The WSRT HALOGAS Survey: Kinematics of Ionized Gas in Edge-on HALOGAS Galaxies Cat Wu, Rene Walterbos, Richard Rand, Robert Benjamin, George Heald and the HALOGAS team Extra-planar gas often shows a decrease in rotational velocity with increasing height above the disk (e.g. Rand 1997, Swaters et al. 1997, Heald et al. 2007). This can be explained by the galactic fountain model (Shapiro & Field, 1976), in which supernovae in the disk drive disk-halo cycling, or accretion of low-angular momentum material from infalling gas clouds. Computer models suggest that a combination of the two is the most likely scenario (e.g. Collins et al. 2002, Heald et al. 2006). Fraternali & Binney (2008) modeled the halos of NGC 891 and NGC 2403. Their models in which 80-90% of extra-planar gas originates from galactic fountain flows and the remaining 10- 20% from accretion produce velocity gradients that match observations. The goal of this project is to measure velocity gradients of the ionized gas in a large number of halos, which can then be used as a constraint for models of the origin of halo gas. Because we are observing in optical, our measurements are likely not affected by warps which generally occur further out in radial distance. Some of our targets overlap with the HALOGAS survey, which is investigating cold gas accretion in spiral galaxies with WSRT observations. We restrict our targets to edge-on galaxies, and our data provide an optical complement to HALOGAS data. We present our ionized halo gas velocities for NGC 4244 and NGC 4631 and show a comparison with HI data. ** Add basic info on 4244 and 4631 Explain - how measured gradients Make table: R, gradient, maximum height of measured emission Compare gradients to literature Ha contour plot? See if can do lines instead of color Oplot Ha velocites on HI plot HI specs – exp time, resolution Multi-slit spectroscopy Wavelength calibration: We take arc frames with a hydrogen lamp, which produces one Hα line per slit. Each line is given an artificial wavelength based on dispersion and pixel separation so the arc can be treated as a single, continuous spectrum. Target spectra are wavelength calibrated with this arc. Velocity calculation: A Gaussian is fit to the Hα emission profile of each slit at several heights, and the centroid is taken as the observed wavelength. The difference between this and the artificial wavelength of the arc line for that slit along with the rest- frame wavelength of Hα gives the recessional velocity of the Hα- emitting gas from the Doppler shift equation. • series of uniform, parallel slits with 3.75' x 4' FOV • narrowband filter prevents neighboring spectra from overlapping • measure a particular spectral line in multiple slits simultaneously Arranging the slits parallel to the minor axis allows us to measure velocities of Hα emitting gas as a function of height above the plane in several radial distance bins in a single exposure. axes...errors Sim-pro; skyplot Explain – projection effect of looking through galaxy produces high/low velocity tail; centroids from Gaussian not ideal Observations Introduction Hα image with field overlaid Single, flat-fielded spectrum Combined spectrum, skylines subtracted Hoopes et al. 1999 NGC 4244 NGC 4631 SINGS 2007 Field NGC 4244, south NGC 4631, central Dates Feb & Apr 2008 Feb & Apr 2009 Exposure time 5.8 hours 5.75 hours Slit size 2” x 4' 1.5” x 4' Number of slits 16 slits, 15” apart 11 slits, 22.5” apart Dispersion 0.80 Å/pix 0.58 Å/pix Resolution 109 km/s 65 km/s Measuring Velocities Rand, R.J. 1997, ApJ, 474, 129 Swaters, R.A., Sancisi, R. & van der Hulst, J.M. 1997, ApJ, 491, 140 Shapiro, P.R. & Field, G.B. 1976, ApJ, 205, 762 Collins, J.A., Benjamin, R.A. & Rand, R.J. 2002, ApJ, 578, 98 References Heald, G.H., Rand, R.J., Benjamin, R.A. & Bershady, M.A. 2006, ApJ, 647, 1018 Fraternali, F. & Binney, J.J. 2008, MNRAS, 386, 935 Hoopes, C.G., Walterbos, R.A.M. & Rand, R.J. 1999, ApJ, 522, 629 SINGS Fifth Enhanced Data Release, 2007 Spectra are binned spatially by 3 pixels and were taken with the ARC 3.5m telescope at Apache Point Observatory, NM. Velocity Gradients and Comparison to HI Results NGC 4244 south NGC 4631 central Velocity plots for 4631 go here
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The WSRT HALOGAS Survey:Kinematics of Ionized Gas in Edge-on HALOGAS Galaxies
Cat Wu, Rene Walterbos, Richard Rand, Robert Benjamin, George Heald and the HALOGAS team
Extra-planar gas often shows a decrease in rotational velocity with increasing height above the disk (e.g. Rand 1997, Swaters et al. 1997, Heald et al. 2007). This can be explained by the galactic fountain model (Shapiro & Field, 1976), in which supernovae in the disk drive disk-halo cycling, or accretion of low-angular momentum material from infalling gas clouds. Computer models suggest that a combination of the two is the most likely scenario (e.g. Collins et al. 2002, Heald et al. 2006). Fraternali & Binney (2008) modeled the halos of NGC 891 and NGC 2403. Their models in which 80-90% of extra-planar gas originates from galactic fountain flows and the remaining 10-20% from accretion produce velocity gradients that match observations.
The goal of this project is to measure velocity gradients of the ionized gas in a large number of halos, which can then be used as a constraint for models of the origin of halo gas. Because we are observing in optical, our measurements are likely not affected by warps which generally occur further out in radial distance. Some of our targets overlap with the HALOGAS survey, which is investigating cold gas accretion in spiral galaxies with WSRT observations. We restrict our targets to edge-on galaxies, and our data provide an optical complement to HALOGAS data. We present our ionized halo gas velocities for NGC 4244 and NGC 4631 and show a comparison with HI data. ** Add basic info on 4244 and 4631
Explain - how measured gradientsMake table: R, gradient, maximum height of measured emissionCompare gradients to literature
Ha contour plot? See if can do lines instead of colorOplot Ha velocites on HI plotHI specs – exp time, resolution
Multi-slit spectroscopy
Wavelength calibration: We take arc frames with a hydrogen lamp, which produces one Hα line per slit. Each line is given an artificial wavelength based on dispersion and pixel separation so the arc can be treated as a single, continuous spectrum. Target spectra are wavelength calibrated with this arc.
Velocity calculation: A Gaussian is fit to the Hα emission profile of each slit at several heights, and the centroid is taken as the observed wavelength. The difference between this and the artificial wavelength of the arc line for that slit along with the rest-frame wavelength of Hα gives the recessional velocity of the Hα-emitting gas from the Doppler shift equation.
• series of uniform, parallel slits with 3.75' x 4' FOV • narrowband filter prevents neighboring spectra from overlapping• measure a particular spectral line in multiple slits simultaneously
Arranging the slits parallel to the minor axis allows us to measure velocities of Hα emitting gas as a function of height above the plane in several radial distance bins in a single exposure.
axes...errors
Sim-pro; skyplot
Explain – projection effect of looking through galaxy produces high/low velocity tail; centroids from Gaussian not ideal
Observations
Introduction
Hα image withfield overlaid
Single, flat-fielded spectrum
Combined spectrum,skylines subtracted
Hoopes et al. 1999
NGC 4244
NGC 4631
SINGS 2007
Field NGC 4244, south NGC 4631, centralDates Feb & Apr 2008 Feb & Apr 2009
Exposure time 5.8 hours 5.75 hoursSlit size 2” x 4' 1.5” x 4'
Number of slits 16 slits, 15” apart 11 slits, 22.5” apartDispersion 0.80 Å/pix 0.58 Å/pixResolution 109 km/s 65 km/s
Measuring Velocities
Rand, R.J. 1997, ApJ, 474, 129Swaters, R.A., Sancisi, R. & van der Hulst, J.M. 1997, ApJ, 491, 140Shapiro, P.R. & Field, G.B. 1976, ApJ, 205, 762Collins, J.A., Benjamin, R.A. & Rand, R.J. 2002, ApJ, 578, 98
References Heald, G.H., Rand, R.J., Benjamin, R.A. & Bershady, M.A. 2006, ApJ, 647, 1018Fraternali, F. & Binney, J.J. 2008, MNRAS, 386, 935Hoopes, C.G., Walterbos, R.A.M. & Rand, R.J. 1999, ApJ, 522, 629SINGS Fifth Enhanced Data Release, 2007
Spectra are binned spatially by 3 pixels and were taken with the ARC 3.5m telescope at Apache Point Observatory, NM.
Velocity Gradients and Comparison to HI
Results NGC 4244south
NGC 4631central
Velocity plots for 4631go here
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