posterCASCA15
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The Mass-Metallicity Relation of Globular Cluster Systems in Massive Elliptical Galaxies The mass-metallicity relation (MMR) of globular cluster systems in massive elliptical galaxies predicts that as the globular clusters become more luminous, their metallicity increases as well. Using data obtained from the CTIO 4m telescope, we present an analysis of the MMR in the giant elliptical galaxy, M49. The globular cluster sample of M49 comprises 2,417. PSF-photometry is used to measure their photometric properties. M49 shows no obvious MMR slope. We argue that the orbital environment; mainly, the accretion of gas by the cluster as it orbits the galaxy is the culprit for the presence or absence of the MMR. Aisha Mahmoud 1 , William E. Harris 1 , Douglas Geisler 2 1: McMaster University, Hamilton, Canada, 2: Universidad de Concepción, Santiago, Chile Motivation & MMR Review Recent photometric studies of extragalactic clusters have revealed a trend between color and magnitude for globular clusters (GCs) of the highest masses (Harris et. al 2006, Strader et. al 2006). These studies show that as the cluster population becomes more massive (higher luminosities), the population becomes redder; this is known as the MMR slope or the ‘blue tilt’. A ‘red tilt’ has been theoretically suggested, but it has not been observed. Most massive galaxies exhibit this phenomenon with the exception of a few. M49 is an example of massive elliptical galaxy that appears to not exhibit a blue tilt (Strader et. al 2006, etc). Here, we intend to revisit the M49 globular cluster system (GCS) and study the causes for the lack of a MMR slope and how it compares to other massive ellipticals such as NGC 4696. Observations & Calibration Techniques M49 was observed using the Blanco 4-m in C and R filters at the Cerro Tololo Inter-American observatory on the night of October 16, Images have an image scale of 0.54” per pixel. 2013. The raw images were processed using several IRAF (Image Reduction and Analysis Facility) packages. Color-Magnitude Diagrams (CMD) The data sample was in two different areas of equal size which we will refer as inner and outer area. Both areas were split into a grid of size of 0.2 in color by 0.4 in magnitude. All grids were compared to each other and if a match was found in both the inner and outer area box then the object was removed from the sample, see figure 1 where the red box indicates the region where the MMR should be present. To determine the limiting color of the Mechanisms of the Mass-Metallicity Relation in Massive Ellipticals Summary Deep C and T1 photometry using the CTIO 4m telescope has been done in the massive elliptical galaxy, M49. While self- enrichment is still a likely culprit if the MMR, accretion of gas as the GC orbits the dwarf-galaxy progenitor is also an important piece of the MMR puzzle. The mass of the cluster and the metallicity are the most influential parameters in the accretion set up. The velocity also plays a role. While self-enrichment is still a possibility for the MMR, more tests, especially for other massive elliptical galaxies, are essential in constraining the parameters used in this simulation. References Self-Enrichment • First suggested by Strader & Smith (2008) and Bailin & Harris (2009). • The basic premise is that GCSs start out by having an initial metallicity dictated by their birth-cloud, yet this proto-cloud can undergo internal self-enrichment by supernovae feedback. • If self-enrichment is the main and only mechanism responsible for the MMR, the MMR should not vary significantly from galaxy to galaxy. This model does not work well with M49. • Mieske & Baum (2007) proposed the gravitational capture of metal-rich field stars as likely metal-enriching candidates. • Field stars belonging to giant elliptical galaxies are substantially more metal-rich than the members of the metal-poor subpopulation of GCs. • N-body simulations showed that this process is far too inefficient to account for the MMR. Gravitational Capture of Field Stars Mass Accretion via Orbit Transit • Similar theories proposed by Bekki et al. 2007 and Maxwell et al. 2014. • Maxwell et al. 2014 suggests that as the GC orbits the dwarf-galaxy where it formed, it will accrete matter and trigger star formation. This mechanism could potentially enrich the GC with a cocktail of abundances via AGB stars. • Since each GC will have a unique accretion history, this can help explain why some galaxies do not show an MMR. This project focuses on the Maxwell et al. 2014 approach. GCS population, which we define as the color where the contributions of the blue and red populations are the same, we binned the dataset by color (figure 2) and ran a Gaussian Mixture Model routine (Muratov & Gnedin 2010). The metal-poor and metal-rich subpopulations of the GCS are clearly observable in figure 2. The limiting color of (C-T1) = 1.519 is shown in green in figure 1b. To confirm the existence or non-existence of a MMR slope we applied a quadratic polynomial. The equations of best fit for the metal-poor and the metal-rich populations were -0.00126(C-T1) 2 + 1.87(C-T1) and -0.00051(C-T1) 2 + 1.62x shown in blue and red in figure 3. Fig. 1: Color-Magnitude Diagram of M49. Red box indicates the region where the MMR would be present. Fig. 2 Color Binned Histogram of M49. The histogram shows two populations with peaks at (C-T1) blue = 1.32 and (C-T1) red = 1.81. Fig. 3: Higher Polynomial Fit to CMD of M49. The solid blue and red lines show the least squares fit while the dashed green line represents the limiting color of the sample. In order to test the theory suggested by Maxwell et al. (2014), we recreated different scenarios of mass and metallicity accretion to determine the most likely event for M49. Since we cannot measure directly the parameters needed to simulated the observed metallicity changes, in the following setup, we treat the cluster mass, the metallicity and the accretion time as free parameters. We chose the initial formation mass of the GCs to be in the order of 10 3-6 M , relative velocities of 20-30km/s and assuming a cloud of 300pc gives a time interval of about 1000 years for the accretion processes. Figure 4 shows the metallicity evolutionary tracks for GCS with peak metallicities comparable to those of M49. Figures 5 compares the modeled data to that of M49. It appears that the best-fit model for the metal poor subpopulation is that of a cloud with a metallicity of [Fe/H] = -0.65 (teal line). A similar approach was done with NGC 4696, a massive elliptical with a prominent MMR slope (figure 6). These plots show that while this model properly fit the the metallicity tracks of the brightest clusters, it does not fully explain the whole trajectory of the GCS (below a magnitude of T1 ~ 21 and I ~23). This could imply that a mixture of enrichment via AGB ejecta and previous SNe self-enrichment are both responsible in the formation of a MMR. Fig. 5: CMD of M49 overplotted with the simulated metallicity lines. Fig. 6: CMD of NGC 4696 overplotted with the simulated metallicity lines. Fig. 4: Simulated metallicity lines over an accretion period of 1,000 years. The accreted metallicity is left as a free parameter. Each line represents a different accreted metallicity. The initial metallicity, mass and velocity are constant. J. Bailin and W. E. Harris. Stochastic Self-Enrichment, Pre-Enrichment, and the Formation of Globular Clusters. , 695:1082–1093, April 2009 K. Bekki, S. W. Campbell, J. C. Lattanzio, and J. E. Norris. Origin of abundance inhomogeneity in globular clusters. , 377:335–351, May 2007. W. E. Harris, B. C. Whitmore, D. Karakla, W. Okoń, W. A. Baum, D. A. Hanes, and J. J. Kavelaars. Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters. , 636:90– 114, January 2006 A.J. Maxwell, J. Wadsley, H. M. P. Couchman, and A. Sills. The Celestial Buffet: multiple populations and globular cluster formation in dwarf galaxies. , 439:2043– 2049, April 2014 S. Mieske and H. Baumgardt. On the efficiency of field star capture by star clusters. , 475:851–857, December 2007 A. L. Muratov and O. Y. Gnedin. Modeling the Metallicity Distribution of Globular Clusters. , 718:1266–1288, August 2010. J. Strader, J. P. Brodie, L. Spitler, and M. A. Beasley. Globular Clusters in Virgo Ellipticals: Unexpected Results for Giants and Dwarfs from Advanced Camera for Surveys Imaging. , 132:2333–2345, December 2006.
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Transcript of posterCASCA15
The Mass-Metallicity Relation of Globular Cluster Systems in Massive Elliptical Galaxies
The mass-metallicity relation (MMR) of globular cluster systems in massive elliptical galaxies predicts that as the globular clusters become more luminous, their metallicity increases as well. Using data obtained from the CTIO 4m telescope, we present an analysis of the MMR in the giant elliptical galaxy, M49. The globular cluster sample of M49 comprises 2,417. PSF-photometry is used to measure their photometric properties. M49 shows no obvious MMR slope. We argue that the orbital environment; mainly, the accretion of gas by the cluster as it orbits the galaxy is the culprit for the presence or absence of the MMR.
Aisha Mahmoud1, William E. Harris1, Douglas Geisler2 1: McMaster University, Hamilton, Canada, 2: Universidad de Concepción, Santiago, Chile
Motivation & MMR Review Recent photometric studies of extragalactic clusters have revealed a trend between color and magnitude for globular clusters (GCs) of the highest masses (Harris et. al 2006, Strader et. al 2006). These studies show that as the cluster population becomes more massive (higher luminosities), the population becomes redder; this is known as the MMR slope or the ‘blue tilt’. A ‘red tilt’ has been theoretically suggested, but it has not been observed. Most massive galaxies exhibit this phenomenon with the exception of a few. M49 is an example of massive elliptical galaxy that appears to not exhibit a blue tilt (Strader et. al 2006, etc). Here, we intend to revisit the M49 globular cluster system (GCS) and study the causes for the lack of a MMR slope and how it compares to other massive ellipticals such as NGC 4696.
Observations & Calibration Techniques M49 was observed using the Blanco 4-m in C and R filters at the Cerro Tololo Inter-American observatory on the night of October 16, Images have an image scale of 0.54” per pixel. 2013. The raw images were processed using several IRAF (Image Reduction and Analysis Facility) packages.
Color-Magnitude Diagrams (CMD) The data sample was in two different areas of equal size which we will refer as inner and outer area. Both areas were split into a grid of size of 0.2 in color by 0.4 in magnitude. All grids were compared to each other and if a match was found in both the inner and outer area box then the object was removed from the sample, see figure 1 where the red box indicates the region where the MMR should be present. To determine the limiting color of the
Mechanisms of the Mass-Metallicity Relation in Massive Ellipticals
Summary Deep C and T1 photometry using the CTIO 4m telescope has been done in the massive elliptical galaxy, M49. While self-enrichment is still a likely culprit if the MMR, accretion of gas as the GC orbits the dwarf-galaxy progenitor is also an important piece of the MMR puzzle. The mass of the cluster and the metallicity are the most influential parameters in the accretion set up. The velocity also plays a role. While self-enrichment is still a possibility for the MMR, more tests, especially for other massive elliptical galaxies, are essential in constraining the parameters used in this simulation.
References
Self-Enrichment
• First suggested by Strader & Smith (2008) and Bailin & Harris (2009).
• The basic premise is that GCSs start out by having an initial metallicity dictated by their birth-cloud, yet this proto-cloud can undergo internal self-enrichment by supernovae feedback.
• If self-enrichment is the main and only mechanism responsible for the MMR, the MMR should not vary significantly from galaxy to galaxy. This model does not work well with M49.
• Mieske & Baum (2007) proposed the gravitational capture of metal-rich field stars as likely metal-enriching candidates.
• Field stars belonging to giant elliptical galaxies are substantially more metal-rich than the members of the metal-poor subpopulation of GCs.
• N-body simulations showed that this process is far too inefficient to account for the MMR.
Gravitational Capture of Field Stars Mass Accretion via Orbit Transit
• Similar theories proposed by Bekki et al. 2007 and Maxwell et al. 2014.
• Maxwell et al. 2014 suggests that as the GC orbits the dwarf-galaxy where it formed, it will accrete matter and trigger star formation. This mechanism could potentially enrich the GC with a cocktail of abundances via AGB stars.
• Since each GC will have a unique accretion history, this can help explain why some galaxies do not show an MMR. This project focuses on the Maxwell et al. 2014 approach.
GCS population, which we define as the color where the contributions of the blue and red populations are the same, we binned the dataset by color (figure 2) and ran a Gaussian Mixture Model routine (Muratov & Gnedin 2010). The metal-poor and metal-rich subpopulations of the GCS are clearly observable in figure 2. The limiting color of (C-T1) = 1.519 is shown in green in figure 1b. To confirm the existence or non-existence of a MMR slope we applied a quadratic polynomial. The equations of best fit for the metal-poor and the metal-rich populations were -0.00126(C-T1)2 + 1.87(C-T1) and -0.00051(C-T1)2 + 1.62x shown in blue and red in figure 3.
Fig. 1: Color-Magnitude Diagram of M49. Red box indicates the region where the MMR would be present.
Fig. 2 Color Binned Histogram of M49. The histogram shows two populations with peaks at (C-T1)blue = 1.32 and (C-T1)red = 1.81.
Fig. 3: Higher Polynomial Fit to CMD of M49. The solid blue and red lines show the least squares fit while the dashed green line represents the limiting color of the sample.
In order to test the theory suggested by Maxwell et al. (2014), we recreated different scenarios of mass and metallicity accretion to determine the most likely event for M49. Since we cannot measure directly the parameters needed to simulated the observed
metallicity changes, in the following setup, we treat the cluster mass, the metallicity and the accretion time as free parameters. We chose the initial formation mass of the GCs to be in the order of 103-6M¤, relative velocities of 20-30km/s and assuming a cloud of 300pc gives a time interval of about 1000 years for the accretion processes. Figure 4 shows the metallicity evolutionary tracks for GCS with peak metallicities comparable to those of M49. Figures 5 compares the modeled data to that of M49. It appears that the best-fit model for the metal poor subpopulation is that of a cloud with a metallicity of [Fe/H] = -0.65 (teal line). A similar approach was done with NGC 4696, a massive elliptical with a prominent MMR slope (figure 6). These plots show that while this model properly fit the the metallicity tracks of the brightest clusters, it does not fully explain the whole trajectory of the GCS (below a magnitude of T1 ~ 21 and I ~23). This could imply that a mixture of enrichment via AGB ejecta and previous SNe self-enrichment are both responsible in the formation of a MMR.
Fig. 5: CMD of M49 overplotted with the simulated metallicity lines. Fig. 6: CMD of NGC 4696 overplotted with the simulated metallicity lines.
Fig. 4: Simulated metallicity lines over an accretion period of 1,000 years. The accreted metallicity is left as a free parameter. Each line represents a different accreted metallicity. The initial metallicity, mass and velocity are constant.
J. Bailin and W. E. Harris. Stochastic Self-Enrichment, Pre-Enrichment, and the Formation of Globular Clusters. , 695:1082–1093, April 2009 K. Bekki, S. W. Campbell, J. C. Lattanzio, and J. E. Norris. Origin of abundance inhomogeneity in globular clusters. , 377:335–351, May 2007. W. E. Harris, B. C. Whitmore, D. Karakla, W. Okoń, W. A. Baum, D. A. Hanes, and J. J. Kavelaars. Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters. , 636:90– 114, January 2006 A.J. Maxwell, J. Wadsley, H. M. P. Couchman, and A. Sills. The Celestial Buffet: multiple populations and globular cluster formation in dwarf galaxies. , 439:2043– 2049, April 2014 S. Mieske and H. Baumgardt. On the efficiency of field star capture by star clusters. , 475:851–857, December 2007 A. L. Muratov and O. Y. Gnedin. Modeling the Metallicity Distribution of Globular Clusters. , 718:1266–1288, August 2010. J. Strader, J. P. Brodie, L. Spitler, and M. A. Beasley. Globular Clusters in Virgo Ellipticals: Unexpected Results for Giants and Dwarfs from Advanced Camera for Surveys Imaging. , 132:2333–2345, December 2006.
