The star forming galaxy, M94, was observed in four filters, V, I, Hcontinuum (Hc), and H. From these observations a star formation rate and V-I color profile were produced and presented. These observations where able to accurately measure the color out to ~110" and provide star formation rates with ~16% accuracy. Some attempts at correcting the H
line is still in our passband), at low galactic extinction (E(B-V)=0.018 taken from NED (Nasa/Ipac Extagalactic Database)), and it is a very bright object (mag=8.99 from NED).
For a long time, H line emission has been used as a tracer of star formation rates (SFR). The flux in the H line can be measured by counting the number of ionizing photons given by, 
where h is Planck's Constant, v is frequency, and Lv is monochromatic luminosity. Then, by assuming Case B recombination (is the low density limit) and ionization equilibrium, the emitted H becomes 
where 
are the effective recombination coefficients for H and Case B recombination, respectively (Osterbrock 1974). In 1983, Kennicutt & Kent adopted an initial mass function (IMF) and produced an equation for the SFR which is directly proportional to L(H).
Telesco & Gezari (1992) point out the importance of dust in star formation regions. Dust, if profound in M94, will have the effect of decreasing the L(H) and cause measurements of the SFR to be correspondingly too low. For this reason, I adopt a method for correcting the L(H) by Scoville et al. (2001) where they found:
where 
and 
(Tomita et al. 2000). At the redshift of M94, z=0.001027 (NED), the doublet of [NII] contaminates the H line flux.
This study, in addition to demonstrating the power of a guide star, is able to illustrate the telescope's ability to find detail structure and morphology on small angular scales. The study assumes H0=70 km/s/Mpc and q0=0.
on May 16, 2002 UT, using the 0.25 meter Great Ohio Telescope (GOT). Both nights enjoyed the most transparent skies possible. Table 1 is a log of observations taken.
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Table 1. Log of observations |
filter using F(H)=9.11x10-10 erg s-1cm-2 from Lame & Pogge (1994).
Over the two nights, the CCD camera was maintained at a comfortable -18 and -10 degrees Celsius, respectively. The GOT has a gain=2.9 electrons/ADU, readnoise=11.8 electrons RMS, and a plate scale=0.7 "/pix. The GOT's camera was mounted upside-down the first night and right-side-up the second. On the second night, an abnormal gradient across the chip was discovered, possibly due to ice on the CCD camera window; this may have contaminated the flat-field images (there was no detectable gradient in the processed images).
As previously mentioned, several stars from the Landolt catalogue where taken for color calibration; and, due to strange point-spread functions of those images IRAF was unable to produce a color calibration. Thus, I wrote routines in IDL (Interactive Data Language) to analyze the data. These routines were also employed to calibrate the H filter with the M57 data. Below, figure 1 displays the color calibration fits.
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| Figure 1a. V and I, Figure 1b. H calibration data.
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As on the two different nights the camera was in two different orientations, care was needed when registering the H and V-I images. I carefully identified a feature common to both, then centered the frames on this feature, found a star in the field (that I was sure in all frames) and calculated the angle of rotation around the feature until the star overlapped in all images. In order to produce an H line emission map, it was necessary to scale the Hc image down to the level of the H. This was done by finding the brightest pixel in a star that was common to H and Hc and calculating that ratio. I slightly tinkered with this number until, in the Hc subtracted image, the star values where ~0.0. I finally settled on Hc/H=3.63; it is on the order of the ratio of the FWHM of the filters.
line emission, a large envelope of blue V-I color, and an H line emitting bulge. Figure 2 is the combined, background subtracted, and smoothed false two color composite image.
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Figure 2. False two color composite in V and I. Note: for all images North is up and East is left. |
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Figure 3. V-I color profile. |
Color analysis was not the primary goal of the project, just an exciting side note. The H measurements were the heart and soul of the project. Kennicutt et al. (1983) assumed an IMF of:
luminosity to reduce to a single constant:
line emission map is displayed as figure 4.
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Figure 4. H+[NII] line emission.
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+[NII] emission is much more prevalant to the west. Also, within the bulge there is an H+[NII] line emitting component. This perhaps suggests some nuclear star formation. It is often believed that bulges are dominated by old stellar populations; figure 4 suggests M94 is different. There is more structure and noticeable features when the H+[NII] is overplotted on the V-I color map, see figure 5.
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Figure 5. V-I in grayscale with H+[NII] line emission contours overplotted (in arbitrary units).
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+[NII] emission ring; many filamental structures coming from the bulge are also visible. I measured the F(H) by adding the counts in a circular aperture of radius 65.1" centered on (0,0) (see figure 4) and calibrated it according to figure 1b. I adopt [NII]/H=0.5 from Lehnert & Heckman (1994). As it was difficult to know the color of the background stellar population, so assumptions for (V-I)intrinsic were based on the expected shape of the color profile. I assume (V-I)intrinsic=1.35 for all radii. Propagating the photon errors, I find L(H)=2.9+/-0.4x107Lsun which corresponds to an:
) agrees reasonably well with the published values. Kennicutt & Bell (2001) found for M94 that L(H)=1.74x107Lsun (these values differ by 50%). This difference (as it is beyond the scope of my error bars) suggests a systematic problem.
Most of the difficulty arises when trying assume the dust distribution. Typically, dust is most highly concentrated in regions of high star formation. Even though the dust tends to redden, the underlying stellar population is too blue to see a noticable reddening effect. The observation was slightly sensitive to the adopted (V-I)intrinsic color. Ultimately, the dust was determined to be negligible as the galaxy is face on. Another major systematic problem was the inability to well calibrate the photometry; figure 1 illustrates this difficulty and the scatter in the calibration data.
Despite the errors and difficulty in reddening corrections, the determined SFR was accurate and close to the published values. Furthermore, this study helps to push the knowledge of color as a function of radius and morphology.