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9. IRREGULAR GALAXIES

9.1. Very Large HI Envelopes

Irregular galaxies can be intriguing, as is shown by observations of the giant magellanic irregular galaxy NGC 4449, whose stellar mass is 1.8 × 109 M (Muñoz-Mateos et al. 2015), i.e., 89% of that of NGC 300. A large Hi envelope around this galaxy had already been discovered by van Woerden et al. (1975), using the 100 m Effelsberg telescope. A 3 × 3-point mosaic with the VLA of this galaxy has been made by Hunter et al. (1998). The small irregular galaxy DDO 125 is present as the southernmost blob in the images in Fig. 15. A further faint dwarf, or rather an extended stellar stream, discussed more recently by Rich et al. (2012); Martínez-Delgado et al. (2012) and Toloba et al. (2016), is indicated by a cross in the Hi image: there is no immediate connection between this object and the Hi features. The outer envelope shows a regular velocity gradient from north to south, while the body of the main galaxy seen in the visible light shows a regular velocity gradient at a position angle of ∼60 in the opposite sense. Lelli et al. (2014) remark that if for NGC 4449 the rotation velocity of the outer envelope is used this galaxy falls on the baryonic Tully-Fisher relation. This suggests that it is the outer envelope which traces the dark halo, and that the inner parts, including the visible galaxy, are not yet settled.

Figure 15

Figure 15. Top: Hi image (left) and velocity field (right) of the Magellanic irregular galaxy NGC 4449 (adapted with permission from Hunter et al. 1998). The southernmost blob is the companion galaxy DDO 125. The cross is the position of the stellar stream as given in Rich et al. (2012). Bottom: details of the central parts of this galaxy (reproduced with permission from Lelli et al. 2014): a V-band optical image (left), the Hi image (centre), and a position-velocity diagram (right) at a position angle of 60 along the green line indicated in the Hi image showing the inversion of the line-of-sight velocities in the inner parts

Galaxies with such large Hi envelopes (more than five times larger than the optical radius) are relatively rare, and only a handful of other cases have been discussed. Blue compact dwarfs can sometimes be surprisingly large in Hi, as is the case for NGC 2915, which has a relatively regular Hi disk (cf. Meurer et al. 1996; Elson et al. 2010; Elson et al. 2011a; b, see Fig. 16, upper panels), UGC 5288 (van Zee 2004), NGC 3741 (Begum et al. 2005), ADBS 113845+2008 (Cannon et al. 2009), IZw18 (Lelli et al. 2014), and IIZw40 (Brinks and Klein 1988). Note that DDO 154 also has a very large Hi size compared to its optical size (Carignan and Freeman 1988; de Blok et al. 2008).

Figure 16

Figure 16. Top: Hi image (left) and velocity field (right) of the blue compact dwarf NGC 2915, made from data kindly supplied by E. Elson, as reported in Elson et al. (2010). The 3.6 µm image in the middle is on the same scale. Middle: Hi image (left) and velocity field (right) of NGC 4214, from the THINGS project, and a 3.6 µm image. Bottom: Hi image (left) and velocity field (right) of Holmberg II, from the ANGST project, and a 3.6 µm image. The THINGS, ANGST and 3.6 µm data were downloaded from the NED.

Some galaxies look a bit surprising, such as NGC 4214, shown in Fig. 16 (middle panels). If the Hi is in a circular disk seen in projection, there is a large misalignment of ∼ 55 between the position angle of its major axis, and the one derived from the velocity field. Lelli et al. (2014) fit a tilted ring model to the observations of this galaxy, and derive a variable inclination, which tends to 0 in the outer parts. This seems hard to square with the apparent axial ratio of the Hi distribution.

Also shown in Fig. 16 (lower panels) is the galaxy Holmberg II, imaged already by Puche et al. (1992), and later in the THINGS and VLA-ANGST projects. From the velocity field, a clear warp can be inferred, and the Hi image also shows a tail towards the west side. Nevertheless, several analyses have been performed on this galaxy. One of the striking aspects in the Hi distribution is the presence of holes, also discussed for bright galaxies, such as M101 by van der Hulst and Sancisi (1988), and NGC 6946 by Boomsma et al. (2008). Puche et al. (1992) remark that Hi holes in late-type dwarf galaxies are larger than the Hi holes in large spiral galaxies.

9.2. Velocity Dispersions in Dwarf Irregular Galaxies

A number of recent surveys concern almost uniquely dwarf irregular galaxies. In particular, the ANGST survey (Ott et al. 2012) has observed or re-observed a number a dwarfs with the VLA at high spectral resolution. An extensive discussion of Hi gas velocity dispersions based on these observations is given in Stilp et al. (2013a, b). The dispersions have again been calculated on the basis of “super-profiles”, derived after derotating the data cube and stacking the individual profiles. The general shape of the profiles can be described by a double Gaussian, with a narrow centre of order 7.2 − 8.5 km/s and a wider wing of order 20 − 25 km/s. The latter is presumably due to the influence of star formation, while the former is attributed to turbulence. Stilp et al. (2013a) do not think it is possible to discriminate between the “cold neutral medium” (CNM) and the “warm neutral medium” (WNM), as done by Ianjamasimanana et al. (2012) for the THINGS data, since the typical velocity dispersions expected for the typical temperatures associated with these do not match. They also argue that the data cubes with robust weighting should be used, and that the signal-to-noise threshold matters.

Interestingly, thicknesses of the gas layer have been derived, based on a hydrostatic equilibrium used already in Ott et al. (2001) and even before that by Puche et al. (1992). Banerjee et al. (2011) studied the flaring of the gas layer in four of the THINGS dwarf galaxies, again using the hydrostatic equilibrium approach. For DDO 154, the disk flares to ∼ 1 kpc at a radius of ∼ 5 kpc. This is to be compared with an overall thickness derived by Stilp et al. (2013b) of 708 ± 139 pc based on the method discussed by Ott et al. (2001), and a thickness of 650 pc calculated by Angus et al. (2012) to get a more or less acceptable MOND fit (cf. Sect. 8). Very recently, Johnson et al. (2017) also argue that the Hi disks in galaxies in the LITTLE THINGS survey are thick, rather than thin.

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