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11.1. Halpha Imaging

The standard waveband for galaxy classification, the B-band, is sensitive enough to the extreme population I component that the degree of resolution of spiral arms into star-forming complexes is part of the classification. The B-band, however, also includes a substantial contribution from the older stellar background. One way to isolate only the star-forming regions in a galaxy is imaging in Halpha, which traces HII regions. Apart from showing the distribution of star formation (modified by extinction), Halpha imaging also traces the rate of global photoionization, which in turn directly traces the rate of formation of stars more massive than about 10Modot (Kennicutt, Tamblyn, & Congdon 1994). The initial mass function (IMF), either assumed or constrained in some way (using, for example, broadband colors), allows Halpha fluxes to be converted to the total star formation rate over all stellar masses.

Halpha imaging often shows well-organized patterns of HII regions that follow structures like spiral arms and especially rings. Images of six early-to-intermediate-type ringed galaxies are shown in Figure 36 (Crocker, Baugus, & Buta 1996). The way Halpha imaging usually works is a galaxy is imaged in or near its redshifted Halpha wavelength, and then in a nearby red continuum wavelength. The net Halpha image is the difference between the Halpha image and the scaled red continuum image. Often, the Halpha filter used is broad enough to include emission from [NII] 6548 and 6584. For each of the galaxies shown in Figure 36, the left image is the red continuum image, while the right image is the net Halpha image. Of the four barred spirals shown (NGC 1433, 7329, 6782, and 7267), three show no HII regions associated with the bar. These three (NGC 1433, 7329, and 6782) all have conspicuous inner rings which are lined with HII regions. As shown by Crocker, Baugus, & Buta (1996), the distribution of HII regions around inner rings is sensitive to the intrinsic shape of the ring. When the ring is highly elongated, the HII regions concentrate around the ends of the major axis (which coincide with the bar axis; NGC 6782), while when the ring is circular, the HII regions are more uniformly spread around the ring (NGC 7329). Inner ring shapes do not correlate well with maximum relative bar torques in a galaxy (dVA), but Grouchy et al. (2010) have shown that when local forcing is considered instead, ring shapes and bar strengths are well-correlated.

Figure 36

Figure 36. Red continuum (left) and net Halpha (right) images of six early-to-intermediate-type galaxies. The galaxies are (left to right): top row: NGC 7702, 1433; middle row: NGC 7329, 6782; bottom row: NGC 6935, 7267. From Crocker, Baugus, & Buta (1996).

The case of NGC 7267 is different in that most of its Halpha emission is concentrated in the bar. Martin & Friedli (1997) have argued that star formation along galactic bars could provide clues to gas flow in the inner regions of galaxies and the fueling of starbursts and AGN. These authors present models which suggest an evolution from a pure Halpha bar, to an Halpha bar with ionized gas in the center, to a gas-poor bar with strong nuclear or circumnuclear star formation. This suggests that the bar of NGC 7267 is younger than those in NGC 1433, 7329, and 6782.

The two other galaxies shown in Figure 36 are nonbarred or only weakly-barred. NGC 6935 is an interesting case where a nonbarred galaxy has a strong ring of star formation. Grouchy et al. (2010) found that the star formation properties of inner rings, but not the distribution of HII regions, are independent of the ring shapes and bar strengths in a small sample. The case of NGC 7702 is different from the others. This is a late S0 (type S0+) showing a very strong and largely stellar inner ring. The ring shows little ionized gas and appears to be in a quiescent phase of evolution.

11.2. Ultraviolet Imaging

The best global imaging of nearby galaxies at ultraviolet wavelengths has been obtained with the Galaxy Evolution Explorer (GALEX, Martin et al. 2005), which provided detailed images of galaxies of all types at wavelengths of 0.225 and 0.152µm. These images reveal young stars generally less than approx 100 Myr old but are affected by dust extinction. There is a strong correlation between the UV morphology and the Halpha morphology (Figure 37). Like Halpha, UV fluxes from galaxies can be used to estimate star formation rates once extinction is estimated, and are particularly sensitive to the ratio of the present to the average past star formation rate. UV imaging is an effective way of decoupling the recent star formation history of a galaxy from its overall, long-term star formation history.

Figure 37

Figure 37. Comparison of a near-UV image (0.225µm) with an Halpha image for the nearby spiral galaxy M83.

Figure 38 shows near-UV (0.225µm) images of eight galaxies over a range of Hubble types. The two earliest types, NGC 1317 and 4314 (both Sa) show a near-UV morphology dominated by a bright circumnuclear ring of star formation. The SB(r)b galaxy NGC 3351 shows a conspicuous inner ring of star formation and little emission from its bar region. In contrast, the SBc galaxy NGC 7479 shows strong near-UV emission from its bar. The late-type galaxies NGC 628 [type SA(s)c] and NGC 5474 [type SA(s)m] are typical of their types, but most interesting is NGC 4625. A key finding of GALEX was extended UV emission well beyond the optical extent of some galaxies. NGC 4625 is an example where the main optical part [type SABm] is only a small fraction of the extent of the UV disk (Gil de Paz et al. 2005).

The final object shown in Figure 38 is NGC 5253, a basic example of what de Vaucouleurs classified as I0 (section 5.3). The inner region is a bright boxy zone of star formation, and even the extended disk is prominent.

Figure 38

Figure 38. GALEX near-UV images of eight galaxies: (left to right) Top row: NGC 1317, 4314, 3351, and 7479; bottom row: NGC 628, 5474, 4625, and 5253

11.3. Atomic and Molecular Gas Morphology

Related to star formation morphologies are the distributions of atomic and molecular gas. Far from being randomly-distributed clouds of interstellar material, atomic and molecular gas morphologies can be highly organized, well-defined patterns. Recent high quality surveys have provided some of the best maps of these distributions in normal galaxies. Atomic hydrogen is mapped with the 21cm fine structure emission line, which has the advantage of not being affected by extinction and for being sufficiently optically thin in general to allow total HI masses to be derived directly from HI surface brightness maps. In addition, HI maps provide information on the kinematics and dynamics of the ISM, as well on the existence and distribution of dark matter in galaxies. Molecular hydrogen is generally mapped using the 12CO J=1-0 rotational transition at a wavelength of 2.6mm, under the assumption that CO and hydrogen mix in a roughly fixed proportion.

Although numerous maps have been made of the HI distribution in nearby galaxies, the most sophisticated and detailed survey made to date is the "The HI Nearby Galaxies Survey" (THINGS, Walter et al. 2008). The earliest surveys had shown that HI is a tracer of spiral structure in galaxies, and the THINGS provides some of the highest quality maps revealing this correlation as well as other characteristics. From a morphological point of view, HI maps tend to reveal: (1) enhanced surface brightness in star-forming features such as spiral arms, rings, and pseudorings; (2) extended gaseous disks, such that the HI extent can exceed the optical extent by several times; and (3) supernova-driven and star-formation driven, wind-blown holes.

Figure 39 shows the HI morphologies of eight THINGS galaxies ranging from the Sab galaxy NGC 4736 to the Sm galaxy DDO 50 (Holmberg II). Bright Sc galaxies like NGC 628 (M74) and NGC 5236 (M83) show HI distributions that extend well beyond the optical disks. These extended patterns can include large spirals as in NGC 628. In M81 and M83, the HI traces the optical spiral structure well. Large rings or pseudorings are seen in NGC 2841 and NGC 4258, while M81 shows an intermediate-scale ring of gas that is much less evident optically. The bright star-forming inner ring in NGC 4736 is well-defined and easily distinguished in HI, but the galaxy's diffuse stellar outer ring is more of an asymmetric spiral zone.

Figure 39

Figure 39. HI morphologies (Walter et al. 2008) of eight galaxies as compared to optical B-band morphologies. Left panels: NGC 628 (M74), NGC 4258 (M106), NGC 4736 (M94), and DDO 154. Right panels: NGC 2841, NGC 3031 (M81); NGC 5236 (M83), and DDO 50 (Ho II).

Most interesting in HI maps are the obvious holes where there appears to be a deficiency of neutral gas compared to surrounding regions. Especially large holes are seen in the HI morphology of the late-type dwarf DDO 50. These holes are thought to be regions cleared by the stellar winds and explosions of massive stars contained or once contained within them. The holes are 100pc to 2kpc in size, and have different systematic properties in early and late-type galaxies in the sense that holes may last longer in late-type dwarfs owing to the lack of serious shear due to strong differential rotation (Bagetakos et al. 2009). Holes may also be found outside the standard isophotal optical angular diameter.

The dwarf galaxy DDO 154 shown in Figure 39 has one of the largest ratios of HI to optical diameter, a factor of 6 at least according to Carignan & Purton (1998), who also estimated that 90% of the mass of the galaxy is in the form of dark matter. The HI and optical morphologies bear little resemblance to one another. Another example of strongly uncorrelated HI and optical morphologies is NGC 2915, a very gas-rich galaxy whose optical morphology is a blue compact dwarf while its at least 5 times larger HI morphology includes a prominent outer spiral with no optical counterpart (Meurer et al. 1996), leading to the concept of a purely "dark spiral." Bertin & Amorisco (2010) consider a general intepretation of such large outer HI spirals, especially those seen in galaxies like NGC 628 and NGC 6946: the spirals represent short trailing waves that carry angular momentum outwards from corotation and, at least in the gaseous component, penetrate a normal barrier at the OLR of the stellar disk pattern and go far out into the HI disk. The short trailing waves are thought to excite the global spiral arms seen in the main optical body of the galaxy (where the prominent optical spirals of NGC 628 and NGC 6946 are found). How NGC 2915 fits into this picture is unclear since the main stellar body of this galaxy is not a grand-design spiral.

Galaxies whose HI disks do not extend much beyond the optical light distribution are also of interest. NGC 4736 in Figure 39 is an example. The large, nearby ringed barred spiral NGC 1433 was found by Ryder et al. (1996) to have neutral hydrogen gas concentrated in its central area, its inner ring, and in its outer pseudoring, with no gas outside the visible disk light and a lower amount of gas in the bar region compared to the ring regions. Higdon et al. (1998) showed that in the ringed, barred spiral NGC 5850, neutral gas is concentrated in the inner ring and in the asymmetric outer arm pattern, with little or no emission detected outside this pattern. The asymmetry led Higdon et al. to propose that NGC 5850 has possibly experienced a high speed collision with nearby NGC 5846.

In galaxy clusters, it is well-known that environmental effects can truncate an HI disk so that it is smaller than the optical disk light. This is dramatically illustrated in the high resolution VLA HI maps of Virgo Cluster galaxies by Chung et al. (2009), who found that galaxies within 0.5 Mpc of the cluster core have severely truncated HI disks typically less than half the size of the optical standard isohotal diameter, D25. A variety of earlier studies had already shown these galaxies to be HI-deficient compared to field galaxies of similar types. As noted in section 10.2, an interaction between a cluster galaxy's ISM and the intra-cluster medium can account for these unsual modifications of HI morphology. Chung et al. also provide evidence for this interaction in some morphologies that appear to be gas stripping in progress.

The Berkeley-Illinois-Maryland Survey of Nearby Galaxies (BIMA SONG; Regan et al. 2001; Helfer et al. 2003) provided some of the highest quality CO maps of normal galaxies. CO emission is often seen in intermediate (Sab-Sd) galaxies, which have a high enough gas content and metallicity to allow the 12CO J=1-0 2.6mm emission line to be detectable. The CO distributions of eight such galaxies from this survey are shown in Figure 40. These display some of the range of molecular gas morphologies seen. CO traces the inner spiral arms of NGC 628, 1068, and 4535, and is seen along the bar of NGC 2903. A common CO morphology is a large-diameter ring of giant molecular clouds (GMCs), without a central CO concentration, as seen in NGC 2841 and 7331. The rings are the peaks of exponentially-declining distributions. The coherent inner molecular gas ring in NGC 7331 appears more like a typical resonance ring, and has an estimated molecular gas mass of 3.4 × 109 Modot (Regan et al. 2004). The CO distribution in NGC 2403 appears to be concentrated in individual GMCs, with little diffuse emission, while that in NGC 3351 is characterized by a small central bar aligned nearly perpendicular to the galaxy's primary bar. Helfer et al. (2003) show that the Milky Way, M31, and M33 have CO morphologies that are consistent with the range of morphologies found by BIMA SONG.

Figure 40

Figure 40. CO morphologies (Helfer et al. 2003) of eight galaxies as compared to optical B-band morphologies. Left panels: NGC 628 (M74), NGC 2403, NGC 2903, and NGC 4535. Right panels: NGC 1068 (M77), NGC 2841; NGC 3351 (M95), and NGC 7331.

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