Proper Motion of Leo I: Constraining the Milky Way Mass

Transcription

1 Hubble Space Telescope Cycle 18 GO Proposal 900 Proper Motion of Leo I: Constraining the Milky Way Mass Principal Investigator: Dr. Sangmo Tony Sohn Institution: Space Telescope Science Institute Electronic Mail: Scientific Category: RESOLVED STELLAR POPULATIONS Scientific Keywords: Astrometry, Dwarf Galaxies, Galactic Halo, Resolved Stellar Populations Instruments: ACS, WFC3 Proprietary Period: 12 Orbit Request Prime Parallel Cycle Abstract The mass of the Milky Way is one of the most poorly established Galactic parameters. One important reason for this problem stems from the uncertainty about the bound/unbound status of one particular object, Leo I, which has an unusually large radial velocity (v_hel = 283 km/s) at extreme distance (261 kpc). We propose to resolve this issue by measuring at high-precision the absolute proper motion of Leo I with HST. ACS/WFC F814W data that can be used as first epoch already exist in the archive. For the second epoch data, we request to obtain 6 more orbits with the same instrument and filter, yielding a 5 year time baseline. With proven techniques that use many compact background galaxies as astrometric reference sources, the predicted transverse velocity accuracy is only 26 km/s at the distance of Leo I. Our results will be accurate enough to address whether or not Leo I is bound. We will then perform new equilibrium modeling of the Milky Way satellite system to obtain an improved estimate of the Milky Way mass. We will also perform new N-body simulations of Leo I, constrained by the measured velocity, to fit simultaneously the orbit, extra-tidal features, and unusual star formation history. This will independently constrain the Milky Way mass. Only HST can achieve the required accuracy, and considerable progress can be made on these important issues with only a small investment of HST time.

2 Description of the Observations 1 Astrometry with HST High accuracy astrometric science with HST is now a well-established field. For example, one of us was involved in the derivation of the most accurate PM catalogofanyglobular cluster (Anderson & van der Marel 2010 measured 170,000 PMs for ω Cen with 70µas/yr median per star accuracy, i.e. 1.6 km/s),andinthemostaccuratepmdeterminationto date for any Local Group galaxy (Kallivayalil et al and Kallivayalil, van der Marel & Alcock 2006 achieved 36µas/yr accuracy for the Magellanic Clouds, as independently verified by Piatek et al. 2008). These observations constrain the presence of an intermediate-mass black hole in ω Cen (van der Marel & Anderson 2010) and the LMC/SMC orbit about the Milky Way (e.g., van der Marel, Kallivayalil, & Besla 2009). Studies of galaxy velocities and orbits require absolute PMs, measured with respect to stationary reference sources in the field of view (FOV). Past studies of MW satellites have focused on fields centered on a single background quasar (e.g., Kallivayalil et al. 2006; Piatek et al. 2007). This limits the accuracy (per field) to the precision with which a single source can be centroided (of order 0.01 pixel), making it impossible todetermineusefulpmsbeyond the nearest MW satellites. However, it is possible to get higher accuracy by using compact background galaxies. While their individual positional accuracy is somewhat poorer than for apointsource(quasar),theirmuchlargernumberprovidesanoverwhelming N advantage in averaging. Also, background galaxies make it possible to do absoluteastrometryon any field, with no need for a known background quasar; and background galaxies sample the whole detector, reducing systematic errors. Mahmud & Anderson (2008) developed a technique that demonstrates the feasibility of this approach, and applied it to the Ultra Deep Field. We adopted and improved their software and are using this technique in an ongoing HST Cycle 17 program (GO-11684; PI: van der Marel) to measure the PM of M31. We have fully analyzed the 20% of the data that have been obtained sofar,andextrapolation indicates that we will achieve 10µas/yr accuracy. The error for Leo I we are aiming for in the present proposal is 21µas/yr, as described in the Scientific Justification, which is well within reach of HSTs demonstrated capabilities. 2 Measurement Strategy Technique: To measure the movement of stars with respect to background galaxies we start by creating a co-added frame based on all individual exposures of a field (similar to MultiDrizzle, but better optimized for astrometry). SExtractor is then run on the co-added frame to detect and classify all sources into point and extended sources. The extended sources are manually inspected to find relatively bright and compact background galaxies. For the stars in the field an epsf (effective Point Spread Function; Anderson & King 2003) is created. Similarly, for each individual background galaxy a GSF iscreated(atemplateof the galaxy that takes into account the galaxy morphology, the PSF,andthepixelbinning). For each individual exposure j =1,...,M we fit all stars with the epsf and all galaxies with their GSF. The inferred positions are corrected to a geometrically undistorted frame using the available distortion solutions (Anderson & King 2004) and usingsix-parameterlinear 6

3 transformations to correct for any time-dependent linear skew variations (Anderson 2007). For each background galaxy l we then determine the difference dr lj between its position and the average position of those stars in its immediate vicinity. The average dr l of this quantity over all exposures j has random error dr l = σ l / M,where σ l is the RMS scatter. We will apply this approach to the same background galaxies and stars in the images from both epochs. The measured reflex displacement of each individual background galaxy l implies a PM of the stars in Leo I equal to µ l =( dr l 1 dr l 2 )/ T,where T is the ( σ 2 l1 /M 1)+( σ 2 l2 /M 2) / T, time baseline between the epochs. The random error is µ l = where M 1 and M 2 are the number of exposures for each epoch. The final PM result µ and its random error µ are obtained by taking the weighted average over the results µ l ± µ l for all galaxies l =1,...,N. Let D be the distance our target in Mpc. A PM of 1 mas/yr then corresponds to 4740D km s 1.Letσdenote the typical random positional accuracy per background galaxy, per exposure, per 1D coordinate, in milli-arcsec; and let T be expressed in years. Then the final PM error per field, per 1D coordinate, in unitsofkms 1 is µ =(4740D/ T ) (σ 2 /N )(1/M 1 +1/M 2 ) (2) Control of Systematic Effects: The ACS/WFC geometric distortion calibration is stable with time to < pixels (Anderson 2007; van der Marel et al. 2007). We compare the position of each background galaxy only to its local neighboring stars, and we then average results for all background galaxies on the frame. These actions further reduce any geometric distortion residuals. Also, CCD data suffer from imperfect CTE thatincreaselinearlywith time and detector y position. Massey et al. (2010) introduced a pixel-based correction scheme for CTE on ACS/WFC. J. Anderson recently developed a further improved and generalized algorithm that applies a time-dependent pixel-by-pixel correction directly to the flat-fielded images (Anderson & Bedin 2010). He kindly made the software available to us, and we will use it for this program. Any remaining CTE-induced shifts are removed by our comparison of every background galaxy to its local neighboring stars (at the same y position). We tested this approach on the M31 images from our Cycle 17 program 11684, and find that it effectively elimates any artificial CTE-induced motions. 3 Observing Strategy Archival First Epoch Data: The PM measurement of Leo I requires two epochs of data separated by an appreciable amount of time. We have found thefirstepochdata, suitable for our PM measurement, in the HST archive. These ACS/WFC data were taken in February 2006 under a science program for studying the star formation history of Leo I. They include F435W and F814W data with exposures equivalent to 3.5 and3orbits,respectively. The resulting broad-band colors will aid in separating the target stars from foreground stars and background galaxies. In addition to the ACS/WFC images, there are two other epochs of WFPC2 data in the archive that were obtained with F814W and overlaps with the ACS/WFC field. Those were obtained in March 1994 and June 2004, and the latter observations were specifically obtained for PM studies but no resultshavebeenpublished so far. We have inspected and analyzed the WFPC2 data and concluded that they are 7

4 unsuitable to be used as data for another epoch. In particular, owing to the fact that WFPC2 has more than 5 times lower sensitivity, twice the pixel sizes, andhalfthefieldof view than the ACS/WFC, the PM error reached by using the WFPC2 is far larger than the level our science goals require, in spite of the > 10 year baseline. Proposed Second Epoch Data: For the second epoch it is sufficient to observe only in F814W, which provides the best astrometric handle on the background galaxies. Individual exposures will be sub-pixel dithered and will last half an orbit. We estimated the final PM error for Leo I using equation (3), with D =257kpc, T =5yrsandM 1 =6(thelatter being the number of 1700 sec exposures in the first epoch in F814W). Values of N and σ were estimated as described below. We adjusted the number of second-epoch exposures M 2 to achieve the PM error required to reach our goal presented in thescientificjustification. This yields M 2 =12half-orbitexposuresof 1300s, i.e., we need 6 orbits for our program. Building Galaxy Templates: Our proposed second epoch data will be deeper than our first epoch data, and will use more dithering. We will therefore build our galaxy templates (GSFs) from the second epoch data. Once the galaxy templates are constructed, the quality of the first epoch data will be sufficient for measuring the galaxy positions. Expected Accuracy: To demonstrate the feasibility of our project, we performed ade- tailed analysis of Archival data for another dwarf galaxy, namely Tucana. This galaxy has images of similar quality as what we expect from our second epoch observations of Leo I. The left panels of Figure 2 show sample background galaxies and results from its ACS/WFC F814W imaging. The right panel of Figure 2 shows the RMS scatter σ l,foreachofthe N = 100brightestcompactreferencegalaxiesinthefield. Theinverse-quadrature weightedaverage positional uncertainty is σ =2.1mas (per background galaxy, per 1D coordinate, per half-orbit exposure). From detailed inspection of the Leo I field we find that the density and properties of background galaxies are similar as for the Tucana field. We therefore used the N and σ for Tucana in our exposure time estimates for Leo I. There are many more stars than background galaxies in the field. The PM errors are therefore dominated by the positional accuracy of the background galaxies, with the stars making only a minimal contribution. Coordinated Parallel Observations: The epoch 1 data were obtained to study the stellar populations and star formation history in Leo I. During our epoch 2 observations with ACS/WFC of the galaxy center, the WFC3/UVIS camera will be pointed 6arcminaway in the outer halo. We propose to use WFC3/UVIS in coordinated parallel (CPAR) mode with the F606W and F814W filters to construct a color-magnitude diagram of the outer galaxy halo. This will be used to determine stellar population gradients and the stellar halo density profile, to constrain formation scenarios. The CPAR data are not needed for our primary science, but will yield maximally efficient use of HST. One of us has similarly used the HST CPAR mode for other programs (e.g., NGC 1569 and 4449; Rys et al. 2010). Special Requirements We request the same telescope orientation as for the epoch 1observations. Thiswillmaximizetheoverlappingareabetween the two epochs, will minimize differential geometric distortion correction errors, andwillyieldsimilarspatialpsf variability in both epochs. 8

5 Hubble Space Telescope Cycle 22 AR Proposal 939 The COS Cold Absorber Puzzle: Understanding the Metallicity and Phase of the Circumgalactic Medium Scientific Category: QUASAR ABSORPTION LINES AND IGM Scientific Keywords: Galaxy Formation And Evolution, Galaxy Halos, Interstellar And Intergalactic Medium, Metal Absorption Systems Total Budget Amount: Medium UV Initiative: Yes Theory: Yes Abstract Grasping the way gas gets in and out of galaxies is fundamental to our understanding of galaxy formation and evolution, and COS observations are slowly providing a better picture of these gas flows. Recent COS observations of the circumgalactic medium (CGM) have revealed two puzzles: the bimodal metallicity distribution of Lyman Limit Systems (LLSs) proximate to galaxies (Lehner et al. 2013) and the low volume density for cold absorbers in the CGM (Werk et al. 2014). We propose to address both of these issues through the execution and analysis of grid-based cosmological hydrodynamics simulations of unprecedented resolution. By modeling galaxies at very high resolution using a physically-motivated, momentum-based feedback method, we will perform the first grid-based studies of the CGM at this scale, allowing us to identify analogs to the observed cold absorber population responsible for these puzzles. We will trace the simulated cold absorbers through time to understand their origins, producing a full picture of how they acquire and expel their gas.

6 Coordinated Observations N/A. Justify Duplications We will re-observe a previously imaged field to measure proper motions. Thisusedoesnotconstituteaduplication. Past HST Usage and Current Commitments PI Summary: PI Sohn works as a postdoc at STScI under the supervision of CoI vander Marel. PI Sohn has not been PI on any HST Programs. In the last four cycles he was a CoI on GO/10901, for which data analysis is still ongoing. CoI Summary: CoI van der Marel has been PI on one HST Program in the past four cycles: GO/11684 (Cycle 17, 9 orbits) The First Proper Motion Measurement for M31. Status: Only the first two orbits executed (in Jan 2010), with the remainder expected by Summer In the past four Cycles, CoI van der Marel has been CoI on twelve HST Programs. The 5 programs for which all data were obtained as expected by early 2010(10585,10586,10885, 11201, 11988) have produced 10 (refereed) publications and 3 NASA press releases. In addition, the recent papers Anderson & van der Marel (2010) and van der Marel & Anderson (2010) on ω Cen were based on unfunded HST archival analysis and will lead to a NASA press release (in prep. at proposal submission time). van der MarelistheformerLeadof ACS and WFPC2 at STScI, and current lead of the Telescopes group. He has (co-)authored some 50 papers based on HST data over the course of his career. He has 50% of his time for scientific research and uses HST funding to support a group of postdocs and students. This will allow efficient completion of the present project, if approved, without interference from other time commitments. CoI Majewski is CoI of the HST Treasury Program GO/10775 to conduct an Advanced Camera for Surveys (ACS) survey of Galactic globular clusters (GCs). Data were collected for 65 GCs, and so far nine refereed articles have come from them, with several more in preparation. CoI Besla has expertise in modeling the orbits of unbound satellites from HST PM measurements (e.g., Besla et al 2007) and in simulating tidal effects in galaxy interactions. 9

7 Analysis Plan Consistent with the HST UV Initiative, the goal of the proposed research plan is to investigate the physical conditions that give rise to the observed properties of the CGM. Specifically, we will focus on two new observational mysteries: the metallicity bimodality in the CGM and the low gas volume density of L* halos implied by COS observations of lowredshift galaxies. We will address these questions using a suite of state-of-the-art adaptive mesh refinement (AMR) cosmological hydrodynamics zoom-in simulations performed with the Enzo code (Bryan et al., 2014). This suite of simulations will follow the model of Hummels et al. (2013), starting with a fiducial model of an isolated L* galaxy with standard cooling and feedback, and systematically varying the following properties to determine the e ects each have on the conditions of the simulated CGM and their ability to reproduce the observed datasets: 1. Halo Mass and Environment: There will be three sets of initial conditions, one which at z =0willproducea M halo, another a M halo in a group, and lastly a M halo in relative isolation. This way we can examine how halo mass and environment might a ect the conditions in our simulated CGM. 2. Spatial Resolution in Halo: Increase the spatial resolution of the target halo to achieve at minimum (1) 1 comoving kpc and (2) 500 comoving pc spatial resolution through the halo. 3. Feedback Mechanism: Incorporate three di erent feedback models (1) a simple thermal deposition model, (2) a thermal deposition & cooling suppression model (i.e. Hummels et al. (2013)), and (3) a momentum feedback model (i.e. Simpson et al., in prep) to determine the e ect that each feedback model has on enabling a multiphase CGM. 4. Non-equilibrium ionization: We will run a set of simulations using the Dengo nonequilibrium ionization solver (Silvia et al., in prep) to determine the degree to which the CGM is in equilibrium and how it cold-absorber creation. There are several aspects that make this simulation suite noteworthy. No grid code has modeled the CGM in a cosmological simulation in the literature at the proposed resolutions, nor has one employed the momentum feedback model to study its e ects on the CGM. None have investigated the e ects of environment on CGM structure, and none have incorporated non-equilibrium ionization into their models. From these simulations, we will generate synthetic spectra of sightlines through our CGM for direct comparison with the COS datasets used in the Werk et al. (2014) and Lehner et al. (2013) studies. These simulations will be reduced and analyzed using the yt analysis suite (Turk et al., 2011). We will incorporate existing infrastructure (e.g. Egan et al., 2013)for generating realistic synthetic spectra from the Enzo outputs to probe quantities of interest (e.g. ion number density, temperature, metallicity). Once produced, we will investigate how well the data reproduce the conditions in the two observational datasets. PI Hummels has 4

8 Figure 3: A simulated L* galaxy from Hummels et al. (2013) at z = 1 showing H I column density and density-weighted metallicity for a cube with 200 kpc on a side. The contours represent the region of LLSs, consistent with the dataset from Lehner et al. (2013). The distribution of these metallicity sightlines are displayed in Figure 2B. su cient allocated computing time on local and national supercomputer resources through NSF XSEDE to perform the simulations and analysis this project requires. HST Datasets Requested The proposed investigation will produce synthetic spectra for comparison against several HST COS archival datasets. To address the metallicity bimodality and cold CGM absorber problems as described in Lehner et al. (2013), and Werk et al. (2014) we request access to these datasets: 11520: COS-GTO: QSO Absorbers, Galaxies and Large-scale Structures in the Local Universe (PI: Green); 11598: How Galaxies Acquire their Gas: A Map of Multiphase Accretion and Feedback in Gaseous Galaxy Halos, a.k.a.the COS-Halos program (PI: Tumlinson); 11692: The LMC as a QSO Absorption Line System (PI: Howk); and 11741: Probing Warm-Hot Intergalactic Gas at 0.5 <z<1.3 with a Blind Survey for O VI, Ne VIII, Mg X, and Si XII Absorption Systems (PI: Tripp). References Bryan, G. L., et al. 2014, The Astrophysical Journal Supplement, 211, 19 Chen, H.-W., et al. 2010, The Astrophysical Journal, 714, 1521 Egan, H., Smith, B. D., O Shea, B. W., & Shull, J. M. 2013, arxiv: Ferland, G. J., Korista, K. T., Verner, D. A., Ferguson, J. W., Kingdon, J. B., & Verner, E. M. 1998, The Publications of the Astronomical Society of the Pacific, 110, 761 Ford, A. B., Davé, R., Oppenheimer, B. D., Katz, N., Kollmeier, J. A., Thompson, R., & Weinberg, D. H. 2013, arxiv: , 5951 Haardt, F., & Madau, P. 2001, Clusters of galaxies and the high redshift universe observed in X-rays, 64 6

9 Hummels, C. B., Bryan, G. L., Smith, B. D., & Turk, M. J. 2013, Monthly Notices of the Royal Astronomical Society, 728 Kereš, D., Katz, N., Weinberg, D. H., & Davé, R. 2005, Monthly Notices of the Royal Astronomical Society, 363, 2 Lehner, N., et al. 2013, The Astrophysical Journal, 770, 138 Maller, A. H., & Bullock, J. S. 2004, Monthly Notices of the Royal Astronomical Society, 355, 694 Nielsen, N. M., Churchill, C. W., Kacprzak, G. G., & Murphy, M. T. 2013, The Astrophysical Journal, 776, 114 Prochaska, J. X., Weiner, B., Chen, H.-W., Mulchaey, J., & Cooksey, K. 2011, The Astrophysical Journal, 740, 91 Ribaudo, J., et al. 2011, The Astrophysical Journal, 743, 207 Shen, S., Madau, P., Conroy, C., Governato, F., & Mayer, L. 2013, arxiv: Steidel, C. C., et al. 2010, The Astrophysical Journal, 717, 289 Stinson, G. S., et al. 2012, Monthly Notices of the Royal Astronomical Society, 425, 1270 Tripp, T. M., et al. 2011, Science, 334, 952 Tumlinson, J., et al. 2011, Science, 334, , The Astrophysical Journal, 777, 59 Turk, M. J., et al. 2011, The Astrophysical Journal Supplement, 192, 9 Werk, J. K., et al. 2013, The Astrophysical Journal Supplement, 204, 17 Werk, J. K., et al. 2014, arxiv: Management Plan Investigator Roles PI Hummels, a core developer for the enzo simulation code and the yt analysis suite and expert in simulating the CGM and stellar feedback, will manage and perform the bulk of the proposed work including: managing the collaboration, planning and executing all hydrodynamics simulations, generating synthetic observations, interpreting results, comparing against observational datasets, authoring the two publications, and planning meetings with collaborators. Co-I Lehner, a CGM observational specialist, will provide access to the observational datasets used to produce the Lehner et al. (2013) result and will help compare these observations against the synthetic spectra resulting from the simulations. Co-I Robertson, an expert in theoretical galaxy and cosmological evolution and PI on a recent NSF computational grant responsible for building a supercomputer at the University of Arizona, will provide access to the supercomputer ElGato for executing the simulations and will aid in the interpretation of theoretical results and what implications these may have vis-a-vis galaxy evolution, hydrodynamic stability of the halo, and mass and metal budget in the Universe. Co-I Silvia, NSF Fellow and author of the Dengo non-equilibrium cooling library for simulations, will aid in incorporating non-equilibrium chemistry libraries into the sim- 7

10 ulations to follow individual COS ion species such as O VI, Si II, and Si III. Co-I Smith, author of the Grackle cooling library used in various hydrodynamics simulations and specialist on simulated galaxy evolution, will advise on planning and executing the hydrodynamics simulations and assist in fine-tuning the spectral-generation analysis. Co-I Werk, Hubble Fellow and one of the COS-Halos team leads, will provide access to the observational datasets used to produce the Werk et al. (2014) result and will help interpret the theoretical results while comparing them against the COS datasets. Timeline Months 1-3: Set up and begin executing the hydrodynamics simulations with feedback from Co-Is Robertson, Silvia, and Smith. While these are running, begin building and modifying existing synthetic spectra analysis pipeline with assistance from Co-Is Silvia and Smith. Months 4-6: Simulations will be complete. Finish and execute analysis pipeline to generate data for publication(s) addressing metal bimodality and cold absorbers in the halo. Acquire observational datasets from Co-Is Lehner and Werk. Months 7-8: Analyze and compare observational datasets and simulated data. PI Hummels and Co-I Robertson will host a collaborator meeting at the University of Arizona attended by each of the Co-Is to discuss and interpret the results and implications of these studies. Months 9-12: Write and submit paper(s) to the Astrophysical Journal. Make enzo parameter files and yt analysis scripts available to the public on the yt data hub 1,a public resource for datasets and scripts from the numerical hydrodynamics community, and through PI Hummels research webpage (Enzo and yt are open-source codes, so their source code is already public). Budget Narrative We request a medium budget in order to pay for twelve months of salary for PI Hummels to manage and execute the proposed work. Funding is also requested for a half-month of salary for Co-I Lehner to interpret and compare the COS observations with the synthetic spectra. We ask for funds to cover the flights and accommodations for Co-Is Lehner, Silvia, Smith, and Werk to visit the University of Arizona during months 7-8 for 2 days to analyze and collaborate on the proposed studies. We additionally request funds to cover publication charges associated with the paper(s) that result from this proposal