Star Formation in NGC 1097 NGC 1097 is a strongly barred, gas-rich, spiral galaxy ~ 45 million light-years away. The power source of its light is a combination of an active galactic nucleus and on-going star forming regions. NGC 1097 is the ideal laboratory for studying star formation processes. It hosts three distinct environments including the inner ring, bar and spiral arms, where the process of star formation differs for strongly barred galaxies (Sheth et al. 2002; Chandar et al. 2010; de Grijs et al. 2012). By examining the variation of the ages of the star clusters in different regions, we will be able to better comprehend the star formation in distinct environments. Project Goals Star clusters evolve from blue (young) to red (old), although dust effects younger star clusters. Thus, we can compare observed colors to models of cluster evolution such as those produced by Bruzual and Charlot (2003). By doing so, we aim to answer: Which environment hosts the youngest and oldest clusters? Do clusters form in bursts or continuously in each environment? To answer these questions, we analyzed data from the Hubble Space Telescope (HST) of NGC 1097 in four filters: F336W (U), F438W (B), F547M (V) and F814W (I) (HST program 13413, PI: K. Sheth). Loreto Barcos-Muñoz 1, Joanna Corby 1, Brendan Ventura 2, Catherine Grebe 2, David Hatter 2, Do Yeop Kim 2, Tim Spuck 3, Jeff Prillaman 2 and Kartik Sheth 3 Age-dating Star Clusters in the Local Spiral Galaxy NGC 1097 Method In order to understand the age distribution of star clusters, we manually selected 60 clusters, including 20 in each of the three sections of the galaxy (Figure 3). We selected clusters that were visible in all four filters. After identifying the clusters, we performed aperture photometry to calculate their flux. In this process, we performed background subtraction using regions with representative backgrounds in all filters. We fine tuned the aperture and background selection because the results were very sensitive to their size and placement. Finally, we calculated the colors B-V, V-I and U-B for all clusters. Figure 4 shows the resulting colors used to determine the ages of the clusters. Each data point represents a single cluster, and we can determine the ages by comparing them to cluster evolution models from Bruzual and Charlot (2003). The red line accounts for a dust extinction of Av=1. The inclusion of a model accounting for dust extinction is important, as if a cluster is younger, it is assumed to contain dust, while in older clusters, all the dust that used to be in the cluster is assumed to be gone. 1 Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA; 2 Albemarle High School, 2775 Hydraulic Road, Charlottesville, VA 22901, USA; 3 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA Results & Conclusion We determined the ages of all 60 star clusters, obtaining ages from ~2Myr to ~5 Gyr. See Figure 5 for the distribution of cluster ages in the galaxy. In the spiral arm, only young and moderate aged clusters are observed, with the latter mostly located below the arm. The bar contains clusters of all ages from very young to old. Many of the clusters appear to have formed in a major burst of star formation ~100 Myrs ago. The inner ring clusters have the largest diversity in ages. The distribution of ages, particularly in the lower half of the ring, provides evidence for bursts at ~100 Myrs and ~5 Gyrs ago. The wide range of ages of the inner ring clusters may indicate the presence of multiple bursts of star formation. While the distribution of ages in the spiral arm may indicate a more continuous history of star formation. Young clusters are present in all three environments, which indicates on-going star formation occurs in each one of them. A more exhaustive study of the clusters, and an increment of the sample size, could likely demonstrate patterns of cluster migration within the different environments of the galaxy. Implications: The absence of old star clusters from the spiral arm indicates that the spiral pattern very effectively triggers star formation. In the bar and inner ring, there is evidence for some continuous star formation, but most of the measured clusters seem to have been formed in major bursts of star formation, particularly ~100 Myrs ago. References Bruzual, G., & Charlot, S. 2003, MNRAS, 344, 1000; Chandar et al. 2010, 719, 966; De Grijs et al. 2012, ApJL, 758, 22; Sheth et al. 2002, AJ 124, 2581 Figure 4: Color-Color plots of the selected clusters for the different regions of the galaxy: inner ring (magenta), bar (cyan), and spiral arm (yellow). The solid lines represent the cluster evolution models used to predict the ages of the clusters, with no dust extinction (blue) and accounting for a visual dust extinction of one (red). Both models were formed assuming solar metallicity. B-V versus V-I plot (bottom left), U-B versus V-I (top left) and U-B versus V-I (top right). We show the different ages (1Myr, 6.3 Myr, 10 Myr, 100 Myr and 1Gyr) with black dots. Figure 3: I filter images of the regions in NGC 1097 with circular apertures enclosing selected star clusters. Figure 2: B filter image of NGC1097 from the HST. This is the part of the galaxy that we analyzed in this work and that is indicated by the red rectangle in Figure 1. Figure 5: I filter image of NGC 1097 with selected clusters color coded by age. Figure 1: Optical image of NGC The red rectangle approximately indicates the coverage of the data presented here (Image Credit: Martin Pugh). Inner RingBarSpiral Arm

Abstract The goals of this project were to determine if the oldest and youngest star clusters in the spiral galaxy NGC 1097 formed in the regions of the spiral arm, bar, or inner ring, as well as whether clusters tended to form continuously throughout time or in bursts. The data analyzed in this project was recorded by the Hubble Space Telescope, which created images of NGC 1097 using five filters, of which four were used in this project. Aperture photometry was performed to determine the age of twenty clusters from each region. Each cluster’s flux, or amount of light emitted, was measured in each wavelength and, after performing background subtraction, then converted to magnitudes, or intensity. The data was then plotted against extinction lines, models used to determine the approximate ages of each cluster. The results indicated that star cluster formation in the spiral arm seemed to occur continuously, while in the other two regions there is evidence of both continuous formation and bursting, particularly around one million years ago. There is evidence of new star formation in all three regions, with the spiral arm notably containing no clusters older than one billion years old. The bar has a wide range of cluster ranges, and the inner ring contains both the oldest and the youngest clusters observed. The lack of older star clusters in the spiral arm suggests that the spiral pattern of the galaxy effectively triggers star formation as it revolves around the center.

Further Research Due to the enormous amount of data concerning NGC 1097 obtained by the Hubble Space Telescope, we can still profit greatly from additional analysis of the galaxy and its clusters. As stated previously, a more exhaustive analysis of star cluster age and place distribution in each region of NGC 1097 is necessary for the purpose of refining the results observed in the relatively small sample size observed throughout the course of this project. Additionally, this project did not incorporate data obtained using the H-alpha filter. This filter is useful for detecting gamma radiation in NGC 1097, which researchers could analyze in the future to help indicate age of star clusters, as younger clusters emit more light in that range of wavelengths. The distribution and movement of dust throughout the galaxy can also indicate the amount of activity and size of the supermassive black hole at the center of the galaxy. Lastly, the trends observed and conclusions made about star cluster formation in NGC 1097 can be compared to that of other galaxies, helping to further our understanding of the universe.