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Gas rich galaxies from the FIGGS survey
Jayaram N. Chengalur1 , Ayesha Begum2 , Igor D. Karachentsev3, Margrita Sharina3 , and Serafim Kaisin3
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arXiv:0711.2153v1 [astro-ph] 14 Nov 2007

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National Centre for Radio Astrophysics, TIFR, Pune University Campus, Ganeshkhind, Pune chengalur@ncra.tifr.res.in Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge Sp ecial Astrophysical Observatory, Nizhnii Arkhys 369167, Russia

1 Introduction
The FIGGS (Faint Irregular Galaxy GMRT Survey) is aimed at creating a multi-wavelength observational data base for a volume limited sample of the faintest gas rich galaxies. As described in more detail in the contribution by Begum et al. in these proceedings, the galaxies form an HI flux and optical diameter limited subsample of the Karachentsev et al.(2004) catalog of galaxies within 10 Mpc. The sample consists of 65 galaxies with MB > - 14.5 with median MB -13 and a median HI mass 3 в 107 M . HI aperture synthesis data (from the Giant Meterwave Radio Telescope - GMRT) has been obtained for all galaxies in the sample. Because the GMRT has a hybrid configuration (see Swarup et al. (1991)) images at a variety of spatial resolutions (ranging from 40" to 3" ) can be made from a single GMRT observation run. Galaxies in the FIGGS survey have substantially lower MHI and LB that typical of galaxies in earlier aperture synthesis surveys. The GMRT observations also used a velocity resolution (1.6 kms-1 ), that is 4 times better than most earlier interferometric studies of such faint dwarf galaxies. This high velocity resolution is crucial to detect large scale velocity gradients, which cannot be clearly distinguished in lower velocity resolution observations (see e.g. Begum et al. 2003a, 2003b, 2004a, 2004b, and for contrast Lo et al. 1993). In this paper we discuss two very gas rich galaxies that were observed as part of the FIGGS survey, viz. NGC 3741 and AndIV.

2 Two extremely gas rich galaxies
NGC 3741 (MB galaxy have bee thought to be a HST imaging Fe -13.13) has MHI /LB 5.8. GMRT observations of this n presented in Begum et al. (2005). And IV, was originally satellite of the Andromeda (M31) galaxy. However, based on rguson et al. (2000) argue that it is likely to be a background


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galaxy that happens to lie in pro jection close to the disk of M31. Consistent with this interpretation, they derive a distance of 6.11 Mpc for it (using the Tip of the Red Giant Branch technique) ­ this places And IV beyond the confines of the local group. The HI velocity measured for this galaxy (i.e. 234 km/s; Braun et al. (2003)) is also substantially different from that of the nearest portion of the disk of M31. The galaxy has blue magnitude of -12.37, which implies that MHI /LB 13.

3 HI in NGC 3741 and And IV

40 38

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Fig. 1. The GMRT moment 0 image of And IV (at 44" resolution), overlayed on the DSS image.

The GMRT observations (Begum et a. 2005) showed that NGC 3741 had an HI disk that extends to 8.3 times its Holmberg radius. This makes it probably the most extended gas disk known. Our observations allowed us to derive the rotation curve (which is flat in the outer regions) out to 38 optical scale lengths. NGC 3741 has a dynamical mass to light ratio of 107 and is one of the "darkest" irregular galaxies known. Follow up WSRT observations are presented in Gentile et al. (2007). For AndIV, the GMRT observations show that its gas disk extends out to 6 Holmberg radii. Fig. 1 shows the integrated HI emission from AndIV at 44 в 38 resolution, overlayed on the digitised sky survey (DSS) image. Fig. 2 shows the velocity field of AndIV at 26 в 23 resolution. The velocity field is regular and a large scale velocity gradient, consistent with systematic rotation, is seen across the galaxy. From the rotation curve Fig. 3 the ratio of the dynamical mass to the blue luminosity is Mdyn /LB 237!.


Gas rich galaxies from the FIGGS survey

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Fig. 2. The GMRT moment 1 image of (velocity field) And IV (at 26" resolution).

Fig. 3. Rotation curve of AndIV as derived from the GMRT data.

These very large dynamical mass to blue luminosity ratios naturally lead one to ask whether extremely gas rich dwarf galaxies have abnormally small baryon fractions, i.e. have they just been inefficient at forming stars, or did they end up with less than the typical baryon fraction? The ratio of baryonic to dark matter is expected to systematically vary with halo mass, since small halos are both inefficient at capturing hot baryons (for e.g. during the epoch of reionization) and also because small halos are less able to prevent energy input from star bursts from leading to escape of baryons (see e.g. Gnedin et al. 2002). In Fig. 4 we show the baryon fraction (as determined at the last measured point of the rotation curve) for a sample of galaxies with well measured rotation curves. The average cosmic baryon fraction is shown as a


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horizontal line. As can be seen, there is a large scatter in baryon fraction, and there is no systematic trend for a lower baryon fraction in smaller galaxies. In particular although gas rich galaxies (shown as solid points in the figure), have somewhat extreme baryon fractions, they lie within the range of that observed for galaxies in general. As such these galaxies have got their "fair share" of baryons, but for some reason have been unable to convert them into stars.

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Cosmic Baryon Fraction

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-1.5 -12 -14 -16 M_B -18 -20 -22

Fig. 4. Baryon fraction (within the last measured p oint of the a function of blue luminosity for a sample of galaxies with well curves. The cosmic baryon fraction is shown as a horizontal l galaxies, viz. DDO154, NGC3741, ESO 215 G? 009 (Warren et IV are shown as solid p oints.

rotation curve) as measured rotation ine. Four gas rich al. 2004) and And

References
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Gas rich galaxies from the FIGGS survey 12. K. Y. Lo, W. L. Sargent, K. Young: AJ, 106, 507 (1993) 13. B. E. Warren, H. Jerjen, B. S. Koribalski: AJ, 128, 1152 (2004)

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