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Understanding Star Formation in Members of the Toomre Sequence Through UV Imaging

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Understanding Star Formation in Members of the Toomre Sequence Through UV Imaging Hubble Space Telescope Cycle 11 General Observer Proposal Understanding Star Formation In Members Of The Toomre Sequence Through UV Imaging Principal Investigator: Mr. Joshua Schlieder Institution: SUNY Stony Brook USA/NY Electronic mail: [email protected] Scientific category: GALAXIES Scientific keywords: INTERACTING AND MERGING GALAXIES, GALAXY FORMATION AND EVOLUTION, GALAXY MORPHOLOGY AND STRUCTURE, STAR FORMATION Instruments: ACS Proprietary period: 12 Cycle 11 primary orbits: 6 Cycle 11 parallel orbits: 0 Abstract The merging of disk galaxies is believed to be responsible for triggering numerous re- sponses across all regions of the galaxies involved, ultimately resulting in the formation of a single elliptical galaxy. One global effect that has been well documented through observation is the formation of massive stars. I propose to further investigate this effect utilizing the high resolution and UV capabilities of the HST ACS High Resolution Camera. A sample of merging galaxies from the Toomre sequence will be observed at near UV wavelengths in order to characterize the evolution and morphology of induced massive star formation throughout the dynamical lifetime of the interaction sequence. The necessity of the HST capabilities in these proposed observations are two fold; needing UV imaging to isolate young massive star clusters and high resolution to resolve them within these distant galaxies. Mr. Joshua Schlieder Understanding Star Formation In Members Of The Toomre Sequence Through UV Imaging Investigator Institution Country PI: Mr. Joshua Schlieder SUNY Stony Brook USA/NY CoI: Total number of investigators: 2 Observing Summary: Configuration,mode,aperture Total Target RA DEC V spectral elements orbits Flags NGC 4676 12 46 11 30 43 38 14.1 ACS/HRC IMAGING 1 F220W NGC 7592 23 18 22 -04 24 57 15.0 ACS/HRC IMAGING 1 F220W NGC 3509 11 04 24 04 49 43 13.5 ACS/HRC IMAGING 1 F220W NGC 2623 08 38 24 25 45 17 14.0 ACS/HRC IMAGING 1 F220W NGC 3921 11 51 07 55 04 43 13.1 ACS/HRC IMAGING 1 F220W NGC 7252 22 20 45 -24 40 42 15.0 ACS/HRC IMAGING 1 F220W Grand total orbit request 6 2 Mr. Joshua Schlieder Understanding Star Formation In Members Of The Toomre Sequence Through UV Imaging Scientific Justification The merging of disk galaxies has been extenisvely studied since the nineteen fifties including observations across the electromagnetic spectrum and numerical simulations. Recent studies strongly suggest that some fraction of elliptical galaxies are the result of the merge of disk galaxies. This idea was illustrated through observation by the Toomre brothers in the late nineteen seventies. The Toomre Sequence is a sample of eleven interacting and merging disk galaxies arranged according to the assumed time before or after merging. The pairs or remnants of pairs were originally chosen because they display tidally dis- rupted tails containing gas, dust and sometimes young stars. The exhibition of these tails indicates that the two galaxies are gravitationally interacting and that they have had or will have a direct encounter. In this scenario, as the galaxies interact their orbits decay and they lose energy and angular momentum to dynamical friciton. The ultimate result is that the galaxies coalesce to form a single elliptical galaxy. This selection criterium and its subsequent effect allows the Toomre Sequence to be called an evolutionary sequence. It should be noted that the Toomre Sequence was based on the apparent degree of interaction of assumedly normal disk galaxies prior to the encounter and was imaged in low resolution on photographic plates. For this reason, the exact time between systems displaying different stages of interaction is not known, although the entire merge process is believed to take approximately five hundred million years. The merging of disk galaxies is believed to be responsible for triggering many of the global responses visible in the members of the Toomre Sequence. Tidal dispution, elongation, AGN activity and star formation are apparent in the sequence, with star formation being one of the most striking induced features. Star formation in merging spiral galaxies is believed to be the result of gas density perturbations caused by tidal forces concentrating large volumes of hydrogen in small regions, thereby accelerating the natural gravitational collapse of dense clouds into stars. The vast expanses of gas are observed to preferentially produce short lived massive stars with peak wavelengths in the ultra-violet. In interacting galaxy pairs this starburst has been observed occuring in the disks, nuclei and tidal tails of the galaxies. The formation and evolution of galaxies remains one of the great mysteries of astro- physics. Retrieving information about the star formation taking place in merging galaxy pairs is essential for understanding merger evolution and the subsequent formation of ellip- tical galaxies. Gaining an understanding of these mergers may lead to insights about galaxy evolution and formation as whole and what role merging galaxies play. For these reasons I propose a high resolution, UV survey of selected members of the Toomre Sequence. The main goal of this proposed investigation is to characterize the evolution and spatial distribu- tion of massive star formation in interacting galaxy pairs as a function of interaction stage. Since O and B stars are short lived observing them during a given stage of interaction is an indicator that star formation is ongoing in that stage. A subsequent question that may be answered by such a survey is what effect does interaction stage have on the magnitude of massive star formation within the merging pair? The targets were chosen to exhibit key steps in the merge sequence, including two disk 3 Mr. Joshua Schlieder Understanding Star Formation In Members Of The Toomre Sequence Through UV Imaging galaxies at first approach, a stage where the galaxies are in direct contact and the final remnant elliptical galaxy. The angular extents of the targets are larger than the HRC field of view, and multiple images of each target must be acquired to generate an image mosaic. Each image will be broken into two 200 second exposures, as is standard practice to allow for the later removal of cosmic rays. The long exposure time is necessary to obtain signal to noise ratios greater than 10 in these diffuse targets. The F220W (Near UV) filter on the WFC/HRC Filter Wheel 2 will be used in the imaging, the filter has a central wavelength of 2220 Angstroms. At 2200 Angstroms the HRC has a quantum efficiency of 16 %, this also necessitates longer exposure times. The resolution of the HST ACS High Resolution Camera will allow young star clusters to be spatially resolved and their position in the galaxies to be determined. Ultra-violet observations will reveal high contrast between the star clusters in questions and the rest of the galactic populations, thereby isolating them for study. The obscuration of UV wavelengths by dust in the galaxy disks may be of some concern, however, it is important to note that the interacting pairs in the Toomre sequence were selected based on optical observations and are seen face on. For this reason it can be said that the systems in this sample will suffer less from extinction due to dust than other candidate interacting pairs. Observations of the members of the Toomre Sequence spanning the entire visible part of the spectrum exist in the HST archive, and could be combined with the data from this proposed survey to determine the spectral energy distribution of merging galaxies as a function of interaction stage. This archival data may also be useful in the event that dust extinction becomes a problem and which stars are red in color and which actually fit the proposed sample but are reddened by dust must be determined. A Survey of this type will benefit astronomy in general by adding to the understanding of galaxy formation and evolution and the role that interacting galaxies play in this process. Observations of massive star clusters in the nuclei of merging galaxies may reveal informa- tion about supermassive black holes and the development of active galactic nuclei. The observations of the systems in the late stages of the Toomre Sequence may reveal signatures that point to disk galaxy mergers as the predecessor of a particular type of elliptical galaxy. The proposed survey may benefit the study of interacting galaxies in general by elucidating the actual time scale between interaction stages in the Toomre Sequence along with adding to the base of information that has been acquired in relation to this particular sample of interacting galaxies. References: Toomre, A., & Toomre, J. 1972, ApJ, 178, 623 Laine et. al. 2003, AJ, 126, 2717 Description of the Observations The dithering of the images was calculated from the size of the HRC field of view (29" X 26") while the exposure time was estimated using the ACS Imaging ETC website with the requirement that the signal to noise ratio be greater than 10 for imaging a 15th mag.(V) 4 Mr. Joshua Schlieder Understanding Star Formation In Members Of The Toomre Sequence Through UV Imaging spiral galaxy with HRC using the F220W filter at a redshiftof 0.02 (the average redshift of the sample). Eventhough the brightest targets are 12th mag. this exposure time will produce count numbers well within the saturation limit of the ccd. The minimum readout time of the ACS HRC is 45 seconds, and the telescope will slew while the chip is read out. Tables 1 - 6 detail the observing procedure for each of the targets and table 7 shows the total amount of time needed for the proposed observations of each target when the readout time and a 5 minute acquisition allowance has been factored in.
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