1. Ewan O’Sullivan (SAO) thanks to: J. Vrtilek, L. David, S. Giacintucci, S. Raychaudhury, A. Zezas, and others

2. Boston, 20 Nov 201415 years of science with Chandra Holm 124 HI in galaxies HCG 16 HI between galaxies NGC 4636: no HI in group core… Increasing Mass, tidal interactions, HI deficiency How are HI-rich, spiral dominated groups transformed into HI-poor, hot gas rich elliptical dominated systems? Is hot gas mainly accreted and gravitationally heated, as is the case in clusters? …but rich in ~1 keV hot gas Verdes-Montenegro et al. (2001) Mishra et al. (2013) Kilborn et al. (2009)

3.  Only a handful of spiral-rich groups with X-ray emitting IGM are known.  Typically faint: L X ≈ few x10 41 erg s -1  All have tidally interacting galaxy populations and disturbed X-ray halos.  Is IGM formation linked to galaxy transformation? Boston, 20 Nov 201415 years of science with Chandra HCG 70 Dos Santos & Mamon (1999) HCG 16 SCG 0018-4854 Trinchieri et al (2008) Stephan’s Quintet Trinchieri et al. (2003)

4. VLA HI on DSS optical Chandra 0.5-2 keV  Hot gas detected by ROSAT and XMM but morphology uncertain  137.5 ks Chandra observation  irregular ridge of 0.3 keV gas  X-ray ridge, H I filament linking galaxies  group not yet relaxed? Boston, 20 Nov 201415 years of science with Chandra

5. Boston, 20 Nov 201415 years of science with Chandra NGC838 NGC839 VLA 1.4 GHz on DSS GMRT 610 MHz on Chandra 0.5-2 keV  2 northern spirals: Seyferts, low SFR  3 southern spirals: starbursts, SFR=7-20 M  /yr  HI and hot gas in ridge are densest around NGC 838 and NGC 839  SF has been ongoing for 4-5x10 8 yr  ~10 10 M  of hot gas ejected in that period  ~20% of IGM mass  If other group members have ejected gas at a similar rate in the past, ~40% of IGM could have come from starburst galaxies.

6. 15 years of science with Chandra  900 km/s collision between infalling spiral and H I filament visible in radio, X-ray, IR  Unexpected physics in shock ridge: cooling via H 2 emission (Cluver et al. 2010)  Mass of ~0.7 keV IGM (~3x10 10 M  ) ≈ H I deficit (2x10 10 M  )  Shocked H I may make up a significant fraction of IGM Boston, 20 Nov 2014 Williams et al (2005)

7. Boston, 20 Nov 201415 years of science with Chandra HI contours on TGSS 150MHz TGSS 150 MHz contours on DSS optical  3x10 9 M  of HI in 100 kpc filament extending across NGC 5903 ( Appleton et al. 1990 )  ~60 kpc wide radio structure, ~7 Jy @150 MHz, steep spectral index α=1.5±0.08 Appleton et al (1990) Gopal-Krishna et al (2012)

8. Boston, 20 Nov 201415 years of science with Chandra  ~40 ks XMM, low-level flaring throughout  Disturbed 0.7 keV IGM correlated with HI!  Radio ridge correlated with X-ray spur.  Hot gas mass ≈ HI deficit (3x10 10 M  ) Collision shock as in Stephan’s Quintet?  No clear high-velocity intruder galaxy  collision in plane of sky? Plans: GMRT HI observations (high-resolution maps) Hα imaging (trace warm gas content) Deep Chandra pointing (155 ks) approved in cycle 16 150 MHz contours XMM 0.3-3 keV

9. 1. Only a handful of spiral-rich groups with a hot IGM have been studied with Chandra and/or XMM. 2. HCG 16: demonstrates that starburst winds can inject significant quantities of hot gas into the IGM in spiral-only groups. Up to ~40% of IGM observed in this unrelaxed group may have been contributed by galaxy winds. 3. Stephan’s Quintet and NGC 5903: demonstrate that collisional shocks can contribute significantly to building IGM by directly heating H I. 1. Deeper soft X-ray surveys needed to identify more examples (eROSITA?), low-frequency radio surveys can help us find shocked systems (LOFAR?) 2. Deep Chandra follow-up necessary to study interactions in detail (to resolve wind structures, shocks, filaments, and determine the physical state of the gas) – another 15 years please! Boston, 20 Nov 201415 years of science with Chandra

10. Boston, 20 Nov 201415 years of science with Chandra HCG 16 falls on or above the L:T relation. Stephan’s Quintet and NGC5903 fall below the relation, but inside scatter of group population.