III. Environment and star formation

An additional galaxy property which influences star formation has been shown to be the density of the environment in which the galaxy lives. Though the community is in  agreement as to what the additional parameter is, its effects are still a bit unclear.  There are now a large number of papers devoted to understanding the affects of galaxy environment on SFR (e.g. Moss and Whittle 1993, Vilchez  1995, Maia et al.  1994, Balogh et al.  1998, Gavazzi et al. 1998).   These studies do not agree on how environment affects star formation.  While some surveys show that spirals in clusters show reduced star formation (e.g.. Balogh et al.  1998, see  paper 1), some show no difference between the two populations (e.g.. Gavazzi et al. 1998), and some show increased star formation in cluster spirals (e.g.. Maia et al. 1994).   Each method is sensitive to a different type of star formation, which may explain why the results are different.  The studies which show reduced star formation in cluster galaxies tend to be more sensitive to widespread star formation over the whole disk, while those which find an increase in star formation of cluster galaxies tend to be more sensitive to central bursts of star formation.   A large scale study still needs to be done which determines  how the environment affects only one type of star formation.

Gavazzi et al.  (1998) found no significant difference between the SFR's of cluster spirals and isolated spirals when they compared the H-alpha equivalent width of galaxies in two density samples using narrow-band photometry, which is best at sampling diffuse star formation in the disk.  Figure 2 below shows the H-alpha equivalent width frequency distributionGavazzi et al. (1998).  While it does appear that the cluster galaxies tend to have lower equivalent widths, the authors did not have a sufficiently large sample to call the difference statistically significant.  Three possible explanations for this result are that environment does not affect star formation, that a significant number of their spirals thought to be members of the Coma cluster are actually field galaxies projected into the cluster, or that their sample of only ~100 galaxies in total was not enough to do accurate enough statistics to address the problem.
 

On the other hand, Maia et al. (1994) found a significant difference in two samples of different densities with a smaller number of galaxies than Gavazzi et al. (1998).  Maia et al. (1994) found that, despite the smaller number of late type spirals in clusters, the cluster galaxies' spectra showed a significantly higher median H-alpha equivalent width, as shown in figure 3.  The difference between the results of Gavazzi et al. (1998) and Maia et al. (1994) could be the difference in the type of star formation observed.    Gavazzi used narrow band H-alpha imaging which sampled the entire galaxy, while Maia's spectra only sampled the brightest portion of the galaxy.  Therefore Gavazzi was more sensitive to sustained disk star formation. and Maia was more sensitive to central bursts of star formation.  The question raised is:  is there a higher rate of central bursts of star formation in cluster spirals than those in the field?  Neither of these data sets has a large enough number of spiral galaxies with the proper data required to answer the question with certianty.
 
 
 
 
 

Moss and Whittle (1993), in a survey of  ~200 galaxies, compared both the strength and the distribution of H-alpha emission by examining images taken through a prism.  They found that, even in clusters, galaxy-galaxy interactions seem to play a more important role in producing star formation than galaxy-cluster interactions.   They found that  "the associations between H alpha emission, its distribution, and galaxy distortions, arise because of tidal perturbations in the galaxy-galaxy interactions.  These perturbations cause central bursts of star formation which we detect as compact emission.  Diffuse emission on the other hand results from normal star formation in the galaxy disk, and this may be detected whether or not the galaxy is experiencing a tidal perturbation." (Moss and Whittle  1993)

Galaxy-galaxy interactions are more common in the cluster environment, and cluster galaxies tend to have gas deficient disks (see Haynes and Giovanelli  1986).  Therefore, it makes sense that galaxy-galaxy interactions in clusters tend to cause central bursts of star formation.  The higher percentage of strong diffuse H alpha detected from the disks of spirals in the field could be explained with the same reason: that the field spirals have more gas in their disks.  This higher rate of diffuse emission in the field is exactly what Moss and Whittle (1993) found.  The problem is that this survey did not compare its findings with observations of the galaxies' gas densities to look for a correlation between gas density and SFR.  Such a correlation was expected, and looked for, but was not seen by Kennicutt in 1983 (Kennicutt and Kent  1983, Kennicutt  1983).   Their data is shown in figure 2 below.

Fig. 4.  Emission equivalent width is plotted against hydrogen content as derived from published catalogs.  Points to the lower left have large uncertainties in both parameters.  Only a very weak correlation, if any, can be seen.  (Kennicutt and Kent  1983)

The result that the gas distribution of similar galaxies in different environments brings about the differences in SFR makes sense, but is unfortunately not in agreement with previous studies.  For example, Kennicutt and Kent (1983) showed that there was actually only a very weak correlation between H I content and star forming activity, and Balogh et al. (1998) find that all star formation is reduced in cluster galaxies compared to galaxies in the field of the same morphological type.   However Balogh et al. (1998) do not attempt to determine the distribution of the star formation in the galaxies of their survey, which could be key in understanding the type of star formation triggered in the field vs. that triggered in clusters.  They simply determine the relative SFRs of galaxies from their [O II] emission line, and their morphologies by their bulge to total luminosity ratio.  In fact, their findings do show a correlation between SFR and environment (see Paper 1), but they are not consistent with the morphology-SFR relation.  The SFRs they find for intermediate galaxies (0.4 < B/T < 0.7) are less than those of bulge galaxies (B/T > 0.7).   These  trends are not consistent with the current morphology-SFR relation found by other groups, which has a rather smooth increase (though with high dispersion) in star formation from early to late type galaxies.  In fact, they may indicate that this survey suffered from exactly the problem that Kennicutt and Kent (1983) warned about: spectra undersampling the line strengths of the disks.

Succinctly, many groups have measured the effects of environment on SFR using different methods, each yielding different results.  Maia et al  (1994) find stronger H alpha emission from galaxies in clusters.  Moss and Whittle find stronger compact H alpha emission in cluster spirals, but stronger diffuse H alpha emission in isolated spirals.  Balogh et al find stronger [O II] emission in galaxies in clusters.  Kennicutt  (1983) showed that spectral determination of SFRs tends to select the compact star forming galaxies as the ones with the high SFRs because spectral measurements undersample the galaxy.  He also showed that the [O II] line strength is not as accurate a tracer of  SFR as H alpha (Kennicutt 1992).  What causes the huge discrepencies in the results of the different surveys?  Sloan may be able to shed some light on this issue.  Unlike previous surveys, Sloan will have the [O II] line strengths as well as the H alpha line strengths for the galaxies surveyed.  Sloan will also provide spectra of the brightest portion of each galaxy so that we will know the spectra are most sensitive to central bursts of star formation.

Clearly, we would like to do a study which compares two samples of galaxies.  One sample will contain a large number of galaxies covering a small range of quantitative morphological types located in clusters.  The other sample will contain a large number of galaxies of the same small range in quantitative morphological types, but located low density regions.  The two samples must have their SFRs determined in the same, systematic, spectroscopic, way, sampling only the brightest region of each galaxy.  Then we may determine the infuence that environment has on central starbursts in disk galaxies.  This type of study requires photometry of a large number of  galaxies to determine quantitative morphological type, and spectra of the brightest region in each of those galaxies to determine the frequency of star formation bursts.  I think this type of study will be possible using Sloan data.