Multiple Populations in Integrated Light Spectroscopy of Intermediate Age Clusters
Nate Bastian, Christopher Usher, Sebastian Kamann, Carmela Lardo, Søren S. Larsen, Ivan Cabrera-Ziri, William Chantereau, Silvia Martocchia, Maurizio Salaris, Ricardo P. Schiavon, Randa Asa'd, Michael Hilker
aa r X i v : . [ a s t r o - ph . GA ] S e p Mon. Not. R. Astron. Soc. , 1–7 (2019) Printed 5 September 2019 (MN L A TEX style file v2.2)
Multiple Populations in Integrated Light Spectroscopy ofIntermediate Age Clusters
Nate Bastian , Christopher Usher , Sebastian Kamann , Carmela Lardo , Søren S. Larsen ,Ivan Cabrera-Ziri ⋆ , William Chantereau , Silvia Martocchia , , Maurizio Salaris ,Ricardo P. Schiavon , Randa Asa’d , and Michael Hilker Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK Laboratoire d’astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, CH-1290 Versoix, Switzerland Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching bei München, Germany Physics Department, American University of Sharjah, P.O. Box 26666, Sharjah, UAE
Accepted. Received; in original form
ABSTRACT
The presence of star-to-star light-element abundance variations (a.k.a. multiple populations,MPs) appears to be ubiquitous within old and massive clusters in the Milky Way and all studiednearby galaxies. Most previous studies have focussed on resolved images or spectroscopy ofindividual stars, although there has been significant effort in the past few years to look formultiple population signatures in integrated light spectroscopy. If proven feasible, integratedlight studies offer a potential way to vastly open parameter space, as clusters out to tens ofMpc can be studied. We use the NaD lines in the integrated spectra of two clusters with similarages (2 − ∼ × M ⊙ ) in the LMC and G114(1 . × M ⊙ ) in NGC 1316. For NGC 1978, our findings agree with resolved studies ofindividual stars which did not find evidence for Na spreads. However, for G114, we find clearevidence for the presence of multiple populations. The fact that the same anomalous abundancepatterns are found in both the intermediate age and ancient GCs lends further support to thenotion that young massive clusters are effectively the same as the ancient globular clusters,only separated in age. Key words: galaxies - star clusters
Resolved studies of ancient globular clusters (GCs) in the MilkyWay (MW) and nearby galaxies, both photometrically and spec-troscopically, have shown that all display star-to-star light-elementabundance variations within them, known as multiple populations(MPs). The origin of these variations is still unknown, in part dueto the restricted parameter space where such studies are possible.In the Galaxy, the GC population spans a wide range of metallic-ities but is limited in the age range that can be probed. The Largeand Small Magellanic Clouds offer cluster populations with a widerrange of ages, but a more limited range of masses (especially atyounger ages). Resolved studies with the necessary precision toprobe MPs are not currently possible outside the Local Group, andare even severely limited in M31 and M33. Opening up studies ofmultiple populations to an increased volume in the local universe ⋆ Hubble fellow would allow access to a much larger portion of parameter space.This is particularly important as recent studies have suggested thatcluster mass (e.g., Carretta et al. 2010; Schiavon et al. 2013; Miloneet al. 2017), age (e.g., Martocchia et al. 2018a; 2019) and metallicity(Pancino et al. 2017) all play a role in the manifestation of multiplepopulations within clusters.One way to search for MPs in integrated light is to estimate theaverage abundance of various elements within clusters, and com-pare the derived mean abundance pattern to expectations based onresolved abundance work in MW GCs (e.g., Colucci et al. 2012;2014; Sakari et al. 2013; 2016; Larsen et al. 2012; 2017; 2018).While each GC that has been studied in detail differs in the exactnature of its MPs, there are broad trends that can be used to infertheir presence (cf. Gratton et al. 2012). In particular, the expectationis that the mean abundance of N, Na, and Al should be larger thanscaled-solar abundances, while C, O and potentially Mg should besomewhat depleted relative to the field stars of similar metallicity.Most integrated light studies to date that have searched for MPs © 2019 RAS
Bastian et al. have used relatively weak atomic lines at high or medium spec-tral resolution, which limits such studies to nearby galaxies (a fewMpc) for ancient GCs (e.g., Larsen et al. 2017) and 10s of Mpc forbrighter young clusters whose integrated light are dominated by redsupergiants (Cabrera-Ziri et al. 2016; Lardo et al. 2017).Here, we use an alternative method, which has been success-fully applied to integrated light studies of galaxies (e.g., Jeong etal. 2013) and ancient globular clusters in M31 (Schiavon et al. 2013),and apply it to two intermediate age (2 − − . = − .
5, and based on the sub-giant branch could rule out a significantage spread ( <
20 Myr) within the cluster. G114 in NGC 1316has previously been the focus of multiple studies and has an ageof 2 . ± . . × M ⊙ (Bastian et al. 2006). Both clustershave low extinction values, with A V ( G ) =0 (Bastian et al. 2006;§ 2) and A V (NGC 1978) = .
22 (Martocchia et al. 2018). The maincluster properties are given in Table 1.This paper is organised as follows: in § 2 we introduce theobservations and models used, while in § 3 we present our mainresults and discuss their implications. We present our conclusionsin § 4.
We use the same VLT Ultraviolet and Visual Echelle Spectrograph(UVES) spectrum of NGC 1316:G114 that was used to measure thevelocity dispersion of the cluster in Bastian et al. (2006 - ProgramID: 073.D-0305(B)). We refer the interested reader to that paperfor details of the data reduction and spectral details. Briefly, thewavelength range covered was 4200 − R ∼ , ∼ Galaxy Cluster Age Mass σ v r (Gyr) (M ⊙ ) (km/s) (km/s)LMC NGC 1978 2.2 3 × . ×
42 1292
Table 1.
Properties of the clusters studied in the present work. NGC 1978has [Fe/H] = − . VLT/MUSE in program 094.B-0298 (PI: Walcher). The clusterG114 in visible in the central two pointings of the mosaic, whichwe reduced and combined using the standard MUSE pipeline (Weil-bacher et al. 2012; 2014). The final data cube consists of 6 individualexposures, with an integration time of 150s each. G114 is visiblein all exposures, hence the effective exposure time for the cluster is900s. The average seeing in the final data cube is 0.8". The spectrumof G114 was extracted by summing the fluxes of all spaxels within adistance of 0 . ′′ from the visually determined cluster location. Thecontribution of the host galaxy was accounted for by averaging thefluxes of all spaxels in an annulus between 1 . ′′ and 2 . ′′ distancefrom said location and subtracting the resulting spectrum from thespectrum extracted for G114. We verified that the final result didnot depend sensitively on the extraction radii that were used. Theextracted MUSE spectrum has S/N (per Å) of 30 in the NaD regionof G114.For NGC 1978 we use the integrated light spectrum from theWAGGS project (WiFeS Atlas of Galactic Globular cluster Spectra).The observations and reductions are described in detail in Usher etal. (2017). In short, the spectrum covers the wavelength range from3300Å to 9050Å, with a resolution of R = F435W-F814W=1.90 and
F555W-F814W=1.03 . Comparing themeasured colours (for
F555W-F814W ) to those of Goudfrooij etal. (2012) shows excellent agreement. Comparing these colours toexpectations of a 3 Gyr, solar metallicity SSP from Bruzual &Charlot (2003; 2016 edition), which have 2.01 and 1.2 for
F435W-F814W and
F555W-F814W , respectively, shows good agreement.Both model colours are 0 . The models were developed in the same way as the integratedlight models of early type galaxies presented in Chantereau et al.(2018), which were based on the stellar models used in Martoc-chia et al. (2017). We refer the interested reader to these papers formore details. In summary, we use the MIST isochrones (Dotter etal. 2016, Choi et al. 2016), at a given age and metallicity, to samplethe distribution of luminosity and effective temperature of stars atall evolutionary stages. Stellar model atmospheres were computedfor each selected point in the HR-diagram using ATLAS12 (Kurucz © 2019 RAS, MNRAS , 1–7
Ps in Integrated Light α -enhanced abundances. The first of these, whichwe will identify as ’intermediate’ uses [N/Fe] = [Na/Fe] = [Al/Fe]= 0.5, [C/Fe] = [O/Fe] = -0.09, and [Mg/Fe] = -0.07. The second ofthese, termed ’extreme’ adopts [N/Fe] = [Na/Fe] = [Al/Fe] = 1.0,[C/Fe] = [O/Fe] = -0.70, [Mg/Fe] = -0.44. These values were cho-sen to keep the C+N+O, Ne+Na and Mg+Al sums constant. Boththe isochrones and the spectral synthesis adopted the solar scaledabundance of Asplund et al. (2009, Y = 0.27, Z = 0.0142) with[Fe/H] = . − . T eff distributions of scaledsolar isochrones remain unchanged. This is a justified assumptionas long as the CNO sum remains unchanged (e.g., Cassisi et al.2013), which is true in our models. We adopted a Kroupa (2001)stellar initial mass function with a lower mass limit of 0 . ⊙ .However, the lowest mass model spectra that was produced was for0 . ⊙ . We checked the impact on the NaD lines in our models bythe adopted initial mass function (comparing a Kroupa 2001 to aSalpeter 1955 distribution) and the differences were less than a fewpercent, consistent with what has been found by other authors (e.g.,Conroy & van Dokkum 2012; Jeong et al. 2013).The models were initially computed for R=200,000 and werethen convolved with the velocity dispersion or resolution measuredfor each of the clusters σ G114 =
42 km/s (Bastian et al. 2006) andR=6800 for NGC 1978 (Usher et al. 2017). We computed specificmodels for each of the clusters, adopting the parameters for age andmetallicity as discussed in § 1. The impact of adopting models withdifferent ages and metallicities is explored in Appendix A.One of the drawbacks of using integrated light spectroscopy isthat it suffers from a degeneracy between the severity of the abun-dance variations (i.e., how enhanced or depleted a given element is)and the fraction of stars that display the anomalies (e.g., Schiavon etal. 2013). In the Galaxy, we see that the fraction of stars that displaythe chemical anomalies increases with increasing present day clus-ter mass (Milone et al. 2017) and also the extent of the variations(e.g., the maximum spread seen in a given element within the clus-ter) also varies with increasing mass (e.g., Bastian & Lardo 2018).Using integrated spectroscopy, we only get a (light-weighted) meanabundance for the full population for each element studied.
In Fig. 1 we show the spectral region of NaD for NGC 1316:G114(top) and NGC 1978 (bottom), along with three models for solarscaled (red), intermediate (green), and extreme (blue) abundancepatterns and we list the corresponding Na enhancement in the leg-end. We see that the solar scaled abundance models predicted thatNaD would be much weaker than observed for G114. The observa-tions are best reproduced (in terms of line profile) with the interme-diate abundance models. Interpolating from the models shown, wefind that G114 has an [Na/Fe] value of 0 . ± .
1, based on fittingthe line profile. This is our primary evidence that Na is enhancedwithin this cluster, hence that it contains multiple populations. We note that the cores of the lines in G114 are observed to be slightlystronger than predicted by the best fitting model, which may indicatethe presence of NLTE effects that are not taken into account in themodelling (Mashonkina et al. 2000).We look for validation of this result using the weaker Na linesat 5683Å and 5888Å, which are not affected by ISM absorption andwhere NLTE effects are expected to be weaker. The results for G114are shown in Fig. 2. While the signal-to-noise in these lines is muchlower than in the NaD lines, we see again that the observed lines arestronger than would be expected based on solar-scaled abundances,hence that Na is enhanced. In this case the best fitting model iscloser to the extreme abundance pattern, although due to the lowerS/N here than in the NaD region we consider the two measurementsto be consistent.In contrast to G114, the spectrum of NGC 1978 is well repro-duced by the solar-scaled abundance of Na (at LMC metallicity).This is in agreement with high resolution spectroscopy of individ-ual stars that also found little or no Na spread within this cluster(Mucciarelli et al. 2008). While this cluster does host a spread in N(Martocchia et al. 2018a,b) it does not in Na, at least not above themeasurement uncertainties. The fact that we do not find enhancedNa in this cluster lends further support to our analysis method, andhence to the conclusion that there is a significant Na spread withinG114.
We have also analysed the NaD line in the extracted MUSE spectrumof NGC 1316: G114 and the results are shown in Fig. 3, alongwith the three MP models (smoothed to the MUSE resolution).As was found with the UVES spectrum, the observations suggestthe presence of MPs with the intermediate model (or slightly moreenhanced) providing the best fit. Hence, in the absence of strongISM absorption, low resolution spectra are also able to be used todetermine whether Na spreads are present.In addition to Na, there are a number of elements that are seento vary within globular clusters. One such element is Al which isseen to vary, to first order, in an equivalent way and amount as Na(e.g., Carretta et al. 2009b). There are a few of Al lines in the redpart of the optical spectrum that, in principle, can be used to eitherconfirm or refute the Na spreads found in the present work. Theline(s) with the strongest difference between the solar-scaled (i.e.,primordial) abundance and the intermediate or extreme abundancepatters are two lines close together at 6696.01Å& 6698.67Å, thatappear as a single line in low resolution spectra.We show this portion of the MUSE spectra for NGC 1316:G114in Fig. 4. We see some evidence for [Al/Fe] to be enhanced, a furtherindication that MPs are present in this cluster. However, we note that,even for the strongest of the Al lines, the lines are relatively weakand model uncertainties within this region (and at this differencelevel) make this detection tentative. Higher S/N, and possibly higherresolution, spectra will be required to confirm this.Unfortunately, the S/N of the WAGGS NGC 1978 spectrum istoo low to carry out an equivalent test on that cluster.
We have presented a technique to identify and quantify the presenceof multiple populations in the integrated spectra of massive stellarclusters, that is, at least in principle, applicable at any age. Thetechnique makes use of the strong NaD lines, along with dedicated © 2019 RAS, MNRAS , 1–7
Bastian et al. N o r m a li s ed F l u x UVESNGC 1316:G114Age = 3 Gyr[Na/Fe]=0[Na/Fe]=0.5[Na/Fe]=1.05875 5880 5885 5890 5895 5900 5905 5910Wavelength [A]0.40.60.81.0 N o r m a li s ed F l u x WAGGSNGC 1978Age = 2.2 Gyr[Na/Fe]=0[Na/Fe]=0.5[Na/Fe]=1.0
Figure 1. Top:
The NaD line of the ∼ Bottom:
The same as thetop but now for the ∼ . N o r m a li s ed F l u x UVESNGC 1316:G114[Na/Fe]=0.0[Na/Fe]=+0.5[Na/Fe]=+1.0
Figure 2.
Spectral region of NGC 1316:G114 including the Na doublet at5683Å and 5688Å. The observed spectra have been box-car smoothed by 5pixels for clarity. These lines are not affected by ISM absorption. N o r m a li s ed F l u x NGC 1316:G114Age = 3 Gyr[Na/Fe]=0[Na/Fe]=0.5[Na/Fe]=1.0
Figure 3.
The NaD spectral features for NGC 1316:G114 observed withMUSE. The models are for an age of 3 Gyr and [Fe/H] = . [Al/Fe]=0.0[Al/Fe]=+0.5[Al/Fe]=+1.0MUSENGC 1316:G114 AlFe Fe Fe Ca
Figure 4.
The portion of the MUSE spectra of NGC 1316:G114 of the Allines near 6697Å. Additionally, we show the primordial, intermediate andextreme SSP models convolved to the MUSE resolution. There is tentativeevidence for Al enhancement, further strengthening the argument the MPsare present in this cluster. stellar population modelling, to look for enhanced [Na/Fe]. Whilethe NaD lines can be affected by ISM absorption, this effect can bemitigated by using high resolution spectroscopy (as ISM absorptionis typically very narrow) or by studying clusters with little or noforeground extinction. We have applied the technique to two clusterswith similar ages, NGC 1978 (2 . ∼ × M ⊙ and 1 . × M ⊙ ,respectively.NGC 1978, in the LMC, has been studied previously, usinghigh-resolution spectroscopy of individual stars, where little or nospread in [Na/Fe] was found (Mucciarelli et al. 2008). Using ourtechnique, we come to the same conclusion, namely that there isno evidence for significant Na enhancement within this cluster. ForNGC 1316:G114, the much more massive cluster, we find clearevidence for the presence of multiple populations, as Na appears tobe quite enhanced, with [Na/Fe] = . ± . © 2019 RAS, MNRAS , 1–7 Ps in Integrated Light potential cause for this apparent contradiction is that mass is knownto play a key role in the manifestation of MPs (e.g., Carretta etal. 2010; Schiavon et al. 2013; Milone et al. 2017). We note thatthe ancient GCs in the MW, with masses comparable to NGC 1978,typically do have detectable Na spreads ( > . ∼ × that of typical Galactic GCs) its abun-dance variations are correspondingly stronger. Hence, our resultssupport the previous suggestions that cluster mass, and potentiallyage, play important roles in setting the (observed) MP properties ofclusters.The technique used here has the potential to significantly openup parameter space in the study of multiple populations. By studyingthe integrated spectra of other high mass clusters formed in galacticmergers and starbursts with ages between ∼
100 Myr and 5 − ACKNOWLEDGMENTS
We thank the anonymous referee for helpful suggestions. NBgratefully acknowledges financial support from the Royal Society(University Research Fellowship). NB, CU, and SK gratefully ac-knowledge financial support from the European Research Council(ERC-CoG-646928, Multi-Pop). CL acknowledges financial sup-port from the Swiss National Science Foundation (Ambizionegrant PZ00P2_168065). Support for this work was provided byNASA through Hubble Fellowship grant HST-HF2-51387.001-A awarded by the Space Telescope Science Institute, which is oper-ated by the Association of Universities for Research in Astronomy,Inc., for NASA, under contract NAS5-26555. WC acknowledgesfunding from the Swiss National Science Foundation under grantP400P2_183846.
REFERENCES
Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47,481Bastian, N., Saglia, R. P., Goudfrooij, P., et al. 2006, A&A, 448, 881Bastian, N., & Lardo, C. 2018, ARA&A, 56, 83Bruzual, G., & Charlot, S. 2003, MNRAS, 344, 1000Cabrera-Ziri, I., Lardo, C., Davies, B., et al. 2016, MNRAS, 460, 1869Carretta, E., Bragaglia, A., Gratton, R., et al. 2009, A&A, 505, 139Carretta, E., Bragaglia, A., Gratton, R. G., et al. 2010, A&A, 516, A55Cassisi, S., Mucciarelli, A., Pietrinferni, A., Salaris, M., & Ferguson, J.2013, A&A, 554, A19Chantereau, W., Usher, C., & Bastian, N. 2018, MNRAS, 478, 2368Choi, J., Dotter, A., Conroy, C., et al. 2016, ApJ, 823, 102Colucci, J. E., Bernstein, R. A., Cameron, S. A., & McWilliam, A. 2012,ApJ, 746, 29Colucci, J. E., Bernstein, R. A., & Cohen, J. G. 2014, ApJ, 797, 116Conroy, C., & van Dokkum, P. 2012, ApJ, 747, 69Dotter, A. 2016, ApJS, 222, 8Goudfrooij, P., Mack, J., Kissler-Patig, M., Meylan, G., & Minniti, D. 2001,MNRAS, 322, 643Goudfrooij, P. 2012, ApJ, 750, 140Gratton, R. G., Carretta, E., & Bragaglia, A. 2012, A&ARv, 20, 50Forbes, D. A., Bastian, N., Gieles, M., et al. 2018, Proceedings of the RoyalSociety of London Series A, 474, 20170616Jeong, H., Yi, S. K., Kyeong, J., et al. 2013, ApJS, 208, 7Kroupa, P. 2001, MNRAS, 322, 231Kurucz, R. L. 1970, SAO Special Report, 309, 309Kurucz, R. L. 2005, Memorie della Societa Astronomica Italiana Supple-menti, 8, 14Kurucz, R. L., & Furenlid, I. 1979, SAO Special Report, 387, 387Kurucz, R. L., & Avrett, E. H. 1981, SAO Special Report, 391, 391Lardo, C., Cabrera-Ziri, I., Davies, B., & Bastian, N. 2017, MNRAS, 468,2482Larsen, S. S., Brodie, J. P., & Strader, J. 2012, A&A, 546, A53Larsen, S. S., Brodie, J. P., & Strader, J. 2017, A&A, 601, A96Larsen, S. S., Brodie, J. P., Wasserman, A., & Strader, J. 2018, A&A, 613,A56Mashonkina, L. I., Shimanski˘i, V. V., & Sakhibullin, N. A. 2000, AstronomyReports, 44, 790Martocchia, S., Bastian, N., Usher, C., et al. 2017, MNRAS, 468, 3150Martocchia, S., Cabrera-Ziri, I., Lardo, C., et al. 2018a, MNRAS, 473, 2688Martocchia, S., Niederhofer, F., Dalessandro, E., et al. 2018b, MNRAS, 477,4696Martocchia, S. et al. 2019, MNRAS, in pressMilone, A. P., Piotto, G., Renzini, A., et al. 2017, MNRAS, 464, 3636Mucciarelli, A., Carretta, E., Origlia, L., et al. 2008, AJ, 136, 375Pancino, E., Romano, D., Tang, B., et al. 2017, A&A, 601, A112Prugniel, P., & Soubiran, C. 2001, A&A, 369, 1048Sakari, C. M., Shetrone, M., Venn, K., McWilliam, A., & Dotter, A. 2013,MNRAS, 434, 358Sakari, C. M., Shetrone, M. D., Schiavon, R. P., et al. 2016, ApJ, 829, 116Salpeter, E. E. 1955, ApJ, 121, 161Sbordone, L., Salaris, M., Weiss, A., & Cassisi, S. 2011, A&A, 534, A9Schiavon, R. P., Caldwell, N., Conroy, C., et al. 2013, ApJ, 776, L7Usher, C., Pastorello, N., Bellstedt, S., et al. 2017, MNRAS, 468, 3828Weilbacher, P. M., Streicher, O., Urrutia, T., et al. 2012, SPIE, 8451, 84510BWeilbacher, P. M., Streicher, O., Urrutia, T., et al. 2014, Astronomical DataAnalysis Software and Systems XXIII, 485, 451© 2019 RAS, MNRAS , 1–7
Bastian et al. N o r m a li s ed F l u x NGC 1316:G114Age=2.4 GyrAge=3 GyrAge=3.6 Gyr5875 5880 5885 5890 5895 5900 5905 5910Wavelength [A]0.40.60.81.0 N o r m a li s ed F l u x NGC 1316:G114Age = 3 Gyr[Fe/H]=-0.2[Fe/H]=0.0[Fe/H]=+0.2
Figure A1.
The NaD spectral features for G114 observed with UVES fordifferent combinations of model parameters.
Top:
The effect of uncertaintieson the age of G114 (at [Fe/H]=0.0).
Bottom:
The effect of uncertainties onthe metallicity of G114.
APPENDIX A: UNCERTAINTIES REGARDING G114
We have created a number of synthetic models in an attempt toreproduce the spectrum of G114. Specifically, we investigate theuncertainties involved if the adopted properties of G114 (3 Gyr,solar metallicity) were altered by 20%. The results are shown inFig. A1. We find that at an age of ∼ & + . = . = + . < . N o r m a li s ed F l u x UVESNGC 1316:G114[Fe/H]=-0.2[Fe/H]=0.0[Fe/H]=+0.2
Figure A2.
The same as the bottom panel of Fig A1 but now for the Na linesat 5683Å and 5688Å. N o r m a li s ed F l u x NGC 1316:G114Age = 3 Gyr[Fe/H]=-0.2[Fe/H]=0.0[Fe/H]=+0.2 N o r m a li s ed F l u x NGC 1316:G114Age = 3 Gyr[Fe/H]=-0.2[Fe/H]=0.0[Fe/H]=+0.2
Figure A3.
Two regions of the UVES spectrum of G114 centred on Fe Lickindices, Fe5709and Fe5782 in the top and bottom panels, respectively. Wealso show the same models used in the bottom panel of Fig. A1, for threedifferent [Fe/H] values (at fixed age). The data have been boxcar smoothedby three pixels for clarity. The best fitting model is the [Fe/H] = . = + . , 1–7 Ps in Integrated Light This paper has been typeset from a TEX/ L A TEX file prepared by theauthor. © 2019 RAS, MNRAS000