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Searching for Ongoing Accretion Using the Green Bank Telescope

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Searching for Ongoing Accretion Using the Green Bank Telescope Searching for ongoing accretion using the Green Bank Telescope D.J. Pisano (West Virginia University) with W.J.G. de Blok, A. Leroy, F. Bigiel, F. Walter, E. Brinks, K. Keating, N. Pingel, K. Rabidoux, G. Heald, F.J. Lockman, J.C. Mihos Comparing the evolution of " HI content and SFR Noterdaeme et al. 2012 Perez-Gonzalez et al. 2005 While the HI content of galaxies has remained relatively constant since z ~ 2-5, the SFR was 10x higher at z ~ 1. Galaxies must have accreted gas from the IGM. Ongoing accretion of gas onto nearby galaxies? M 101 NGC 925 Sancisi et al. 2008 Ongoing accretion of gas onto nearby galaxies? M 101 The accre8on rate of these HI clouds onto galaxies is only about 10% of the star formaon rate in these galaxies. Will galaxies stop forming stars or are we missing NGC 925 gas accre8on? Sancisi et al. 2008 z=5.52 z=3.24 Hot/Cold Flows • Many simulations predict that gas is accreted by galaxies in two forms (e.g. Birnboim & Dekel 2003, Keres et al. 2005, 2009). • Hot flows are gas that is shock- heated to the virial temperature; T > 105 K. • Cold flows remain below Tvir, < 105 K, and falls onto galaxy along filaments. • At z=0, cold mode should be 11 dominant for Mhalo # 10 M¤ Green =hot and in low density environments. Green =cold Keres et al. 2005 H HI HI Hydrogen around galaxies • Some of HI is condensed at high NHI, the rest is diffuse with low NHI. • Low NHI filaments have sizes of ~ 25 kpc. • These HI filaments would be seen as Lyman limit systems in absorption. • This HI can be detected in emission with current radio telescopes and sufficient time. Popping et al. 2009 How can we observe cold flows locally? • Most of the previous HI maps were made with interferometers (VLA, WSRT). These are very sensitive to clumpy HI clouds. • To detect faint, diffuse HI, we need to use a single-dish radio telescope (like the GBT). How do we determine the origin of HI around nearby galaxies? • Simulations and theory predict that cold flows should be more common around low mass galaxies in low density environments at low redshift. • We need to observe a large sample of nearby galaxies in a range of environments with a range of masses and SFRs. Walter et al. 2008 While THINGS VLA observations are excellent for studying the inner disks of galaxies at high resolution, they are insensitive to HI extended over large scales and at low column densities. " " HALOGAS is more sensitive, but single-dish observations are the most sensitive to low column density HI. GBT THINGS Survey Goals • Provide total power HI, 21cm, data to combine with VLA/WSRT data for studies of star formation in outer disks. • Combined GBT+VLA/WSRT data will improve sensitivity for studying the HI distribution and kinematics of outer disks. • Study the low column density HI environments of galaxies including identifying signatures of past tidal interactions and cold flows for galaxies with a range of masses, SFRs, and environments. GBT Observations • Green Bank Telescope (GBT) has unmatched combination of sensitivity and resolution as well as very low sidelobes! • Mapped 4 ☐°around 21 THINGS galaxies at 9.2’ resolution between January-March 2011. THINGS galaxies at D = 2-15 Mpc. • Recently completed mapping additional 20 galaxies from HALOGAS. – maps are 70-513 kpc on a side depending on galaxy – 5-40 kpc beamsize depending on galaxy • Spent 10 hours per galaxy for a rms sensitivity of ~15-20 mK per 5.2 km/s 18 -2 channel or 5σ, 20 km/s detection limit of NHI ~ 2x10 cm . • Data taken in frequency-switched mode, but can improve sensitivity by using edges of maps as “OFF” position. • The remaining 13 galaxies have already been observed by Chynoweth et al. (2008, 2009) and Mihos et al. (2012). Walter et al. 2008 Walter et al. 2008 NGC 6946" (D ~ 6 Mpc) THINGS data: Greyscale ranges from Boomsma et al. 2008, 192 hour WSRT map 20-21.6 -2 NHI~10 cm lowest contour = 1.25x1019 cm-2 Many HVCs seen associated with disk. NGC 6946 Channel Maps A putative HI filament persists at the 2$ level across at least 12 channels (~ 60 km/s). If we convolve the interferometer map with the GBT beam, a filament appears, but at a column density 10x fainter! NGC 6946 HI filament A single ON-OFF pointed observation confirms the reality of the HI filament, although not its continuity. NGC 6946 • NGC 6946 is a void galaxy in a group. Local galaxy density is 0.07 Mpc-3 (Tully 1988) • Optical in blue, WSRT HI data in green, GBT HI data in red. • GBT can detect HI down to NHI = 1018 cm-2. 18 • Filament has peak NHI = 2x10 cm-2 and FWHM = 48 km/s. • The filament smoothly connects in position and velocity with NGC 6946 and companions. • Some emission from filament could be due to stray radiation. • Filament could be a cold flow, Pisano (2014) but is more likely to be a tidal Boomsma et al. (2008) stream. No visible stellar counterpart. NGC 2403 • Fraternali et al. 2001 found an anomalous velocity HI cloud in their WSRT data. • Our GBT data (de Blok et al. 2014) reveal a more diffuse, extended cloud connected spatially and kinematically. • Stick around for the next talk! M101" (D ~ 7.4 Mpc) Mihos et al. 2012 M101 is lopsided in the interferometer map and in the GBT map, but in the opposite direction. There is a possible connection with a nearby companion. Sensitivity is about 5x1017 cm-2. NGC 925" (D ~ 9.3 Mpc) UGC 2023 NGC 925 UGC 2023 3σ, 20 km/s sensitivity ~1018 cm-2. Can see the tidal features near NGC 925, but no connection with companion. Contours at 1, 3, 6, 10...600x1018 cm-2. Absence of low N features probably real, HI See signs of extended HI around NGC but may be due to distance of source. 925, but no filamentary structures. NGC 891" (D ~ 9.3 Mpc) Oosterloo et al. 2007 No signature of a filament connected NGC 891 to its companion 17 -2 at the NHI = 5x10 cm level . NGC 2997 Pisano (2014) 17 -2 No discrete features at the NHI = 5x10 cm level, but HI covers 23% more area at the 1018 cm-2 level. NGC 2997 Pisano (2014) GBT: solid line GMRT+ATCA: dashed line The HI flux of NGC 2997 as measured by the GBT agrees within 10% of interferometer data. GBT THINGS data for the full sample (almost) NGC 628 DDO 154 NGC 2403 NGC 2841 NGC 2903 NGC 3184 NGC 3198 NGC 3351 NGC 3521 NGC 3627 NGC 3631 NGC 4449 NGC 4826 NGC 5055 NGC 5194 NGC 5236 NGC 5457 NGC 6946 NGC 7331 NGC 7793 GBT THINGS data for the full sample (almost) NGC 628 DDO 154 NGC 2403 NGC 2841 NGC 2903 Based on a first look, only those THINGS galaxies NGC 3184 undergoingNGC 3198 an interactionNGC 3351 appearNGC 3521 to have diffuseNGC 3627 gas 18 -2 features with NHI %10 cm . NGC 3631 NGC 4449 NGC 4826 NGC 5055 NGC 5194 NGC 5236 NGC 5457 NGC 6946 NGC 7331 NGC 7793 GBT THINGS data for the full sample (almost) NGC 628 DDO 154 NGC 2403 NGC 2841 NGC 2903 Nevertheless, there is #22% more HI seen in the NGC 3184 GBTNGC 3198 data, thanNGC 3351 in the VLA/WSRTNGC 3521 data.NGC 3627 Some of this is due to the galaxy extending beyond the VLA’s primary beam (field of view). NGC 3631 NGC 4449 NGC 4826 NGC 5055 NGC 5194 NGC 5236 NGC 5457 NGC 6946 NGC 7331 NGC 7793 GBT THINGS data for the full sample (almost) NGC 628 DDO 154 NGC 2403 NGC 2841 NGC 2903 Galaxies are ~20% larger at ~1018 cm-2 in GBT data NGC 3184 as comparedNGC 3198 to smoothedNGC 3351 VLANGC 3521 data. ConsistentNGC 3627 sizes when comparing between GBT & WSRT for preliminary comparison. NGC 3631 NGC 4449 NGC 4826 NGC 5055 NGC 5194 NGC 5236 NGC 5457 NGC 6946 NGC 7331 NGC 7793 NGC 3198: Going even deeper • Barely any sign of HI at 1017 cm-2 around 3198 or 2403. Conclusions • We have mapped low column density HI emission around 41 THINGS and HALOGAS galaxies with the GBT to provide total power data for studies of HI in galaxies and identify tidal features and cold flows around galaxies. • Diffuse HI clouds/filaments seen associated with NGC 6946 & NGC 2403 (next talk). The remainder of the galaxies have extended HI distributions, but no discrete features unless a tidal interaction is ongoing. • On average, the GBT data is revealing 22% more HI than seen by the interferometers. • The HI traced by the GBT covers ~20% more area than seen by the VLA. Similar sizes when comparing to WSRT data. • Future observations of MHONGOOSE galaxies will be easier with FLAG (phased array feed for the GBT). Focal L-band Array for the GBT • Tests of FLAG on the 20m yielded Tsys/η ~ 50 K. • Using new dipoles & cryogenic system on GBT, the goal is Tsys/η ~ 35 K. • Mapping speed 3-5 times faster than current single-pixel feed. .
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