PoS(PRA2009)050 http://pos.sissa.it/ † ∗ [email protected] Speaker. A footnote may follow. Gas accretion is a vitalcent years, support the for study of evolution gaseouscomplexes, and haloes analogous surrounding the to disk feeding the of has galacticaccretion. shown High-Velocity the Clouds, formation. However, presence that the of In can gas accretion re- bewhich rates direct estimated are evidence from one of these order gas features ofproblem consistently can magnitude be give lower overcome values, than if most what ofthe is the effects accretion needed it is has to “hidden” on feed and the visiblethe the kinematics only disk of star indirectly through the through formation. galactic halo fountains gas. sweeps This Invides up an this ambient explanation second gas for scheme, causing the the it missing gas to gasthe expelled accrete. accretion halo from and gas, This also in model reproduces particular pro- the the peculiar vertical kinematics rotation of gradient. ∗ † Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

Panoramic Radio Astronomy: Wide-field 1-2 GHzJune research on 2-5 galaxy 2009 evolution Groningen, the Netherlands Filippo Fraternali Gas accretion onto galaxies: modelsobservations vs University of Bologna E-mail: PoS(PRA2009)050 Filippo Fraternali 4 and now the stellar = z , thus a roughly similar rate of 1 − yr

M 2 A further piece of evidence for gas accretion comes from observations of the so-called Damped Is the postulated accretion detected in the local Universe? A number of local galaxies show Galaxies form and evolve through the acquisition of gas from the surrounding intergalactic The history of the Milky Way and star-forming galaxies in general differ sub- signs of interaction withcan gas-rich be dwarf revealed companions inthese or neutral systems have gas can peculiar (HI) be considered kinematical surveyscompanions as features such with minor HI that as mergers tails at tails and different bridges orvisible stages. companions indicating large but Some that peculiar infalling an features have in obvious interaction gas their optical isof HI complexes. taking structure interactions and place, (Sancisi kinematics, All others which et have are al. reminiscent no than 2008). 10% In of these the minor mergers main the galaxy. companions have HI The masses Milky less Way and the Magellanic Clouds are in this class of Lyman Alpha (DLA) systems,population long recognized of to (gas-rich) be spiralsurveys the galaxies. such analogues as HIPASS at (Barnes A et high al. comparisonnot 2001) changed between indicate of significantly that DLA the the throughout amount studies the local ofby Hubble and neutral a time gas local factor (Zwaan in 2 galaxies et galaxy between has al. redshiftWay 2005), 4 has at and most now. not This it changed is has consistent substantiallygalaxies declined with over is the the fact remarkably that same different the timeframe. to SFR in that the The of Milky behaviour the of stellar the mass gas as mass between in Gas accretion onto galaxies: models vs observations 1. Introduction space however the mechanisms guidinggas this accretion process onto are galaxies still is Star largelyleast in Formation. unknown. the In solar A the neighbourhood, major has Milky been effect WayBinney remarkably the of et constant Star-Formation al. over rate the 2000). (SFR), last 10 at Given Gyrsin that (Twarog the 1980; a current couple SFR of would gigayears, exhaustthe the Milky it gas Way seems must in that be the replaced solar theto neighbourhood by gas explain accretion. the consumed Steady discrepancy by accretion between star ofbourhood the metal-poor formation observed and gas in stellar the is metallicity a also prediction distribution disk required of in2003). galaxy the closed-box like solar chemical neigh- evolution models (Tinsley 1981; Matteucci stantially from the history ofUV the or global IR star-formation luminosities density, derived (e.g.factor using 10 Hopkins SFR from & tracers redshift about such Beacom 1 as 2006).ical to surveys now, The and are it universal however appears dominated SFR fairly by showsi.e. constant large a beyond galaxies galaxies redshift decline that such 1. of as belong Cosmolog- ellipticals a to andepoch the SOs than so-called at the red-sequence high less and , massive thatbins blue-sequence stopped of galaxies. forming galaxies Indeed with the at different masses breakdown anclining shows of SFR earlier that and the Milky-Way spirals SFR type of density galaxies latertype in have type galaxies a even we very a can slowly slowly assume de- rising that SFRgas the (Panter accretion current et is SFR al. required is 2008). and about For 2 should Milky-Way be detected in the local Universe. mass has built up by a factor more than 10 in a galaxy2. like the Gas Milky Way. accretion from minor mergers PoS(PRA2009)050 − × 1 2 . . Filippo Fraternali has been found falling in with

M 8 10 × 3 , the mean accretion rate from these interactions will be around 0

M 9 (bottom right). − 8 1 ), where a large HI complex of about 2 1 Top left: total HI map for M 101 (contours) overlaid on a DSS image. Top right: high-velocity gas ) probably destined to be accreted. At advanced stages of the interaction-accretion process

A first estimate of the accretion events in local galaxies has been made using the sample of M 8 galaxies provided by WHISPabout (van 25% of der 300 Hulst spirals and etgalaxies irregulars are al. show evidence undergoing of now 2001). minor or interactions.and have we If From undergone 25% assume in of an lifetimes these the of inspection field HI the recent masses of observed past of features this some order of 10 kind sample about of 1 tidal Gyr, given interaction that the gas features have Figure 1: complex (contours) overlaid on thevelocity optical diagram at image. constant declination Bottom (see left:complex. horizontal line From global in Sancisi HI top et profile. right al. panel) 2008. Bottom showing the right: high-velocity HI Position- velocities of up to 150 km/ssuggestive with of respect a to collision the galaxy with disk ahole (van dwarf observed der companion in Hulst that Fig. & may Sancisi have 1988). gone This through is the disk leaving the Gas accretion onto galaxies: models vs observations phenomena and the Magellanic10 Stream (Bruns et al.the accreting 2005) dwarf is may the beM no gas 101 longer component (Fig. visible (about or 1 easy to identify unambiguously. An example is PoS(PRA2009)050

M 9 10 × 2 , column 9) . 1 Filippo Fraternali contours) for the edge-on + , the position-velocity plot along 2 halo with a mass of 1 I shows the two ways of detecting extra-planar 2 4 disk rotation curve). = , which also includes the Milky Way). For edge-on galaxies the halo gas 1 . Thus the measured accretion rate is about an order of magnitude lower than the SFR. 1 Two methods of detecting extra-planar gas. Left: total H I map (blue − yr

The detection of the extra-planar gas in nearby galaxies is a difficult task given the low surface The main kinematic feature of extra-planar gas is its decreasing rotational velocity with in- M 2 . (Oosterloo, Fraternali & Sancisi 2007). On the right panel of Fig. HI. The total HI map of NGC 891 (left) shows an extended H is spatially separated from the diskbe gas, separated whilst thanks for to its galaxies seen distinct kinematics. at intermediate Fig. inclinations it can Figure 2: galaxy NGC 891 overlaid onobtained the with optical the image Westerbork Synthesis (orange)sky, Radio (data it Telescope). from surrounds The Oosterloo, the extra-planar Fraternali whole gasdiagram & is galactic along clearly Sancisi the disk major separated 2007, with axis on a of the The filament the extra-planar intermediate extending gas inclination up galaxy is to NGC kinematically 2403more 20 (from separated slowly kpc. Fraternali than from et the the Right: al. disk disk 2001). (white position-velocity gas; dots it is seen as a faint component rotating brightnesses involved. In the last decade15 or galaxies so, deep (see HI Table observations have been collected for about the major axis of NGC 2403of shows the a disk broad (called component the of “beard”). gas at velocities lower thancreasing the height rotation above the plane, it2003). is said Such to a be velocity “lagging”and gradient yet behind has it the been is disk an gas estimated important (Matthewspresence for constraint & of only for Wood this a models gradient of few is extra-planarwhich galaxies also gas are (Table the not formation main edge-on. (see reason Extra-planar Section gas why 5).phase. is extra-planar possibly The gas ubiquitous Optical can and studies it be of is detected also nearby in observed edge-on in galaxies the galaxies ionised show that roughly half of them have extended 3. Halo gas in spiral galaxies Gas accretion onto galaxies: models vs observations 0 PoS(PRA2009)050 (8) (9) (10) (11) (15) (16) (12) (13) (14) (21) (20) (19) Ref. (4,5) (6,7) (1,2,3) (17,18) (Heald I a e e e e e c i 25 31 12 10 13 8 15 - - 22 − − − − − Filippo Fraternali ∼ − ∼ − ∼ − > ∼ not measured directly: e b from the models of Greisen d 2 h l . References: (1) Kalberla & . /yr) (km/s/kpc) --- - -

0

from the counter-rotating cloud M M h from complex C and other clouds ≈ 5 b > 4 - - m . from the sum of the various extra-planar 2 3 g g g f /yr) ( 01 10 - - 6 6 7 . . . .

− − 0 SFR Acc. rate Gradient M > 1 . ∼ extrapolated from the inner 100 pc (Levine et al. c )(

tot M HI 9 1.9 0.18 - 8.7 5.1 - - M 5 SFR of only massive stars ) (10 m l 1 1.1 1 1 0.5 0.05 3 3 0.35 - -

f . 4 . . halo 1 9.4 1.2 0.2 4 4 1 M 2.9 6.7 2.2 - - 0 0 0 8 HI − ∼ ∼ > ∼ > ∼ > > M estimated using optical lines (Heald et al. 2006a); i flat 185 0.8300 2.5 4.4 ∼ ∼ yr; 8 Physical properties of extra-planar gas in spiral galaxies 10 ) (km/s) (10 75 80 ◦ - 220 × ( 55 110 9063 230 130 12 3 4.1 3.2 3.8 1.3 0.2 0.1 77 226 24 200 0.8 6.1 1 8686 310 200 - 9.1 7 67 120 5.9 6.7 0 3284 226 150 1.4 4 8.0 3 5 2 1.2 18 88 110 7538 113 175 2 ∼ ∼ Table 1: mass in Grossi et al. without their correction for the ionised fraction; I Sc Sc Sc Sc Sb Sb Sb Sb Sb Sb Sd Scd Scd Scd Scd Scd calculated from the FIR luminosity using the formula in Kewley et al. 2002; from the H g d The strategy to estimate gas accretion is to look for gas components (usually at very anomalous Gradient in rotation velocity with height (from the flat part of the rotation curve); M 83 NGC 253 NGC 891 NGC 2403 NGC 2613 NGC 5746 NGC 5775 NGC 2997 NGC 3044 NGC 4559 Galaxy Type incl v UGC 7321 M 33 NGC 6503 NGC 6946 M 31 Milky Way a Gas accretion onto galaxies: models vs observations with known distances without correction for the ionised fraction; derived from the average lag divided by an estimate of the halo thickness; 2008); clouds; Dedes (2008); (2) Wakker et al. (2007); (3)Reakes Levine & et Newton al. (1978); (2008); (4) (7) Thilker Grossi et et(10) al. al. Oosterloo (2004); et (2008); (5) Walterbos al. (8) et (2007); Miller, al. (11) Bregman (1994); Fraternali & (6) al. et Wakker (1997); (2009); al. (15) (9) Barbieri (2002); Boomsma et (12) al. et Chavez (2005); et al.(19) (16) al. Greisen (2005); Rand (2001); et & (13) al. Benjamin Hess (2009); (2008); et (20) (17) Boomsma al. Irwin et (2009); et al. (14) al. (2008); Lee (1994); (21) et (18) Matthews Heald & et Woodlayers (2003). al. of (2006a). diffuse ionised gas (Rossaet & Dettmar al. 2003) 2006b). and with similar kinematics to the H 4. Accretion from halo gas velocities) which are incompatible with an internal originThe (galactic large fountain, majority Shapiro of & Field the 1976). extra-planar gas studied so far has actually a very regular kinematics that using an infall time-scale of 1 et al. (2009), average between central and outer parts; PoS(PRA2009)050 filaments. I erg, i.e. about . They are both . In NGC 2403, Filippo Fraternali 55 2 1 − 10 mass located in pro- yr (column 8), they are × I

1 1 M ∼ 8 . ) extending up to about 20 kpc from

M 7 10 × , and they should be corrected for helium and I 6 ) and for NGC 2403: 0 . 6 3 1 accretion model is that it predicts that most of the ∼ + HI M and generally 1 order of magnitude lower than the SFRs. These 2kpc from the plane. Clearly the fountain clouds in the model . yr. The resulting rates are shown in Table 1 5 8 − ). yr = 10 1 ). NGC 2403 also has a filament with a similar H z

− 2 M 7 1 . 10 × (see the blue curves in Fig. highlights this problem for NGC 891. The points are rotation velocities derived 1 3 9kpc and − . 3 ). This points to a tight connection between disk and halo components. yr 2

= z M One implication of the above fountain The first features that deserve attention in the search for gas accretion are H How can the fountain clouds loose part of their angular momentum? Fraternali & Binney The result that the rate of gas accretion onto galaxies which is directly observed is much lower From the above structures we can estimate the rate of gas accretion by assuming typical in- supernovae. A second type of feature that has been found in these new deep surveys are clouds 5 extra-planar gas is producedextragalactic. by the This is galactic in fountainanomalous agreement and velocity with clouds only the and a metallicity star small of forming the fraction regions IVCs (about (e.g. and 10%) Boomsma the is et clear al. links 2008). between A second im- the plane of the disk (Fig. this model is also able to2008). reproduce the observed radial inflow of the halo gas (Fraternali & Binney rotate too fast (have a larger angular momentum) than the extra-planar gas(2008) in consider the the data. possibility thatthe fountain halo. clouds In sweep this up schemetheir ambient path ambient gas through gas the as condenses halo they onto andlow travel the eventually through angular fall fountain down clouds, momentum into these the about latter disk.momentum the grow of If along z-axis the the ambient fountain then gas gas. has thisis The relatively process tuned only free to produces parameter reproduce a ofaccretion the reduction the rate turns rotation model in out is curves to the be the of very accretion angular similarof the to rate, about the extra-planar which 3 SFR. For gas. NGC 891 we Remarkably, found a the best-fit accretion required rate gas at heights than expected implies that mostpossible indirect of evidence the of accretion this missing shouldof gas the be extra-planar accretion, gas. somewhat provided The by “hidden”. steepness the of(e.g. rotation this Fraternali I gradient velocity & describe is Binney gradient not 2006; here reproduced Heald by ethalf galactic al. fountain or 2006b) models as less). they Fig. tend to predict shallower values (a factor 5. Gas accretion induced by galactic fountain Gas accretion onto galaxies: models vs observations follows closely the kinematics ofpanel the of disk Fig. (see for instance the p-v diagram forNGC 891 NGC 2403, has right a long massive filament ( jection outside the bright optical disk,very partially similar visible to in Complex the C p-vthat diagram in they in our come Fig. from Galaxy. the10 The disk energy through needed a to galactic fountain format is very these anomalous of filaments velocities the assuming that order endproduced up in in any kind the of region galactic of fountain counter-rotation. and These they clouds are cannot most likely be falling direct times evidence of of gas few accretion. typically of the order 0 directly observed accretion ratespossibly include ionised only gas fractions. H However, itformation appears (column difficult 7, to Table reconcile them with the rates of star PoS(PRA2009)050 Filippo Fraternali , very similar to the star formation rate of 1 − yr

M 3 7 ∼ Indirect evidence for gas accretion. Rotation velocities (black points) of the H I extra-planar gas in [1] Barbieri C.V., Fraternali F., Oosterloo T. et[2] al. 2005, Barnes A&A, D.G., 439, et 947 al. 2001, MNRAS,[3] 322,486 Binney, J., Dehnen, W., Bertelli, G., 2000,[4] MNRAS 318, Boomsma 658 R., Oosterloo T.A., Fraternali F. et[5] al. 2005, Boomsma A&A, R., 431, Oosterloo 65 T.A., Fraternali F. et[6] al. 2008, Bruns A&A, C., 490, et 555 al. 2005, A&A,[7] 432, 45 Chaves T.A. & Irwin J.A. 2001,[8] ApJ, 557, 646 Fraternali F., Oosterloo T., Sancisi R., van[9] Moorsel G. Fraternali 2001, F., ApJ, van Moorsel 562, G., L47 Sancisi R., Oosterloo T. 2002, AJ, 123, 3124 Figure 3: NGC 891 at 3.9 kpc (left) andand 5.2 predictions kpc (right) from from two the plane models:fountain compared clouds to a sweep the pure disk up galactic rotation and curveThe fountain accrete (dotted model accretion ambient line) (red gas rate line during required their above) to passage and through a produce the model this halo where fit (blue the line is below). Gas accretion onto galaxies: models vs observations [10] Fraternali F. & Binney J.J. 2006,[11] MNRAS, 366, 449 Fraternali F. & Binney J.J. 2008,[12] MNRAS, 386, 935 Kalberla P.M.W., Dedes L. 2008, A&A, 487,[13] 951 Kewley L.J., Geller M.J., Jansen R.A., Dopita M.A. 2002, AJ, 124, 3135 plication is that it doesits not angular require momentum about that the thematerial. z-axis accreting Finally, is this gas less model is than predicts in an aboutportional any accretion to half rate particular the the of phase angular the rate. but momentum order Interestingly,throughout only of of this the the that the appears Hubble SFR time disk to and as be it in a reconcilesmass general, general the in pro- requirement observed galaxies cosmic for at star galaxies low formation and history high with redshifts the (Hopkins, gas McClure-Griffiths & GaenslerReferences 2008). NGC 891 (from Fraternali & Binney 2008). PoS(PRA2009)050 Filippo Fraternali 8 (Cambridge: Cambridge University Press) (astro-ph/0306034) Gas and galaxy evolution. ASP Conference series, vol 240, p 451 T.C., Ivezic Z., Richter P., Schwarz U.J. 2007, ApJ, 670, L113 Gas accretion onto galaxies: models vs observations [14] Greisen E.W., Spekkens K., van Moorsel G.A.[15] 2009, AJ, Grossi 137, M., 4718 Giovanardi C., Corbelli E.[16] et al. Heald 2008, G.H., A&A, Rand 487, R.J., 161 Benjamin R.A.[17] et al. Heald 2006a, G.H., ApJ, Rand 636, R.J., 181 Benjamin R.A.,[18] Bershady M.A. Hess, 2006b, Kelley ApJ, M.; 647, Pisano, 1018 D. J.;[19] Wilcots, Eric M.; Hopkins Chengalur, A.M., Jayaram McClure-Griffiths N. N.M., 2009, Gaensler ApJ, B.M.[20] 699, 2008, 76 ApJ, Hopkins 682, A.M., L13 Beacom J.F. 2006, ApJ,[21] 651, 142 Irwin J.A. 1994, ApJ, 429, 618 [22] Levine E.S., Heiles C., Blitz L.[23] 2008, ApJ, Lee 679, S.-W., 1288 Irwin J.A. 1997, ApJ,[24] 490, 247 Matthews L.D. & Wood K. 2003,[25] ApJ, 593, 721 Matteucci, F., 2003, in “Origin and Evolution of the Elements”, ed. A.[26] McWilliam & Miller M. E.D., Rauch Bregman J.N. & Wakker[27] B.P. 2009, ApJ, Oosterloo 692, T., 470 Fraternali F., Sancisi R. 2007,[28] AJ, 134, Panter 1019 B., Jimenez R., Heavens A.F., Charlot[29] S. 2008, Rand MNRAS, R.J., 391, Benjamin 1117 R.A. 2008, ApJ,[30] 676, 991 Reakes M.L., Newton K. 1978, MNRAS,[31] 185, 277 Rossa J., Dettmar R.-J. 2003, A&A,[32] 406, 505 Sancisi R., Fraternali F., Oosterloo T., van[33] der Hulst Shapiro T. P.R. 2008, & A&ARv, Field 15, G.B. 189 1976,[34] ApJ, 205, 762 Tinsley, B.M., 1981, ApJ, 250, 758 [35] Thilker D.A., Braun R., Walterbos R.A.M.[36] et al. 2004, Twarog B.A. ApJ, 1980, 601, ApJ, L39 242, 242 [37] van der Hulst J.M., Sancisi R.[38] 1988, AJ, van 95, der 1354 Hulst J.M., van Albada T.S., Sancisi R. 2001. Hibbard JE,[39] Rupen M, Wakker van B.P., York Gorkom D.G., JH Howk (eds). C., Barentine J.C., Wilhelm R., Peletier R.F., van[40] Woerden H., Beers Walterbos R.A.M., Braun R. 1994, ApJ,[41] 431, 156 Zwaan M.A., van der Hulst J.M., Briggs F.H. et al. 2005, MNRAS, 364, 1467