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2. Sample selection

Our sample of early-type galaxies has been extracted from the second release of the WFPC2-HDF frames, in the V606 band. Due to the efficiency curve of the WFPC2, this filter usually ensures a better S/N ratio with respect to the other available filters (U300, B450 and I814) even in the case of high redshift galaxies, as we verified by comparing the V606 and I814 images.

2.1. Selection criteria

The sample selection is based on the compilation provided by the ESO-STECF-HDF Group, obtained through the automated SExtractor algorithm (Bertin & Arnouts 1996). Among the other things, the catalog includes, for each object in the field, the total V606 magnitude in the STMAG system, the number of pixels (tex2html_wrap_inline1472) detected above the background using a threshold of tex2html_wrap_inline1474 of the noise, and a star/galaxy (s/g) classifier, ranging from 0 to 1, which gives the probability that the object is stellar (s/g=1 means "star''). After a preliminar inspection of the frames, supplemented by several fast average-tracing luminosity profiles of faint and/or small objects, we decided to set the following limits for inclusion in our preliminar sample: (1) tex2html_wrap_inline1480 26.5; (2) tex2html_wrap_inline1482 200; (3) tex2html_wrap_inline1484 0.6.

As for the first two limits, they concern essentially the technical "feasibility'' of the morphological analysis. Although at this stage we preferred not to be too much severe in the selection, these limits provide completeness criteria to our sample. The choice of the third limit turned out not to be critical, probably due to the very shape of the star profiles. Nevertheless, in this case we preferred to be more conservative, in order to prevent the inclusion of stars in our sample.

2.2. Morphological screening

We found 372 objects of the SExtractor catalog matching the above limits in the three chips of the WFC (the catalog does not include the PC). 29 objects from the chip tex2html_wrap_inline1486 (PC) were added later to this list by simple visual inspection, so that the total preliminar sample turned out to be of 401 objects.

We used the "imexam'' IRAF's tool, together with the " Saoimage'' display facility, to estimate the morphological type of each object, thus producing a first screening of the galaxies in the sample. Apart from some cases, the angular resolution did not allow us to give precise Hubble types. Moreover, many faint galaxies appear heavily disturbed by the presence of close (often multiple) companions or peculiar structures, which obviously make even more difficult the classification. Nevertheless, most of the "early-type'' candidates turned out to be easily recognizable on the basis of the light concentration. The doubious cases (mainly S0s and Sa galaxies) were retained at this stage and were examined later in a quantitative way (see next section).

After this selection we were left with 134 galaxies. In order to check the reliability of our classifications, we compared them with those given by Van den Bergh et al. (1996) and by Statler (1996, private communication). Due to the different selection criteria, the three samples do not overlap exactly each other. Nevertheless, for the common objects the agreement was fairly good (tex2html_wrap_inline1492 with Van den Bergh et al. and tex2html_wrap_inline1494 with Statler). To be more accurate, we decided to provisionally include in our sample all galaxies matching our selection criteria which, contrary to our estimates, were classified as "early-type'' by Van den Bergh et al. (1996) and/or by Statler (1996). In this way 28 more galaxies enriched our sample, which become of 162 objects.

2.3. Final sample

We performed the detailed surface photometry of this sample, producing luminosity and geometrical profiles of each object. The next Section outlines the techniques we used to extract the morphological information from this very peculiar observational material. From the analysis of the luminosity and geometrical profiles, 34 objects of the sample have been recognized to be " disk-dominated'' objects (likely Sa galaxies), whereas 28 more objects showed peculiar or unclassifiable profiles. These galaxies were excluded from the final sample of "bona fide'' early-type galaxies.

The final sample of 99 galaxies is reported in Table 1 (click here), where Col. (1) gives the FOCAS list identification (Williams et al. 1996), Cols. (2) and (3) the J2000 coordinates tex2html_wrap_inline1498 and tex2html_wrap_inline1500, Col. (4) our morphological classification, Cols. (5) the morphological type (if available) given by van den Bergh et al. (1996).

   

FOCAS-ID Coordinates (2000) MorphologyLum.Prof. tex2html_wrap_inline1502 tex2html_wrap_inline1504 tex2html_wrap_inline1506 tex2html_wrap_inline1508 tex2html_wrap_inline1510
tex2html_wrap_inline1498 tex2html_wrap_inline1500Our V.d.Bergh class (STMAG) (tex2html_wrap_inline1516) (tex2html_wrap_inline1518)
ID_4_942_0 tex2html_wrap_inline1520 tex2html_wrap_inline1522 E E 1 24.75(.33) 0.54 0.18 0.25 15
ID_4_926_2 tex2html_wrap_inline1524 tex2html_wrap_inline1526 m p 3 25.36(.17) 0.19 0.04 0.17 60
ID_4_928_1 tex2html_wrap_inline1528 tex2html_wrap_inline1530 S0 E 1 23.61(.15) 0.36 0.10 0.19 50
ID_4_878_2 tex2html_wrap_inline1532 tex2html_wrap_inline1534 E E 2 25.79(.19) 0.22 0.35 0.35 8
ID_4_878_11 tex2html_wrap_inline1536 tex2html_wrap_inline1538 E Sap 2 23.66(.14) 0.22 0.12 0.14 30
ID_4_822_0 tex2html_wrap_inline1540 tex2html_wrap_inline1542 E E 1 25.42(.27) 0.33 0.08 0.10 5
ID_4_858_13 tex2html_wrap_inline1544 tex2html_wrap_inline1546 E E 1 25.18(.23) 0.15 0.14 0.19 11
ID_4_767_0 tex2html_wrap_inline1548 tex2html_wrap_inline1550 S0 E 2 25.16(.15) 0.19 0.36 0.39 6
ID_4_661_1 tex2html_wrap_inline1552 tex2html_wrap_inline1554 E E 2 24.96(.17) 0.22 0.18 0.23 20
ID_4_639_1 tex2html_wrap_inline1556 tex2html_wrap_inline1558 E Et? 1 25.12(.14) 0.17 0.08 0.16 47
ID_4_937_0 tex2html_wrap_inline1560 tex2html_wrap_inline1562 E E 1 25.50(.19) 0.18 0.19 0.22 30
ID_1_95_0 tex2html_wrap_inline1564 tex2html_wrap_inline1566 E - 1 25.03(.13) 0.13 0.28 0.30 12
ID_4_602_0 tex2html_wrap_inline1568 tex2html_wrap_inline1570 E E 1 25.30(.18) 0.16 0.24 0.28 15
ID_4_804_0 tex2html_wrap_inline1572 tex2html_wrap_inline1574 E Ep? 1 25.40(.16) 0.20 0.23 0.27 8
ID_4_774_3 tex2html_wrap_inline1576 tex2html_wrap_inline1578 E - 1 25.95(.18) 0.14 0.20 0.21 26
ID_4_581_11 tex2html_wrap_inline1580 tex2html_wrap_inline1582 E/S0 - 1 25.77(.15) 0.14 0.19 0.25 40
ID_4_845_0 tex2html_wrap_inline1584 tex2html_wrap_inline1586 E Sa 2 25.08(.16) 0.20 0.37 0.37 3
ID_4_554_1 tex2html_wrap_inline1588 tex2html_wrap_inline1590 Ep Ep 2 25.58(.20) 0.17 0.29 0.33 38
ID_4_493_0 tex2html_wrap_inline1592 tex2html_wrap_inline1594 S0 E 1 23.26(.24) 0.44 0.33 0.33 7
ID_4_727_0 tex2html_wrap_inline1596 tex2html_wrap_inline1598 E Ep? 2 23.43(.10) 0.21 0.49 0.48 3
ID_4_565_0 tex2html_wrap_inline1600 tex2html_wrap_inline1602 m Sap 3 23.46(.12) 0.31 0.18 0.28 65
ID_4_744_0 tex2html_wrap_inline1604 tex2html_wrap_inline1606 E - 1 22.33(.18) 0.63 0.12 0.16 50
ID_2_82_1 tex2html_wrap_inline1608 tex2html_wrap_inline1610 E E/m 2 24.92(.15) 0.18 0.27 0.28 19
ID_4_752_1 tex2html_wrap_inline1612 tex2html_wrap_inline1614 E - 1 22.97(.22) 0.90 0.18 0.18 3
ID_1_56_0 tex2html_wrap_inline1616 tex2html_wrap_inline1618 E - 1 26.38(.19) 0.13 0.15 0.21 7
ID_4_579_0 tex2html_wrap_inline1620 tex2html_wrap_inline1622 E E/* 1 25.53(.14) 0.13 0.06 0.09 93
ID_1_37_1 tex2html_wrap_inline1624 tex2html_wrap_inline1626 m - 3 25.75(.18) 0.13 0.50 0.46 28
ID_2_163_0 tex2html_wrap_inline1628 tex2html_wrap_inline1630 E - 1 25.94(.19) 0.18 0.25 0.26 15
ID_4_555_2 tex2html_wrap_inline1632 tex2html_wrap_inline1634 E/S0 E 1 24.79(.21) 0.32 0.17 0.24 20
ID_4_368_0 tex2html_wrap_inline1636 tex2html_wrap_inline1638 E/S0 - 1 26.07(.22) 0.24 0.26 0.25 25
ID_2_80_0 tex2html_wrap_inline1640 tex2html_wrap_inline1642 E - 2 25.64(.18) 0.18 0.26 0.28 8
ID_1_100_0 tex2html_wrap_inline1644 tex2html_wrap_inline1646 E - 1 26.37(.14) 0.13 0.12 0.29 22
ID_1_35_0 tex2html_wrap_inline1648 tex2html_wrap_inline1650 E - 1 24.35(.16) 0.21 0.24 0.26 14
ID_4_516_0 tex2html_wrap_inline1652 tex2html_wrap_inline1654 E Et 1 25.30(.15) 0.18 0.13 0.20 40
ID_4_497_0 tex2html_wrap_inline1656 tex2html_wrap_inline1658 E - 1 26.05(.17) 0.13 0.12 0.14 20
ID_4_520_0 tex2html_wrap_inline1660 tex2html_wrap_inline1662 E/S0 - 1 26.07(.19) 0.14 0.11 0.18 15
ID_2_61_0 tex2html_wrap_inline1664 tex2html_wrap_inline1666 E E 2 25.91(.15) 0.15 0.26 0.31 10
ID_4_254_0 tex2html_wrap_inline1668 tex2html_wrap_inline1670 E E/* 1 23.64(.19) 0.55 0.27 0.17 45
ID_1_47_0 tex2html_wrap_inline1672 tex2html_wrap_inline1674 E - 2 25.17(.13) 0.15 0.38 0.37 16
ID_4_322_2 tex2html_wrap_inline1676 tex2html_wrap_inline1678 E Et 2 25.59(.16) 0.16 0.23 0.28 5
ID_2_251_0 tex2html_wrap_inline1680 tex2html_wrap_inline1682 E/S0 - 1 22.96(.17) 0.52 0.05 0.14 110
ID_4_471_0 tex2html_wrap_inline1684 tex2html_wrap_inline1686 E E 1 23.11(.07) 0.22 0.08 0.12 45
ID_4_289_0 tex2html_wrap_inline1688 tex2html_wrap_inline1690 E E 1 25.73(.14) 0.16 0.11 0.20 49
ID_2_201_0 tex2html_wrap_inline1692 tex2html_wrap_inline1694 E Et 1 24.45(.16) 0.14 0.15 0.24 15
ID_2_272_0 tex2html_wrap_inline1696 tex2html_wrap_inline1698 E - 2 25.98(.16) 0.17 0.26 0.28 34
ID_2_363_0 tex2html_wrap_inline1700 tex2html_wrap_inline1702 E - 2 25.94(.15) 0.15 0.25 0.33 17
ID_2_121_111 tex2html_wrap_inline1704 tex2html_wrap_inline1706 S0 - 1 21.48(.11) 0.60 0.04 0.23 30
ID_2_412_11 tex2html_wrap_inline1708 tex2html_wrap_inline1710 E/S0 Sap 1 25.42(.22) 0.17 0.27 0.42 7
ID_4_260_112 tex2html_wrap_inline1712 tex2html_wrap_inline1714 E E 2 25.08(.13) 0.18 0.30 0.28 13
ID_2_449_1 tex2html_wrap_inline1716 tex2html_wrap_inline1718 E E 2 24.11(.22) 0.22 0.24 0.30 15
ID_2_173_0 tex2html_wrap_inline1720 tex2html_wrap_inline1722 E E 2 23.74(.29) 0.72 0.18 0.35 8
ID_2_537_12 tex2html_wrap_inline1724 tex2html_wrap_inline1726 E Et 2 25.60(.15) 0.20 0.32 0.35 12
ID_3_51_0 tex2html_wrap_inline1728 tex2html_wrap_inline1730 E E/* 1 25.93(.17) 0.17 0.08 0.14 5
ID_2_236_2 tex2html_wrap_inline1732 tex2html_wrap_inline1734 E Et 1 24.81(.10) 0.16 0.03 0.13 65
ID_2_180_0 tex2html_wrap_inline1736 tex2html_wrap_inline1738 E Sa 2 24.65(.15) 0.20 0.50 0.47 4
ID_4_274_0 tex2html_wrap_inline1740 tex2html_wrap_inline1742 S0 - 2 26.02(.25) 0.17 0.23 0.24 10
ID_2_264_2 tex2html_wrap_inline1744 tex2html_wrap_inline1746 E E 1 22.80(.11) 0.21 0.24 0.26 30
ID_2_456_1111 tex2html_wrap_inline1748 tex2html_wrap_inline1750 S0 - 1 18.93(.05) 0.52 0.37 0.39 7
ID_3_229_0 tex2html_wrap_inline1752 tex2html_wrap_inline1754 E E/* 2 24.96(.24) 0.24 0.10 0.12 15
ID_2_590_1 tex2html_wrap_inline1756 tex2html_wrap_inline1758 E - 2 25.89(.21) 0.16 0.18 0.16 55
ID_4_109_0 tex2html_wrap_inline1760 tex2html_wrap_inline1762 E p 2 25.29(.12) 0.16 0.38 0.40 10
ID_3_143_0 tex2html_wrap_inline1764 tex2html_wrap_inline1766 m Sap 3 22.85(.15) 0.32 0.55 0.65 4
Table 1: The sample

 

FOCAS-ID Coordinates (2000) MorphologyLum.Prof. tex2html_wrap_inline1502 tex2html_wrap_inline1504 tex2html_wrap_inline1506 tex2html_wrap_inline1508 tex2html_wrap_inline1510
tex2html_wrap_inline1498 tex2html_wrap_inline1500Our V.d.Bergh class (STMAG) (tex2html_wrap_inline1516) (tex2html_wrap_inline1518)
ID_3_243_0 tex2html_wrap_inline1786 tex2html_wrap_inline1788 E/S0p Ep 1 25.52(.21) 0.21 0.32 0.34 14
ID_2_456_22 tex2html_wrap_inline1790 tex2html_wrap_inline1792 S0 Ep 1 25.15(.12) 0.17 0.36 0.50 6
ID_3_321_1 tex2html_wrap_inline1794 tex2html_wrap_inline1796 S0 - 1 23.05(.13) 0.43 0.34 0.36 5
ID_2_373_0 tex2html_wrap_inline1798 tex2html_wrap_inline1800 E E 1 25.58(.18) 0.16 0.27 0.33 9
ID_2_725_0 tex2html_wrap_inline1802 tex2html_wrap_inline1804 E E 2 25.50(.15) 0.17 0.52 0.44 12
ID_2_693_1 tex2html_wrap_inline1806 tex2html_wrap_inline1808 E E 2 25.22(.15) 0.18 0.18 0.25 22
ID_3_659_2 tex2html_wrap_inline1810 tex2html_wrap_inline1812 Ep E/S0 1 25.69(.15) 0.13 0.18 0.17 23
ID_3_902_1 tex2html_wrap_inline1814 tex2html_wrap_inline1816 E/Sa - 2 26.25(.22) 0.16 0.18 0.20 42
ID_2_531_0 tex2html_wrap_inline1818 tex2html_wrap_inline1820 E E 3 24.06(.15) 0.29 0.10 0.07 15
ID_3_586_0 tex2html_wrap_inline1822 tex2html_wrap_inline1824 E E/Sa 1 25.77(.36) 0.36 0.14 0.19 20
ID_2_646_0 tex2html_wrap_inline1826 tex2html_wrap_inline1828 E E 2 25.14(.21) 0.20 0.42 0.38 5
ID_2_849_0 tex2html_wrap_inline1830 tex2html_wrap_inline1832 E Ep 2 25.60(.26) 0.19 0.32 0.30 15
ID_3_625_0 tex2html_wrap_inline1834 tex2html_wrap_inline1836 m p 3 25.39(.13) 0.15 0.50 0.49 3
ID_3_696_0 tex2html_wrap_inline1838 tex2html_wrap_inline1840 E E 2 23.77(.09) 0.19 0.46 0.45 10
ID_2_637_0 tex2html_wrap_inline1842 tex2html_wrap_inline1844 E E/* 1 25.18(.15) 0.18 0.11 0.14 80
ID_3_886_0 tex2html_wrap_inline1846 tex2html_wrap_inline1848 E E 2 25.21(.18) 0.14 0.16 0.21 8
ID_2_726_1 tex2html_wrap_inline1850 tex2html_wrap_inline1852 E - 1 26.75(.19) 0.11 0.23 0.18 35
ID_2_635_0 tex2html_wrap_inline1854 tex2html_wrap_inline1856 E - 1 25.99(.15) 0.17 0.24 0.24 29
ID_2_591_2 tex2html_wrap_inline1858 tex2html_wrap_inline1860 E - 2 25.05(.15) 0.18 0.31 0.30 10
ID_3_670_1 tex2html_wrap_inline1862 tex2html_wrap_inline1864 E E 2 25.73(.21) 0.14 0.28 0.26 20
ID_2_643_0 tex2html_wrap_inline1866 tex2html_wrap_inline1868 m E+E 3 25.03(.17) 0.22 0.65 0.62 5
ID_2_973_2 tex2html_wrap_inline1870 tex2html_wrap_inline1872 E - 2 25.88(.18) 0.18 0.23 0.19 10
ID_3_118_1 tex2html_wrap_inline1874 tex2html_wrap_inline1876 E E/* 1 24.59(.17) 0.14 0.08 0.09 10
ID_2_898_0 tex2html_wrap_inline1878 tex2html_wrap_inline1880 m Sa 3 25.14(.20) 0.27 0.49 0.50 3
ID_3_743_0 tex2html_wrap_inline1882 tex2html_wrap_inline1884 E - 2 25.61(.20) 0.19 0.11 0.14 93
ID_3_266_0 tex2html_wrap_inline1886 tex2html_wrap_inline1888 E E 1 25.19(.19) 0.22 0.30 0.31 3
ID_3_180_1 tex2html_wrap_inline1890 tex2html_wrap_inline1892 E/S0 E 1 23.94(.21) 0.51 0.18 0.19 70
ID_2_966_0 tex2html_wrap_inline1894 tex2html_wrap_inline1896 E E 2 25.83(.19) 0.18 0.24 0.24 40
ID_3_904_0 tex2html_wrap_inline1898 tex2html_wrap_inline1900 E E 1 25.11(.13) 0.17 0.08 0.14 70
ID_3_815_1 tex2html_wrap_inline1902 tex2html_wrap_inline1904 E/S0 E 1 24.29(.20) 0.41 0.21 0.22 11
ID_3_355_0 tex2html_wrap_inline1906 tex2html_wrap_inline1908 E E 1 24.16(.20) 0.39 0.04 0.08 5
ID_3_726_0 tex2html_wrap_inline1910 tex2html_wrap_inline1912 E - 2 26.15(.18) 0.18 0.29 0.28 12
ID_3_363_1 tex2html_wrap_inline1914 tex2html_wrap_inline1916 E - 1 26.23(.20) 0.11 0.05 0.04 5
ID_3_813_0 tex2html_wrap_inline1918 tex2html_wrap_inline1920 E - 1 26.02(.18) 0.17 0.04 0.08 135
ID_3_748_0 tex2html_wrap_inline1922 tex2html_wrap_inline1924 E - 1 26.23(.17) 0.15 0.23 0.20 16
ID_3_888_1 tex2html_wrap_inline1926 tex2html_wrap_inline1928 E Sap 1 25.57(.18) 0.17 0.12 0.18 42
ID_3_790_1 tex2html_wrap_inline1930 tex2html_wrap_inline1932 E - 1 22.60(.14) 0.40 0.10 0.13 20

Table 1: continued


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