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Electronographic Photometry of NGC 3379
Electronographic Photometry of NGC 3379 M. A. R. HARDWICK, A. B. HARRISON and B. L. MORGAN Astronomy Group, Blackett Laboratory, Imperial College, University of London, England
INTRODUCTION In this paper photometric data are presented for the elliptical galaxy NGC 3379. The results were derived from electronographic exposures of the galaxy which were scanned with an automated microdensitometer. The surface brightness profiles and integrated magnitudes in the standard V and B system are compared with those obtained photoelectrically by other observers. The ( B - V) colour profile reveals a reddened nucleus.
THEELLIPTICAL GALAXY NGC 3379 NGC 3379 is an elliptical galaxy in the Leo cluster, designated as EO by Hubble' and E l by de Vaucouleurs.' In the Morgan3 system, which takes into account integrated colours as well as morphology, it is classified as kED1, the k denoting spectral dominance of K-giants and the ED1 indicating that it is an early-type elliptical. Photoelectric observations of NGC 3379 have been made by Burkhead and K a l i n ~ w s k i ,Miller ~ and Prendergast' and de Vaucouleurs.6 Photographic observations have been made by Hubble,' Redman and Shirley' and Fish.' These studies have sought to determine accurate luminosity profiles and to derive radial colour gradients. Radial velocity studies indicate that NGC 3379 belongs to a group including at least six other galaxies. The companion galaxy NGC 3384 is a member of this group and its close proximity to NGC3379 has led Burkhead and Kalinowski to search for observational evidence of a tidal interaction between the two. They report an anomalously high U surface brightness between the two galaxies and the rapid €all-off in the (B + V) profile which they observe near their detection limit, might also indicate some physical interaction. Assuming a Hubble constant of 50 km sec-' Mpc-', NGC 3379 is at a 329
330
M. A. R. HARDWICK. A. B. HARRISON AND B. L. MORGAN
distance of 14-9Mpc. The irregular galaxy to the South-East is at a distance of 24.1Mpc and is therefore not likely to be a member of the same group.
OBSERVATIONS The observations described here were obtained using a Spectracon camera at the Newtonian focus of the Helwan Institute’s 74 in. telescope at Kottamia, Egypt. The Spectracon camera, which was designed and built at Imperial College, is shown mounted on the telescope in Fig. 1. A guiding eyepiece can be scanned over a circular field of radius 75 mm around the Spectracon. A “flip-in’’ mirror enables the object field to be located and accurately positioned on the photocathode. The Newtonian focal ratio is f/4-9, corresponding to a plate scale of 22.3 arcsec mm-’. This is readily resolved by the Spectracon and gives a blackening rate adequate for the surface photometry of the outer regions of galaxies. The Spectracon had an S-11 photocathode and the images were recorded on Ilford G-5 nuclear emulsion. Standard €? and V filters were employed. Table I lists details of the electronographs on which the results are based.
FIG. 1. The Spectracon camera mounted at the Newtonian focus of the 74 in. telescope of the Helwan Institute, Kottamia, Egypt.
ELECTRONOGRAPHIC PHOTOMETRY OF NGC 3379
331
TABLEI Observational data Plate number
Filter
Exposure
B 40 min V 20 min Seeing: 4 arcsec (visual estimate) Emulsion : Ilford G-5 Spectracon : AS-9 (S.11 photocathode) K 22e K 22f
Date 13/14 March 1978 13/14 March 1978
Figure 2 shows a reproduction of a Palomar Schmidt survey plate containing the field.
DATAREDUCTION
Preparation Two-dimensional scans of the electronographs were made using an automated Joyce-Loebl microdensitometer which measures the density at each sample point and records the result digitally on magnetic tape for subsequent computer processing. The microdensitometer scanning aperture, which defines the pixel size, was a square of side 50 k m . This
FIG. 2. Reproduction from the Palomar sky survey chart of the area around NGC 3379. The superimposed rectangle represents the image area of the photocathode.
332
M. A. R. HARDWICK. A. B. HARRISON AND B. L. MORGAN
corresponds to 1.1arcsec in the image, about one quarter of the diameter of the seeing disc during these observations. The scans were displayed as isophotal contour maps and a rectangular area around the galaxy and extending over most of the photocathode was selected for further analysis. A number of extraneous objects, such as foreground stars, were rejected. Rectangular zones enclosing them were discarded by specifying the elements in the data array corresponding to vertices of the rectangles. Pixels which fell within these “forbidden” areas were labelled and ignored in subsequent calculations. The data values of the picture points were not replaced by average values as this would have biased later statistical analysis.
Luminosity Profiles Luminosity profiles were obtained in the following manner. The pixel with the largest measured density was assumed to be the centre of the object and the distance of each pixel from this point was calculated. This measure was used to ascribe each pixel to an annulus. The computer stored the number of corresponding array elements for each annulus, the sum of the data values and the sum of the squares of the data values. From these sums the average, mean error and standard deviation of density readings in the individual annuli were computed. Since NGC 3379 is an EO or E l galaxy, the variation of surface brightness as a function of position angle was expected to be small and elements of the array in an annulus whose data values differed by more than 2.6 standard deviations from the average were rejected. This enabled small areas contaminated by dust or emulsion defects to be eliminated. The average value for each annulus was then recomputed. The density level due to the sky brightness was found by taking the weighted average density reading of a number of adjacent annuli at as large a radius as possible. This value was subtracted from the average values obtained for the individual inner annuli to yield surface brightness as a function of radius. Since the image of NGC3379 is comparable in size to the photocathode the adopted sky density value is inevitably contaminated by galaxy light. However, the effect of this small overestimation in the sky brightness is important only in the outermost regions of the galaxy. The B luminosity profile is somewhat more affected since the centre of the galaxy image falls closer to the centre df the photocathode than in the V exposure, thus requiring that the sky estimate be made at a slightly smaller radius. The contribution to the total relative luminosity from an annulus was found by multiplying the average value for that annulus by its area. The
ELECTRONOGRAPHIC PHOTOMETRY OF NGC 3379
333
luminosity within a chosen radius was determined by summing the contributions from the enclosed annuli.
Calibration No standard stars were observed for direct calibration; instead the photoelectric integrated magnitude determinations made by other observers4.'0.11 were employed. A preliminary estimate was made of the integrated magnitude within a particular radius. This was used in conjunction with the relative luminosity measured from the electronograph within the same radius to calibrate the results. Using this calibration the average difference between the electronographic and photoelectric measures was calculated. The preliminary estimate was then revised so that the average difference became zero. For the exposure taken with the V filter the mean error between the electronographic and photoelectric results is 0.02 magnitudes, and for the B filter it is 0.03 magnitudes. These errors are of a similar order or are less than those quoted for the photoelectric results. The differences between the electronographic and photoelectric measures are shown in Figs. 3 and 4.
+ A
+
A
0
0
0
100
Radius (arcsec 1
FIG. 3. Differences between photoelectric and electronographic results for integrated V magnitudes of NGC 3379 plotted as a function of radius. 0, Burkhead and Kalinowski; A, de Vaucouleurs; f, Sandage.
334
o'21
M. A. R. HARDWICK, A. B. HARRISON AND B. L. MORGAN
+
0.I
+
+
A
i
0 A
0
1
0
25
,
,
,
,
1
1
,
50
,
,
1
1
75
1
1
1
1 0
Radius ( orcsec)
FIG. 4. Differences between photoelectric and electronographic results for integrated B magnitudes of NGC 3379 plotted as a function of radius. 0, Burkhead and Kalinowski; A d e Vaucoulers: +, Sandage.
RESULTS Figures 5 and 6 show the distribution of surface brightness in V and B respectively. The crosses indicate observations, the curves are fits of the various luminosity laws to the data and are discussed below. The V and B profiles are in good agreement with the observations of Burkhead and Kalinowski4 and of Miller and Prendergast5 except at large radii where the B profile is likely to be slightly affected by galaxy light. Early attempts by R e y n o l d ~ ' ~and , ~ ~later Hubble' to fit empirical luminosity laws to the observed surface brightness distribution of galaxies gave acceptable results for the data then available. The law suggested by Reynolds and Hubble was:
where So is the central surface brightness. More recent observations have shown that this law falls off too slowly at large radii. Oem1erI4 in an attempt to overcome this difficulty has proposed a modified version of Hubble's law which introduces an exponential cut-off factor. The law
0.5
1.0
I
1
1.5
2.0
2.5
Log radius arcsec
FIG. 5 . The V surface brightness of NGC 3379 as a function of radius. Crosses are observed values, curves are the fits of luminosity laws: (1) Hubble; (2) Oemler; ( 3 ) d e Vaucouleurs; (4) King
t
28t 0.5
t 10
1.5
2.0
2.5
Log radius arcsec
FIG. 6 . The B surface brightness of N G C 3379 as a function of radius. Crosses are observed values, curves are the fits of luminosity laws: (1) Hubble; (2) Oemler: (3) d e Vaucouleurs; (4) King.
336
M. A. R. HARDWICK, A. B. HARRISON AND 0. L. MORGAN
becomes:
So exp (- r/a)’ (1 + r / P l 2 De Vaucouleurs’ has suggested a law of the form: S=
Log (SlS,) = -3.33 [(r/re)1’4- 11
(3) where re is the radius which encloses half the total luminosity and S, is the surface brightness at that radius. Kingls has proposed a law of the form: where and
S = k(l/A - l/B)*, A = [l+(r/r,)*]’/’ B = [l +(r,/r,)’]”’;
(4)
r, and r, are core and tidal radii respectively. The best fits of these laws to the observations between 15 and 200 arcsec are plotted in Figs. 5 and 6. Data points at radii less than 15 arcsec were not used in the calculation because of the poor seeing at the time of the observations; those beyond 200 arcsec were excluded because of the possible errors introduced by incorrect estimate of the sky brightness. These best fits were determined by a computer program which varied the parameters of the law being fitted until a minimum value of reduced chi-squared, a measure of the goodness of fit, was obtained. This fitting was done in linear space, i.e., to observations of intensity as a function of radius rather than in the magnitude-log radius space of Figs. 5 and 6 . It can be seen that despite the differences of form of the various laws they all closely match the observations in the range 15 to 100 arcsec. At larger radii none of the laws is markedly better than the others in describing the surface brightness profile, although Hubble’s is marginally the worst. The total magnitudes found by integrating the luminosity profiles are 9-35 in V and 10.27 in B. These results compare well with those of Burkhead and Kalinowski and of Miller and Prendergast. The B and V luminosity profiles each show a change of gradient at a radius of about 50 arcsec. This feature is also present in both Burkhead and Kalinowski’s and Miller and Prendergast’s data. Since this gradient change occurs at about the same radius in B and V, it may be of a dynamical origin. Both the integrated and local (B - V) curves are shown in Fig. 7. They display a reddened nucleus with a rapidly increasing colour gradient within a radius of 20arcsec, rising to a maximum value of 1.3 at the centre. The integrated ( B - V) profile remains close to a value of 0.92 from 120 to 200 arcsec. The local ( B - V) profile which is a more
ELECI’RONOGRAPHICPHOTOMETRY OF NGC 3379
337
“4
0
eeoo
1 0 Rodnus (arcsec)
FIG. 7. Integrated and local ( B - V) colour index as a function of radius: 0, local; -, integrated.
sensitive indicator of colour variations at larger radii, falls to a value of 0.75 at 80 arcsec. Values at greater radii are probably influenced by errors in the B sky brightness level. Larson16 has described a number of simplified models of spherical galaxies that reveal a rapidly increasing metallicity gradient in nuclear regions. For the model that provides the best overall fit to the observed photometric properties of NGC 3379, he predicts a dynamical boundary at a radius of 1.5 kpc or 21 arcsec separating “Nuclear” and “Halo” regions. The model suggests that stars in the halo region were formed during the initial evolution of the galaxy and are therefore metal-poor. In the nuclear region the continuing inflow of gas enriched with the debris of supernovae leads to the formation of metal-rich stars. Outside the nuclear region the model shows a constant metallicity and in a later paper” Larson and Tinsley predict a value of 0.95 for (B - V) in the outer region of the model.
CONCLUSIONS Integrated magnitudes and surface brightness and colour profiles have been derived from electronographs of the galaxy NGC 3379. They are found to agree well with the photoelectric results of other observers. Fits
338
M. A. R. HARDWICK, A. B. HARRISON AND B. L. MORGAN
of various luminosity laws indicate that in this case, they can all be made to match the data closely over the radius range 15 to 100arcsec. The integrated colour magnitude is in good agreement with Larson’s best models for NGC 3379. A reddened nucleus is detected coinciding with the region over which Larson predicts metal enrichment. ACKNOWLEDGMEWS The authors wish to thank the Director and staff of the Helwan Institute. Cairo, for their invaluable assistance in obtaining the observations described in this paper. A. B. H. was in grateful receipt of a Science Research Council Studentship.
REFERENCES 1. Hubble, E., Astrophys. J. 64, 321 (1962). 2. de Vaucouleurs, G. and de Vaucouleurs, A,, In “Reference Catalogue of Bright Galaxies”. Published by the University of Texas Press (1964). 3. Morgan, W. W., Publ. Astron. SOC. Pac. 71, 394 (1959). 4. Burkhead, M. S. and Kalinowski, J. K., Astron. J. 79, 835 (1974). 5 . Miller, R. H. and Prendergast, K. H., Astrophys. J. 136, 713 (1962). 6. de Vaucouleurs, G., Ann. Astrophys. 11, 247 (1948). 7. Nubble, E., Astrophys. J. 71, 231 (1930). 8. Redman, R.’O. and Shirley, E. G., Mon. Not. R. Astron. SOC.98, 613 (1938). 9. Fish, R. A., Astrophys. J. 139, 284 (1964). 10. de Vaucouleurs, G. and de Vaucouleurs, A., Mem. R. Astron. SOC. 77, 1 (1972). 11. Sandage, A. R., Astrophys. J. 183, 711 (1973). 12. Reynolds, J. H., Mon. Not. R. Astron. SOC.74, 132 (1913). 13. Reynolds, J. H., Mon. Not. R. Astron. SOC.80, 746 (1920). 14. Oemler, A,, Astrophys. J. 209, 693 (1976). 15. King, I., Astron. J. 67, 471 (1962). 16. Larson, R. B., Mon. Not. R. Astron. SOC. 166, 585 (1974). 17. Larson, R. B. and Tinsley, B. M., Astrophys. J. 192, 293 (1974).
INTRODUCTION In this paper photometric data are presented for the elliptical galaxy NGC 3379. The results were derived from electronographic exposures of the galaxy which were scanned with an automated microdensitometer. The surface brightness profiles and integrated magnitudes in the standard V and B system are compared with those obtained photoelectrically by other observers. The ( B - V) colour profile reveals a reddened nucleus.
THEELLIPTICAL GALAXY NGC 3379 NGC 3379 is an elliptical galaxy in the Leo cluster, designated as EO by Hubble' and E l by de Vaucouleurs.' In the Morgan3 system, which takes into account integrated colours as well as morphology, it is classified as kED1, the k denoting spectral dominance of K-giants and the ED1 indicating that it is an early-type elliptical. Photoelectric observations of NGC 3379 have been made by Burkhead and K a l i n ~ w s k i ,Miller ~ and Prendergast' and de Vaucouleurs.6 Photographic observations have been made by Hubble,' Redman and Shirley' and Fish.' These studies have sought to determine accurate luminosity profiles and to derive radial colour gradients. Radial velocity studies indicate that NGC 3379 belongs to a group including at least six other galaxies. The companion galaxy NGC 3384 is a member of this group and its close proximity to NGC3379 has led Burkhead and Kalinowski to search for observational evidence of a tidal interaction between the two. They report an anomalously high U surface brightness between the two galaxies and the rapid €all-off in the (B + V) profile which they observe near their detection limit, might also indicate some physical interaction. Assuming a Hubble constant of 50 km sec-' Mpc-', NGC 3379 is at a 329
330
M. A. R. HARDWICK. A. B. HARRISON AND B. L. MORGAN
distance of 14-9Mpc. The irregular galaxy to the South-East is at a distance of 24.1Mpc and is therefore not likely to be a member of the same group.
OBSERVATIONS The observations described here were obtained using a Spectracon camera at the Newtonian focus of the Helwan Institute’s 74 in. telescope at Kottamia, Egypt. The Spectracon camera, which was designed and built at Imperial College, is shown mounted on the telescope in Fig. 1. A guiding eyepiece can be scanned over a circular field of radius 75 mm around the Spectracon. A “flip-in’’ mirror enables the object field to be located and accurately positioned on the photocathode. The Newtonian focal ratio is f/4-9, corresponding to a plate scale of 22.3 arcsec mm-’. This is readily resolved by the Spectracon and gives a blackening rate adequate for the surface photometry of the outer regions of galaxies. The Spectracon had an S-11 photocathode and the images were recorded on Ilford G-5 nuclear emulsion. Standard €? and V filters were employed. Table I lists details of the electronographs on which the results are based.
FIG. 1. The Spectracon camera mounted at the Newtonian focus of the 74 in. telescope of the Helwan Institute, Kottamia, Egypt.
ELECTRONOGRAPHIC PHOTOMETRY OF NGC 3379
331
TABLEI Observational data Plate number
Filter
Exposure
B 40 min V 20 min Seeing: 4 arcsec (visual estimate) Emulsion : Ilford G-5 Spectracon : AS-9 (S.11 photocathode) K 22e K 22f
Date 13/14 March 1978 13/14 March 1978
Figure 2 shows a reproduction of a Palomar Schmidt survey plate containing the field.
DATAREDUCTION
Preparation Two-dimensional scans of the electronographs were made using an automated Joyce-Loebl microdensitometer which measures the density at each sample point and records the result digitally on magnetic tape for subsequent computer processing. The microdensitometer scanning aperture, which defines the pixel size, was a square of side 50 k m . This
FIG. 2. Reproduction from the Palomar sky survey chart of the area around NGC 3379. The superimposed rectangle represents the image area of the photocathode.
332
M. A. R. HARDWICK. A. B. HARRISON AND B. L. MORGAN
corresponds to 1.1arcsec in the image, about one quarter of the diameter of the seeing disc during these observations. The scans were displayed as isophotal contour maps and a rectangular area around the galaxy and extending over most of the photocathode was selected for further analysis. A number of extraneous objects, such as foreground stars, were rejected. Rectangular zones enclosing them were discarded by specifying the elements in the data array corresponding to vertices of the rectangles. Pixels which fell within these “forbidden” areas were labelled and ignored in subsequent calculations. The data values of the picture points were not replaced by average values as this would have biased later statistical analysis.
Luminosity Profiles Luminosity profiles were obtained in the following manner. The pixel with the largest measured density was assumed to be the centre of the object and the distance of each pixel from this point was calculated. This measure was used to ascribe each pixel to an annulus. The computer stored the number of corresponding array elements for each annulus, the sum of the data values and the sum of the squares of the data values. From these sums the average, mean error and standard deviation of density readings in the individual annuli were computed. Since NGC 3379 is an EO or E l galaxy, the variation of surface brightness as a function of position angle was expected to be small and elements of the array in an annulus whose data values differed by more than 2.6 standard deviations from the average were rejected. This enabled small areas contaminated by dust or emulsion defects to be eliminated. The average value for each annulus was then recomputed. The density level due to the sky brightness was found by taking the weighted average density reading of a number of adjacent annuli at as large a radius as possible. This value was subtracted from the average values obtained for the individual inner annuli to yield surface brightness as a function of radius. Since the image of NGC3379 is comparable in size to the photocathode the adopted sky density value is inevitably contaminated by galaxy light. However, the effect of this small overestimation in the sky brightness is important only in the outermost regions of the galaxy. The B luminosity profile is somewhat more affected since the centre of the galaxy image falls closer to the centre df the photocathode than in the V exposure, thus requiring that the sky estimate be made at a slightly smaller radius. The contribution to the total relative luminosity from an annulus was found by multiplying the average value for that annulus by its area. The
ELECTRONOGRAPHIC PHOTOMETRY OF NGC 3379
333
luminosity within a chosen radius was determined by summing the contributions from the enclosed annuli.
Calibration No standard stars were observed for direct calibration; instead the photoelectric integrated magnitude determinations made by other observers4.'0.11 were employed. A preliminary estimate was made of the integrated magnitude within a particular radius. This was used in conjunction with the relative luminosity measured from the electronograph within the same radius to calibrate the results. Using this calibration the average difference between the electronographic and photoelectric measures was calculated. The preliminary estimate was then revised so that the average difference became zero. For the exposure taken with the V filter the mean error between the electronographic and photoelectric results is 0.02 magnitudes, and for the B filter it is 0.03 magnitudes. These errors are of a similar order or are less than those quoted for the photoelectric results. The differences between the electronographic and photoelectric measures are shown in Figs. 3 and 4.
+ A
+
A
0
0
0
100
Radius (arcsec 1
FIG. 3. Differences between photoelectric and electronographic results for integrated V magnitudes of NGC 3379 plotted as a function of radius. 0, Burkhead and Kalinowski; A, de Vaucouleurs; f, Sandage.
334
o'21
M. A. R. HARDWICK, A. B. HARRISON AND B. L. MORGAN
+
0.I
+
+
A
i
0 A
0
1
0
25
,
,
,
,
1
1
,
50
,
,
1
1
75
1
1
1
1 0
Radius ( orcsec)
FIG. 4. Differences between photoelectric and electronographic results for integrated B magnitudes of NGC 3379 plotted as a function of radius. 0, Burkhead and Kalinowski; A d e Vaucoulers: +, Sandage.
RESULTS Figures 5 and 6 show the distribution of surface brightness in V and B respectively. The crosses indicate observations, the curves are fits of the various luminosity laws to the data and are discussed below. The V and B profiles are in good agreement with the observations of Burkhead and Kalinowski4 and of Miller and Prendergast5 except at large radii where the B profile is likely to be slightly affected by galaxy light. Early attempts by R e y n o l d ~ ' ~and , ~ ~later Hubble' to fit empirical luminosity laws to the observed surface brightness distribution of galaxies gave acceptable results for the data then available. The law suggested by Reynolds and Hubble was:
where So is the central surface brightness. More recent observations have shown that this law falls off too slowly at large radii. Oem1erI4 in an attempt to overcome this difficulty has proposed a modified version of Hubble's law which introduces an exponential cut-off factor. The law
0.5
1.0
I
1
1.5
2.0
2.5
Log radius arcsec
FIG. 5 . The V surface brightness of NGC 3379 as a function of radius. Crosses are observed values, curves are the fits of luminosity laws: (1) Hubble; (2) Oemler; ( 3 ) d e Vaucouleurs; (4) King
t
28t 0.5
t 10
1.5
2.0
2.5
Log radius arcsec
FIG. 6 . The B surface brightness of N G C 3379 as a function of radius. Crosses are observed values, curves are the fits of luminosity laws: (1) Hubble; (2) Oemler: (3) d e Vaucouleurs; (4) King.
336
M. A. R. HARDWICK, A. B. HARRISON AND 0. L. MORGAN
becomes:
So exp (- r/a)’ (1 + r / P l 2 De Vaucouleurs’ has suggested a law of the form: S=
Log (SlS,) = -3.33 [(r/re)1’4- 11
(3) where re is the radius which encloses half the total luminosity and S, is the surface brightness at that radius. Kingls has proposed a law of the form: where and
S = k(l/A - l/B)*, A = [l+(r/r,)*]’/’ B = [l +(r,/r,)’]”’;
(4)
r, and r, are core and tidal radii respectively. The best fits of these laws to the observations between 15 and 200 arcsec are plotted in Figs. 5 and 6. Data points at radii less than 15 arcsec were not used in the calculation because of the poor seeing at the time of the observations; those beyond 200 arcsec were excluded because of the possible errors introduced by incorrect estimate of the sky brightness. These best fits were determined by a computer program which varied the parameters of the law being fitted until a minimum value of reduced chi-squared, a measure of the goodness of fit, was obtained. This fitting was done in linear space, i.e., to observations of intensity as a function of radius rather than in the magnitude-log radius space of Figs. 5 and 6 . It can be seen that despite the differences of form of the various laws they all closely match the observations in the range 15 to 100 arcsec. At larger radii none of the laws is markedly better than the others in describing the surface brightness profile, although Hubble’s is marginally the worst. The total magnitudes found by integrating the luminosity profiles are 9-35 in V and 10.27 in B. These results compare well with those of Burkhead and Kalinowski and of Miller and Prendergast. The B and V luminosity profiles each show a change of gradient at a radius of about 50 arcsec. This feature is also present in both Burkhead and Kalinowski’s and Miller and Prendergast’s data. Since this gradient change occurs at about the same radius in B and V, it may be of a dynamical origin. Both the integrated and local (B - V) curves are shown in Fig. 7. They display a reddened nucleus with a rapidly increasing colour gradient within a radius of 20arcsec, rising to a maximum value of 1.3 at the centre. The integrated ( B - V) profile remains close to a value of 0.92 from 120 to 200 arcsec. The local ( B - V) profile which is a more
ELECI’RONOGRAPHICPHOTOMETRY OF NGC 3379
337
“4
0
eeoo
1 0 Rodnus (arcsec)
FIG. 7. Integrated and local ( B - V) colour index as a function of radius: 0, local; -, integrated.
sensitive indicator of colour variations at larger radii, falls to a value of 0.75 at 80 arcsec. Values at greater radii are probably influenced by errors in the B sky brightness level. Larson16 has described a number of simplified models of spherical galaxies that reveal a rapidly increasing metallicity gradient in nuclear regions. For the model that provides the best overall fit to the observed photometric properties of NGC 3379, he predicts a dynamical boundary at a radius of 1.5 kpc or 21 arcsec separating “Nuclear” and “Halo” regions. The model suggests that stars in the halo region were formed during the initial evolution of the galaxy and are therefore metal-poor. In the nuclear region the continuing inflow of gas enriched with the debris of supernovae leads to the formation of metal-rich stars. Outside the nuclear region the model shows a constant metallicity and in a later paper” Larson and Tinsley predict a value of 0.95 for (B - V) in the outer region of the model.
CONCLUSIONS Integrated magnitudes and surface brightness and colour profiles have been derived from electronographs of the galaxy NGC 3379. They are found to agree well with the photoelectric results of other observers. Fits
338
M. A. R. HARDWICK, A. B. HARRISON AND B. L. MORGAN
of various luminosity laws indicate that in this case, they can all be made to match the data closely over the radius range 15 to 100arcsec. The integrated colour magnitude is in good agreement with Larson’s best models for NGC 3379. A reddened nucleus is detected coinciding with the region over which Larson predicts metal enrichment. ACKNOWLEDGMEWS The authors wish to thank the Director and staff of the Helwan Institute. Cairo, for their invaluable assistance in obtaining the observations described in this paper. A. B. H. was in grateful receipt of a Science Research Council Studentship.
REFERENCES 1. Hubble, E., Astrophys. J. 64, 321 (1962). 2. de Vaucouleurs, G. and de Vaucouleurs, A,, In “Reference Catalogue of Bright Galaxies”. Published by the University of Texas Press (1964). 3. Morgan, W. W., Publ. Astron. SOC. Pac. 71, 394 (1959). 4. Burkhead, M. S. and Kalinowski, J. K., Astron. J. 79, 835 (1974). 5 . Miller, R. H. and Prendergast, K. H., Astrophys. J. 136, 713 (1962). 6. de Vaucouleurs, G., Ann. Astrophys. 11, 247 (1948). 7. Nubble, E., Astrophys. J. 71, 231 (1930). 8. Redman, R.’O. and Shirley, E. G., Mon. Not. R. Astron. SOC.98, 613 (1938). 9. Fish, R. A., Astrophys. J. 139, 284 (1964). 10. de Vaucouleurs, G. and de Vaucouleurs, A., Mem. R. Astron. SOC. 77, 1 (1972). 11. Sandage, A. R., Astrophys. J. 183, 711 (1973). 12. Reynolds, J. H., Mon. Not. R. Astron. SOC.74, 132 (1913). 13. Reynolds, J. H., Mon. Not. R. Astron. SOC.80, 746 (1920). 14. Oemler, A,, Astrophys. J. 209, 693 (1976). 15. King, I., Astron. J. 67, 471 (1962). 16. Larson, R. B., Mon. Not. R. Astron. SOC. 166, 585 (1974). 17. Larson, R. B. and Tinsley, B. M., Astrophys. J. 192, 293 (1974).