The spatial and temporal relationships between stellar age, kinematics, and
chemistry are a fundamental tool for uncovering the physics driving galaxy
formation and evolution. Observationally, these trends are derived using
carefully selected samples isolated via the application of appropriate
magnitude, colour, and gravity selection functions of individual stars;
conversely, the analysis of chemodynamical simulations of galaxies has
traditionally been restricted to the age, metallicity, and kinematics of
`composite' stellar particles comprised of open cluster-mass simple stellar
populations. As we enter the Gaia era, it is crucial that this approach
changes, with simulations confronting data in a manner which better mimics the
methodology employed by observers. Here, we use the \textsc{SynCMD} synthetic
stellar populations tool to analyse the metallicity distribution function of a
Milky Way-like simulated galaxy, employing an apparent magnitude plus gravity
selection function similar to that employed by the RAdial Velocity Experiment
(RAVE); we compare such an observationally-motivated approach with that
traditionally adopted - i.e., spatial cuts alone - in order to illustrate the
point that how one analyses a simulation can be, in some cases, just as
important as the underlying sub-grid physics employed.Comment: Accepted for publication in PoS (Proceedings of Science): Nuclei in
  the Cosmos XIII (Debrecen, Jul 2014); 6 pages; 3 figure

Gibson, B. K.

Macfarlane, B. A.

Miranda, M. S.

English

arXiv

Maider Sancho Miranda

B. A. Macfarlane

Brad K Gibson

Crossref

Observationally-Motivated Analysis of Simulated Galaxies

arXiv.org e-Print Archive

© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. The spatial and temporal relationships between stellar age, kinematics, and chemistry are a fundamental tool for uncovering the physics driving galaxy formation and evolution. Observationally, these trends are derived using carefully selected samples isolated via the application of appropriate magnitude, colour, and gravity selection functions of individual stars; conversely, the analysis of chemodynamical simulations of galaxies has traditionally been restricted to the age, metallicity, and kinematics of 'composite' stellar particles comprised of open cluster-mass simple stellar populations. As we enter the Gaia era, it is crucial that this approach changes, with simulations confronting data in a manner which better mimics the methodology employed by observers. Here, we use the SYNCMD synthetic stellar populations tool to analyse the metallicity distribution function of a Milky Way-like simulated galaxy, employing an apparent magnitude plus gravity selection function similar to that employed by the RAdial Velocity Experiment (RAVE); we compare such an observationally-motivated approach with that traditionally adopted - i.e., spatial cuts alone - in order to illustrate the point that how one analyses a simulation can be, in some cases, just as important as the underlying sub-grid physics employed

Gibson, Brad K.

MacFarlane, Ben A.

Miranda, Maider S.

Repository@Hull - Worktribe

arXiv:1502.00444v1  [astro-ph.GA]  2 Feb 2015Observationally-Motivated Analysis of SimulatedGalaxiesMaider S. Miranda∗ †University of Central LancashireE-mail: msancho@uclan.ac.ukBen A. MacFarlaneUniversity of Central LancashireE-mail: bmacfarlane@uclan.ac.ukBrad K. GibsonUniversity of Central LancashireE-mail: brad.k.gibson@gmail.comThe spatial and temporal relationships between stellar age, kinematics, and chemistry are a funda-mental tool for uncovering the physics driving galaxy formation and evolution. Observationally,these trends are derived using carefully selected samples isolated via the application of appropri-ate magnitude, colour, and gravity selection functions of individual stars; conversely, the analysisof chemodynamical simulations of galaxies has traditionally been restricted to the age, metallic-ity, and kinematics of ‘composite’ stellar particles comprised of open cluster-mass simple stellarpopulations. As we enter the Gaia era, it is crucial that this approach changes, with simula-tions confronting data in a manner which better mimics the methodology employed by observers.Here, we use the SYNCMD synthetic stellar populations tool to analyse the metallicity distri-bution function of a Milky Way-like simulated galaxy, employing an apparent magnitude plusgravity selection function similar to that employed by the RAdial Velocity Experiment (RAVE);we compare such an observationally-motivated approach with that traditionally adopted - i.e.,spatial cuts alone - in order to illustrate the point that how one analyses a simulation can be, insome cases, just as important as the underlying sub-grid physics employed.XIII Nuclei in the Cosmos7-11 July, 2014Debrecen, Hungary∗Speaker.†Special thanks to our collaborators S. Pasetto, D. Kawata, C. Brook, G. Stinson, and C. Few.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/Observationally-Motivated Analysis of Simulated Galaxies Maider S. Miranda1. IntroductionAn inherent assumption underpinning the analysis of galaxy simulations is that they providean analogous framework to that of galaxies in nature, in the sense that the chemo-kinetics of themassive stellar-like particles in simulations can be compared directly with the individual stars inobservational samples. Considering that the typical mass of the ‘composite’ stellar particles in cos-mological, hydrodynamical, simulations of Milky Way-like galaxies is of order∼10 5±1 M⊙, ratherthan the∼100±1 M⊙ encountered in nature, it is not unreasonable to question the importance of thismismatch in ‘resolution’; graphically, this ‘mismatch’ can perhaps best be appreciated by referringto Fig 1. Further, observers construct samples of stars based upon their respective and individualcolour, apparent magnitude, chemistry, and gravity, while simulators are limited primarily to theintegrated luminosities and chemistry of the coeval, open cluster-scale, simple stellar populationsused to represent the ‘composite’ stellar particles.Figure 1: Left panel: A real colour-magnitude diagram comprised of ∼104 stars drawn from NGC 6397 [9].Right panel: How the same ∼104 stars are represented by ‘composite’ stellar particles within a simulation.In what follows, we present a pilot study for our long-term programme aimed at developingpowerful, and flexible, tools for better transforming simulations into the observer’s plane. Here,we employ the synthetic colour-magnitude diagram package SYNCMD [5] to replace compositestellar particles with individual stars in a self-consistent fashion, thereby allowing stellar samplesto be drawn by an observer situated within a simulation using apparent magnitude and gravity cri-teria. The impact of this observationally-motivated selection function upon the inferred metallicitydistribution of an analogous ‘solar neighbourhood’, compared with the traditional approach (i.e.,selecting composite stellar particles spatially), will be assessed, and our future directions outlined.2Observationally-Motivated Analysis of Simulated Galaxies Maider S. Miranda2. MethodologyThe characteristics of the ∼L∗ Milky Way-analogue analysed for this work (MaGICC-g1536)are detailed in [4]. The simulation was realised with the smoothed particle hydrodynamics codeGASOLINE [10], employing the MaGICC (Making Galaxies in a Cosmological Context) stel-lar feedback prescription [3]. To transform the simulation into the observer’s plane, we use theSYNCMD (synthetic colour-magnitude diagram tool) developed by [5]. Using the age, metallicity,and initial mass function underpinning stars in simulations, SYNCMD decomposes each particleinto its individual constituent (or ‘synthetic’) stars (of order 10 5 ‘synthetic’ stars per ‘compos-ite’ stellar particle, corresponding to ∼10 11 individual stars for MaGICC-g1536), by mapping thecomposite particles onto the Padova isochrones ( [1]; [2]).3. ResultsFor this pilot study, we situate the observer at an analogous ‘solar neighbourhood’, much aswe did in [4]. From that vantage point, we ‘view’ the simulated galaxy, first isolating compositestellar particles within d=10 kpc of the observer and which avoid the galactic plane (i.e., enforcinga galactic latitude restriction of |b|>20 ◦ from the observer’s position). Such galactic plane restric-tions mimic the typical constraint imposed upon ground-based, optically-selected, all-sky surveys,such as RAVE [8].For each composite stellar particle passing the spatial cut, we employ SYNCMD to distributethe underlying ∼10 5 individual ‘synthetic’ stars, in the appropriate numbers (weighted by theinitial mass function), into each ‘bin’ of the associated colour-magnitude diagram (CMD). Aftertaking into account the distance modulus of each such star, the relevant colour-apparent magnitudediagram, from the observer’s vantage point, results. The upper panel of Fig 2 shows the resulting(I,V−I) CMD for the aforementioned stars passing the initial spatial cut; colour-coding here is bydensity of stars populating each CMD bin, as noted in the legend.Having transformed our composite stellar particles into individual synthetic stars, each withcolours, luminosities, and gravities drawn from the Padova isochrones, we are now in a positionto apply observationally-motivated selection criteria to the simulation. For this pilot study, we em-ploy criteria not dissimilar to that of RAVE [8]. We will show the results of three experiments: (i)including all stars with an I-band magnitude in the range 9<I<12, regardless of gravity, which isreflected in the white horizontal lines in the upper panel of Fig 2; (ii) including both the aforemen-tioned I-band selection, and a surface gravity cut aimed at isolating main sequence and sub-giantbranch (MS+SG) stars (log g>3.5); and (iii) including both the aforementioned I-band selection,and a surface gravity cut aimed at isolating giant branch (GB) stars (logg<3.5). The bottom leftand right panels of Fig 2 correspond to the colour-apparent magnitude diagrams associated with(ii) and (iii), respectively.To assess the impact of our observationally-motivated selection criteria in a quantitative sense,for this pilot study we examine the resulting metallicity distribution function (MDF) for each ofthe aforementioned SYNCMD (i.e., ‘synthetic’) experiments (referred to as (i), (ii), and (iii) in theprevious paragraph), and contrast those with the MDF inferred from using only the ‘composite’stellar particles.3Observationally-Motivated Analysis of Simulated Galaxies Maider S. MirandaFigure 2: Upper: CMD for the synthetic stars passing the spatial cut, colour-coded by density. The twolines represent our I-band selection cut. Bottom left: As for the upper, but including gravity cut for mainsequence + sub-giants. Bottom right: As for the upper, but including gravity cut for giant branch stars.The resulting four MDFs are shown in Fig 3: the thick solid line corresponds to the sub-set of‘composite’ stellar particles passing the initial spatial cut, while the thin solid line corresponds tothe case in which also an I-band cut has been applied. One can immediately appreciate that the dis-tributions, while similar globally, differ in detail; formally, the MDF inferred from the compositesimulation particles is significantly less skewed, with significantly lower kurtosis, than the counter-part derived employing a RAVE-like I-band selection cut (skewness=−1.2 vs−1.5; kurtosis=1.4 vs2.5). The differences become even more obvious when further constraining the observational cuts4Observationally-Motivated Analysis of Simulated Galaxies Maider S. Mirandabeyond just apparent magnitude, to isolate main sequence plus sub-giant branch (MS+SG) stars(blue-dashed histogram in Fig 3) or giant branch (GB) stars (red-dashed histogram in Fig 3). Inthose cases, the skewness ranges from−2.2 (MS+SG) to−1.3 (GB), while the kurtosis ranges from7.1 (MS+SG) to 1.7 (GB). Differences in the mean and variance also range from 0.1 to 0.2 dex,across the four experiments.Figure 3: Comparative metallicity distribution function of the composite particles from our simulated galaxy(thick black line) and results from the decomposition into individual stars made with SYNCMD (thin blackline). We have further decomposed this synthetic population into main sequence + sub-giants (MS+SG, bluedashed line) and giant branch stars (GB, red dashed line), according to the logg restrictions described in § 3.In and of themselves, these quoted moments of the MDFs might not be enlightening, but acomparison with the corresponding moments from the broad suite of simulations described by [6]proves interesting. In [6], five variants of the same simulated disk galaxy were presented, employ-ing different initial mass functions, different treatments of radiation pressure from massive stars,different treatments of metal diffusion, and different treatments of the thermal cooling timescale.These five simulations were identical in every other consideration; as Table 2 in [6] shows, theresulting ranges in ‘solar neighbourhood’ MDF skewness and kurtosis vary from −0.9 to −1.8(skewness), and 0.9 to 3.8 (kurtosis). In other words, the impact of simply how one observes asimulation (i.e., observationally-motivated, as we have done here, or simple spatial cuts, as hasbeen done previously by essentially the entire field) can be as quantitatively important as any of5Observationally-Motivated Analysis of Simulated Galaxies Maider S. Mirandathe sub-grid physics treatments within the simulations themselves!In some sense, the results shown here should not be shocking, and are already appreciatedby the observational community. For example, for the bright apparent magnitude cut employedhere, the main sequence and sub-giant branch stars all lie within 1−2 kpc of the simulation ob-server, while the intrinsically brighter giants probe much greater distances. Without an a posteriorimatching volume-limited selection criterion for the two samples (MS+SG vs GB), the different ef-fective distances of the samples mean that different metallicities and ages are encountered due to theinterplay between the radial+vertical metallicity gradients and the spatially varying age-metallicityrelations.4. SummaryWe have transformed a cosmological hydrodynamical simulation of an ∼L∗ Milky Way-likegalaxy from the simulator’s space to the observer’s plane, making use of the synthetic colour-magnitude diagram tool. Having replaced ∼10 6 ‘composite’ stellar particles with ∼10 11 ‘syn-thetic’ stars, we have employed various spatial, apparent magnitude, and surface gravity cuts tothe individual synthetic stars, and viewed the simulation much as an observer would do in nature.This pilot study was not meant to be exhaustive, but instead, to illustrate that under very commonconditions, selecting stars based upon actual observable characteristics, rather than just using mas-sive composite simulated stellar particles, can lead to inferred metallicity distributions which differby as much as any of the various sub-grid physics treatments employed by simulators. While anenormous literature and community has evolved which is predicated upon understanding the sub-tleties of these sub-grid physics implementations, we have shown that it is equally as importantto simply view the simulation ‘correctly’ as it is to get the sub-grid physics ‘right’. In the nextphase of our work, we will also include the effect of foreground reddening, as well as explorethe impact of magnitude-limited vs volume-limited sample selection upon not only the MDF, butalso various chemistry-chemistry projections, the age-metallicity relation, and metallicity gradientdeterminations.References[1] G. Bertelli, et al., A&A, 484, 815 (2008)[2] G. Bertelli, et al., A&A, 508, 355 (2009)[3] C. B. Brook, et al., MNRAS, 424, 1275 (2012)[4] B. K. Gibson, et al. A&A, 554, A47 (2013)[5] S. Pasetto, et al., A&A, 545, 14 (2012)[6] K. Pilkington, et al. MNRAS, 425, 969 (2012)[7] E. E. Salpeter, et al., ApJ, 121, 161 (1955)[8] M. Steinmetz, et al., AJ, 132, 1645 (2006)[9] R. R. Strickler, et al., ApJ, 699, 40 (2009)[10] J. W. Wadsley, et al., New Astronomy, 9, 137 (2004)6

Observationally-motivated analysis of simulated galaxies

University of Hull Institutional Repository

arXiv:1502.00444v1  [astro-ph.GA]  2 Feb 2015Observationally-Motivated Analysis of SimulatedGalaxiesMaider S. Miranda∗ †University of Central LancashireE-mail: msancho@uclan.ac.ukBen A. MacFarlaneUniversity of Central LancashireE-mail: bmacfarlane@uclan.ac.ukBrad K. GibsonUniversity of Central LancashireE-mail: brad.k.gibson@gmail.comThe spatial and temporal relationships between stellar age, kin matics, and chemistry are a funda-mental tool for uncovering the physics driving galaxy formation and evolution. Observationally,these trends are derived using carefully selected samples isolated via the application of appropri-ate magnitude, colour, and gravity selection functions of individual stars; conversely, the analysisof chemodynamical simulations of galaxies has traditionally been restricted to the age, metallic-ity, and kinematics of ‘composite’ stellar particles comprised of open cluster-mass simple stellarpopulations. As we enter the Gaia era, it is crucial that thisapproach changes, with simula-tions confronting data in a manner which better mimics the methodology employed by observers.Here, we use the SYNCMD synthetic stellar populations tool to analyse the metallicity distri-bution function of a Milky Way-like simulated galaxy, employing an apparent magnitude plusgravity selection function similar to that employed by the RAdial Velocity Experiment (RAVE);we compare such an observationally-motivated approach witthat traditionally adopted - i.e.,spatial cuts alone - in order to illustrate the point that howone analyses a simulation can be, insome cases, just as important as the underlying sub-grid physics employed.XIII Nuclei in the Cosmos7-11 July, 2014Debrecen, Hungary∗Speaker.†Special thanks to our collaborators S. Pasetto, D. Kawata, C. Brook, G. Stinson, and C. Few.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licen. http://pos.sissa.it/Observationally-Motivated Analysis of Simulated Galaxies Maider S. Miranda1. IntroductionAn inherent assumption underpinning the analysis of galaxysimulations is that they providean analogous framework to that of galaxies in nature, in the sense that the chemo-kinetics of themassive stellar-like particles in simulations can be compared directly with the individual stars inobservational samples. Considering that the typical mass of the ‘composite’ stellar particles in cos-mological, hydrodynamical, simulations of Milky Way-likegalaxies is of order∼10 5±1 M⊙, ratherthan the∼100±1 M⊙ encountered in nature, it is not unreasonable to question the importance of thismismatch in ‘resolution’; graphically, this ‘mismatch’ can perhaps best be appreciated by referringto Fig 1. Further, observers construct samples of stars based upon their respective and individualcolour, apparent magnitude, chemistry, and gravity, whilesimulators are limited primarily to theintegrated luminosities and chemistry of the coeval, open cluster-scale, simple stellar populationsused to represent the ‘composite’ stellar particles.Figure 1: Left panel: A real colour-magnitude diagram comprised of∼104 stars drawn from NGC 6397 [9].Right panel: How the same∼104 stars are represented by ‘composite’ stellar particles within a simulation.In what follows, we present a pilot study for our long-term programme aimed at developingpowerful, and flexible, tools for better transforming simulations into the observer’s plane. Here,we employ the synthetic colour-magnitude diagram package SYNCMD [5] to replace compositestellar particles with individual stars in a self-consistent fashion, thereby allowing stellar samplesto be drawn by an observer situated within a simulation usingapparent magnitude and gravity cri-teria. The impact of this observationally-motivated selection function upon the inferred metallicitydistribution of an analogous ‘solar neighbourhood’, compared with the traditional approach (i.e.,selecting composite stellar particles spatially), will beassessed, and our future directions outlined.2Observationally-Motivated Analysis of Simulated Galaxies Maider S. Miranda2. MethodologyThe characteristics of the∼L∗ Milky Way-analogue analysed for this work (MaGICC-g1536)are detailed in [4]. The simulation was realised with the smoothed particle hydrodynamics codeGASOLINE [10], employing the MaGICC (Making Galaxies in a Cosmological Context) stel-lar feedback prescription [3]. To transform the simulationnto the observer’s plane, we use theSYNCMD (synthetic colour-magnitude diagram tool) developed by [5]. Using the age, metallicity,and initial mass function underpinning stars in simulations, SYNCMD decomposes each particleinto its individual constituent (or ‘synthetic’) stars (ofrder 105 ‘synthetic’ stars per ‘compos-ite’ stellar particle, corresponding to∼10 11 individual stars for MaGICC-g1536), by mapping thecomposite particles onto the Padova isochrones ( [1]; [2]).3. ResultsFor this pilot study, we situate the observer at an analogous‘solar neighbourhood’, much aswe did in [4]. From that vantage point, we ‘view’ the simulated galaxy, first isolating compositestellar particles withind=10 kpc of the observer and which avoid the galactic plane (i.e., enforcinga galactic latitude restriction of|b|>20 ◦ from the observer’s position). Such galactic plane restric-tions mimic the typical constraint imposed upon ground-based, optically-selected, all-sky surveys,such as RAVE [8].For each composite stellar particle passing the spatial cut, we employ SYNCMD to distributethe underlying∼10 5 individual ‘synthetic’ stars, in the appropriate numbers (weighted by theinitial mass function), into each ‘bin’ of the associated colour-magnitude diagram (CMD). Aftertaking into account the distance modulus of each such star, the elevant colour-apparent magnitudediagram, from the observer’s vantage point, results. The upper anel of Fig 2 shows the resulting(I,V−I) CMD for the aforementioned stars passing the initial spatial cut; colour-coding here is bydensity of stars populating each CMD bin, as noted in the legend.Having transformed our composite stellar particles into individual synthetic stars, each withcolours, luminosities, and gravities drawn from the Padovaisochrones, we are now in a positionto apply observationally-motivated selection criteria tothe simulation. For this pilot study, we em-ploy criteria not dissimilar to that of RAVE [8]. We will showthe results of three experiments: (i)including all stars with an I-band magnitude in the range 9<I<12, regardless of gravity, which isreflected in the white horizontal lines in the upper panel of Fig 2; (ii) including both the aforemen-tioned I-band selection, and a surface gravity cut aimed at isolating main sequence and sub-giantbranch (MS+SG) stars (logg>3.5); and (iii) including both the aforementioned I-band selection,and a surface gravity cut aimed at isolating giant branch (GB) stars (logg<3.5). The bottom leftand right panels of Fig 2 correspond to the colour-apparent magnitude diagrams associated with(ii) and (iii), respectively.To assess the impact of our observationally-motivated selection criteria in a quantitative sense,for this pilot study we examine the resulting metallicity distribution function (MDF) for each ofthe aforementioned SYNCMD (i.e., ‘synthetic’) experiments (referred to as (i), (ii), and (iii) in theprevious paragraph), and contrast those with the MDF inferred f om using only the ‘composite’stellar particles.3Observationally-Motivated Analysis of Simulated Galaxies Maider S. MirandaFigure 2: Upper: CMD for the synthetic stars passing the spatial cut, colour-coded by density. The twolines represent our I-band selection cut. Bottom left: As for the upper, but including gravity cut for mainsequence + sub-giants. Bottom right: As for the upper, but including gravity cut for giant branch stars.The resulting four MDFs are shown in Fig 3: the thick solid line corresponds to the sub-set of‘composite’ stellar particles passing the initial spatialcut, while the thin solid line corresponds tothe case in which also an I-band cut has been applied. One can immediately appreciate that the dis-tributions, while similar globally, differ in detail; formally, the MDF inferred from the compositesimulation particles is significantly less skewed, with significantly lower kurtosis, than the counter-part derived employing a RAVE-like I-band selection cut (skewness=−1.2 vs−1.5; kurtosis=1.4 vs2.5). The differences become even more obvious when furtherconstraining the observational cuts4Observationally-Motivated Analysis of Simulated Galaxies Maider S. Mirandabeyond just apparent magnitude, to isolate main sequence plus sub-giant branch (MS+SG) stars(blue-dashed histogram in Fig 3) or giant branch (GB) stars (red-dashed histogram in Fig 3). Inthose cases, the skewness ranges from−2.2 (MS+SG) to−1.3 (GB), while the kurtosis ranges from7.1 (MS+SG) to 1.7 (GB). Differences in the mean and variancealso range from 0.1 to 0.2 dex,across the four experiments.Figure 3: Comparative metallicity distribution function of the composite particles from our simulated galaxy(thick black line) and results from the decomposition into individual stars made with SYNCMD (thin blackline). We have further decomposed this synthetic population into main sequence + sub-giants (MS+SG, bluedashed line) and giant branch stars (GB, red dashed line), according to the log restrictions described in § 3.In and of themselves, these quoted moments of the MDFs might not be enlightening, but acomparison with the corresponding moments from the broad suite of simulations described by [6]proves interesting. In [6], five variants of the same simulated disk galaxy were presented, employ-ing different initial mass functions, different treatments of radiation pressure from massive stars,different treatments of metal diffusion, and different treatments of the thermal cooling timescale.These five simulations were identical in every other consideration; as Table 2 in [6] shows, theresulting ranges in ‘solar neighbourhood’ MDF skewness andkurtosis vary from−0.9 to−1.8(skewness), and 0.9 to 3.8 (kurtosis).In other words, the impact of simply how one observes asimulation (i.e., observationally-motivated, as we have done here, or simple spatial cuts, as hasbeen done previously by essentially the entire field) can be as qu ntitatively important as any of5Observationally-Motivated Analysis of Simulated Galaxies Maider S. Mirandathe sub-grid physics treatments within the simulations thems lves!In some sense, the results shown here should not be shocking,and are already appreciatedby the observational community. For example, for the brightapparent magnitude cut employedhere, the main sequence and sub-giant branch stars all lie within 1−2 kpc of the simulation ob-server, while the intrinsically brighter giants probe muchgreater distances. Without anposteriorimatching volume-limited selection criterion for the two samples (MS+SG vs GB), the different ef-fective distances of the samples mean that different metallici ies and ages are encountered due to theinterplay between the radial+vertical metallicity gradients and the spatially varying age-metallicityrelations.4. SummaryWe have transformed a cosmological hydrodynamical simulation of an∼L∗ Milky Way-likegalaxy from the simulator’s space to the observer’s plane, making use of the synthetic colour-magnitude diagram tool. Having replaced∼10 6 ‘composite’ stellar particles with∼10 11 ‘syn-thetic’ stars, we have employed various spatial, apparent magnitude, and surface gravity cuts tothe individual synthetic stars, and viewed the simulation much as an observer would do in nature.This pilot study was not meant to be exhaustive, but instead,to illustrate that under very commonconditions, selecting stars based upon actual observable characteristics, rather than just using mas-sive composite simulated stellar particles, can lead to inferred metallicity distributions which differby as much as any of the various sub-grid physics treatments employed by simulators. While anenormous literature and community has evolved which is predicated upon understanding the sub-tleties of these sub-grid physics implementations, we haveshown that it is equally as importantto simply view the simulation ‘correctly’ as it is to get the sub-grid physics ‘right’. In the nextphase of our work, we will also include the effect of foreground reddening, as well as explorethe impact of magnitude-limited vs volume-limited sample selection upon not only the MDF, butalso various chemistry-chemistry projections, the age-metallicity relation, and metallicity gradientdeterminations.References[1] G. Bertelli, et al.,A&A, 484, 815 (2008)[2] G. Bertelli, et al.,A&A, 508, 355 (2009)[3] C. B. Brook, et al.,MNRAS, 424, 1275 (2012)[4] B. K. Gibson, et al.A&A, 554, A47 (2013)[5] S. Pasetto, et al.,A&A, 545, 14 (2012)[6] K. Pilkington, et al.MNRAS, 425, 969 (2012)[7] E. E. Salpeter, et al.,ApJ, 121, 161 (1955)[8] M. Steinmetz, et al.,AJ, 132, 1645 (2006)[9] R. R. Strickler, et al.,ApJ, 699, 40 (2009)[10] J. W. Wadsley, et al.,New Astronomy, 9, 137 (2004)6

Observationally-Motivated Analysis of Simulated Galaxies

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