1. 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)

2. Outline Introduction Sample Definition Aim Results & Conclusions
Summary
Future Prospects

3. 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.

4. 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.

5. 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.

6. 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

7. 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)

8. 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

9. 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)

10. 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%).

11. 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.

12. Relative HI content vs. HI-to-optical diameter ratio

13. 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

14. 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)

15. 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).

16. Future Prospects (cont.)
R-band optical diameter instead of B-band optical diameter.

17. Thank You