Investigating the relationship between the HI and optical diameter of a sample of LVHIS galaxies Nicolette Pekeur (UCT) Supervisors: Prof. Renee Kraan-Korterweg (UCT), Prof. Erwin de Blok (UCT) and Dr. Baerbel Koribalski (ATNF)
Outline Introduction Sample Definition Aim Results & Conclusions Summary Future Prospects
Introduction Not clear how small-scale galaxy processes determine the large-scale structure of a galaxy. Not even clear how Mpc environment is correlated with the kpc HI disk. Examining how the HI disk of a galaxy depends on various galaxy properties, presents a first step towards understanding how it is formed.
Sample definition Subsample of the Local Volume HI Survey (LVHIS; Koribalski et al., 2008). HI selected (HIPASS; Barnes et al., 2001) All morphological types. Volume-limited (D≤10 Mpc-LV; dwarf galaxies). Observations with ATCA sensitivity ~1 mJy and resolution ~1’. Motivation: Test previous studies that selects against faint galaxies. Pilot study.
Aim Investigate the relationship between the HI and optical diameter of 19 galaxies from LVHIS. The HI diameter is also correlated with the HI mass, HI mass- to-light ratio and environment.
Results HI properties were obtained at 3 HI column density levels (NHI=2.5E19 (~3σ), 8.0E19 and 2.5E20 cm2). Only review results obtained for the outermost HI diameter (NHI=2.5E19 cm-2). HI- vs. Optical Diameter HI Mass HI Mass-to-Light Ratio
HI Diameter vs. Optical Diameter The HI diameter (DHI) measured at NHI=2.5E19 cm-2. Optical diameter (Dopt), defined at μB=25 and 26 mag arcsec-2, D25B and D26B. log(DHI)=0(.55 ±0.13)log(D25B)+(1.48±0.29) log(DHI)=(0.66±0.21) log(D26B)+(1.11±0.52)
Conclusion I The HI and optical diameter is strongly correlated. DHI> Dopt for all galaxies. NHI (cm-2) μB (mag arcsec-2) DHI/Dopt Sample 2.5×1019 25 3.2±0.5 LVHIS 1.25×1020 3.3±1.5 WHISP1 26 2.0±0.3 1019 26.5 2.4±0.1 FIGGS2 References: 1. Swaters et al. (2002) The Westerbork HI Survey of Spiral and Irregular galaxies I. 2. Begum et al. (2008) FIGGS: Faint Irregular Galaxies GMRT Survey- Overview, Observations and First Results
HI Mass vs. HI Diameter Examine the HI mass (MHI) as a function of HI diameter by computing the HI mass inside the diameter. log (MHI) = (2.00±0.16) log(DHI)+(6.15 ±0.18)
Conclusion II log(MHI)=(1.99±0.11) log(DHI)+(6.08±0.06) HI mass is proportional to the square of the HI diameter. The fitted relation is, log(MHI)=(2.00±0.16) log(DHI)+(6.15±0.18) FIGGS galaxies, log(MHI)=(1.99±0.11) log(DHI)+(6.08±0.06) MHI α D2HI the HI surface density is constant when it is averaged over the HI disk. ΣHI=(1.8±0.2) M pc-2 Spread in ΣHI is due to local density variations presence of a central bar (~60%).
HI mass-to-light ratio Measures the relative amounts of HI gas and stars (relative HI content). Comparing the relative HI content with the HI-to-optical diameter ratio determine effect of star formation and the star formation rate on the HI diameter.
Relative HI content vs. HI-to-optical diameter ratio
Conclusion III More extended galaxies have higher HI mass-to-light ratios. MHI/LB (solar units) Sample 0.9±0.2 LVHIS 1.5 WHISP1 5.0 FIGGS2 References: 1. Swaters et al. (2002) The Westerbork HI Survey of Spiral and Irregular galaxies I. 2. Begum et al. (2008) FIGGS: Faint Irregular Galaxies GMRT Survey- Overview, Observations and First Results
Summary Results agree with literature. Pilot study LVHIS data is reliable. What are the HI properties of all LVHIS galaxies? (Koribalski et al., 2008/2009)
Future Prospects HI properties of all LVHIS galaxies means: Reduce statistical error increasing the sample size. Majority of galaxies studied here, are late-type galaxies. Expand our results to all morphological types (with detectable HI). Definitive results for environmental effects. Most of the galaxies are isolated. MeerKAT and ASKAP much more complete LV census. Estimated increase in the number of LV members, ~4 times the current known value (Staveley-Smith, 2008).
Future Prospects (cont.) R-band optical diameter instead of B-band optical diameter.
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