Local Turbulent Disks: analogs of high-redshift...

Local Turbulent Disks: analogs of high-redshift vigorouslystar-forming disks and laboratories for galaxy assembly?

Abstract Kinematical investigations at redshifts 1<z<3 with Keck and VLT have revealed that star-forming high-redshiftgalaxies exhibit very high internal velocity dispersions. Dynamical data collected with integral fieldspectrographs (IFS) suggest that the level of rotational support in these systems follows a trend in mass, withcompact dispersion-dominated Lyman Break Galaxies at lower stelar masses and large dynamically unstableturbulent disks at stellar masses larger than 10^10 solar masses. The high velocity dispersion of these youngdisks results in a large characteristic scale for star-forming clusters thereby also explaining their 'clump cluster'morphology. Galaxies like these were thought to be absent from the local Universe. As part of a IFS campaignto observe the most H-alpha luminous galaxies in SDSS, we have discovered a sample of very rare objectsseemingly identical to these high-z turbulent disks. In this proposal we seek imaging in H-alpha of thirteen localdisk galaxies in our sample, using the ACS tunable-wavelength ramp filters. Our goal is to measure the sizedistribution of the star-forming complexes in these objects, with ten times the typical physical resolution ofHST observations of high-z galaxies, in order to test the idea that they are indeed dynamically unstableturbulent disks caught in the process of formation. In synergy with existing high resolution HST imaging of thelocal analogs of low-mass dispersion-dominated galaxies at high redshift (Lyman Break Analogs), our proposedobservations of the local counterparts to large turbulent disks at high redshift will help to paint a completepicture of local analogs of high-z star-forming galaxies.

Principal Investigator: Dr. Ivana Damjanov

Institution: Harvard University

Electronic Mail: [email protected]

Scientific Category: UNRESOLVED STELLAR POPULATIONS AND GALAXY STRUCTURE

Scientific Keywords: Dynamics, Galaxy Disks, Galaxy Formation And Evolution, Galaxy Morphology AndStructure, Star Formation

Instruments: ACS

Proprietary Period: 12

Orbit Request Prime Parallel

Cycle 20 13 0

Hubble Space Telescope Cycle 20 GO Proposal 1004

Investigators:

Target Summary:

Observing Summary:

Investigator Institution CountryPI Dr. Ivana Damjanov Harvard University USA/MACoI Dr. Andrew Green The Australian Astronomical Observatory AUSCoI Prof. Karl Glazebrook Swinburne University of Technology AUSCoI Dr. Roberto G. Abraham University of Toronto CANCoI Dr. Patrick J. McCarthy Carnegie Institution of Washington USA/DCCoI Dr. Erin Mentuch University of Texas at Austin USA/TXCoI Dr. Gregory Brian Poole Swinburne University of Technology AUSCoI Dr. Peter J. McGregor Australian National University AUSNumber of investigators: 8

Target RA Dec MagnitudeHFLUXHZ.10-1 10 21 42.4700 +12 45 18.72 Halpha luminosity = 10^42.37 erg/sHFLUXHZ.13-1 13 50 22.7500 -02 51 57.53 Halpha luminosity = 10^42.58 erg/sHFLUXHZ.14-3 14 54 35.3600 -02 00 49.72 Halpha luminosity = 10^42.16 erg/sHFLUXHZ.4-1 04 12 19.7400 -05 54 48.67 Halpha luminosity = 10^42.36 erg/sHFLUXLZ.13-5 13 30 7.0100 +00 31 53.39 Halpha luminosity = 10^42.10 erg/sMFLUXHZ.10-1 10 19 40.2900 -00 26 24.75 Halpha luminosity = 10^42.23 erg/sHFLUXHZ.14-1 14 54 28.3300 +00 44 34.37 Halpha luminosity = 10^41.93 erg/sHFLUXHZ.20-2 20 44 2.9200 -06 46 57.94 Halpha luminosity = 10^42.26 erg/sHFLUXHZ.8-5 08 54 18.7400 +06 46 20.60 Halpha luminosity = 10^42.05 erg/sHLUMAZ.10-2 10 44 31.7600 +12 09 25.20 Halpha luminosity = 10^42.52 erg/sELFLUXLZ.4-3 04 02 42.5100 -06 44 47.76 Halpha luminosity = 10^41.36 erg/sHFLUXLZ.15-3 15 34 35.4000 -00 28 44.50 Halpha luminosity = 10^41.76 erg/sMFLUXLZ.13-1 13 26 39.4200 +01 30 1.48 Halpha luminosity = 10^41.94 erg/s

Dr. Ivana Damjanov : Local Turbulent Disks: analogs of high-redshift vigorously star-formingdisks and laboratories for galaxy assembly?

Karl Glazebrook

Target Config Mode and Spectral Elements Flags OrbitsHFLUXHZ.10-1 ACS/WFC RampFilter FR782N 1

ACS/WFC RampFilter FR647MHFLUXHZ.13-1 ACS/WFC RampFilter FR782N 1



ACS/WFC RampFilter FR647MHFLUXLZ.13-5 ACS/WFC RampFilter FR716N 1

ACS/WFC RampFilter FR647MMFLUXHZ.10-1 ACS/WFC RampFilter FR782N 1




ACS/WFC RampFilter FR647MHLUMAZ.10-2 ACS/WFC RampFilter FR782N 1

ACS/WFC RampFilter FR647MELFLUXLZ.4-3 ACS/WFC RampFilter FR716N 1

ACS/WFC RampFilter FR647MHFLUXLZ.15-3 ACS/WFC RampFilter FR716N 1

ACS/WFC RampFilter FR647MMFLUXLZ.13-1 ACS/WFC RampFilter FR716N 1

ACS/WFC RampFilter FR647MTotal prime orbits: 13

Dr. Ivana Damjanov : Local Turbulent Disks: analogs of high-redshift vigorously star-formingdisks and laboratories for galaxy assembly?

Scientific Justification

Summary: Summary: We request 13 orbits to image a unique sample of nearby galaxies withvery unusual star-formation and kinematic properties. These systems appear to closely resemblemassive highly-turbulent disk galaxies identified at z ⇠ 2 by a number of independent groups.While several attempts to use HST to study local analogs to high-z galaxies have already beenmade (e.g., the ‘Lyman Break Analogs’ (LBAs)), galaxies in our sample are much better matchedin physical parameters to high-z large disk systems revealed by AO-fed integral field spectroscopy(Forster Schreiber et al. 2009). Our galaxies have similar sizes (1–10 kpc) and are rotationally sup-ported (v/� > 1) with resolved dispersions of 40–90 km/s. In contrast, LBAs are much smaller anddispersion-dominated — they are a better match to lower-mass Lyman Break Galaxies at high-z(Law et al. 2009). We propose to image 13 galaxies at 0.05 < z < 0.15 to test the idea that oursample represent the ‘true’ analogs to high-z star-forming disks. Imaging in H↵ will determine ifthey contain structures equivalent to the star-forming ‘clump clusters’ that are prevalent in assem-bling disks at high-redshifts. We also seek to elucidate the mechanisms by which local turbulentdisk galaxies obtain their gas. To date HST has only observed local analogs of low-mass dispersiondominated galaxies at high-redshift. This is a unique opportunity to observe analogs of large rotatingdisks and complete our picture of high-redshift galaxy assembly.

How do galaxies get their gas?: In the standard hierarchical model massive galaxies assemblethrough stochastic processes such as mergers. In the local universe it has seemed that mergers dom-inate at high star-formation rates (SFRs, e.g., Freedman Woods et al. 2010 and references therein).It has recently been suggested, however, that at high redshifts galaxies can be built up through adi↵erent process, namely that of smooth gas accretion along cold flows (Dekel et al. 2009; Ocvirket al. 2008; Keres et al. 2008). Although the latter process is only beginning to be understood, itnow seems that both mechanisms may be important for galaxy assembly. Bournaud & Elmegreen(2009) argue that the smooth process dominates in most high-z galaxies, and that the observationalsignature of this process is an abnormally high velocity dispersion (i.e., turbulent dynamically hotdisks) combined with highly clumped star-formation (so that the galaxies resemble ‘clump clus-ters’). Therefore, it seems that galaxies get their gas by both processes: first by smooth accretionresulting in large, unstable star forming clumps, and then later by merging and/or migration ofclumps via dynamical friction and tidal torques (Genzel et al. 2011), which drive the formation ofthe bulge, and ultimately the progression to a stellar spheroid/elliptical galaxy.

Kinematics of high-z star-forming galaxies: At present, the state-of-the-art data obtainedby Keck and VLT has resulted in a range of findings for the internal dynamical state of star-forming galaxies at high redshift. Using Keck with the OSIRIS IFS operating in AO-assisted mode,Law et al. (2009) targeted 13 z = 2 � 3 UV-selected galaxies and obtained sub-kpc resolution (1kpc= 0.100 at z ⇠ 2). While all objects show high intrinsic velocity dispersions (⇠ 60 � 100 km/s,a factor of about 10 higher than typically found in z = 0 disks) throughout the galaxy, very fewshowed evidence of significant rotational gradients about a preferred kinematic axis. In contrast,Forster Schreiber et al. (2009) used the VLT + SINFONI IFS (in both AO-assisted and naturalseeing modes) to assemble a very large sample of 63 galaxies (SINS sample, defined using a mixtureof UV, sub-mm, and K-band selection) and found similarly high velocity dispersions but a greaterincidence of rotation. Law et al. (2009) galaxies are more compact and tend to lie at lower stellarmasses compared to the SINS sample where the proportion of disk-like systems increases at highermasses. Thus it seems plausible that there is a trend of rotation with stellar mass which could make

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the various results consistent. The presence of massive star-forming disks suggests a scenario inwhich gas is accreted at high rates via cold flows along cosmic filaments, giving rise to disk galaxieswith dynamical instabilities because of the extreme gas-richness (e.g. Bournaud et al. 2007).

Pushing to higher resolution with gravitational lensing: Recently Jones et al. (2010) wereable to push to higher resolution by using gravitational lensing. These researchers used the KeckAO system + OSIRIS IFS to analyze the resolved dynamics of six strongly lensed star-forminggalaxies at 1.7 < z < 3.1. A global kinematical analysis indicates that most systems are unstablehigh-dispersion disks (consistent with the ‘cold flow’ picture noted above). However, since thesegalaxies are magnified by a factor of about ten, Jones et al. (2010) were able to probe the struc-ture of their star-formation complexes down to a resolution of about 200 pc in the rest-frame H↵and [OIII]�5007. They discovered that the giant star-forming HII regions in these young forminggalaxies have diameters around 500 pc and star-formation densities about two orders of magnitudehigher than those observed in most local spirals. In the local Universe star-formation complexesof comparable luminosity are only seen in mergers such as NGC4038/39 (The Antennae). Suchmassive star-formation complexes also seem to be correspond to those seen in the ‘clump cluster’galaxies at high redshift. Figure 1 (taken from Jones et al. 2010) presents the star-formation ratevs. size diagram for giant HII regions in their high-z sample and compares these with the corre-sponding measurements in a sample of local galaxies. While the lensing data are limited (they onlyachieve high spatial resolution of 100 pc in one dimension, and reconstruction is dependent on thelensing model) this does seem to confirm a scenario of triggering star formation at high redshiftsby fragmentation of dynamically unstable systems.

Enter Project DYNAMO: Over the last few years the proposers have been undertaking ProjectDYNAMO (DYnamics of Newly Assembled Massive Objects) using the AAT SPIRAL and Stromlo2.3m WiFeS integral field spectrographs (Green et al. 2012, submitted). These natural-seeing IFSswere used to study 95 nearby z ⇠ 0.1 galaxies in order to match the spatial resolution (in physicalkpc) and field of view of the AO-assisted IFSs on Keck, VLT and Gemini being used to studyhigh-z galaxies. Our project targeted the most extreme H↵-luminous galaxies in the entire SDSS(excluding AGN) in order to match the high SFRs of the objects being studied at high redshift andassemble a sample whose H↵ dynamics could be used to understand high-zobservations. Galaxieswith H↵ luminosities analogous to the high-z observed systems are rare and we had to search thefull SDSS sample out to z ⇠ 0.15 in order to find them. Our surprising discovery (front cover ofNature: Green et al. 2010) was that the DYNAMO sample galaxies were even more like the high-zobjects than we had any right to expect: at high H↵ luminosities, despite their rareness, we findessentially all objects to have high internal velocity dispersions up to 100 km/s. Note we use theaverage flux weighted mean dispersion �m following Law et al. (2009) which is insensitive to thelarge-scale ordered motion. One of the factors that may cause low intrinsic velocity dispersions to beoverestimated when measured in this way is beam smearing due to atmospheric seeing (Davies et al.2011). However, our analyses show no correlation between �m and the spatial resolution of observa-tions, confirming that the variation in velocity dispersion is dominated by physical e↵ects (Green etal. 2010, 2012). While some of high �m DYNAMO objects have merger-like large-scale kinematics,at least half the sample have kinematics very similar to high-z disks suggesting we have indeeddiscovered local turbulent disks. Interestingly, the scaling between star-formation rate and meanvelocity dispersion is essentially the same at high and low-redshift (Figure 2) indicating that the twophysical processes may be intimately linked. This last point is not at all obvious when considering

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just the high-z data alone as only the luminous subset can be observed. It is vitally important toconfirm if these objects are true analogs of high-z turbulent disks; our HST observations would thenprovide a key window in to the physics driving the fundamental star-formation processes of galaxies.

Local Turbulent Disks and Lyman Break Analogs: A sample of supercompact UV luminousgalaxies remarkably similar to high-z Lyman Break Galaxies in terms of global properties (thatinclude high SFRs and high velocity dispersions) has been found at z < 0.2 and studied in greatdetail (e.g., Overzier et al. 2009, Goncalves et al. 2010). These Lyman Break Analogs (LBAs)are extremely rare and present the opportunity to study the evolution of star-forming galaxies bycomparing samples at di↵erent redshifts. Based on spatially resolved kinematical properties, sizes,and stellar masses, the majority of these objects resemble compact dispersion-dominated high-zsample of Law et al. (2009). If so, then are the local turbulent DYNAMO disks the analogs ofhigh-z dynamically unstable massive disks found in the SINS sample (Figure 3)?

Do dynamically unstable turbulent disks really exist in the local Universe?: Our aimnow is to learn more about the nature of these objects, and in particular to understand if we re-ally can use these systems as laboratories to understand galaxy formation at high-redshifts. Thefirst step is to confirm that these systems really are close analogs to the high-z galaxies describedpreviously, and the obvious way forward is to image a sample of them in H↵ with ACS ramp fil-ters to see if the star-formation is concentrated in giant high-intensity star-forming clumps, as theturbulent disk hypothesis would predict. At z > 1 these ‘clump clusters’ are 500–1000 pc in size(e.g. Elmegreen et al. 2008, Jones et al. 2010), and the 100 pc resolution of ACS at z ⇠ 0.1would be very similar to the lensed samples mentioned above, but without the complexities of in-terpretation and in a sample with a well-defined selection and two-dimensional 100 pc resolution.Size measurements will easily show us whether their HII regions fall on the lower or upper locus ofFigure 1. We will be able to artificially redshift these images to simulate high-z observational limits.

Sample and requested observations: We request time to image ten rotationally supportedgalaxies with high velocity dispersion and three disk galaxies with low velocity dispersion (as acontrol) in H↵. Our goal is to measure the sizes of HII regions in this sample. The field of view ofthe ACS ramp filters is ideal for our purposes, since the galaxies are typically 10–15 arcsec in size.(Note that the quasi-monochromatic field of view is 25⇥ 60 arcsec for FR716n and 35⇥ 65 arcsecfor FR782n; the ramp filter can be rotated so H↵ is in-band over this area). In addition to imagingon-band in H↵, for each galaxy we also request a short o↵-band integration in the FR647M rampfilter to allow for continuum subtraction. The proposed sample size is modest but large enough toallow us to determine the systematics of the population without severe biases from small numberstatistics. In terms of size, stellar mass, and kinematical properties our sample is a close match tothe massive turbulent disks in the SINS sample (Figure 3). Our existing imaging is all from SDSS,and its spatial resolution of 2.4 kpc is clearly inadequate (see Figure 1) for our goal of using thesizes of the star-formation complexes to test the turbulent disk hypothesis. It is also a poor matchto the spatial resolution of the best high-z observations (e.g. the lensed Jones et al. 2010 data).

Why not do this using AO imaging?: This project is not feasible with AO. At these redshiftsH↵ corresponds to visible wavelengths, where AO does not work. Furthermore, relatively few ofthese rare galaxies lie near a bright natural guide star needed to supply a laser guide star systemwith tip-tilt correction. Even if these problems were somehow surmounted existing AO systems havea fairly non-uniform PSF over the ⇠ 10 � 15 arcsec size of a typical galaxy, and we must observethe entire disk with a uniform high-resolution PSF. Therefore this project absolutely requires HST.

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References:Bournaud, F., & Elmegreen, B. G. 2009, 694, L158Bournaud, F., Elmegreen, B. G. , & Elmegreen, D. M. 2007, ApJ, 670, 237Brinchmann, J., et al. 2004, MNRAS, 351, 1151Davies, R., et al. 2011, ApJ, 741, 69Dekel, A., et al. 2009, Nature, 457, 451Elmegreen, B.G., Bournaud, F., & Elmegreen, D. M. 2008, ApJ, 688, 67Epinat, B., et al. 2009, A&A, 504, 789Forster Schreiber, N. M., et al. 2009, ApJ, 706, 1364Freedman Woods, D., et al. 2009, AJ, 139, 1857Genzel, R., et al. 2011, ApJ, 733, 101Goncalves, T. S., et al. 2010, ApJ, 724, 1373Green, A. W., et al., 2010, Nature, 467, 684Green, A. W., et al. 2012, ApJ, submittedJones, T. A., et al. 2010, MNRAS, 404, 1247Kau↵mann, G., et al. 2004, MNRAS, 353, 713Keres, D., et al. 2009, MNRAS, 395, 160Law, D.R., et al. 2009, ApJ, 697, 2057Lemoine-Busserolle, M., & Lamareille, F. 2010, MNRAS, 402, 2291Ocvirk, P., Pichon, C., & Teyssier, R. 2008, MNRAS, 390, 1326Overzier, R. A., et al. 2009, ApJ, 706, 203Tremblay, G. R., et al. 2009, ApJS, 183, 278Tremonti, C., et al. 2004, ApJ, 613, 898

16 Jones et al.

Figure 7. Size versus dynamical mass of individual star-formingregions within our high-redshift sample, compared to local starclusters and Hii regions. The high-redshift Hii regions have sizeand mass consistent with the largest star-forming complexes inthe local universe. Local data are taken from Harris (1996) andPryor & Meylan (1993) for Galactic globular clusters; Haseganet al. (2005), Hilker et al. (2007) and Evstigneeva et al. (2007)for ultra-compact dwarf galaxies; McCrady & Graham (2007) forM82 clusters; Fuentes-Masip et al. (2000) for giant Hii regions;and Bastian et al. (2006) for star-forming complexes in the An-tennae. The dashed line represents a model for regions which areoptically thick to far-infrared radiation and have undergone adi-abatic expansion (Murray 2008). Dynamical mass for the high-

redshift data is calculated as Mdyn = C R�2

G with assumed C = 5appropriate for a uniform-density sphere. The mass error barsaccount for statistical uncertainty in R and �.

termine the dynamical state of galaxies which would be toopoorly sampled to distinguish between rotation, merging, ordispersion-dominated kinematics without gravitational lens-ing.

The lensed sample allows a unique study of relativelyfaint and small galaxies at high resolution. The median lu-minosity of the sample is half of the characteristic L� forz = 3 LBGs and well below that of other high-redshift IFUstudies. We find that the kinematics of the sub-L� popu-lation are in general agreement with larger and more lu-minous galaxies at similar redshifts: four of the six sys-tems are clearly rotating with V sin i/� = 0.5–1.3, onehas a velocity gradient consistent with rotation within thesmall H�-bright region of the arc detected with OSIRIS,and one is a major merger. All have high velocity disper-sion � = 50–100 km s�1, consistent with all other resolvedobservations of z > 2 star-forming galaxies and compa-rable to the value � � 50 km s�1 for stars which formedin the Galactic disk at z � 2 (Sparke & Gallagher 2000).Galaxies with a larger radius of detected nebular emis-sion tend to have higher velocity shear and V/� and lower�SF R; these trends are consistent with other observations.The case of MACS J0744+3927 with inclination-correctedV = 180+100

�40 km s�1 clearly demonstrates that some small(R = 1.0 ± 0.2 kpc), fast-rotating field disk galaxies are al-

Figure 8. H↵-inferred star formation rate as a function of Hiiregion size. Red circles represent individual Hii regions from ourlensed sample. Cl 0949+5152 was observed in [Oiii] with the H↵flux taken to be 1.2 times the [Oiii]�5007 flux as determined fromlong slit NIRSPEC observations. The value for MACSJ2135-0102was determined from the H� flux assuming case B recombination.The star formation rates are all derived from H↵ luminositiesusing the Kennicutt (1998) prescription. Red squares are fromthe lensed z = 4.92 galaxy presented by Swinbank et al. (2009),black points are Hii regions in local spiral galaxies from Gonzalez-Delgado and Perez (1996), and blue open circles are star clustercomplexes in the interacting galaxies NGC 4038/4039 from Bas-tian et al. (2006). The vectors A and B are explained in the text.The dashed line shows a fit to the data from local spiral galax-ies, and the dotted line is the same but increased by a factor of100� in SFR density. The high-redshift star-forming regions havesignificantly higher density of massive stars than Hii regions innearby spirals, but are comparable to the brightest star clustercomplexes found in the local universe.

ready in place by z = 2. Well-sampled velocity fields demon-strate that the ubiquitous giant clumps are often embed-ded in common rotating systems and are not independentmerging components. In contrast, non-lensed clumpy sys-tems with velocity shear have been interpreted as mergersor remained ambiguous. These results demonstrate the needof high-resolution observations to distinguish rotating sys-tems and mergers, as well as to probe the < 1 kpc scale ofstar formation at high redshift.

The observations presented here represent the best cur-rent probe of the scale of star formation at z � 2 and, assuch, can be compared to theoretical predictions. Most ofthe star formation in our sample occurs within giant Hiiregions of diameter 0.3–1 kpc comparable to the largest lo-cal Hii regions, with star formation rate density comparableto the most vigorous local starbursts and � 100� higherthan in typical spiral galaxies. This o�set in �SF R cannotbe explained by the di�erent resolution or sensitivity of low-and high-redshift observations. Only one of our six objectsis a clear major merger with both merging components bro-ken into high-SFR clumps, suggesting that star formationepisodes in sub-L� high redshift galaxies are triggered byToomre instability independent of major merger events. Wenote that the merging system demonstrates no enhancement

c� 0000 RAS, MNRAS 000, 000–000

Figure 1: Figure taken from Jones et al. (2010) showing the star formation rate as a function ofHII region size in nearby galaxies (bottom locus) and for lensed galaxies in the distant Universe(upper locus). A few of the most massive star formation complexes found in the local universe(which occur in mergers, such as NGC4038/39, i.e. the Antennae) also occupy the upper locus,showing that they approach the level of star formation at high redshift. The dashed line is a fit todata from local spiral galaxies, while the dotted line is the same but increased by a factor of 100in star-formation rate density.

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Figure 2: (Reproduced from Green et al. Nature, 2010, see that paper for full details andanalysis of systematic errors). Mean local velocity dispersion as a function of total H↵ luminosity(bottom axis) and star-formation rate (SFR, top axis) for z ⇠ 0.1 galaxies in the DYNAMOsurvey (black circles, with red borders for those with non-disk kinematics), compared with high-zgalaxies from a number of AO IFU surveys (other points). The right hand axis (and browndashed line) shows the rapidly declining space density of local H↵ luminous galaxies. There is astrong correlation of velocity dispersion with SFR and local galaxies with SFRs similar to thehigh-z ones that have been observed with IFUs show similarly high velocity dispersions,suggesting a fundamental scaling of velocity dispersion with star-formation.

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Figure 3: Size vs. velocity shear to velocity dispersion ratio for two z < 0.3 samples ofstar-forming galaxies with high velocity dispersions: LBAs (points, Goncalves et al. 2010) andlocal turbulent disks (drawn from the DYNAMO sample: perturbed rotators (triangles) androtating disks (squares), based on the classification from Green et al. 2012), and for z ⇠ 2star-forming objects from Law et al. 2009 and SINS samples. Open circles present a controlsample of three relaxed DYNAMO disks with �m < 50 km/s. Our local turbulent disks lie in theregion of dynamically unstable disks at z ⇠ 2 (red crosses) and have the following properties: 1)stellar masses M⇤ > 1010 M�, larger than 75% of the LBAs, 2) median half-light radius ⇠ 2 largerthan for the LBA sample, and 3) these are rotationally supported objects with high velocitydispersions (�m > 50 km/s, vshear/�m > 1). In terms of global structural and dynamical propertiesLBAs and local turbulent disks present two distinct populations of local star-forming galaxies.

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Figure 4: Montage of chosen high-dispersion DYNAMO targets in our main sample.Columns show (a) SDSS 3-color images with 22x11 arcsec IFS FoV marked, (b) H↵ IFSvelocity map, (c) H↵ intensity map, (d) H↵ velocity dispersion map with meandispersions indicated. In all case star-forming complexes are essentially unresolvedfrom the ground.

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Description of the Observations

Sample selection: The DYNAMO sample was selected from SDSS DR4 and the JHU-GarchingValue Added Catalogues (Brinchmann et al. 2004, Kau↵mann et al. 2004, Tremonti et al. 2004).Targets were selected to be star-forming according to the definition of Brinchmann et al. 2004, whichis based on the BPT diagram. Redshift windows were choosen to avoid night sky absorption andemission contamination. Galaxies were chosen to span the full range in H↵ luminosities sampled bySDSS, but with a particular emphasis on extremely luminous galaxies. There are 124,061 galaxiesin the parent sample in our redshift windows, but only 2804, or about 0.78% have H↵ luminositiesabove 1041.8 (after applying an aperture correction of 0.659 dex). This represents a comovingvolume density of 4.1⇥ 10�5 Mpc�3. Objects become even more rare as luminosity increases, withonly about 200 out of 360,000 having log LH↵ > 42.5 at any redshift in SDSS, corresponding to aspace density around 3 ⇥ 10�7Mpc�3. (We have observed 14 of these that lie within our redshiftwindows). This space density is about three orders of magnitude below the corresponding value oftheir high-z counterparts: Dekel et al. (2009) give a space density of 2 ⇥ 10�4 Mpc�3 for galaxieswith SFRs of 100�200 M� yr�1 (which includes BX/BM and sBzK galaxies that are found to haveclumpy structures).

Since almost all local and high-z surveys aimed to study spatially resolved dynamical andstructural properties of star-forming galaxies have been focused on the rest-frame H↵ observations,this wavelength has become a gold standard for targeting star-forming clumps and comparing theirproperties across a wide range of redshifts. That is why we propose to use narrow-band H↵ imagingwith FR716n and FR782n ACS ramp filters to study the morphology of star-forming clumps atz ⇠ 0.1. In addition, the field of view of the ACS ramp filters is ideal for our purposes, since thegalaxies are typically 10–15 arcsec in size, as noted in the Science Justification section.

All galaxies in our sample of ten high velocity dispersion (�m > 50 km/s) galaxies with0.05 < z < 0.15 are objects with clear evidence for regular disk rotational motions (vshear/�m¿1, seeFigure 4). All these objects have H↵ luminosity H↵ > 10.41.9 erg/s. This luminosity cut was chosento match the typical luminosities of the populations reported in Forster Schreiber et al. (2009),Law et al. (2009), Epinat et al. (2009) and Lemoine-Busserolle & Lamareille (2010), as shownin Figure 2. The sample covers a factor of ⇠ 10 in stellar mass (1010.1–1010.9 M�), matching themassive turbulent disks in the z ⇠ 2 SINS sample. The dust extinctions (derived from H↵/H�) arealso comparable with A(H↵) ' 0.8 mag.

The remaining three objects in our control sample were chosen from the subset of lower lumi-nosity DYNAMO galaxies (H↵ < 10.41.9 erg/s) with low velocity dispersion (�m < 50 km/s) andshow clear evidence for rotation. Our purpose in imaging a control sample is to make sure we havea sample of well-understood objects imaged through the ramp filters, since these filters have notbeen used very often on z ⇠ 0.15 galaxies with ACS.

Required depth and integration times: Our central goal is to measure the sizes of HII regionsin the proposed sample. Therefore we estimated the required integration times by computing thetime needed to obtain a signal-to-noise of around fifteen per clump, assuming the simple case ofa ‘clump cluster’ like object made up of ten or so luminous blobs. Based on our experience withdetermining the sizes of morphological structures, a signal-to-noise of about fifteen is adequate forestimating sizes of compact high surface brightness components. The least luminous galaxy in oursample (one of the control sample objects) has L(H↵) ⇠ 1041 erg/s. Assuming this is divided into

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ten clumps, a typical clump has L(H↵) = 1040 erg/s. Using the luminosity distance to z = 0.15,and assuming (to be conservative) a clump size of 500 pc (corresponding to an angular size of 0.2arcsec at z = 0.15) the STScI ITC reports that a 30 min integration will give SNR=14 per clumpin a 0.2 arcsec aperture for a 2% ramp filter. We note that our calculation produces numbers inagreement with the SNR reported by Tremblay et al. (2009) for their program of emission lineimaging of low-z radio galaxies, which were undertaken with the same ramp filters proposed here.

The calculation above is rather conservative, and more luminous objects will have clumps withhigher signal-to-noise (for example, at the L(H↵) = 1041.9 erg/s luminosity used to define our sampleof candidate turbulent disks, subdivision of this flux over ten clumps results in a signal-to-noise ofaround 80 per clump in 30 min). But of course our expectation is that real clumps will span quite alarge dynamic range of luminosity. Our calculation shows that with a 30 min integration we oughtto be able to resolve clumps over a dynamic range of 10 or so for brighter galaxies, which will beimportant for analyzing their distribution on the size–SFR diagram (see Figure 1).

Short integration o↵-band images in FR647M will be used for continuum subtraction. SinceH↵ flux is expected to be very bright, continuum subtraction should be relatively straightforward.The ITC indicates we have adequate SNR in 900s. Thus the total requested integration time pergalaxy is 45 min (30 min in the narrow ramp filter, 15 min in the medium ramp filter, both withCR-SPLIT=2). This is well matched to a typical HST orbit, so we expect to image one galaxy/orbit.

Other Observations: All galaxies in our sample have SDSS 5-band images and have alreadybeen observed with either the AAT + SPIRAL IFS or the Stromlo 2.3m + WiFeS IFS providinghigh-resolution two-dimensional optical spectral mapping (R=10,000) in natural seeing. The datahave been fully reduced and further observations (such as ALMA and Herschel observations) areplanned. A subsample of our galaxies is a part of the approved OT2 (Priority 2) Herschel imagingprogram aimed to determine dust mass and temperature in 30 objects from the DYNAMO sample.

Special Requirements

None.

Coordinated Observations

None.

Justify Duplications

None.

Past HST Usage

9760 - “ACS Imaging of the Gemini Deep Deep Survey Fields: Galaxy Assembly at z ⇠ 1.5”. Datafrom this program has now appeared in six GDDS papers:1. Savaglio, S., et al. 2005, ApJ, 635, 3602. Abraham, R. G., et al. 2007, ApJ, 669, 1843. McCarty, P. J., et al. 2007, ApJL, 664, 194. Damjanov, I., et al. 2009, ApJ, 695, 1015. Mentuch, E., et al. 2009, ApJ,706, 1020

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6. Damjanov, I., et al. 2011, ApJL, 739, 44

10406 - “NICMOS Imaging of Massive Galaxies at z ⇠ 2”. Data from this program has now ap-peared in four GDDS papers:1. McCarty, P. J., et al. 2007, ApJL, 664, 192. Damjanov, I., et al. 2009, ApJ, 695, 1013. Mentuch, E., et al. 2009, ApJ,706, 10204. Damjanov, I., et al. 2011, ApJL, 739, 44

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