Newsletter

Spiral Galaxy Morphology and the Link with Rotation Curve Type

If such a correlation does exist, it would provide a physical basis for this classification scheme. The Hubble classification scheme, however, breaks down in the near-infrared, where it has been shown that neither spiral arm pitch angle nor K band bulge-to-disk ratio correlate well with Hubble type.

Figure 2 - Rotation curve type (1=declining, 2=flat, 3=increasing) versus spiral arm pitch angle.

Figure 3 - Shear rate (A/omega) versus spiral arm pitch angle.

We have therefore investigated the properties of arm morphology in images of spiral galaxies (e.g. pitch angle) and dynamical properties from their rotation curves (e.g. shear rate) in order to determine if there is a physical basis for the dust-penetrated class.

We observed 8 galaxies at UKIRT with UFTI in the K-band to a depth of 21.5 magnitudes per square arcsec (S/N=3), between August 1st and 4th 2001. The galaxy sample was selected from a large sample of galaxies with measured H-alpha rotation curves. We selected galaxies from this sample using the following criteria: major axis, a < 1.5 arcmin, the ratio of the major to the minor-axis, a/b < 3.0 and declination, d > -20°. Figure 1 shows two example galaxies from the sample.

The galaxies are first de-projected to a face-on orientation, by orientating the major axis vertically and stretching the minor axis until it is the same as the major axis. This method therefore assumes zero intrinsic ellipticity in spiral galaxies.

Spiral arm pitch angle is then measured using a Fast Fourier Transform (FFT) technique developed by Ivanio Puerari (see Schröder et al. 1994, A&AS, 108, 41, for an in-depth discussion of this method).

The shear rate is measured from the rotation curves, from which the first Oort constant, A, and the rotational velocity, omega, can be easily calculated, and thus the shear rate is measured from their ratio.

Figure 2 shows a correlation between rotation curve type and pitch angle. Figure 3 shows a correlation between shear rate and pitch angle. This implies that the dust-penetrated classification scheme has a physical basis, which depends on the link between arm morphology and disk dynamics. However, these correlations are only 94% significant and the errors are ~20%. We therefore need more (deeper) data, and we recently acquired more time to observe galaxies to a depth of K=21.5 with S/N=5. These data will hopefully be acquired in March this of year.

In the Autumn 2001 UKIRT Newsletter, there was an article on Sakurai's Object. This unique star was discovered by a Japanese amateur astronomer in 1995 when it brightened dramatically in the optical. It appeared to be a star going through some astronomically short-lived evolutionary phase and was "re-born" in what appeared to be a nova. UKIRT observations covering 1-2.5µm, taken over the period 1996 to 2001, show quite dramatic changes in the near-IR spectrum.

Astronomy is always an amazing science in terms of its excitement and the unknown, and we report here on the next star likely to be following in the footsteps of Sakurai's Object. V445 Puppis was also discovered by a Japanese amateur astronomer in late 2000 when the "nova" reached an optical visual magnitude near 9. The object, if a typical classical nova, would be classified as a very slow nova based on its time scale for optical fading.

During October 2001, one of the authors (SBH) and graduate student Mark Huber were observing with CGS4 on UKIRT. They obtained spectra of V445 Pup as part of an observational campaign to understand the evolution of this star. We were uncertain as to the chances of collecting spectra as, just that month, the optical magnitude of V445 Pup was reported to be fainter than I=19.5.

Figure 1 - CGS4 and Michelle spectra of V445 Pup.

Figure 1 - CGS3 (upper) and Michelle (lower) spectra of the reddened field star Elias 3-16. The horizontal scale is wavelength in microns; the vertical scale isan arbitrary flux scale.

CGS3, the previous UKIRT mid-infrared instrument, provided much of the available ground-based data on the silicates formed in outflows from late-type stars [7] and in bright YSOs [8,9,10]. But it lacked the sensitivity required to give signal to noise high enough to allow the presence or absence of fine structure in other environments to be conclusively demonstrated [1,2].

The arrival of Michelle makes such investigations possible, and we have begun to exploit its capabilities. In January 2002, we used Michelle's low-N grating to obtain moderate-resolution spectra of the silicate band in YSOs and field stars in the field of the Taurus molecular cloud, one half of a survey directed at the detailed differences in fine structure between the molecular cloud and diffuse-ISM environments. The instrument worked extremely well, producing excellent spectra of objects which had previously been quite out of reach. As an example, Figure 1 compares the best previous spectrum of the field star Elias 16 (obtained in 1993 by the authors with CGS3) with some of the data obtained this January. The spectra look similar, but it is impressive to note that the upper spectrum took a total of 4.5 hours of exposure time with CGS3, while the lower one required just 20 minutes with Michelle. It is much better sampled and already competes well in terms of signal-to-noise. At this early stage of the data reduction, we have only included a small fraction of the available pairs in the new spectrum. Combining this sensitivity and the flexibility of many different spectral resolutions, Michelle can be expected to contribute fundamentally important information on silicate mineralogy.