What What can can we we learn learn from from present present- - PDF document

04.10.2017 What What can can we we learn learn from from present present- -day day HVCs HVCs about about cold cold- -gas gas accretion accretion of of galaxies galaxies Gerhard Hensler Gerhard Hensler Dept Dept. . of of Astrophysics Astrophysics, , University University of of Vienna Vienna Bastian Sander, Vienna/Magdeburg Bastian Sander, Vienna/Magdeburg Wolfgang Wolfgang Vieser Vieser, Kiel/ , Kiel/Munich Munich Sylvia Sylvia Ploeckinger Ploeckinger, Vienna/Leiden , Vienna/Leiden 1 The The Milky Milky Way Way is is bombarded bombarded by by infalling infalling High- High -velocity velocity Clouds Clouds; 20 ; 20- -40% 40% sky sky coverage coverage 2 1

04.10.2017 Properties of HVCs Properties of HVCs � HI-detection with galactocentric velocities <-200 km/s � Masses of ~10 8 …. 10 4 M � � ~50% of the sky are covered with infalling gas clouds. � Their origin is not yet solved, but obviously various: Some from the Mag. Stream and MW satellites or even of cosmological origin 3 Infalling clouds are divided according to their velocities into: � Intermediate-vel. clouds (IVC): -50 > v GSR [km/s] > -100 � High-velocity clouds (HVCs): -100 > v GSR [km/s] > -200 � Ultra-HVCs: v GSR < -200 km/s v GSR 2

04.10.2017 5 Properties of HVCs Properties of HVCs � HI-detection with galactocentric velocities <-200 km/s � Masses of ~10 8 …. 10 4 M � � ~50% of the sky are covered with infalling gas clouds. � Their origin is not yet solved, but obviously various: Some from the Mag. Stream and MW satellites � Very massive entities exist very close to the gal. plane � Full of substructures; no central mass concentration. 6 3

04.10.2017 Massive HVCs Massive HVCs H H exhibit exhibit complex complex internal internal structures structures C C Simon et al. (2006) ApJ, 640 Fox et al. 2004 Cores Cores and and Head Head- -tail tail Structures Structures Smaller HVCs (~10 4 M � ) show � core-halo structures ⇒ hydrostatic stratification ⇒ self-gravity � head-tail structures ⇒ ram-pressure stripping and seem to survive, decelerated and stretched by interactions with the hot static halo gas. Brüns et al. 2000 8 4

04.10.2017 Properties of HVCs Properties of HVCs � HI-detection with galactocentric velocities <-200 km/s � Masses of ~10 8 …. 10 4 M � � ~50% of the sky are covered with infalling gas clouds. � Their origin is not yet solved, but obviously various: Some from the Mag. Stream and MW satellites � Very massive entities exist very close to the gal.plane � Full of substructures; no central mass concentration. � Mostly, head-tail structure ⇒ drag 9 Properties of HVCs Properties of HVCs � HI-detection with galactocentric velocities <-200 km/s � Masses of ~10 8 …. 10 4 M � � ~50% of the sky are covered with infalling gas clouds. � Their origin is not yet solved, but obviously various: Some from the Mag. Stream and MW satellites � Very massive entities exist very close to the gal.plane � Full of substructures; no central mass concentration. � Mostly, head-tail structure ⇒ drag � Metal-poor � Normal ISM phases � No stellar content! heating processes prevent cooling? � B field? � Core-halo structure ⇒ self-gravitating! 10 5

04.10.2017 Are Are our our assumptions assumptions of of (2004) ApJ, 615 numerical HVC numerical HVC models models correct correct? ? Massive clouds passing through hot gas are exposed to ram pressure, Kelvin-Helmholtz and Rayleigh-Taylor instabilities HVCs are NOT homogeneous! Without self-gravity clouds are disrupted! (see also Heitsch & Putman, 2009, Apj, 698) Self-gravity is an urgent ingre-dient to be included to num. models! Motivation to care about HVCs especially? Motivation to care about HVCs especially? � Gas infall feeds galaxies! Cold streams? � SFR-gas relations? � P ext – H 2 /HI relation? � How is the gas accreted? Streams vs. clouds! HVCs? 12 6

04.10.2017 How does the “cold accretion” scenario work? How does the “cold accretion” scenario work? Dekel et al. (2009) Nature, 457: Colours refer to inflow rate per solid angle of point-like tracers at the centres of cubic-grid cells. Box side length is 320 kpc. Keres & Hernquist (2009) ApJ, 700: Example of the instability developing from an overdense filament in a hot dilute halo gas 13 leading to cloud formation. Motivation to care about HVCs especially? Motivation to care about HVCs especially? � Gas infall feeds galaxies! Cold streams? � Pext – H2/HI relation? � SFR-gas relations? � How is the gas accreted? Streams vs. clouds! HVCs? � Where do MWG HVCs come from? 3 sources: 1. Intergalactic stripped galaxy gas 2. tidally stripped satellites 3. Primordial clouds � Do they represent the link to missing Λ CDM subhalos, i.e. DM dominated? (see Poeckinger & Hensler, 2012, A&A, 547) � How do HVCs interact with the gaseous galactic halo? Deceleration? Halo Magnetic fields? (see Jelinek & Hensler, 2011, Comp. Phys. Comm., 182) � What stabilizes them against disruption by RTI & KHI? � Role of Heat conduction + Gravity? (see Vieser & Hensler, 2007, A&A, 472) � Do they reach the galactic disks? Gas replenishment? Triggered SF in gas disks? (see Izumi et al., 2014) 14 7

04.10.2017 Cresci et al. (2010) Nature, 467 Signatures as local metal deficiency in high-z galaxies are indicating low-Z/primordial gas infall. Motivation to care about HVCs especially? Motivation to care about HVCs especially? � Gas infall feeds galaxies! Cold streams? SFR-gas relations? � How is the gas accreted? Streams vs. clouds! HVCs? � Where do MWG HVCs come from? 3 sources: 1. Intergalactic stripped galaxy gas 2. tidally stripped satellites 3. Primordial clouds � Do they represent the link to missing Λ CDM subhalos, i.e. DM dominated? (see Poeckinger & Hensler, 2012, A&A, 547) � How do HVCs interact with the gaseous galactic halo? Deceleration? Halo Magnetic fields? (see Jelinek & Hensler, 2011, Comp. Phys. Comm., 182) � What stabilizes them against disruption by RTI & KHI? � Heat conduction + Gravity? (see Vieser & Hensler, 2007, A&A, 472) � Do they reach the galactic disks? Gas replenishment? Triggered SF in gas disks? (see Izumi et al., 2014) � Star formation in HVCs? Not observed! But SF in some RPS clouds! Why? Under which conditions? 16 8

04.10.2017 At ram-pressure stripped clouds star formation is possible: Under which conditions? VCC1217 NGC4569 ⇒ HVCs bridge the research fields from ISM (+ SF) to galaxy evolution and cosmology 18 9

04.10.2017 Without self-gravity, RTI grows and facilitates ram pressure to destruct the cloud: Self-gravity is an urgent ingredient to be included! BUT: Is ram pressure the only effect of the hot halo gas on HVCs? Between hot gas and warm/cold clouds thermal conduction plays a prominent role: at rest, only energy exchange leads to • evaporation or condensation. in motion: KHI is suppressed. • 19 Do HVCs survive fast motion through hot gas? Very low- mass models: 210 M � , Self-grav.+cooling+therm.cond.; KHI+ RP lead to destruction 2005, MN, 342 and many other authors (see in Vieser & Hensler, 2007, A&A, 472, 141) 10

04.10.2017 3D Cloud Models 3D Cloud Models mass (10 3 M � ) size (pc) [Z/Z � ] T c,cl /K self- heat./ thermal V rel grav. cool. conduct. [km/s] 62.4 – 87.5 41.16 – 48.42 0.1 - 1.0 230 + – + – class. 15.1 – 19.9 11.34 – 27.0 0.1 - 1.0 1150 + – + – saturated depend. ICM, Z depend. ICM, - 167, Z 250, 333 � Core-halo structure � Radiative heating-cooling balance � Thermal conduction (see Vieser & Hensler, 2007a+b, A&A, 472 + A&A, 475) � Self-gravity! Sander & Hensler (2017) in prep. 21 Big HVC in 3D Big HVC in 3D Most of the HVC remains gravitationally bound. 4 M M=8.75 M=8.75 � 10 10 4 M � , , Z=0.1 Z Z=0.1 Z � v v ICM ICM =167 km/s =167 km/s 6 K , T ICM T ICM =5.6 =5.6 � 10 10 6 K , n n ICM ICM =0.7 =0.7 � 10 10 3 3 cm cm - -3 3 for 50 Myrs 22 11

04.10.2017 Destruction Destruction vs. vs. Survival Survival: : the the mass mass dependence dependence 4 M M=8.75 M=8.75 � 10 10 4 M � , , vs. vs. 4 M M=2.0 M=2.0 � 10 10 4 M � , , Less massive HVCs are disrupted! 23 Sander & Hensler (2017) in prep. Cloud Cloud deceleration deceleration included included Deceleration leads to H-T structure. 25 12

04.10.2017 Decelerated Decelerated big big HVCs HVCs survive survive with with accretion accretion! ! 26 Sander & Hensler (2017) in prep. Velocity effect Velocity effect Higher velocity leads to: � stronger Bernoulli effect � HT structure, � higher mass loss, � stronger central concentration Jeans unstable?? Sander & Hensler (2017) in prep. 13