Table Of Content

The SKA view of the Neutral Interstellar Medium in Galaxies 5 W.J.G. de Blok∗1,2,3, F. Fraternali4,3, G.H. Heald1,3, E.A.K. Adams1, A. Bosma5, 1 0 Bärbel S. Koribalski6 and the HI Science Working Group 2 1ASTRON n Postbus2,7990AA,Dwingeloo,TheNetherlands a J 2DepartmentofAstronomy,Astrophysics,CosmologyandGravityCentre, 6 UniversityofCapeTown,PrivateBagX3,Rondebosch,7701SouthAfrica 3KapteynAstronomicalInstitute,UniversityofGroningen ] A Postbus800,9700AV,Groningen,TheNetherlands 4DepartmentofPhysicsandAstronomy,UniversityofBologna, G viaBertiPichat6/2,I-40127Bologna,Italy . h 5AixMarseilleUniversite,CNRS,LAM(Laboratoired’AstrophysiquedeMarseille) p UMR7326,F-13388Marseille,France - o 6AustraliaTelescopeNationalFacility,CSIROAstronomy&SpaceScience r t P.O.Box76,Epping,NSW1710,Australia s a E-mail: [email protected] [ 1 Twomajorquestionsingalaxyevolutionarehowstar-formationonsmallscalesleadstoglobal v 1 scaling laws and how galaxies acquire sufficient gas to sustain their star formation rates. HI 1 observations with high angular resolution and with sensitivity to very low column densities are 2 someoftheimportantobservationalingredientsthatarecurrentlystillmissing. Answerstothese 1 0 questions are necessary for a correct interpretation of observations of galaxy evolution in the . 1 high-redshiftuniverseandwillprovidecrucialinputforthesub-gridphysicsinhydrodynamical 0 simulationsofgalaxyevolutions. Inthischapterwediscusstheprogressthatwillbemadewith 5 1 theSKAusingtargetedobservationsofnearbyindividualdiskanddwarfgalaxies. : v i X r a AdvancingAstrophysicswiththeSquareKilometreArray June8-13,2014 GiardiniNaxos,Italy ∗Speaker. (cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/ NeutralISMinGalaxies W.J.G.deBlok 1. TheGalacticGasCycle Galaxyevolutionisdrivenbytheflowofgasintogalaxies,thetransformationofgasintostars, andtheexpulsionofgasduetothesubsequentstellarevolutionprocesses. Atomicneutralhydrogen (HI) is an excellent tracer — and often the main constituent — of this gas, and can be observed readily in the 21-cm line. The SKA will be able to directly trace the gradual transformation from primordialhydrogenintogalaxiesovercosmictime. However,tocorrectlyinterpretthisevolution, direct detailed observations of the sub-kpc-scale physical processes that cause this transformation areessentialforunderstandingtheastrophysicsofgalaxyevolution. Two very important processes in this cosmic evolution are star formation and gas accretion. Thefirstoneenrichestheinterstellarmedium(ISM)withmetals,injectsenergyintoit,and,through theejectionofgas,alsoenrichestheintergalacticmedium(IGM).Thesecondprocessdeliversgas from outside galaxies (either primordial or previously ejected) into the star-forming disks, which ensuresgalaxiescankeepformingstarsoveraHubbletime. Starformationthusplaysacentralroleingalaxyevolution, yetlittleisknownaboutthenec- essary conditions for star formation to occur. Global, kpc-scale relations, such as the Kennicutt- Schmidt Law (Kennicutt 1989, 1998), have established that there is a direct connection between the(molecular)gassurfacedensityandthestarformationrate(SFR)surfacedensity. Thisrelation hasbeenfoundtoholdforbothspiralgalaxies,aswellaslate-typedwarfgalaxies(see,e.g.,Leroy et al. 2008 and Bigiel et al. 2008 for a recent analysis). How the underlying astrophysics at the scalesofindividualgascloudsandcloudcomplexesleadstosucharelationonmoreglobalscales remains,however,ill-understood. The initial phase of this star-formation sequence will presumably be the cooling of gas to temperaturesbelow∼104 K,whereitcancollapseandformclouds. Observingthiscold,clumpy ISM and distinguishing it from the warmer, more diffuse phase requires imaging at high spatial and velocity resolution. To date, few HI observations of nearby galaxies resolve the individual cold gas complexes (see, e.g., Braun et al. 2009 and Kim et al. 2003 for examples in the Local Group). TheSKAwillhavesufficientsensitivityattherequiredhighresolutiontodothisformany galaxies. This enables studies of the properties of the cold gas in galaxies at the desired cloud- size resolution over a wide range of galaxy properties and environments. In addition, ALMA can observe the molecular components of these clouds and complexes, and this complementarity will lead to a revolutionary improvement in our understanding of star formation — and the conditions leadingtoit—indifferentenvironments. A related question is: how can galaxies sustain their star formation over a Hubble time? In general, local galaxies only have enough gas to sustain their SFR for few Gyr, and they must thus acquire gas from somewhere else (see Sancisi et al. 2008 for a recent overview). Numerical simulationssuggestgasflowsinfromtheIGM(or“cosmicweb”), throughaprocesscalled“cold accretion” (e.g., Kereš et al. 2005). “Cold” in this context means that the gas has not been shock- heatedas itenteredthe galaxyhalo. Thiscold accretionprocessis discussedinmore detailinthe chapterbyPoppingetal.(2014)elsewhereinthisvolume. Sofarthereislittledirectobservational evidenceforsignificantcoldaccretion. Theobservedcoldgasaccretioningalaxiesseemstobean orderofmagnitudetoolowtoexplainthecurrentstarformationrates(SFR)ingalaxies(Sancisiet al.2008;Putmanetal.2012). Ifcoldaccretionisthedominantprocessbywhichgalaxiesacquire 2 NeutralISMinGalaxies W.J.G.deBlok their gas, then current observational limits indicate it must happen at HI column densities below ∼1018 cm−2. Onlyalimitednumberofgalaxieshavesofarbeenexploredatsufficientsensitivity and spatial resolution to probe this regime (Heald et al. 2011; Putman et al. 2012). SKA will be able to directly detect and map in detail the gas at these column densities and trace its connection withthecosmicweb. Thegalacticfountainprocess(Shapiro&Field1976;Bregman1980;Norman&Ikeuchi1989) links the gas in the star forming disk and that in the halo. Massive stars, through supernova ex- plosionsandstellarwinds,canpushgasoutofthediskandintothehaloofagalaxy. Thiscreates theholesandbubblesfrequentlyobservedinthegasdisksofgalaxies(Bagetakosetal.2011). The expelled gas will cool and eventually rain back on the disk, most likely in the form of HI clouds (Putman et al. 2012). Such clouds have also been observed in a number of other galaxies as part ofanextra-planargascomponent(Sancisietal.2008),andpresumablyformtheequivalentofthe high and intermediate velocity clouds (HVCs and IVCs, respectively) in our Galaxy. It is thought thattheprocessofthesecloudsmovingthroughthehotgaseoushaloofagalaxyprovidesanalter- native mechanism for accretion of gas. Here, hot halo gas cools in a cloud’s wake and is dragged alongasthecloudmovesbackintothedisk(Fraternali2013). Whilethegalacticfountaincloudsthusprovideapossiblemeansforthetransportationofgas into the disk, they also hinder our ability to identify primordial gas clouds that are being accreted from the IGM. A small number can be identified as they are counter-rotating with respect to their host galaxy (Oosterloo et al. 2007), but a better census down to lower HI masses and column densities is needed to properly understand these accretion mechanisms. Prime targets for this are galaxiesinthe‘LocalVolume’,definedasthesphereofradius10MpccenteredontheLocalGroup (Karachentsevetal.2004,2013;Koribalski2008). Thisvolumeincludesaround900galaxies,the majorityofwhicharegas-rich. Distancestothesegalaxiesareknownwithhighaccuracy. The HI contents and sizes of Local Volume galaxies cover more than two orders of magnitudes, ranging fromlow-massdwarfgalaxieswithdiametersoflessthanonekpc(e.g.,LeoT;Ryan-Weberetal. 2008)togranddesignspiralswith HIdiametersof∼100kpc(e.g.,Circinus;Foretal.2012). In summary, the SKA will be a unique instrument that will be able to track the gas in low- redshift galaxies as it accretes onto the disk, forms HI cloud complexes (which in turn produce starsthroughanintermediate, molecularphase), isexpelledthroughfeedbackprocesses, andsub- sequentlyreturnstothedisk. ComplementedbyALMAmolecularlineobservations,thetransfor- mation from neutral atomic and molecular gas to the formation of stars can thus be investigated inexceptionaldetailoveralargerangeingalaxyproperties(see,e.g.,Ottetal.2008andFukuiet al. 2009 for work on the LMC and Stanimirovic´ et al. 2014 for our Galaxy). These processes can bestudiedindetailinavarietyofenvironmentsatlowredshift,thusestablishingtheastrophysical foundations that are needed to interpret observations of galaxies at higher redshifts, and to guide theimplementationof‘sub-grid’physicsintohydrodynamicalsimulationsofgalaxyformationand evolution. IntherestofthissectionwecalculatetheHIcolumndensityandmasssensitivitiesofSKA1- MID,SKA1-SURandSKA2foranumberofobservingscenarios. Thischapterspecificallydeals withhigh-resolutionandhigh-sensitivityobservationsofindividualobjects,i.e.,deepobservations oflimitedareasofsky. The remainder of this chapter then discusses the prospects for obtaining detailed, sensitive 3 NeutralISMinGalaxies W.J.G.deBlok observations of nearby, individual galaxies and the progress that will be made towards a better understandingofthelinkbetweenstarformation,accretion,andgasinthesegalaxies. 1.1 SKAcolumndensityandHImasssensitivities WecalculatecolumndensitysensitivitiesforSKA1-MID,SKA1-SURandSKA2forarange of angular and velocity resolutions and three representative integration times. For the latter we choose: • 10 hours: representing a typical “single-track” observation (as motivated in Sect. 2.3). In terms of science, an observation like this can be used to characterise high-resolution structures in the star forming disks of galaxies, as well as (at lower spatial resolutions) the outer low-column densitydiskdowntoN ∼1018 cm−2; HI •100hours: thislongerobservationwilltypicallyresultindetectionof HI columndensities downto∼1017 cm−2 whereaccretionandfaintoutergascanbedirectlystudied; •1000hours: thisextremelylongobservationallowsdetectionofcolumndensitieswellbelow 1017 cm−2 whereonecandirectlyexplorethelinkofgalaxieswiththecosmicweb. For the angular resolutions we choose representative beam sizes between 1(cid:48)(cid:48) and 30(cid:48)(cid:48). For the velocity resolution we choose values between 1 and 20 km s−1. Using these parameters we calculate the 5σ column density detection limits per velocity channel. Our basis for this are the SKA1-MID and SKA1-SUR natural sensitivities as listed in Table 1 of the SKA Baseline Design Document(Dewdneyetal.2013). ForSKA1-MIDweusethesensitivitylistedfor190SKAdishes combinedwiththe64MeerKATdishes. ForSKA1-SURthesensitivityof60SKAdishescombined with36ASKAPdishesisused. ForSKA1-MIDthe1σ sensitivityisgivenas63µJybeam−1 over 100 kHz for one hour of observing time. For SKA1-SUR this value is 263 µJy beam−1 over 100 kHzafteronehour. Wescalethesesensitivitiesusingtheappropriateintegrationtimeandvelocity resolution. For the sensitivity scaling as a function of angular resolution we use the noise scaling curveshowninthebottompanelofFigure1oftheSKA1ImagingSciencePerformancedocument (Braun2014). Thesescalingstakeintoaccounttheeffectofthespatialtaperingneededtoachieve thedesiredresolution. Inaddition,weapplyanoverallefficiencyfactorof0.9aslistedinAppendix AofBraun(2014). At1.4GHzSKA1-MIDwillhaveafieldofviewof0.5squaredegrees. SKA1- SURwillbeabletomap18squaredegreesduetoitswide-fieldphasedarrayfeeds. The 5σ column density limits for SKA1-MID, SKA1-SUR and SKA2 are given in Table 1. ThesevaluesagreewellwiththoselistedinPoppingetal.(2014)whichwerederivedusingsimula- tions. Notethatacolumndensitylimitisdefinedasthefluxdensitylimittimesthevelocitywidth, so for a fixed observation time and angular resolution, observing with a larger velocity channel width will lead to a higher column density limit. This is therefore distinctly different from the case where an already detected signal is smoothed to a larger channel width giving an improved sensitivity(butatthecostofacoarservelocityresolution). For SKA2, we assume that it has 10 times the natural sensitivity of SKA1-MID, and apply a 50% correction factor to take into account tapering and weighting. We assume this factor is independentofangularresolution. In addition to column densities, we also list in Table 2 the limiting HI masses that can be detected using the various setups. Here we assume the sources are unresolved and have a top-hat velocityprofilewithapeakfluxequalto5σ andavelocitywidthof50kms−1. Optimalsmoothing 4 NeutralISMinGalaxies W.J.G.deBlok (i.e.,achannelwidthequaltothevelocitywidth)isassumed. Masslimitsforothervelocitywidths canbededucedbysimplyscalingwiththewidthoftheprofile. The science described here focuses on HI observations of low-redshift galaxies (z<0.1) so only frequency band 2 (950-1760 MHz on SKA1-MID and 650-1670 MHz on SKA1-SUR) is needed. 2. Gas,StarFormationandDarkMatteratHighResolution 2.1 GasandStars The transformation of gas into stars is one of the most important processes in galaxy evolution. Understandingtheconditionsthatdeterminetheefficiencyofthisprocess,andtheassociated physics, is the goal of many observational and theoretical studies. They also form important in- put into numerical models of galaxy formation and evolution. This requires knowledge of these processesoveralargerangeinscales: fromgalaxy-sizedscaleswheregasistransportedfromthe diskofthegalaxyintothehaloandback, tokpc-sizedscaleswheregascloudsarecollapsing, via sub-kpc scales where neutral gas cools and turns molecular in Giant Molecular Clouds (GMC), to parsec scales where individual stars are formed. The latter, very small, scales can be directly observed in our Galaxy, while processes happening at galaxy scales can be studied in external galaxies. Tyingtogethertheprocesseshappeningatthesetwoextremescalesisamajorchallenge: inourGalaxywelacktheoverview,whileinexternalgalaxieswerarelyhavetherequiredresolu- tionandsensitivitytostudytheseprocessesindetail. Overthelastdecadethenumberofhigh-resolution,multi-wavelengthstudiesofgalaxiesinthe nearbyuniversehasincreaseddramatically. TheHIobservationswereobtainedaspartofdedicated surveyssuchasTHINGS(Walteretal.2008),LittleTHINGS(Hunteretal.2012),VLA-ANGST (Ott et al. 2012), FIGGS (Begum et al. 2008), WHISP (van der Hulst et al. 2001), HALOGAS (Heald et al. 2011), SHIELD (Cannon et al. 2011), and LVHIS (Koribalski 2008). These surveys havemadepossiblenewstudiesoftheconditionsforstarformationonkpcorevensub-kpcscales inalargernumberofgalaxies(seethechapterbyBlythetal.2014foramoreextensivedescription ofrecentHIsurveys). Thesehavemadeitpossibletomakethefirststepstowardsbridgingthegap between the observations at the scales of stars and those at scales of galaxies. With the SKA the nextstepcanbetaken. Kennicutt (1989) studied the gas and Hα content of a number of nearby galaxies and found a power-law relation between the total gas surface density and the Hα-derived SFR. Over the years, similar studies (see e.g., Calzetti et al. 2012; Calzetti 2013), using different measures for the gas surface density, and different star formation tracers (such as ultra-violet, Hα, infra-red or combinationsofthem)foundsimilarrelations,butwithalargespreadintheparameters. Thismay reflectactualvariationsinthephysics,butalargepartofthisspreadisverylikelyalsoduetochoice ofsample,analysisandstarformationtracers. Recentstudiesofthestar-formingdisksofnearbygalaxiesfoundatightlinearrelationbetween themoleculargassurfacedensityandtheSFRsurfacedensity(Leroyetal.2008;Bigieletal.2008). This can be interpreted as stars forming from the molecular ISM at a constant efficiency. Note though that these results do not say anything about the necessary conditions for star formation on 5 NeutralISMinGalaxies W.J.G.deBlok Table1: 5σ HIcolumndensitylimits(cm−2) beam velocityresolution size 1kms−1 5kms−1 10kms−1 20kms−1 SKA1-MID t=10h 1(cid:48)(cid:48) 1.75E+21 3.91E+21 5.53E+21 7.83E+21 3(cid:48)(cid:48) 1.63E+20 3.65E+20 5.16E+20 7.30E+20 10(cid:48)(cid:48) 1.13E+19 2.52E+19 3.57E+19 5.05E+19 30(cid:48)(cid:48) 1.31E+18 2.94E+18 4.16E+18 5.89E+18 t=100h 1(cid:48)(cid:48) 5.53E+20 1.23E+21 1.75E+21 2.47E+21 3(cid:48)(cid:48) 5.16E+19 1.15E+20 1.63E+20 2.30E+20 10(cid:48)(cid:48) 3.57E+18 7.99E+18 1.13E+19 1.59E+19 30(cid:48)(cid:48) 4.16E+17 9.32E+17 1.31E+18 1.86E+18 t=1000h 1(cid:48)(cid:48) 1.75E+20 3.91E+20 5.53E+20 7.83E+20 3(cid:48)(cid:48) 1.63E+19 3.65E+19 5.16E+19 7.30E+19 10(cid:48)(cid:48) 1.13E+18 2.52E+18 3.57E+18 5.05E+18 30(cid:48)(cid:48) 1.31E+17 2.94E+17 4.16E+17 5.89E+17 SKA1-SUR t=10h 1(cid:48)(cid:48) 4.71E+21 1.05E+22 1.49E+22 2.11E+22 3(cid:48)(cid:48) 4.45E+20 9.96E+20 1.40E+21 1.99E+21 10(cid:48)(cid:48) 4.48E+19 1.00E+20 1.41E+20 2.00E+20 30(cid:48)(cid:48) 8.38E+18 1.87E+19 2.65E+19 3.75E+19 t=100h 1(cid:48)(cid:48) 1.49E+21 3.33E+21 4.71E+21 6.67E+21 3(cid:48)(cid:48) 1.40E+20 3.15E+20 4.45E+20 6.30E+20 10(cid:48)(cid:48) 1.41E+19 3.16E+19 4.48E+19 6.33E+19 30(cid:48)(cid:48) 2.65E+18 5.93E+18 8.38E+18 1.18E+19 t=1000h 1(cid:48)(cid:48) 4.71E+20 1.05E+21 1.49E+21 2.11E+21 3(cid:48)(cid:48) 4.45E+19 9.96E+19 1.40E+20 1.99E+20 10(cid:48)(cid:48) 4.48E+18 1.00E+19 1.41E+19 2.00E+19 30(cid:48)(cid:48) 8.38E+17 1.87E+18 2.65E+18 3.75E+18 SKA2 t=10h 1(cid:48)(cid:48) 1.13E+20 2.52E+20 3.57E+20 5.05E+20 3(cid:48)(cid:48) 1.25E+19 2.80E+19 3.97E+19 5.61E+19 10(cid:48)(cid:48) 1.13E+18 2.52E+18 3.57E+18 5.05E+18 30(cid:48)(cid:48) 1.25E+17 2.80E+17 3.97E+17 5.61E+17 t=100h 1(cid:48)(cid:48) 3.57E+19 7.99E+19 1.13E+20 1.59E+20 3(cid:48)(cid:48) 3.97E+18 8.87E+18 1.25E+19 1.77E+19 10(cid:48)(cid:48) 3.57E+17 7.99E+17 1.13E+18 1.59E+18 30(cid:48)(cid:48) 3.97E+16 8.87E+16 1.25E+17 1.77E+17 t=1000h 1(cid:48)(cid:48) 1.13E+19 2.52E+19 3.57E+19 5.05E+19 3(cid:48)(cid:48) 1.25E+18 2.80E+18 3.97E+18 5.61E+18 10(cid:48)(cid:48) 1.13E+17 2.52E+17 3.57E+17 5.05E+17 30(cid:48)(cid:48) 1.25E+16 2.80E+16 3.97E+16 5.61E+16 6 NeutralISMinGalaxies W.J.G.deBlok Table2: HImasslimitsforunresolvedsources beam M /D2 forSKA1-MID M /D2 forSKA1-SUR HI Mpc HI Mpc t=10h t=100h t=1000h t=10h t=100h t=1000h 1(cid:48)(cid:48) 2.6E+03 8.3E+02 2.6E+02 7.1E+03 2.2E+03 7.1E+02 3(cid:48)(cid:48) 2.1E+03 7.0E+02 2.2E+02 6.0E+03 1.9E+03 6.0E+02 10(cid:48)(cid:48) 1.7E+03 5.3E+02 1.7E+02 6.7E+03 2.1E+03 6.7E+02 30(cid:48)(cid:48) 1.8E+03 5.6E+02 1.8E+02 1.1E+04 3.6E+03 1.1E+03 HImasseswerecalculatedassuminga5σ fluxdensitylimitand avelocitywidthof50kms−1. Optimalsmoothingisassumed. thescalesofGMCsorsmaller. Withtheanalysisperformedataresolutionof750pc,anyrelation mustbeinterpretedwithinthecontextof“countingclouds”,i.e.,onehastoassumethattheGMCs haveuniformpropertieswiththeobservedmolecularsurfacedensitydeterminedbythebeamfilling factoroftheseclouds. Higherresolutionsareneededtogobeyondthislimitation. Schruba et al. (2011) found that the linear relation between molecular gas and SFR surface densitiesextendsintotheHI-dominatedregimeoftheouterdisksofgalaxies(seealsoRoychowd- huryetal.2014). Thisissupportedbytherealisationthatstarformationintheouterpartsofdisks is more widespread than originally thought. GALEX UV observations enabled direct detection of O and B stars which would otherwise have escaped detection due to their inability to excite the surrounding ISM enough to produce Hα emission. It is now thought that these extended UV (XUV)disksarefoundin∼30%ofnearbydiskgalaxies(Thilkeretal.2007). Astrikingexample is M83 (NGC 5236) as shown in Fig. 1. The outlying HI structures show up remarkably well in the UV, indicating that star formation is progressing there as well, albeit with a lower efficiency (Bigieletal.2011)thaninthebright,innerregionofthegalaxy. Anextremeexampleofthisisthe dwarf galaxy ESO215-G?009 (Warren et al. 2004). This galaxy has the highest gas-to-light ratio knownforagalaxyinthelow-redshiftuniverse,yet,despitethelargegasreservoir,starformation apparentlyhashalted,isinhibitedorlacksatrigger. The above broad-brush picture illustrates the progress made in the last few years, and men- tionssomeoftheempiricalrelationsthatarenowroutinelyusedasinputfornumericalmodelsthat attempt to explain the variations of star formation efficiency with, e.g., redshift, environment or galaxymass. Itishoweverstilldifficulttolinktheobservationswiththeactualphysicalprocesses drivingthestarformationrate. Forexample,Leroyetal.(2008)testanumberoftheoreticalexpla- nationsproposedintheliteraturelinkingthegasdensityandtheSFR.Theylookedatthediskfree falltime,theorbitaltimescale,theeffectsofcloud-cloudcollisions,theassumptionoffixedGMC starformationefficiency,andtherelationbetweenpressureintheISMandthephasesoftheISM. Their conclusions were that none of these offers a unique explanation for the observed behavior. So, thoughlarge-scalerelationscan beidentified(suchastheKennicutt-SchmidtLaw), theactual physics is a lot more complicated, and happens below the resolutions achieved so far. Observa- tionally,wewillthereforehavetoprobetheISMatscalesbelowthatofGMCsandindividual HI complexes. WealsoneedtohaveabetterunderstandingofthebalancebetweenwarmandcoldHI phases, the efficiency of H formation and the effects of shocks and turbulence. Examples of the 2 importance of high resolution and high sensitivity for the study of turbulence in HI are found in, e.g.,Duttaetal.(2009,2013). 7 NeutralISMinGalaxies W.J.G.deBlok Figure1: Compositemulti-wavelengthimageofM83. Near-andfar-UVfromGALEXareshowninblue. Optical R-band from SDSS in green, J-band from 2MASS in red, HI data from LVHIS is shown in pale- blue, andacombinationofB, RandHα havebeenusedastheluminosityofthestellardisk. Thefieldof viewis1.3◦×1.5◦. (PicturecourtesyB.Koribalski) Young & Lo (1996, 1997), Young et al. (2003), de Blok & Walter (2006), Ianjamasimanana et al. (2012), Warren et al. (2012) and Stilp et al. (2013) all analyzed the HI emission velocity profilesofgalaxiesandfoundevidenceforthepresenceofcoldandwarm HI componentsbyde- composing the velocity profiles into components with a high and a low velocity dispersion. They found that the cold (low velocity dispersion) component is usually located near star-forming re- gions, whereasthewarm(highvelocitydispersion)componenttendstobefoundalongeveryline of sight. This presumably tells us something about the conditions for the warm HI to cool and turn molecular, but the key limitation of these studies was again spatial resolution. The improved capabilities of the SKA will enable these studies to be repeated but now resolving the individual gascomplexes. (EquivalentabsorptionlinestudiesaredescribedinthechaptersbyMorgantietal. 2014andMcClure-Griffithsetal.2014.) Thisshouldthusprovideobservationsofalargenumber of galaxies at sufficient resolution to gauge the ability of and the conditions for the ISM to form GMCsoverawiderangeofgalaxyconditionsandenvironments. Anangularresolutionof1(cid:48)(cid:48)allows100pcphysicalresolutionsoutto20Mpc(i.e.,thedistance of the Virgo cluster). The high-resolution observations of M31 by Braun et al. (2009) (shown in Fig. 2) have a maximum resolution of 50 pc, while the LMC observations by Kim et al. (2003) probescalesof15pc. AssumingSKAcanobserveefficientlyuptoδ ∼+35◦,thengivesaccessto ∼800galaxieswithin20Mpcthathaveindependentdistancemeasurements. Ofthese∼80%have HI detections. With the SKA these can therefore all be studied at a resolution close to that of the Braunetal.(2009)M31observations. Outto3Mpc,wecanstudygalaxiesatthesameresolution 8 2 SKA2: The future NeutralISMinGalaxies W.J.G.deBlok No.2,2009 AWIDE-FIELDHIGH-RESOLUTIONHiMOSAICOFMESSIER31. I. 941 90 K 120 K With 1” beam SKA2   reaches 2.5 x 1020 cm-2 in 10 hours.   50 pc resolution out   B) B) T_ T_ to 11 Mpc sqrt( sqrt( ! Can do THINGS out to  100 Mpc  Transformational… 0 K 0 K Braun et al 2009 Figure3.PeakbrightnessofHiemissiondeterminedat60aFrcisgecuarned26:kmsI−n1tegrateFkdmigHusrIe15mr.esPaopelaukotibofrniMgohf3tnt1heesfscroeonfmtHrailhe5img0%his-sroioefnstohdeleutseturirmovnienyeodrbeagstieoarnbv.oaPutetiao1k5nsbarricbgsyhetcnBaensrsadui6sn et al. (2009). The resolutionofthecentral50%ofthesurveyregion.Peakbrightnessisshownon − asquare-rootscalewhichsaturatesat90K.ThebeamFWHreMsoilsuitnidoincaotefdtihnese obssheorwvnatoionnassqisua∼re-5ro0otpscc.aleTwhehicbheasamturiasteisnadtic12a0teKd.bTyhethbeeaamrrFoWwHiMn tihse bottom-left corner. indicatedinthelowerleftcorner. thelowerleftcorner. SKA2willenablehighlydetailedobservationsofthiskindforhundredsofgalaxies. (Acolorversionofthisfigureisavailableintheonlinejournal.) (Acolorversionofthisfigureisavailableintheonlinejournal.) of the Ki1m20 eKt al. (2w00a3rm),LsMemCi-oobpaseqruveatfioornegs.roTunhderceomarpeo∼nen7t5togamlaaxkieesawwiathrmin–dependent distance measurements withicnoothl–awt darismta“nscaendinwtihche”pwarotuoldf tshigensikfiycaanctlcyesfislilbilnetthoetdheeteScKteAd. This includes, for temperaturedecrement.Clearly,theinterpretationoftheHISA example,theSculptorgroup. temperaturedecrementsinphysicaltermsispoorlyconstrained. There are thereforTehaesliagrgneifistcainndtinvuidmubalerfeoaftugraelsaxsieeesnoiunt ttohethGesibesdoinstaentcaels., and this opens up the exciting prospec(t20o0f5b)eGinaglaacbtilceHtoIScAhasraamctpelreizeextthenedporovpereratibeosutan5d◦ mato2rpkhpoclogies of individual distance,andsohavelengthsaslargeas175pc.Asubsetofthe HI cloudsinothergalaxiesoverawiderangeofenvironments. Combinationwithhigh-resolution GalacticHISAfeatures,suchasthosekinematicallyassociated ALMA observationswsithhotuhledPperrosveiudsesapicraolmaprmre,hiesnosrigvaenpizicedturoeveorfttehnescoofnddeigtiroeensfor and first phases ofstarfor_B)mation. making thecomplexes at least1kpclong. Thedeparture from T 2.2 Dynasqrt(micsandt(DhBeararGukanlMa1c9at9itc1t)eermdgeea-nosnthgaetotmheetnryecteosstharey∼al7ig8n◦minecnlitncaotniodnitioofnMsf3o1r witnessing HISA are essentially eliminated on large scales in M31. Instead, the “sandwich” geometry that can yield large- The high angular resolution that can be achieved with SKA will also be of importance to scale HISA features in the Galaxy would be projected into studiesoftheinternaldynamicsofgalaxiesandthedistributionofdarkmatter. spatially resolved, parallel “slices” of semi-opaque gas of HIstudiesplaydeidffearmenatjosprirnolteeminptehraetu1r9e7.0Wsaenidde1n9t8if0ystthoeefistlaambleisnhtatrhyelporceaslenceofdarkmatter ingasrichgalaxies,mneinciemssaairnypteoakkeberpiguhptntehsesesxeetennidneMd,3fl1awt(itohrtrhiesicnogl)dreortoaptiaoqnuecurvesatlargeradii. featuresthatareresponsibleforlarge-scaleHISAintheGalaxy. More recent work on late-type low surface brightness galaxies showed that pure ΛCDM models We use the term “self-opaque” to emphasis the importance of areindisagreementiwntiethrnoalbospertivcaatlidoenpst,htheeffescot-scianlldeedtecromrien-icnugstphecopnrotrfiolevesrhsaype(sdeofBlok2010). Recent 0 K attempts to solve thtihsepserofbealetumrecs,oanscdenisttriantcetfornomin“trsoeldfu-acbinsogrpfteieodnb”awckhicinhitmheplsietasr formation recipes asubstantialtemperaturecontrastoffeaturesthatoverlapboth used in the numerical simulations of galaxy formation (Oh et al. 2011), so that an initially cuspy Figure4.PeakbrightnessofHiemissiondeterminedat30arcsecand6kms−1 alongthelineofsightandinradialvelocity. resolutionofthecentral50%ofthesurveyregion.PeakbrigdhtanrekssmissahtotewrndonistributioBnyccaonmbpeamrisoodnifitoetdhesukfpficceixetnetnlyto.fRGeasleaacrtcichHoInSAthcisomprpolbexleems, canbeexpectedto asquare-rootscalewhichsaturatesat120K.ThebeamFWcHoMntisininudeicatotedpirnogresst,huensteillf-aobpaeqttueerfiulnadmeernsttaarnydminigniomfathineMse3f1ee(fdobraecxkammpelcehthaeniosnmeshasbeenreached. thelowerleftcorner. runningfrom(α,δ) (00:45:25,+41:36)to(00:45:55,+41:50) (AcolorversionofthisfigureisavailableintheonlinejournAalt.)present,thepredictionsfromtheses∼imulationsseemtobegintoworkforsmallgalaxies,butfor in Figure 6) are often in excess of 10 arcmin in length correspondingtomorethan2kpc.Onesuchlinearfeatureisseen comparable to its own spin temperature together with a single tocrossverynearthelineofsighttothebackgroundcontinuum 9 cooler, opaque feature in the foreground. In this simple case source J004218+412926 ( B0039+412), as seen in Figure 7. = theHISAtemperaturedecrementwouldgivesomeinformation Hiabsorptionmeasurementsforthissourceandseveralothers aboutthespintemperatureoftheforegroundcomponent.More have been published previously in Braun & Walterbos (1992). complicated geometries are easily conceivable. Adding a third Wewillcommentfurtheronthesesourcesbelow.Morecomplex NeutralISMinGalaxies W.J.G.deBlok largeronestheanswerisstillopen. Thisisanactiveareaofresearch,andfurtherimprovementsof thegalaxyformationmodelscanbeexpected(e.g.,diCintioetal.2014). In terms of further HI observations addressing this problem, surveys planned with the SKA precursors will enable the selection of an adequate sample of relatively unperturbed galaxies, for whichfardeeperHIobservationscanbedonewiththeSKA,aswellasmoleculargasobservations with ALMA. These data will yield detailed information on the circular and (equally important) non-circular motions in these galaxies. Combined with more extensive diagnostics of the stellar kinematics using optical integral-field units, this will yield crucial information about the galaxy kinematics, and hence can be used to study the dark matter problem. Moreover, they can also be usedforthepurposesofstudyingthestarformationandgasaccretionindiskgalaxies,asdiscussed inthischapter. 2.3 SKAProspects Here we use the column density limits listed in Table 1 and explore how the increased resolution and sensitivity of SKA makes new science possible. We use the results from the THINGS survey (Walter et al. 2008) as a benchmark. THINGS observed 34 nearby disk galaxies with the combined VLA B, C and D array with a maximum angular resolution of 6(cid:48)(cid:48), or ∼500 pc on av- erage. For each galaxy, observing time was 7h with the VLA B-array, 2.5h with the C-array and 1.5h with the D-array. A total of ∼10h is therefore needed in this setup to produce a complete observation—thismotivatesourchoicefor10hasatypicalobservingtimeinSect.1.1. Withthese parameters,thecolumndensitysensitivityoftheTHINGSobservationsis2.7·1020 cm−2 fora5σ detection over a 5 km s−1 channel at 6(cid:48)(cid:48) resolution (Walter et al. 2008). This angular resolution is effectivelyalsothehighestthatcanbeachievedwiththeVLAB-arrayat1.4GHz,andiscurrently thehighestresolutionatwhich HIcanstillbeobservedroutinely. To compare prospective SKA observations directly with THINGS, we focus here on limits derived with a channel width of 5 km s−1. A 10h integration time at 6(cid:48)(cid:48) with SKA1-MID will achieve a column density limit of 8.1·1019 cm−2. This is close to the 4·1019 cm−2 THINGS sensitivity at 30(cid:48)(cid:48). A 10h observation on SKA1-MID at 6(cid:48)(cid:48) thus enables studies at the resolution of THINGS, but to column density limits that are a factor of 3 deeper, thus probing the HI well outside the star forming disks. The larger sensitivity of SKA1-MID allows tapering to spatial resolutions higher than used by THINGS. For 10h and at 3(cid:48)(cid:48), the sensitivity at is 3.7·1020 cm−2. ThisiscomparabletotheTHINGSsensitivityat6(cid:48)(cid:48) andthereforeallowsstudyofthestar-forming diskattwicethespatialresolution. Forgalaxiesoutto10Mpcthelinearresolutioncorresponding to3(cid:48)(cid:48) isbetterthan∼145pc,whichisstartingtoresolveindividual HIcomplexes. With the current baseline design, 3(cid:48)(cid:48) is an upper limit in terms of resolution on SKA1-MID. Thereisnotenoughsensitivityonthelongerbaselinestopushtheresolutionhigher. Forexample, performingthesame10hobservationdescribedaboveat1(cid:48)(cid:48)givesacolumndensitylimitof3.9·1021 cm−2, which is higher than the maximum face-on column densities typically found in galaxies at scales of hundreds of parsecs (∼1021 cm−2; Bigiel et al. 2008). It is possible that optical depth effects mask higher column densities at much smaller scales (e.g., Braun et al. 2009), but this has been tested only in a limited number of galaxies, and is the kind of project one would tackle with theSKAforalargersample. 10