2 Samples and observations

 

 

Table 1: Details of spectroscopic observing runs

Run	Dates of Observations	Telescope	Wavelength	$\Delta\lambda$	Res	Res
Code			Range ( $\mbox{\AA}$)	( $\mbox{\AA}$/pix)	( $\mbox{\AA}$)	(km s-1)
S1	28/06/93 - 04/07/93	MDM 2.4 m	4290 - 6780	2.43	4.7	110
S2	16/06/94 - 21/06/94	MDM 2.4 m	4440 - 6730	2.24	3.9	93
S3	11/09/94 - 18/09/94	MDM 2.4 m	4675 - 7035	2.31	5.0	118
S4	25/09/94 - 27/09/94	MMT 4.4 m	5000 - 5970	0.81	1.6	40
S5	21/10/94 - 23/10/94	MDM 2.4 m	4295 - 7165	2.81	5.6	133
S6	09/03/95 - 14/03/95	MDM 2.4 m	4305 - 6540	2.19	4.4	103
S7	29/05/95	MDM 1.3 m	4000 - 7100	3.04	5.6	132
S8	05/06/95 - 11/06/95	MDM 2.4 m	4320 - 6565	2.20	4.3	102
S9	01/09/95 - 04/09/95	MDM 2.4 m	4320 - 6565	2.20	4.3	102

 

 

Table 2: CCD and instrument parameters for spectroscopy

	Run Code
	---------------------------------------------------------------------
Parameter	S1, S2, S3,	S4	S5	S7
	S6, S8, S9
Telescope	MDM 2.4 m	MMT 4.4 m	MDM 2.4 m	MDM 1.3 m
Spectrograph	Mark III	Red Channel	Mark III	Mark III
Grating/grism (l/mm)	grism (600)	grating (1200)	grism (600)	grism (600)
Blaze ( $\mbox{\AA}$)	5800	5750	5800	4600
Slit width (arcsec)	1.68	1.00	1.68	2.17
Detector	Tek CCD	Loral CCD	Loral CCD	Loral CCD
Format	$1024 \times 1024$	$800 \times 1200$	$2048 \times 2048$	$2048 \times 2048$
Binning	$1 \times 1$	$2 \times 1$	$2 \times 2$	$2 \times 2$
Readout Noise (e-)	6.0	7.0	5.4	5.4
Gain (e-/ADU)	3.45	2.60	1.94	1.94
Pixel size ($\mu$m)	24	15	30	30
Spatial scale (arcsec/pixel)	0.777	0.300	0.971	1.794

Sample galaxies were selected from the new Automatic Plate Measuring Facility (APM) South Equatorial Strip Catalog, which was made available prior to publication (Raychaudhury & Lynden-Bell 2000). The South Equatorial Strip Catalog ( $-17\hbox{$.\!\!^\circ$ }5 < \delta < +2\hbox{$.\!\!^\circ$ }5$) is an uncharted region in the velocity field, because previously no good galaxy catalog existed for this region, and consequently the peculiar motions of galaxies in this strip had never been mapped. Redshifts had been previously measured for only about 20 of the sample galaxies, and velocity dispersion data existed for only a few of the sample galaxies.

The sample consists of early-type galaxies in three selected directions: (1) 15h10-16h10; (2) 20h30-21h50; (3) 00h10-01h30. The first region is about 20$^{\circ}$ from the direction of the LP bulk flow, the second region is almost perpendicular to the first, and the third is on the opposite side of the sky from the first, close to the direction of the Perseus-Pisces region and the South Galactic Pole. The APM South Equatorial Strip Catalog has a magnitude limit of b_j = 17.0 mag, which corresponds to 15.05 mag in Kron-Cousins R. All candidate galaxies in the three regions were examined on the POSS plates and then on CCD images to verify the morphological type. The original intention was to observe all galaxies down to this magnitude limit. We have both photometric and spectroscopic data for 179 of these galaxies, which resulted in a sample of E/S0 galaxies virtually complete to R = 14.0. The completeness drops for fainter magnitudes; galaxies down to R = 15.0 are included. New observations were carried out for all 179 galaxies, so that the sample has a fully independent data set with homogeneous observations, uniform data reduction, and consistent measurement techniques for all the galaxies.

In addition to the galaxies in the three sample regions, a number of standard galaxies from previous studies were also observed, for comparison purposes, in order to confirm the accuracy of the spectroscopic parameters obtained in this work. Comparison galaxies were selected from the following samples in the literature: Davies et al. (1987); González (1993); McElroy (1995); and J$\o$rgensen et al. (1995). Spectra were also obtained for a sample of 40 galaxies in the Coma cluster, which was used as the calibration cluster for the FP distance-indicator relation. Galaxies were chosen to be E/S0 using earlier studies (e.g. Lucey et al. 1991; J$\o$rgensen et al. 1993), and most lie within about $0.5^{\circ}$ of the point midway between NGC 4889 and NGC 4874.

Spectroscopic observations were done at the 2.4 m Hiltner telescope and the 1.3 m McGraw-Hill telescope of the Michigan-Dartmouth-M.I.T. (MDM) Observatory on Kitt Peak, Arizona, and also at the 4.4 m Multiple Mirror Telescope (MMT) on Mount Hopkins, Arizona. Data were collected during a series of nine observing runs between June 1993 and September 1995. Over 260 galaxy spectra were obtained. In total, 38 nights were allocated for this project, and 25 of them were usable. The main details of the observing runs are summarized in Table 1. (The resolution in $\mbox{\AA}$ given in Table 1 is the instrumental resolution, found from the FWHM of arc lines, whereas the resolution in km s^-1 is the spectrograph resolution or instrumental dispersion.) The spectral range was chosen to cover the Mgb band (around $\lambda_0$ = 5177 $\mbox{\AA}$), the E-band (5270 $\mbox{\AA}$) and the Fe I line (5335 $\mbox{\AA}$). For some runs the H$\beta$ (4861 $\mbox{\AA}$) and Na D (5895 $\mbox{\AA}$) features were also included. All observations were made with a long slit and a CCD detector. The setup and instrument parameters for all runs are shown in Table 2.

All observations on the two telescopes at MDM were made with f/7.5 and using the Mark III spectrograph, which consists of a grism, glass optics, and a CCD detector. Two different detectors were used with the MDM telescopes: the Tektronix TK1024A 1024² CCD, which is a thinned, back-illuminated CCD with a pixel size of 24 $\mu$m, and the Loral 2048² CCD, which is a thick, front-illuminated CCD with a 15 $\mu$m pixel size. Both CCDs are good cosmetically and have low readout noise. For the observations at the MMT we used the Red Channel: a spectrograph consisting of a collimator, a folding flat, a grating, and a CCD detector. The spectrograph was used in the high-throughput long-slit mode with a slit 180 arcsec long. The detector used at the MMT was a Loral $800 \times 1200$ CCD binned by 2 pixels in the narrower spatial direction (perpendicular to the dispersion). The CCD is very good cosmetically, with only a few traps.

The slit was usually oriented with the long axis running North-South. The instrumental resolution FWHM (in pixels and in $\mbox{\AA}$) was determined by fitting a Gaussian to measure the widths of lines in calibration lamp spectra and of night-sky lines. Each galaxy was observed with a sufficiently long exposure time to ensure a high enough signal-to-noise ratio ($S/N \sim$ 20) to enable the central velocity dispersion, as well as the redshift of the galaxy, to be determined accurately from the spectrum. At the MDM 2.4 m telescope, the average integration time needed was 2400 s; the integration times for individual galaxies ranged from 900 s to $2 \times 3600$ s according to the magnitude and surface brightness of the galaxy and the observing conditions. At the MMT, the average integration time per galaxy was 1200 s. A total of 263 spectra were obtained, of 238 galaxies. For some sample galaxies, more than one spectrum was obtained; an observation was repeated in some cases to provide a way to check the accuracy. Spectra were also obtained for KPNO IIDS spectrophotometric flux standard stars (Strom 1979), for use in flux calibration of the spectra. The exposure time was usually 360 s, and one flux standard star was observed per night.

 

 

Table 3: Radial velocity standard stars and template stars

Star	Spectral	S/N	Radial Velocity
	Type	per $\mbox{\AA}$	(km s-1)
HD  4388	K3 III	62	-28.3
HD 12029	K2 III	350	+38.6
HD 20893	K3 III	110	+5.9
HD 22072	DG7	120	+12.2
HD 36003	K0	106	-58.2
HD 38751	G8 III	220	+15.7
HD 51440	K2 III	117	+27.1
HD 64606	G5	137	+93.8
HD 65934	K0	130	+35.0
HD 72324	G9 III	68	+75.2
HD 73665	K0 III	120	+36.9
HD 74377	K0	84	-25.4
HD 90861	K2 III	140	+36.3
HD 132737	K0 III	110	-24.1
HD 165195	G5	50	-0.2
HD 171232	G8 III	142	-35.9
HD 172401	-	102	-73.0
HD 194071	G8 III	127	-9.8
HD 213947	K4 III	280	+16.7
HD 223094	K5 III	375	+19.6

In addition, several stars with known radial velocities were observed during each run. (Values of radial velocity for the standard stars were taken from Wilson 1953; Evans 1970; Abt & Biggs 1972; and Barbier-Brossat & Petit 1987.) These radial velocity standard stars were chosen to be of luminosity class III and in the range of spectral types G5 - K5. Most were K giants fainter than 7th magnitude. The observed stars are listed in Table 3. The same set of stars served as spectral templates for the determination of the velocity dispersions of the galaxies and as radial velocity standards for finding the galaxy redshifts. All the standard stars were trailed up and down the slit so that the slit illumination would be uniform and more similar to that of a galaxy. The exposure time was normally 420 s, which usually resulted in a very high S/N ratio.

Before and after the spectrum of each galaxy or standard star, a comparison spectrum was taken for use during wavelength calibrations. At MDM, Hg + Ne and Ne lamps were exposed for 0.2 s, and at the MMT He + Ne + Cu + Ar lamps were exposed for 120 s. Each night a series of bias frames was taken, as well as spectral flat field frames using an internal continuum flat lamp. Exposure times for flat fields were 30 s at MDM and 2 s at the MMT.