Abstract

In large galaxies molecular hydrogen is a better tracer of star formation than atomic hydrogen as shown recently by Leroy et al. (2008) and Bigiel et al. (2008). Molecular hydrogen content in dwarf galaxies, however, is difficult to constrain, and one must resort to other methods in defining locations of star formation. One intriguing idea involves identifying cold gas through narrow HI emission (e.g., Young et al. 1996, 1997). Cold HI gas is thought to be an intermediate step between the warm, neutral ISM and molecular hydrogen. In this project, we aim to characterize the cold neutral medium via VLA HI line maps from the VLA-ANGST (P.I. Ott) and THINGS (Walter et al. 2008) projects. VLA-ANGST is an NRAO Large program which obtained HI line images of 36 dwarf galaxies within ~3.5 Mpc. These 36 galaxies have HST observations from the ANGST project (P.I. Dalcanton), providing star formation histories from the resolved stellar populations. The high quality VLA observations allow us to spatially decompose line-of-sight spectra across the HI disk into broad and narrow components. Studying the environment immediately surrounding areas of narrow line emission allows us to investigate the relationship between the cold gas and star formation. We present analysis of the HI data, the morphologies of the cold gas components, and a comparison of the cold HI gas to locations of recent star formation as seen in HST optical images.

The Cold ISM of Dwarf GalaxiesSteven R. Warren1, Evan Skillman1, Jürgen Ott2, Adrienne Stilp3, Daniel Weisz1, Kristen Mcquinn1, Julianne Dalcanton3, & Fabian Walter4

1University of Minnesota, 2NRAO, 3University of Washington, 4MPIA

Image Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team

Figures: Sextans A Blue box – The upper left box is a DSS optical image overlaid with the HST WPC2 observations. The upper middle box is the HI integrated intensity map. The rest of the boxes have black integrated HI intensity maps contours overlaid with red dots indicating positions of narrow line detection. Green, blue, and magenta dots are blue helium burning stars whose ages are also listed. the cyan contour is the outline of the HST observations. Green boxes – Top: V-I Color magnitude diagrams for the Southern (left) and Northern (right) HST observations. The colored dots are the selected blue helium burning stars. Bottom: Star formation history over the past 400 Myr produced from the resolved stars.

Figures: Holmberg I Blue box – The upper left box is a DSS optical image. The upper middle box is the HI integrated intensity map. The rest of the boxes have black integrated HI intensity map contours overlaid with red dots indicating positions of narrow line detection. Green, blue, and magenta dots are BHeB stars whose ages are also listed. The cyan contour is the outline of the HST ACS observation. Green boxes – Top: A (F555W, F555W - F814W) Color magnitude diagram. The colored dots are the selected blue helium burning stars. Bottom: Star formation history over the past 400 Myr produced from the resolved stars.

Method

We fit Gaussians to line-of-sight spectra through HI image cubes. Each cube has been smoothed to a 15” beam. Spectra were extracted in steps of half-beam widths (7.5”) across the emission area. A single-Gaussian was fit to each spectra and was compared to a double-Gaussian fit. The reduced chi-squares were compared in a single-tailed F -test. Narrow line detections are those locations where adding a second Gaussian component improved the quality of the fit at the 95% probability level. The narrow line gas typically has velocity widths between 2 – 7 km/s while the broader components and single Gaussian fits have velocity widths between 7 – 15 km/s. The Narrow line gas may be tracing the cold, Neutral ISM. We detect narrow line emission in each of the galaxies analyzed thus far. The distribution of the narrow line gas is different than the global HI distribution. We also find narrow line to total mass ratios of up to ~8%.

FIGURES: Yellow boxes – HI integrated intensity maps with narrow line emission contours over-plotted in blue. Green boxes – Sample spectra for each galaxy. Red solid lines are the sum of the green lines (broad and narrow, two-component fit). The blue, dashed lines are the single-component fits to the data. The residuals are plotted beneath. Blue boxes – HI contours with narrow line emission locations marked as red dots. Each map has been convolved to a 15” beam size.

Holmberg I

Discussion

Blue helium burning (BHeB) stars offer a unique way to age date stellar populations. These stars occupy a distinct area on a color magnitude diagram (CMD) based on their ages (see Dohm-Palmer et al. (1997)). The BHeB stars are marked as colored dots in the above CMDs where green are stars less the 50 Myr old, blue are 50 – 200 Myr old and magenta are 200-400 Myr old. Future work will use these stars to look for spatial and temporal correlations between the areas of star formation and areas of narrow line detections. At first glance, however, the narrow line gas is not in areas of the most recent (≤ 200 Myr) star formation.

References:

•Bigiel, F., Leroy, A., Walter, F., Brinks, E., de Blok, W. J. G., Madore, B., & Thornley, M. D. 2008, aj, 136, 2846 •Dalcanton, J. J., et al. 2009, apjs, 183, 67 •Dohm-Palmer, R. C., et al. 1997, aj, 114, 2527 •Leroy, A. K., Walter, F., Brinks, E., Bigiel, F., de Blok, W. J. G., Madore, B., & Thornley, M. D. 2008, aj, 136, 2782 •Ott, J., Skillman, E., Dalcanton, J., Walter, F., Stilp, A., Koribalski, B., West, A., & Warren, S. ``Connecting Gas

Dynamics and Star Formation Histories in Nearby Galaxies: The VLA-ANGST Survey'‘ 2008, "The Evolution of Galaxies through the Neutral Hydrogen Window", Arecibo, PR, USA, eds. R. Minchin & E. Momjian, AIPC, 1035, 105, arXiv:0805.4605

•Walter, F., Brinks, E., de Blok, W. J. G., Bigiel, F., Kennicutt, R. C., Thornley, M. D., & Leroy, A. 2008, aj, 136, 2563 •Young, L. M., van Zee, L., Lo, K. Y., Dohm-Palmer, R. C., & Beierle, M. E. 2003, ApJ, 592, 111•Young, L. M., & Lo, K. Y. 1997, ApJ, 490, 710•Young, L. M., & Lo, K. Y. 1996, ApJ, 462, 203

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Sextans A Holmberg I

Total HINarrow Line

DDO187 DDO181

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Sextans A

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Conclusions

• VLA-ANGST has obtained VLA HI line imaging of 36 dwarf

galaxies beyond the local group and within ~3.5 Mpc.

• ANGST has produced star formation histories from HST imaging

for ~70 galaxies, including the 36 from VLA-ANGST.

• We detect narrow line emission in each of the four galaxies

analyzed thus far.

• The distribution of the narrow line gas is different than the

global HI distribution.

• We find narrow line to total mass ratios of up to ~8%.

• At first glance the narrow line gas is not in areas of the most

recent (≤ 200 Myr) star formation.

VLA-ANGST is a large NRAO program and is supported by the NSF Collaborative Research Grant 807515.