We present deep near-infrared K_s-band AAT IRIS2 observations of a selected sample of nearby barred spiral galaxies, including some with the strongest known bars. The sample covers a range of Hubble types from SB0- to SBc. The goal is to determine if the torque strengths of the spirals correlate with those of the bars, which might be expected if the bars actually drive the spirals as has been predicted by theoretical studies. This issue has implications for interpreting bar and spiral fractions at high redshift. Analysis of previous samples suggested that such a correlation exists in the near-infrared, where effects of extinction and star formation are less important. However, the earlier samples had only a few excessively strong bars. Our new sample largely confirms our previous studies, but still any correlation is relatively weak. We find two galaxies, NGC 7513 and UGC 10862, where there is a only a weak spiral in the presence of a very strong bar. We suggest that some spirals probably are driven by their bars at the same pattern speed, but that this may be only when the bar is growing or if there is abundant gas and dissipation.
Deep Dive into Do Bars Drive Spiral Density Waves?.
We present deep near-infrared K_s-band AAT IRIS2 observations of a selected sample of nearby barred spiral galaxies, including some with the strongest known bars. The sample covers a range of Hubble types from SB0- to SBc. The goal is to determine if the torque strengths of the spirals correlate with those of the bars, which might be expected if the bars actually drive the spirals as has been predicted by theoretical studies. This issue has implications for interpreting bar and spiral fractions at high redshift. Analysis of previous samples suggested that such a correlation exists in the near-infrared, where effects of extinction and star formation are less important. However, the earlier samples had only a few excessively strong bars. Our new sample largely confirms our previous studies, but still any correlation is relatively weak. We find two galaxies, NGC 7513 and UGC 10862, where there is a only a weak spiral in the presence of a very strong bar. We suggest that some spirals proba
The bar phenomenon is a pervasive and complex aspect of disk galaxies. A bar can be identified in ∼60% or more of present-epoch disk galaxies (Knapen et al. 2000;Laurikainen et al. 2004;Menendez-Delmestre et al. 2007;Marinova & Jogee 2007). Studies of galaxies in the GEMS and GOODS fields suggest that this fraction has been largely constant to at least z=1 (Elmegreen et al. 2004;Jogee et al. 2004). Results from a larger sample in the COSMOS field indicates that the bar fraction is approximately constant out to z=0.84 for the most massive galaxies only, and that smaller and less massive galaxies have a significantly declining bar fraction out to that redshift (Sheth et al. 2008). There is also a slight correlation between the presence of a bar and the presence of a prominent bulge among the high redshift galaxies; this is consistent with the massive galaxies having a constant bar fraction, since those galaxies tend to have a bulge (Sheth et al. 2008). Another issue is the effect of environment on bar fraction. Verley et al. (2007) showed that in a sample of isolated galaxies, a comparable fraction is barred as in samples not selected for isolation. Isolated barred galaxies also were found to have a comparable distribution of bar strengths to a non-isolation-selected sample.
An important question is how the strength of a bar impacts the features seen in a barred galaxy. We are particularly interested in the relation between the strength of a bar and the appearance or strength of a spiral. Is there a correlation between bar strength and spiral arm strength, as suggested by theoretical models? For example, Yuan & Kuo (1997; see also Kormendy & Norman 1979;Elmegreen & Elmegreen 1985) showed that stronger bars excited sharper gaseous density waves than weaker bars, although other parameters also affected the appearance of the waves. The fact that some strong observed bars join to a strong two-armed global spiral suggests that the bars and spirals are closely connected and that a bar strength-spiral strength correlation may be present. These global spirals are so tightly connected to the bar that it would seem the two features have the same pattern speed. Two-armed spirals around strong bars are rather common, representing ≈70% of typical field spirals, unlike non-barred field spirals where only ≈30% are two-armed (Elmegreen and Elmegreen 1982). We consider this bar-spiral correlation as evidence for interaction between the bar and the spiral, but do not know the nature of the interaction.
It could be through various resonances, for example, and the exact resonances would determine the ratio of pattern speeds.
On the other hand, many bars are not connected to global two-armed spirals. There are bars with flocculent blue arms around them, galaxies with tiny bars and long irregular (swing amplified?) types of spirals around them, multiple-armed patterns, and old bars (SB0) with no spiral around them. It is clear that there is a wide variety in bar-disk interactions that do not include driving. There are no complete theoretical models which examine bar-driven density waves that consider both gas and stars.
We suspect that bars may drive spirals only when (a) the bar is young and growing in strength itself, or (b) there is ample gas in the bar-spiral system. Each of these situations provides an “arrow of time” for the spiral to know whether to be leading or trailing (Lynden-Bell & Ostriker 1967). Dissipation, growth, and interactions provide this but a steady state does not (e.g., Toomre 1969Toomre , 1981)). Elmegreen & Elmegreen (1985) suggested that strong bars can grow to extend all the way to corotation and organize the gas clouds along strong outer spiral shocks. The issue of whether bars drive spirals is fundamental to our understanding of galaxy evolution because a close connection between bars and spirals should manifest itself in the fractions of such features seen at high z.
Observationally, one way to evaluate these ideas is to use near-infrared K s -band images to infer the gravitational potential due to the dominant stellar backbone of galaxies. With such potentials, we can derive the relative importance of tangential forces due to bars and spirals. Near-infrared imaging is a necessity because optical images are confused by dust and star formation, whereas the K s -band emphasizes the mass distribution in the old disk (e.g., Block & Wainscoat 1991;Regan & Elmegreen 1997;Block et al. 1999).
In a recent study, Block et al. (2004) analyzed 2.2µm images of 17 galaxies covering a range of bar strengths and Hubble types. A Fourier-based technique was used to separate the bars from their associated spirals and derive separate maximum relative torques (Buta, Block, & Knapen 2003). The bar and spiral strengths, Q b and Q s , are derived from the maximum ratio of the tangential force to the mean background radial force (Combes & Sanders 1981), as obtained after a Fourier decomposition of the galaxy image. To
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