scotland the gas temperature and, crucially, the with effective temperatures around 6 Institute for Astronomy, University of abundance of the element. Unfortunately, 4000 K, and so their spectra are domi- Edinburgh, Scotland these auroral lines are often very weak, nated by absorption from thousands of 7 University of Montpellier, France especially at high metallicity. This means molecular lines. Historically this has that one must attempt to derive abun- meant that high spectral resolving powers dances based on only the strong emission were needed to study individual lines, By studying a galaxy’s present-day lines (e.g., [O III] 500.7 nm), which contain making these stars unsuitable for extra- chemical abundances, we are effec- no temperature information on their own, galactic work due to the large integration tively looking at its star-forming history. by calibrating against either results from times required. Cosmological simulations of galaxy the direct method or from photoionisation evolution make predictions about the models. The limitation of these strong- In order to adapt these techniques to be relative metal contents of galaxies as a line methods is that they have well-known useful at extragalactic distances, in function of their stellar mass, a trend systematic offsets from one another, Davies, Kudritzki & Figer (2010) we identi- known as the mass–metallicity relation. particularly at high metallicity, where two fied a spectral region around 1.2 μm that These predictions can be tested with different calibrations may give results is relatively free of molecular lines, the observations of nearby galaxies. How- ~ 0.8 dex apart (Kewley & Ellison, 2008). dominant features being those of neutral ever, providing reliable, accurate abun- metallic atoms. We then demonstrated dance measurements at extragalactic In order to solve this problem, our team that metallicities relative to Solar (defined distances is extremely challenging. has been pioneering new ways to obtain as [Z]) may be determined from moderate In this project, we have developed a the metallicities of external galaxies by spectral resolutions of only R ~ 3000, technique to extract abundance infor- studying their constituent stars individu- making this technique viable at large dis- mation from individual red supergiant ally. The stars chosen as targets must tances, and well-suited to the latest stars at megaparsec distances. We be: (a) young, so that their abundances multi­object spectrographs (MOSs), such are currently exploiting this technique reflect those of H II regions; and (b) lumi- as the K­band Multi­Object Spectrograph using the unique capabilities of KMOS nous, so that they can be observed at (KMOS) on the Very Large Telescope on the VLT. megaparsec distances. This means that (VLT). Using instrument simulators, we the obvious targets of choice are super- then demonstrated the potential with the giants — massive, post main­sequence E­ELT, showing that we could measure [Z] A well­known correlation exists between stars, which can outshine entire globular at distances of 75 Mpc (Evans et al., 2011). the mass of a galaxy and its metal con- clusters and dwarf galaxies. Substantial tent, whereby more massive galaxies recent progress has been made using tend to be more metal-rich (Lequeux et optical spectroscopy of blue supergiants Building the tools for the job al., 1979). This is understood qualitatively (BSGs; see Kudritzki et al. [2014] and as being due to progressive enrichment references therein). However, to really Spurred on by the potential of this tech- by stellar and supernova nucleosynthesis, take advantage of the next generation of nique, we set about developing the nec- moderated by the infall of pristine gas telescopes such as the European essary analysis tools. The initial task in and outflow of enriched material via Extremely Large Telescope (E-ELT), it is this project was to build a suite of model winds. This enriched gas escapes more crucial to reduce our dependence on spectra from which we can derive high- easily from lower-mass galaxies due optical techniques and move to the near- precision abundances. We generated a 32 The Messenger 161 – September 2015 grid of MARCS (Model Atmospheres in Figure 1. A spectrum of Radiative and Convective Scheme) model 1 an RSG in Per OB1 observed with Subaru atmospheres (Gustafsson et al., 2008), and IRCS in black, and computed under the assumptions of the best­fitting model spherical symmetry and local thermody- (red), at bottom. The namic equilibrium (LTE), which span the 1 other spectra are the same data but degraded parameter ranges in effective tempera- to lower resolution and ture, surface gravity, microturbulence and S re­fit, showing that the RD metal abundances appropriate for RSGs. EE 1 diagnostic lines are still We use these models to compute syn- identifiable down to resolutions of R = 3000. W N thetic spectra with full non-LTE radiation T Taken from Gazak et al. transport for the diagnostic chemical ele- Ck (2014a). ments Fe, Si, Mg and Ti (Bergemann et 1 al., 2012; 2013; 2015). Fitting the models to the data is done by a χ2-minimisation process of matching the observed -NQL@KHRD 1 strengths of the diagnostic metal lines, described in detail in Davies et al. (2015). Road­testing in the Milky Way %D 3H 3H %D 2H 2H 2H The next phase of our project was to demonstrate that our technique works at 6@UDKDMFSGƅL high precision in the Solar Neighbour- hood. For our target we chose the nearby Perseus OB1 association, which contains The added value of the X-shooter data precision absorption-line spectroscopy several RSGs, and which we can safely was that, in addition to the coverage of of the highest possible quality, it is there- assume all have the same metallicity. our spectral window at 1.2 μm, we fore essential that we take the greatest Using high­resolution spectroscopy from obtained spectra covering the whole of care in removing sky emission and telluric the Infrared Camera and Spectrograph the optical and near-infrared, from 0.3– features from our data. After extracting (IRCS) on the Subaru Telescope, we 2.5 μm. This allowed us to verify that the the calibrated datacubes using the obtained a metallicity averaged over all temperatures we obtain from our narrow KMOS data reduction pipeline, we found RSGs in the region consistent with Solar, window in the J-band were consistent that variations in spectral resolution and [Z] = –0.02 ± 0.08, as would be expected with those from fits to the spectral energy wavelength calibration were introducing (Gazak et al., 2014a). Further, by degrad- distributions. The agreement for the noise into the final spectra. In Figure 2, ing the spectral resolution of the data and SMC stars was excellent, while the LMC we demonstrate these variations across repeating the analysis, we showed con- stars were also similar to within the the field of view of one integral field unit clusively that this technique is stable down errors, aside from the two stars in our (IFU), as measured from the sky emission to resolutions of R < 3000, typical of MOS sample with the highest temperatures. lines. We corrected for this effect in a instruments (see Figure 1). process we call KMOGENIZATION, whereby we smooth the spectra at each Beyond the Local Group with KMOS: A spaxel down to a level which is common Extragalactic baby steps: The Magellanic study of NGC 300 to all spaxels to be extracted. This is usu- Clouds ally defined to be R = 3000–3200. The Our first target beyond the Local Group spatially varying wavelength calibration With the technique now proven at Solar was chosen to be the Sculptor Group effects are minimised by only extracting metallicities, the first sub­Solar metallicity member NGC 300. This galaxy is a rela- spectra within a 1 spaxel radius of the systems to be studied were the Large tively nearby (2 Mpc) face-on spiral flux peak. Although this discards a small and Small Magellanic Clouds (LMC and with a strong abundance gradient, as amount of flux, this is more than com­ SMC).
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