3. Observations and reduction

3.1. Observational material

Most images were obtained at the Bernard Lyot 2 m telescope at Pic du Midi Observatory (France) using ISARD imaging all-reflective camera (Lemaitre et al. 1996, in preparation) using a thick Thomson THX CCD with pixels. ISARD gives a scale of 0.248 arcsec per pixel but in the seeing conditions we met this leads to some oversampling at the expense of exposure time. Therefore, the readout of the CCD was systematically done using a pixels on-chip binning mode. Seeing conditions were between 2'' (at worst) in November 92 and an exceptional 0.5'' during a night of February 1994, but the average was around 1.2''. For objects observed during different runs, we selected the frames taken under the best sky absorption/seeing conditions.

Several objects were observed at the 182 cm Cima Ekar telescope at Asiago Observatory (Italy), using a TK 512 CCD (with ) during several runs from August 92 to March 94. The average seeing conditions were around 2''.

We used Johnson B and Gunn R filters. Coupled with the thick Thomson CCD quantum efficiency spectral distribution, we found that the R instrumental band was easy to reduce to the standard Cousins R band with very small color term in the reduction equation. The color term was a little bit larger for the reduction of the B instrumental band to the standard Johnson B band. To derive the reduction equations, Landolt (1983, 1992) equatorial standard stars were observed, as well as, each night, M 67 "dipper asterism'' field. (Schild 1983). The M 67 field was observed at two or three zenith distances to derive the extinction correction. The globular cluster NGC 7790 was also observed in December 1992. High photometric quality nights in February 94 at Pic du Midi allowed to search for the color extinction coefficient, which was found to be not significant () and therefore not taken into account in the galaxy images reduction. Flat-fielding was done with frames taken on evening and morning twilight sky.

3.2. Pre-reduction of the frames

The data reduction was performed in 2 steps. First, the raw images were bias subtracted and flat-fielded. When a complete nightly set of flats was not available (because of observing conditions only favourable during part of the night) we used a procedure creating a mask of the images and we used as a flatfield the averaged and masked sky backgrounds. MIDAS package also provides a way of building a flatfield from the raw image. Both methods were used in order to avoid spurious effects like stars or faint objects in the field being taken into account during the process. The images from Asiago were treated along the same scheme.

In a second step the cosmic rays impacts were removed from the CCD frames. We largely used a standard MIDAS routine for this operation, checking very carefully that the information contained in the images were not altered, especially the centers of the stars and the maxima of brightness in the galaxies, which could be mistaken as cosmic rays impacts by the automated procedure in very sharp seeing conditions.

3.3. Standard stars

Careful aperture photometry was performed on the standard stars of the cluster fields and on the Landolt stars. The following equations were used to reduce the instrumental magnitudes and colors to the Johnson-Cousins system:



at Bernard Lyot telescope



at Cima Ekar telescope.
where b and r are the instrumental magnitudes defined by:

and being the flux in ADU corrected from the sky background, per minute of exposure time.

The reference magnitudes for the M 67 stars were taken from Schild (1985), Gilliland et al. (1992) and Chevalier & Ilovaisky (1991). Odewahn et al. (1992) provided the values for NGC 7790 stars.

The atmospheric absorption coefficients were determined on a nightly basis whenever possible through standard methods. These coefficients were then used to correct the magnitudes and surface brightnesses of the galaxies.

The R Gunn filter includes the line in emission in the rest frame, and all objects having a low redshift, their emission is always present in the measured R flux. This effect is probably not dominant in the presence of a substantial background of evolved stellar population, but may contribute in some cases to produce local red excess inside the galaxies, especially on star-forming complexes that have a large equivalent width in emission. Note however that in the galaxy SBS 1331+493, which contains a knot of high excitation and very large equivalent width in emission lines, the knot appears extremely blue in our photometry.

3.4. Galaxy photometry

3.4.1. Data reduction

The surface photometry of the galaxies was performed with specially designed MIDAS macro-procedures. Basically, we performed isophotal integration of the flux. First, the sky background is evaluated in a number of pixel sub-arrays surrounding the object. Further, the classical method of "equivalent profile'' devised by de Vaucouleurs (1959) and applied by numerous other authors (see e.g. Ables 1971; Fraser 1977 for detailed explanations) has been used. The flux from the galaxy is computed inside isophotes of decreasing brightness , without setting any constraint on the geometrical shape of the isophotes. This method allows to take into account irregular shapes occurring in star-forming regions, detached "islands'' of brightness, etc... and is well adapted to non-axisymmetric objects. The brightness profile produced is called the "equivalent'' profile. Each step of the isophotal integration produces the following information:

• "equivalent'' radius: is defined by:
  
  
  where A is the area of the isophote considered
• surface brightness at :
  
  
  where I and are the respective levels (in ADU per pixel per minute of exposure time) of the isophote and of the sky background. is reduced to the B, R standard system by application of the reduction Eqs. (1) to (4) after correction for atmospheric extinction.
• integrated magnitude at :
  
  
  where F is the total flux inside the isophote of area A (expressed in pixels). is reduced to the standard system after atmospheric extinction correction.

The sky background was checked for each frame to be sufficiently flat in the vicinity of the galaxies to avoid sky-background fitting and subtraction inducing complementary noise around the fainter isophotes. This was done using different sky fitting algorithms and computing the rms on the sky-background. The errors were estimated using the rms of the sky-background mean level around the galaxies. Typically, the errors were of less than a percent when no sky was subtracted and of about after subtraction.

The quality of the isophotal integration critically depends on the local signal-to-noise ratio. The outermost parts of the galaxies are weak extended sources and their signal-to-noise ratio is generally insufficient on the direct images. To extend the photometry down to 1 percent of the sky background or even further, one must smooth the frames in some way. However, efficient smoothing numerical filters (such as the median filter) have generally poor local flux conservation properties, especially when strong brightness gradients are present in the image. Therefore, the isophotal integration was performed in two steps. For the inner parts of the galaxies, we used the direct full resolution frame, producing a first brightness profile. For the outer parts, we applied a gaussian filter of width slightly larger than the seeing disk before integrating and built a second brightness profile. The two brightness profiles were subsequently compared and merged together. This method proved satisfactory in all cases.

The data obtained at Asiago were also treated independently (by M.T.) using a method of ellipse fitting to derive the integrated magnitude and surface brightness profiles. This process fits ellipses to the isophotes without forcing the position of the ellipse centers. It gives results in excellent agreement with those produced by the equivalent de Vaucouleurs profile when applied to regular elliptical-like dwarfs. It fails on the central parts of the irregular-like dwarfs or the dwarf compacts showing several star formation regions surrounded by regular (elliptical or circular) shells.

The results of the isophotal integration are contained in two photometric tables (B and R) for each object. From these tables, the following quantities are derived:

• asymptotic magnitudes and colors: the "asymptotic'' (total) magnitude of a galaxy can be determined only through an extrapolation process, since the flux integration is always limited to some fraction of the sky background in excess of the latter. We used the following procedure: the integrated apparent magnitude was plotted versus and an asymptotic value for this "growth curve'' was searched interactively, with a simple parabolic extrapolation beyond the last three measured points.
• "effective'' radius and brightness: these quantities were introduced by De Vaucouleurs to allow the comparison between various types of galaxies, and hence of brightness distributions, by means of "metric'' data, that is, quantities that are independent of the brightness of the sky background. They are defined as:
  • (effective equivalent radius): the equivalent radius of the isophote containing half the total luminosity of the object (i.e. the value of for which the cumulated magnitude is equal to or ).
    
  • (effective surface brightness): the surface brightness at
• the mean surface brightness inside the effective radius is also an interesting quantity related to the so-called "compactness'' of the galaxy. It is derived from the effective apparent magnitude and the area of the effective isophote. The mean surface brightness inside the effective radius is given by:
  
  
• a concentration index gives an idea of the relative importance of the inner parts of the object with respect to the outer parts as regard to the luminosity. It is also related to the "compactness'' in the visual appearance of the image of the galaxy. The concentration index as defined by de Vaucouleurs (1959) is given by:
  
  where r_0.25 is the radius at 1/4th of the total luminosity.

3.4.2. Errors estimation

The errors on the surface brightness profiles were estimated assuming that the Poisson statistics apply and taking into account the errors on the background subtraction (Saglia 1996).

For the errors on the integrated magnitude, the Poisson statistics do not entirely apply as one has to take into account the extrapolation process and the limitation due to the quality of the observations. The deeper the better, in order to reach the asymptotic magnitude without extrapolating.

3.4.3. B-R color profiles

Ideally, The B-R profiles are derived from the B-R maps obtained from the B and R images of the galaxies. Due to the limited S/N ratios obtained in the outer faint regions on our R band images, in order to analyze the color distribution of our BCDGs, we have adopted a method that uses the SB profiles of the galaxy in B and R band. The principle is to subtract the two SB distributions to built the new B-R distribution. We used the best sampled SB profile (B or R) to interpolate the apparent surface brightnesses as function of the equivalent radius. This allows to sample the surface brightnesses following the same distribution of radius and to subtract them point by point. This method does not exactly reproduce the general variation in colors along the SB profiles when the isophotal integration centers are not the same in both bands, because the interpolated points do not correspond physicaly. However, the differences are smaller than the errors estimated on the color profiles.

The B-R color distributions are shown in the atlas with the estimated errors (we assumed the errors in B and R to be independent, so the ).

3.4.4. Results of the photometry

The results of the photometry are presented in an atlas (Fig. 1 (click here)) and a table. For each galaxy, the isophotal map in B (with three exceptions for which only R images are available) is given and the equivalent surface brightness distributions in B and R are plotted using a linear scale for the radius, as well as the B-R profile at the top of the plot. Table 2 (click here) collects the observable parameters derived from the surface photometry:

Column 1: Markaryan number or SBS name.
Column 2: Asymptotic apparent magnitude in B and asymptotic B-R color.
Column 3: Surface brightness inside the effective radius in .
Column 4: Mean surface brightness inside the effective radius.
Column 5: Equivalent effective radius in arcsec.
Column 6: Equivalent radius at 1/4 of the total luminosity in arcsec.
Column 7: Isophotal equivalent radius at in arcsec.
Column 8: Concentration index in B and R colors.

  

Figure 1: Atlas of contour plots and photometric profiles. For each galaxy, the contour plot of the B image is shown on the left panel, with a surface brightness interval of 0.5 magnitude per square arc second. The threshold surface brightness is indicated in the lower left corner. For Mk 1423, Mk 1426 and SBS 1533+574, the R maps are shown, no B image being available. The solid horizontal bar has a length of 1 kpc. The right-hand panel displays the surface brightness distribution profiles (abscissa: equivalent radius in arc seconds, ordinate: surface brightness in magnitude per square arc second) in B (dots) and R (circles).

 

Object name[B filter] m_B(asympt) r_0.25(B)[''] r₂₄(B)[''] C₂₁(B)

'' [R filter] B-R (asympt) r_0.25(R)[''] r₂₄(R)[''] C₂₁(R)

SBS 0136+328 16 . 91 19.20 22.50 4 . 98 2.38 5 . 68 2.09

'' 1 . 03 16.68 20.40 3 . 08 1.78 9 . 04 1.73

SBS 0940+544C 17 . 47 24.12 23.51 6 . 14 3.59 3 . 92 1.71

'' 0 . 65 22.94 22.30 4 . 81 2.72 7 . 8 1.77

SBS 1006+578 16 . 55 22.38 21.81 4 . 28 2.22 7 . 75 1.92

'' 0 . 88 22.26 21.47 5 . 59 3.31 10 . 22 1.69

SBS 1054+504 16 . 19 22.33 21.37 4 . 11 2.01 7 . 81 2.04

'' 1 . 04 21.73 20.67 4 . 86 2.31 12 . 64 2.11

SBS 1147+520 17 . 29 22.27 21.81 2 . 88 1.58 4 . 85 1.82

'' 1 . 45 20.90 20.37 3 . 01 1.86 8 . 17 1.62

SBS 1331+493 15 . 03 18.77 22.63 12 . 52 7.50 15 . 48 1.67

'' 0 . 61 18.17 22.04 12 . 91 7.73 20 . 09 3.09

SBS 1413+495 16 . 40 20.23 23.78 15 . 78 6.35 8 . 84 2.44

'' 0 . 82 18.01 22.77 9 . 98 3.22 10 . 13 3.09

SBS 1428+457 16 . 02 22.07 21.27 4 . 48 2.61 9 . 44 1.98

'' 1 . 45 20.75 19.86 4 . 56 2.13 13 . 36 2.14

SBS 1533+574 -- -- -- -- --

'' 15 . 36 (m_r) 21.22 20.50 4 . 11 2.01 10 . 28 2.04

MRK 324 14 . 91 19.63 22.07 10 . 40 3.06 9 . 57 3.40

'' 0 . 41 16.56 20.01 4 . 92 2.70 12 . 91 1.82

MRK 900 14 . 38 19.23 22.58 16 . 19 5.63 14 . 31 2.87

'' 1 . 47 18.41 21.70 21 . 75 8.44 36 . 34 2.58

MRK 996 - - - 5 . 74 2.58 15 . 74 2.22

'' - - - 14 . 94 5.92 19 . 66 2.52

Mk 1131 15 . 03 19.76 23.58 19 . 77 9.12 10 . 61 2.16

'' - - - 7 . 34 3.62 22 . 63 2.03

MK 1308 14 . 87 22.68 21.45 8 . 17 2.55 15 . 06 3.21

'' 1 . 14 21.06 20.10 7 . 44 3.12 22 . 51 2.38

MK 1416 16 . 99 22.93 22.07 4 . 14 2.19 6 . 75 1.89

'' 1 . 29 18.87 20.94 4 . 49 2.00 9 . 81 2.24

MK 1418 14 . 37 21.56 20.85 7 . 79 4.90 16 . 95 1.59

'' 1 . 46 20.50 19.81 9 . 10 5.44 28 . 09 1.67

MK 1423 -- -- -- -- --

'' 13 . 76 (m_r) 18.78 21.41 13 . 42 5.84 20 . 39 2.30

MK 1426 -- -- -- -- --

'' 14 . 76 (m_r) 21.32 20.22 4 . 89 2.30 14 . 45 2.12

MK 1434 17 . 36 22.01 20.86 2 . 00 1.06 4 . 79 1.88

'' 1 . 17 20.91 19.80 2 . 10 1.07 6 . 90 1.97

MK 1450 16 . 99 21.68 20.71 2 . 22 1.22 5 . 33 1.82

'' 2 . 16 19.76 18.78 2 . 46 1.37 11 . 17 1.79

MK 1480 16 . 56 21.95 21.11 3 . 25 1.64 6 . 69 1.97

'' 1 . 03 21.60 20.65 4 . 21 2.13 10 . 37 1.98

MK 1481 16 . 78 23.53 22.83 6 . 47 3.70 8 . 05 1.75

'' 0 . 79 22.99 22.24 7 . 13 3.95 10 . 14 1.81

MK 1499 16 . 53 22.43 21.49 3 . 90 1.97 6 . 82 1.98

'' 1 . 10 21.41 21.16 5 . 55 2.47 12 . 40 2.24

 

The magnitudes were corrected from the galactic extinction, using the E(B-V) maps of Burstein & Heiles (1982). The absorption coefficient are computed from the E(B-V) values using the equation given by Savage & Mathis (1979):

Figure 2 (click here) shows the comparison of the measured asymptotic B magnitudes with the catalog photographic magnitudes given by Mazzarella & Balzano (1986), and the magnitudes given by Stepanian (private communication) for SBS objects. The CCD magnitudes appear consistent with the eye estimated magnitudes from the catalogs across the range spanned by the selected objects. The dispersion in the diagram is also consistent with the general accepted accuracy of of the Markaryan catalog apparent magnitudes.

3.5. Comments on individual objects

A large fraction of the galaxies show several knots of star formation activity, e.g. SBS1533+57 which shows at least 4 knots of star formation activity. In most cases, the knots have a different B-R color as their environment. They can be classified from red to blue, this probably reflecting different states of evolution, the brightness of the reddest knots being possibly dominated by red supergiants. Usually the bluest knots are off-centered and may contribute significantly to the luminosity in the blue range.

A substantial fraction of the sample shows outer isophotes that depart conspicuously from axisymmetric shapes. As examples, SBS0940+544C and Mk 1416 which show a comet-like shape. Although it is tempting to associate these departures from axisymmetry with recent gravitational interactions of tidal type with other systems, we have found only two objects with clearly identified companions, namely Mk 1308 associated with a very close-by dwarf Magellanic companion (2 kpc in projected distance), and Mk 1480 whose companion Mk 1481 is at about 11 kpc from its center in projected distance. A number of objects also show distortions in more inner regions, the most common behavior being the "boxy'' nature of isophotes (that may extend to almost rectangular or lozenge shapes in extreme cases) and the rotation of the position angle of the apparent major axis of the isophotes. It has been widely claimed in the literature that boxy isophotes in spheroidal systems may be signatures of past gravitational interactions and/or current merging phenomena seen just before the complete relaxation of the system (Nieto & Bender 1989; Hearnquist & Quinn 1989; Jedrzejewski et al. 1987; Barnes & Hearnquist 1992; Nieto et al. 1994).

  
Figure: Comparison between photographic apparent magnitudes from Byurakan surveys and asymptotic B magnitudes in this work. The sources of photographic magnitudes are Mazzarella & Balzano (1988) (filled hexagon) and Stepanian (private communication, open circle). The right arrows represent Mk 1131 and Mk 996 for which the photometric data are not reliable because of very poor weather conditions during the observing run

Below are listed short descriptions of the objects.

• SBS 0136+328: The most distant and most luminous object in our sample, it shows an elliptical shape with no apparent substructure. The exact nature of the brightness distribution is difficult to assess with certainty because of the limited scale (), the most probable case is a large, dominant r^1/4 bulge plus exponential envelope. The photometry is uneasy due to a bright star projected near the galaxy and producing diffuse light in the background.
• SBS 0940+544C: The faintest object in the sample (M_B = -13.9). Elongated diffuse galaxy, with an extended central region of quite uniform brightness. A Galactic star is superimposed at one extremity. Numerous faint galaxy images in the vicinity, probably distant background objects. The brightness distribution obeys a quasi-pure exponential law.
• SBS 1006+578: Outer contours are roughly rectangular in shape, with a brighter extended knot at the SW edge (there remains a little suspicion that this knot could be a background object) and a weak extension along the major axis. The central star forming region is kidney-shaped, with no apparent structure in it. A strong color gradient is present, the outer envelope being very red. The brightness distribution looks exponential, but in the red band the profile is composite with the outer envelope having a scale length different from the one observed in the central region. This anomalous morphology may result from past tidal interactions, but no neighbors have been identified so far.
• SBS 1054+504: A quite regular elliptical-like galaxy with some isophote rotation between the central part and the outer envelope and evidences for local departure from axisymmetry. The brightness distribution obeys a r^1/4 law, with a clear and regular color gradient, the outer envelope being redder.
• SBS 1147+520: A small galaxy with roughly regular outer isophotes. A conspicuous diffuse component appears at the NW. The central bright star forming region is elongated and kidney-like. The brightness distribution shows a trend to obey a linear relationship between and r^1/4 but the coefficient is not typical of normal spheroidal systems (see discussion in Sect. 4.1.1). A weak color gradient is present.
• SBS 1331+493: Classified as a compact dwarf because of the luminous blue knot located E (Movsesyan et al. 1996): it is a giant starforming region with strong and [OIII] lines and a very weak continuum. The galaxy shows an amorphous shape and the brightness distribution does not obey standard laws. We have constructed a B-R map that shows complex structures in the central region with several faint blue enhancements.
• SBS 1413+495: Elliptic shape with very regular isophotes. Unfortunately the presence of a Galactic star very close-by and the poor seeing conditions do not allow a more accurate description of this object. Bluest galaxy of our sample (B-R = 0.22 in asymptotic value), but this extreme color is due to an extremely blue envelope, the core color being normal.
• SBS 1428+457: The overall shape is a deformed lozenge, whose central part is a very high surface brightness star forming region. The overall color is not very blue, and there is no appreciable color gradient. The brightness distribution roughly follows an exponential law with a central excess of light. A lot of structures are present across the inner regions, four plumes each containing 2-3 resolved knots protrude out of the central bright core. A compact object of neutral color, apparently a superimposed Galactic star, is conspicuous near the S end.
• SBS 1533+574: Only observed in R band. The outer isophotes are elliptic and quite regular. The central region is dominated by 2 bright components, one compact and stellar-like, the other more diffuse and elongated towards the former. Both are off-centered with respect to the outer isophotes. Two secondary components at north form a trapezoidal structure with the main cores. This object may perhaps be linked to or classified as a double nucleus galaxy. The brightness distribution obeys an exponential law with evidence for an outer envelope having a shallower gradient.
• Mk 324: A quite regular elliptical double-nucleated galaxy (Barbieri et al. 1979; Korovyakovskii et al. 1981). The two "nuclei'' differ in angular extent and compactness. Both are off-centered with respect to the outer contours. The brightness distribution follows an r^1/4 law. There is a strong color gradient, the envelope being bluer than the inner region. The photometry presented here has uncertain calibration because of poor atmospheric conditions during the observations.
• Mk 900: Elliptical shape with isophotes showing rotation and boxiness. The central part of the galaxy is elongated and strongly distorted, especially on the R frame. The brightness distribution is consistent with an r^1/4 law in the central region, but there is evidence for an extended envelope of different dynamical behaviour.
• Mk 996: Typical elliptical-like galaxy, with rotating isophotal major axis and surface brightness light profiles described by a pure r^1/4 law. Qualitative photometry and morphology could only be done on this galaxy because of the very poor photometric conditions during the observation.
• Mk 1131: Elliptic shape with off-centered central core. The isophotes are regular ellipses, the brightness distribution is peculiar with a very shallow outer envelope surrounding the steeper central population. The photometric calibration is quite uncertain because of poor weather conditions.
• Mk 1308 (= UM 465): Brightest component of a pair, a faint irregular companion (confirmed as physical neighbour by 6 m telescope spectroscopy) is located at 32'' (2 kpc). The envelope of Mk 1308 is roughly circular in shape but the isophotes exhibit departures from symmetry. The brightness distribution follows an exponential law. Salzer et al. (1989) describe this object as a "nearby dwarf elliptical'' with a "nuclear starburst'' but we shall notice that the brightness distribution in this case rather corresponds to that observed in non-active dwarf spheroidal objects (see e.g. Ichikawa et al. 1986) and not to that observed in giant ellipticals.
• Mk 1416: Comet-like galaxy with elongated boxy isophotes. The brightest region exhibits a double component structure: one component approximately located at the center has a red color while the second component is very blue. An extended "transversal'' structure is visible in the half of the galaxy opposite from the blue compact component. The brightness distribution could be roughly consistent with a superimposition of two exponential laws. A faint red envelope is detectable near the surface brightness limit of the images.
• MK 1418: A nearby dwarf galaxy. The external isophotes are oval. The central bright region is very extended, with several protruding blobs out of a distorted central component surrounding a small bright nucleus. The brightness distribution follows an r^1/4.
• Mk 1423: Only observed in R band. Oval and regular in shape with an asymmetric bright core. The brightness distribution is consistent with a disk plus bulge population superimposition.
• Mk 1426: Only observed in R band. Another, fainter galaxy at 48'' but we have no information about its velocity. The outer shape is elliptic. The central bright starburst region is a double component embedded in an elongated envelope. The brightest component is centered with respect to outer isophotes. The brightness distribution follows an r^1/4 law.
• Mk 1434: Distant dwarf object with lozenge-like outer contours and a jet-like curved extension at one end. The central bright component is ovoid in shape. There is no significant color gradient. The brightness distribution obeys an r^1/4 law.
• Mk 1450: A dwarf compact object with moderately deformed circular isophotes and a unique central star forming component. The brightness distribution obeys an r^1/4 law. A strong color gradient is present, the envelope being very red. Note that the central component is also quite red for a starburst object.
• Mk 1480: In physical pair with Mk 1481 (see below). This galaxy is quite regular with a faint extension opposite to the direction of the companion and boxy isophotes. The bright core is centrally located. The brightness distribution obeys an r^1/4 law, with a strong and regular color gradient, the envelope being much redder than the center.
• Mk 1481: Small irregular object of Magellanic aspect with complicated inner structure of several bright spots embedded in a common envelope. The brightness distribution follows an exponential law with a shallow gradient. The outer envelope is redder than the central parts.
• Mk 1499: Boxy (almost rectangular) inner body embedded in an elliptical extended envelope in which there are some hints of very faint shells corresponding to the small sides of the inner "box''. The inner body itself contains a bright elongated very blue central structure with two components, the brightest being off-centered with respect to the outer contours. This structure is reminiscent of "double nuclei'' objects. The brightness distribution is neither consistent with an exponential nor with an r^1/4 law.