A&A 384, 24-32 (2002)
DOI: 10.1051/0004-6361:20011803
A. Vicari1 - P. Battinelli2 - R. Capuzzo-Dolcetta1 - T. K. Wyder3 - G. Arrabito4
1 - Dipart. di Fisica, Università La Sapienza, P.le A. Moro 5, Roma, Italy
2 -
Oss. Astronomico di Roma, Viale del Parco Mellini 84, Roma, Italy
3 -
California Inst. of Technology MC 405-47 1200 E. Cal. Blvd Pasadena, CA 91125, USA
4 -
Dipart. di Matematica, Università La Sapienza, P.le A. Moro 5, Roma, Italy
Received 28 June 2001 / Accepted 23 November 2001
Abstract
The identification of young star groupings (YSG) in the three spiral galaxies
NGC 3377A, NGC 3507, NGC 4394 is obtained by mean of the statistical
method described in Paper I. We find 83, 90, 185
YSGs, respectively. An identification map of YSGs,
as well as their size distribution, their
B-luminosity function and their surface luminosity density radial
behaviour are presented and compared.
These data, in addition to those in Paper I, constitute a first sample suitable
for seeking correlations among properties of galaxies and their YSGs, which we
briefly discuss here.
Key words: galaxies: spiral - galaxies: general - galaxies: stellar content - stars: formation - methods: statistical
The problem of the identifications of regions of star formation in distant, unresolved, galaxies is quite difficult, because the identification suffers from several biases, like those introduced by differences in observational data and/or in the identification criteria adopted. Hodge (1986) discussed these difficulties when trying to derive general properties of star forming regions.
To overcome these complications we developed an automatic method for the identification of the star forming regions (Young Star Groupings, or YSGs hereafter) in unresolved galaxies (Adanti et al. 1994). The availability of various sets of colours and fluxes allows every pixel of the galaxy image to be represented as a point in the space of the variables (i.e. of fluxes and colours). Principal Component Analysis (PCA) and Cluster Analysis (CA) result in an artificial image of the galaxy, where pixels are grouped into classes according to their relative distances in the space of the variables. As in Battinelli et al. (2000, hereafter Paper I), we performed our classification using U, U-B, B-V and B-R as variables. A first step to a deeper understanding of the link between the YSG's and parent galaxy's properties is to build a homogeneous and "objective'' database. Target galaxies were chosen according to the criteria discussed in Paper I.
In this paper we present data and results of YSGs in three spiral galaxies: NGC 3337A, NGC 3507 and NGC 4394. In Sect. 2 observational data acquisition and reduction are described; in Sect. 3 we present and discuss our identifications in each galaxy. Finally, in Sect. 4, we discuss some properties of the sample of YSGs in all the six galaxies we have studied so far.
Three exposures per filter were taken for each galaxy. The CCDPROC
routine in IRAF was used to subtract
the bias, as measured in the overscan region of each image, as well
as to divide by the appropriate flat field for that night and filter.
The spatial offsets among the images of a particular galaxy were obtained using
the positions of stars within each field. The images were shifted to
a common reference frame using the IMALIGN routine in IRAF. Finally,
the IRAF task IMCOMBINE was used to average the three exposures
in each filter while at the same time rejecting cosmic rays.
NGC 3377A was observed on the night of 1998 November 21, a photometric night, while the NGC 3507 and NGC 4394 images were taken on 1998 December 21 and on 17-18 January 1999 respectively. These last three nights were not photometric, so we obtained shallower exposures of NGC 3507 and of NGC 4394 on a clear night, 16 April 1999, which we used to calibrate these images. The ratio in each filter between the averaged images of the non-photometric nights and the shallower images taken on the last photometric night was determined using stars within the field, if present, or using the radial profile of the galaxy itself. Total exposure times are always 1200 s in the U images and 600 s in the B, V and R bands, with the exception of the U image of NGC 4394, whose exposure time is 2000 s. The seeing of each image is reported in Table 1.
Filter | Date | Exp. time | FWHM | |
---|---|---|---|---|
(d/m/y) | (s) | (arcsec) | ||
NGC 3377A | U< | 21/11/98 | 3![]() |
1.18 |
B | 21/11/98 | 3![]() |
1.18 | |
V | 21/11/98 | 3![]() |
1.40 | |
R | 21/11/98 | 3![]() |
1.18 | |
NGC 3507 | U | 21/12/98 | 3![]() |
1.04 |
B | 21/12/98 | 3![]() |
1.12 | |
V | 21/12/98 | 3![]() |
0.92 | |
R | 21/12/98 | 3![]() |
0.84 | |
NGC 4394 | U | 17/01/99 | 2![]() ![]() |
1.12 |
B | 18/01/99 | 3![]() |
1.06 | |
V | 18/01/99 | 3![]() |
0.90 | |
R | 18/01/99 | 3![]() |
0.92 |
As mentioned in Paper I, the flat fields (one for each night and for each band) may be contaminated by scattered light from the lack of baffling of the 3.5 m APO telescope, leading to an artificial spatial gradient in the sky background. This is more evident in the B and R band images than in the U and B ones. The one exception is NGC 4394 where some of the gradient in the background is due to the presence of the galaxy M 85, just off the field of view.
Calibration was performed using observations of several standard stars
from Landolt (1992). An aperture magnitude for each observation of each
standard star was obtained with an
aperture radius of
,
chosen to match the aperture used by Landolt.
Standards
included 3 stars in the field of PG 1323-086 and 5 stars in the field of PG 1633+099, with color index in the range from -0.9 to 1.1 for U-B, from -0.2 to
1.1 for B-V and from -0.1 to 0.6 for V-R. The standards were mostly observed
at low airmass, less than 1.2, with the entire range being from 1.1 to 2.2.
NGC 3377A is an almost face-on dwarf spiral galaxy, located about
north-west from the E6 giant elliptical NGC 3377, in the Leo I (M96)
group. The radial velocity difference of 120 km s-1 (de Vaucouleurs et al. 1991, hereafter RC3)
between these two galaxies suggests that NGC 3377A may be a
companion of the giant elliptical.
According to Tonry et al. (1997), NGC 3377A is about
10.7 Mpc distant, implying a linear separation from NGC 3377 of only
20 kpc. Sandage et al. (1991) and Knezek et al. (1999) showed that
this very low surface brightness galaxy has various peculiarities. In
particular, in spite of its high
/
ratio (
0.30 in
solar units, which is a typical value for a Sc type spiral), it has a
very low star formation rate, about 0.003
/yr, so that star
formation can continue for more than a Hubble time (Knezek et al. 1999).
The application of our algorithm to this galaxy led us to the
identification of 83 YSGs, whose positions in the galaxy are shown in
Fig. 1 and whose main characteristic parameters are given in Table 2.
We remind the reader that, in order to reduce the contamination by random groups in this list of YSGs candidates,
we adopted (as in Paper I) the procedure described by Battinelli & Demers (1992). This procedure
results in the introduction of a threshold,
,
in the number of pixels in each groups, such
that the contamination by random groups in the sample of all the groups
composed of at least
pixels is
less than 10
.
The computed
are 9, 4, 7 for NGC 3377A, NGC 3507 and NGC 4394, respectively.
Blue magnitudes and surface brightnesses have been corrected for a total
extinction of 0.06 mag (0.04 mag galactic extinction and 0.02 mag internal
extinction) as given in RC3.
As explained in Paper I (Sect. 3), a precise determination of the integrated colours
of YSGs is not easy since they often lie in areas with a lot of "structure''.
N | X | Y | R | B |
![]() |
Dxy | N | X | Y | R | B |
![]() |
Dxy |
1 | 39 | -127 | 6.9 | 24.56 | 24.29 | 72 | 43 | -26 | 12 | 1.51 | 23.65 | 24.14 | 86 |
2 | 99 | -121 | 8.13 | 24.67 | 24.3 | 43 | 44 | 11 | 11 | 0.84 | 23.66 | 24.09 | 86 |
3 | -130 | -96 | 8.42 | 24.4 | 24.33 | 72 | 45 | -163 | 12 | 8.49 | 23.22 | 23.15 | 57 |
4 | -136 | -83 | 8.27 | 24.43 | 24.37 | 72 | 46 | 9 | 12 | 0.83 | 24.19 | 24.21 | 72 |
5 | -166 | -82 | 9.61 | 24.56 | 24.5 | 72 | 47 | 12 | 13 | 0.97 | 23.44 | 24.37 | 115 |
6 | -99 | -71 | 6.35 | 22.07 | 23.55 | 159 | 48 | -21 | 15 | 1.37 | 21.08 | 23.59 | 275 |
7 | -19 | -68 | 3.71 | 24.64 | 24.58 | 57 | 49 | -32 | 14 | 1.86 | 24.36 | 24.1 | 72 |
8 | 63 | -66 | 4.76 | 24.77 | 24.39 | 57 | 50 | -1 | 15 | 0.83 | 21.13 | 23.63 | 231 |
9 | 28 | -65 | 3.69 | 24.65 | 24.39 | 57 | 51 | 2 | 14 | 0.78 | 23.74 | 24.52 | 101 |
10 | -122 | -49 | 6.84 | 24.78 | 24.52 | 72 | 52 | -2 | 14 | 0.78 | 24.91 | 24.53 | 57 |
11 | -21 | -45 | 2.62 | 23.21 | 23.64 | 86 | 53 | -26 | 15 | 1.59 | 21.84 | 23.56 | 173 |
12 | -1 | -31 | 1.63 | 21.43 | 23.63 | 231 | 54 | -31 | 16 | 1.85 | 22.57 | 24.03 | 144 |
13 | -25 | -31 | 2.1 | 24.58 | 24.6 | 72 | 55 | -14 | 16 | 1.15 | 23.97 | 24.29 | 86 |
14 | -24 | -29 | 2 | 23.46 | 23.84 | 86 | 56 | -25 | 17 | 1.58 | 24.47 | 24.31 | 72 |
15 | -21 | -28 | 1.84 | 24.48 | 24.1 | 57 | 57 | -21 | 17 | 1.46 | 22.71 | 23.93 | 130 |
16 | 4 | -26 | 1.39 | 24.28 | 24.22 | 72 | 58 | -19 | 18 | 1.4 | 24.1 | 24.03 | 72 |
17 | -29 | -22 | 1.93 | 24.27 | 24.29 | 72 | 59 | -12 | 18 | 1.15 | 24.52 | 24.14 | 57 |
18 | -24 | -22 | 1.72 | 24.04 | 24.21 | 72 | 60 | -10 | 20 | 1.21 | 24.1 | 24.48 | 101 |
19 | -15 | -20 | 1.35 | 21.38 | 23.83 | 231 | 61 | -12 | 22 | 1.34 | 23.57 | 24.21 | 101 |
20 | 40 | -17 | 2.28 | 25.02 | 24.64 | 57 | 62 | -3 | 24 | 1.28 | 23.73 | 24.46 | 115 |
21 | 25 | -15 | 1.53 | 23.71 | 24.2 | 86 | 63 | -15 | 24 | 1.52 | 22.86 | 24.25 | 130 |
22 | 36 | -14 | 2.03 | 24.02 | 23.95 | 57 | 64 | 0 | 24 | 1.29 | 24.76 | 24.49 | 57 |
23 | 9 | -13 | 0.84 | 23.08 | 23.72 | 101 | 65 | -1 | 25 | 1.34 | 24.13 | 24.31 | 72 |
24 | 33 | -11 | 1.82 | 22.31 | 23.93 | 159 | 66 | 0 | 27 | 1.42 | 22.23 | 24.1 | 188 |
25 | 4 | -11 | 0.65 | 24.18 | 23.91 | 57 | 67 | 8 | 25 | 1.4 | 24.6 | 24.53 | 72 |
26 | -161 | -8 | 8.35 | 24.27 | 24.58 | 72 | 68 | -12 | 28 | 1.61 | 21.29 | 23.8 | 260 |
27 | 3 | -4 | 0.31 | 21.41 | 24.08 | 289 | 69 | 8 | 27 | 1.47 | 23.95 | 24.05 | 72 |
28 | -9 | -4 | 0.54 | 20.87 | 23.34 | 217 | 70 | 9 | 28 | 1.53 | 23.75 | 24.13 | 101 |
29 | 106 | -5 | 5.5 | 23.86 | 23.79 | 57 | 71 | 12 | 29 | 1.64 | 22.5 | 24.07 | 188 |
30 | 8 | -5 | 0.53 | 25.01 | 24.64 | 43 | 72 | -11 | 29 | 1.65 | 24.51 | 24.13 | 57 |
31 | 9 | -1 | 0.49 | 25 | 24.62 | 57 | 73 | -12 | 31 | 1.73 | 24.62 | 24.24 | 57 |
32 | 5 | 0 | 0.3 | 24.23 | 24.33 | 86 | 74 | 9 | 35 | 1.87 | 23.67 | 24.4 | 101 |
33 | 7 | 0 | 0.39 | 24.44 | 24.37 | 72 | 75 | -44 | 43 | 3.24 | 22.96 | 23.89 | 115 |
34 | 7 | 0 | 0.36 | 24.73 | 24.35 | 57 | 76 | -5 | 42 | 2.22 | 24.23 | 24.26 | 72 |
35 | -52 | 1 | 2.7 | 22.51 | 24.04 | 144 | 77 | -92 | 45 | 5.33 | 22.27 | 23.52 | 115 |
36 | 9 | 4 | 0.56 | 24.89 | 24.62 | 72 | 78 | -32 | 49 | 3.07 | 22.23 | 23.82 | 159 |
37 | -22 | 8 | 1.25 | 21.1 | 23.61 | 246 | 79 | -118 | 94 | 7.85 | 24.88 | 24.61 | 57 |
38 | -10 | 8 | 0.69 | 24.54 | 24.48 | 72 | 80 | -154 | 95 | 9.4 | 24.99 | 24.73 | 57 |
39 | -94 | 8 | 4.89 | 24.22 | 24.15 | 57 | 81 | -149 | 115 | 9.8 | 24.13 | 24.44 | 101 |
40 | -54 | 10 | 2.9 | 23.32 | 24.1 | 86 | 82 | 35 | 125 | 6.73 | 24.37 | 24.11 | 57 |
41 | -12 | 10 | 0.87 | 24.29 | 24.66 | 86 | 83 | 77 | 130 | 7.85 | 24.21 | 24.05 | 57 |
42 | -21 | 11 | 1.27 | 24.79 | 24.53 | 57 |
We define the size, Dxy, of an YSG, as the average of the x and y extents; the size distribution is shown in Fig. 4 for the three
galaxies studied in this paper. For NGC 3377A the average size is 87 pc
with a standard deviation of 59 pc. Fitting the high-tail of the size
distribution with the power law
we find
(correlation coefficient r=0.93).
The shaded strips shown in Fig. 4 are the Dxy intervals directly affected
by the introduction of the
threshold for each galaxy. Such shaded areas
are therefore certainly incomplete.
![]() |
Figure 1: Panel a) B band image of NGC 3377A. Panel b) map of the identified YSGs. North is up and East to the left. Coordinates are in arcseconds and the offset is relative to the galactic centre. |
Open with DEXTER |
Knezek et al. (1999) give a multi-color map of NGC 3377A where several
H
"knots'' are evident over the whole optical disk.
A comparison of our YSG map with the H
knots shows
that a large percentage of the latter (about
)
correspond to YSGs. Two out of the six H
sources that have no counterpart in our YSG sample are very close to a bright field star, three
are in the very outskirts of the optical galaxy and just one lies along a spiral branch.
This is a very tiny H
region perhaps related to a YSG too small to be resolved by us.
The most significant difference between our YSGs and
Knezek 's sources is in two big clumps near the center that do not show well localized
H
emission, but are well characterized in our work.
N | X | Y | R | B |
![]() |
Dxy | N | X | Y | R | B |
![]() |
Dxy |
1 | -30 | -75 | 4.76 | 19.88 | 22.26 | 229 | 46 | -65 | -17 | 4.26 | 21.75 | 22.3 | 98 |
2 | 17 | -76 | 4.77 | 22.67 | 22.69 | 81 | 47 | 54 | -17 | 3.73 | 22.18 | 22.67 | 81 |
3 | 5 | -72 | 4.34 | 20.3 | 22.31 | 196 | 48 | -63 | -12 | 4.13 | 21.57 | 22.38 | 114 |
4 | 6 | -69 | 4.19 | 20 | 22.19 | 212 | 49 | -71 | -10 | 4.6 | 22.8 | 22.53 | 65 |
5 | -7 | -59 | 3.56 | 21.88 | 22.37 | 98 | 50 | 30 | -10 | 2.11 | 23.5 | 22.48 | 32 |
6 | -14 | -58 | 3.51 | 21.03 | 22.33 | 147 | 51 | -40 | -9 | 2.64 | 23.58 | 22.32 | 32 |
7 | 4 | -58 | 3.52 | 22.7 | 22.53 | 65 | 52 | -37 | -5 | 2.44 | 23.4 | 22.38 | 32 |
8 | -16 | -58 | 3.54 | 22.77 | 22.61 | 81 | 53 | -36 | -5 | 2.38 | 23.52 | 22.5 | 48 |
9 | -8 | -56 | 3.35 | 20.53 | 22.23 | 163 | 54 | -67 | -4 | 4.34 | 24.02 | 22.77 | 32 |
10 | -15 | -54 | 3.35 | 20.81 | 22.34 | 163 | 55 | -33 | 5 | 2.2 | 16.82 | 21.92 | 1032 |
11 | 8 | -55 | 3.36 | 21.74 | 22.33 | 81 | 56 | 28 | 0 | 1.86 | 19.69 | 22.22 | 294 |
12 | 4 | -55 | 3.31 | 23.8 | 22.54 | 32 | 57 | -38 | -2 | 2.45 | 23.49 | 22.24 | 32 |
13 | -49 | -53 | 4.36 | 21.84 | 22.48 | 98 | 58 | -27 | 3 | 1.77 | 23 | 22.49 | 48 |
14 | 10 | -52 | 3.26 | 22.54 | 22.47 | 65 | 59 | 34 | 9 | 2.25 | 18.94 | 22 | 376 |
15 | -20 | -49 | 3.14 | 21.29 | 22.38 | 163 | 60 | 28 | 10 | 1.92 | 21.95 | 22.12 | 81 |
16 | 1 | -49 | 2.95 | 23.97 | 22.95 | 48 | 61 | 32 | 11 | 2.13 | 21.62 | 22.26 | 98 |
17 | 21 | -49 | 3.31 | 23.18 | 22.67 | 48 | 62 | -63 | 11 | 4.22 | 22.87 | 22.71 | 65 |
18 | -36 | -46 | 3.48 | 23.31 | 22.8 | 65 | 63 | -27 | 13 | 1.97 | 22.56 | 22.18 | 65 |
19 | -46 | -44 | 3.84 | 20.28 | 22.4 | 212 | 64 | 33 | 14 | 2.24 | 20.01 | 22.11 | 196 |
20 | -21 | -43 | 2.83 | 20.56 | 22.33 | 180 | 65 | -25 | 15 | 1.97 | 22.9 | 22.39 | 48 |
21 | -32 | -36 | 2.94 | 17.75 | 22.1 | 622 | 66 | -20 | 18 | 1.77 | 18.38 | 22.09 | 491 |
22 | -26 | -40 | 2.86 | 21.39 | 22.31 | 98 | 67 | 32 | 18 | 2.29 | 20.39 | 22.16 | 180 |
23 | 30 | -40 | 3.2 | 21.68 | 22.37 | 98 | 68 | 33 | 17 | 2.35 | 22.47 | 22.31 | 81 |
24 | -19 | -38 | 2.55 | 21.21 | 22.43 | 130 | 69 | 5 | 20 | 1.24 | 23.13 | 22.47 | 48 |
25 | 10 | -39 | 2.48 | 22.46 | 22.56 | 81 | 70 | 1 | 22 | 1.33 | 21.81 | 22.18 | 81 |
26 | 11 | -36 | 2.33 | 20.29 | 22.22 | 212 | 71 | -13 | 24 | 1.73 | 21.28 | 22.17 | 98 |
27 | -14 | -36 | 2.3 | 23.81 | 22.79 | 48 | 72 | 3 | 24 | 1.46 | 20.19 | 22.11 | 163 |
28 | -10 | -36 | 2.2 | 23.51 | 22.7 | 32 | 73 | 31 | 24 | 2.41 | 20.38 | 21.97 | 147 |
29 | 13 | -34 | 2.27 | 22.44 | 22.61 | 81 | 74 | 0 | 24 | 1.48 | 22.48 | 22.32 | 65 |
30 | 10 | -31 | 2.05 | 20.44 | 22.18 | 163 | 75 | -3 | 25 | 1.53 | 22.73 | 22.07 | 48 |
31 | -59 | -32 | 4.19 | 21.95 | 22.59 | 98 | 76 | 22 | 27 | 2.1 | 22.03 | 22.4 | 81 |
32 | -86 | -32 | 5.78 | 24.33 | 23.08 | 32 | 77 | 29 | 29 | 2.51 | 21.68 | 22.32 | 114 |
33 | 6 | -29 | 1.83 | 23.01 | 22.19 | 48 | 78 | -1 | 34 | 2.07 | 23.32 | 22.81 | 48 |
34 | 6 | -27 | 1.72 | 19.53 | 22 | 262 | 79 | 37 | 36 | 3.14 | 20.58 | 22.17 | 147 |
35 | -36 | -28 | 2.81 | 23.2 | 22.38 | 48 | 80 | -13 | 42 | 2.71 | 18.85 | 21.76 | 278 |
36 | 15 | -26 | 1.91 | 18.52 | 21.68 | 393 | 81 | -9 | 44 | 2.78 | 20.18 | 22.11 | 196 |
37 | 20 | -22 | 1.95 | 18.7 | 21.95 | 376 | 82 | -50 | 45 | 4.39 | 21.18 | 22.33 | 114 |
38 | 25 | -24 | 2.26 | 20.02 | 22.17 | 212 | 83 | -39 | 45 | 3.82 | 23.32 | 22.5 | 32 |
39 | 14 | -24 | 1.78 | 23.11 | 22.09 | 48 | 84 | 2 | 47 | 2.8 | 22.34 | 22.36 | 81 |
40 | -39 | -24 | 2.82 | 23.34 | 22.32 | 32 | 85 | -1 | 49 | 2.97 | 19.51 | 22.18 | 262 |
41 | -39 | -19 | 2.73 | 18.97 | 22.18 | 475 | 86 | 26 | 51 | 3.38 | 20.52 | 22.18 | 147 |
42 | -60 | -22 | 4.03 | 21.24 | 22.2 | 98 | 87 | 0 | 54 | 3.21 | 21.25 | 22.31 | 130 |
43 | -36 | -20 | 2.56 | 23.06 | 22.41 | 48 | 88 | 0 | 56 | 3.37 | 23.34 | 22.52 | 48 |
44 | -35 | -19 | 2.49 | 22.26 | 22.43 | 81 | 89 | 15 | 57 | 3.5 | 23.37 | 22.55 | 48 |
45 | 22 | -19 | 1.93 | 22.01 | 22.25 | 81 | 90 | 16 | 58 | 3.56 | 24.14 | 22.88 | 32 |
![]() |
Figure 2:
Panel a) B band image of NGC 3507. Panel b) map of the identified
YSGs. Orientation of the image is shown. The angle between the y-axis and
North is 70![]() |
Open with DEXTER |
The distribution of the YSGs, shown in Fig. 3, is clearly concentrated
in the ring region and just few YSGs are detected along the spiral arms.
This is supported by the radial distribution of
the surface luminosity density, ,
defined as the total B
luminosity of the YSGs found in an annulus divided by its area (see Fig. 5).
This quantity is clearly related to the high-mass star formation rate.
The concentrated distribution of the YSGs probably reflects the high
abundance of cold gas detected in the rings of galaxies (Wong et al. 2000). Moreover, HII regions were studied by Hodge (1974) in NGC
4394 with a 2.1 m telescope. He also found a clear concentration
of the HII regions in the ring, few of them being distributed in the
outer arms.
Table 4 gives the YSG parameters; the average
YSG diameter is 114 pc with a standard deviation of 77 pc,
while the exponent
of the power law is
(r=0.92).
N | X | Y | R | B |
![]() |
Dxy | N | X | Y | R | B |
![]() |
Dxy | N | X | Y | R | B |
![]() |
Dxy |
1 | 50 | -87 | 7.91 | 21.97 | 22.51 | 108 | 63 | -35 | -18 | 3.42 | 23.21 | 22.84 | 65 | 125 | -33 | 28 | 3.54 | 23.48 | 22.83 | 87 |
2 | -28 | -73 | 6.29 | 20.35 | 22.83 | 347 | 64 | -35 | -15 | 3.29 | 23.25 | 22.88 | 87 | 126 | 7 | 30 | 2.49 | 21.59 | 22.93 | 173 |
3 | 21 | -73 | 5.98 | 21.59 | 22.93 | 173 | 65 | -36 | -14 | 3.37 | 21.17 | 22.84 | 239 | 127 | -34 | 31 | 3.72 | 20.91 | 22.63 | 304 |
4 | -30 | -66 | 5.89 | 22.14 | 22.78 | 130 | 66 | -25 | -14 | 2.49 | 23.53 | 23.15 | 87 | 128 | -20 | 29 | 2.86 | 22.48 | 22.32 | 87 |
5 | -42 | -60 | 6.02 | 21.57 | 22.89 | 173 | 67 | -69 | -14 | 6.06 | 23.3 | 23.23 | 87 | 129 | -29 | 30 | 3.38 | 22.55 | 22.65 | 130 |
6 | 26 | -59 | 5.04 | 23.6 | 23.22 | 87 | 68 | -55 | -11 | 4.82 | 21.47 | 22.93 | 173 | 130 | -36 | 31 | 3.91 | 23.43 | 22.92 | 87 |
7 | 24 | -58 | 4.92 | 22.92 | 23.09 | 108 | 69 | 45 | -11 | 3.91 | 22.64 | 22.95 | 108 | 131 | -42 | 32 | 4.34 | 22.99 | 22.93 | 108 |
8 | 0 | -50 | 3.92 | 21.87 | 22.84 | 152 | 70 | -35 | -11 | 3.2 | 23.47 | 22.96 | 65 | 132 | -41 | 32 | 4.28 | 22.37 | 23.06 | 173 |
9 | 22 | -44 | 3.9 | 20.99 | 22.77 | 239 | 71 | 54 | -9 | 4.63 | 20.62 | 22.75 | 325 | 133 | 3 | 32 | 2.56 | 22.53 | 22.84 | 108 |
10 | 26 | -44 | 4.04 | 21.66 | 22.87 | 173 | 72 | -32 | -10 | 2.93 | 23.2 | 23.04 | 108 | 134 | -20 | 33 | 3.06 | 22.03 | 22.35 | 130 |
11 | -8 | -42 | 3.41 | 21.5 | 22.96 | 217 | 73 | -34 | -8 | 3.03 | 21.09 | 22.62 | 195 | 135 | -22 | 33 | 3.18 | 21.31 | 22.34 | 195 |
12 | -6 | -41 | 3.28 | 21.86 | 22.93 | 195 | 74 | -74 | -8 | 6.43 | 23.03 | 23.05 | 87 | 136 | -37 | 33 | 4.07 | 22.9 | 23 | 87 |
13 | 38 | -42 | 4.54 | 23.82 | 23.32 | 87 | 75 | 42 | -6 | 3.64 | 21.12 | 22.74 | 260 | 137 | -34 | 33 | 3.86 | 22.71 | 22.89 | 108 |
14 | 26 | -39 | 3.74 | 23.65 | 22.99 | 65 | 76 | 40 | -7 | 3.43 | 23.42 | 22.77 | 65 | 138 | -29 | 33 | 3.57 | 23.08 | 22.81 | 108 |
15 | 27 | -38 | 3.7 | 23.29 | 23.02 | 87 | 77 | 59 | -6 | 5.03 | 23.31 | 23.15 | 87 | 139 | 2 | 33 | 2.66 | 23.15 | 22.5 | 65 |
16 | -17 | -37 | 3.34 | 23.09 | 22.83 | 87 | 78 | -54 | -5 | 4.69 | 21.73 | 22.27 | 130 | 140 | -24 | 34 | 3.29 | 22.8 | 22.42 | 87 |
17 | 47 | -36 | 4.87 | 22.24 | 23.01 | 130 | 79 | 51 | -4 | 4.38 | 19.55 | 22.32 | 347 | 141 | -1 | 35 | 2.75 | 21.59 | 22.75 | 217 |
18 | -13 | -37 | 3.16 | 23.31 | 23.05 | 87 | 80 | -44 | -5 | 3.83 | 23.07 | 23 | 108 | 142 | -25 | 35 | 3.4 | 23.04 | 22.67 | 87 |
19 | -16 | -36 | 3.25 | 23.55 | 23.05 | 87 | 81 | 43 | -4 | 3.7 | 23.09 | 22.83 | 108 | 143 | 5 | 35 | 2.79 | 23.52 | 22.87 | 65 |
20 | -10 | -36 | 3.04 | 22.21 | 22.85 | 108 | 82 | -52 | -3 | 4.45 | 21.94 | 23.1 | 173 | 144 | -40 | 35 | 4.33 | 23.12 | 23.06 | 87 |
21 | 12 | -35 | 2.93 | 21.13 | 22.57 | 195 | 83 | -49 | -1 | 4.23 | 23.32 | 23.06 | 87 | 145 | -8 | 35 | 2.82 | 23.25 | 22.87 | 87 |
22 | 5 | -35 | 2.79 | 23.47 | 23.1 | 87 | 84 | -42 | -1 | 3.63 | 22.71 | 22.88 | 108 | 146 | -22 | 35 | 3.3 | 23.05 | 22.68 | 65 |
23 | -17 | -35 | 3.19 | 22.33 | 22.87 | 130 | 85 | 50 | 0 | 4.26 | 21.91 | 22.98 | 173 | 147 | -17 | 36 | 3.11 | 23.35 | 22.84 | 87 |
24 | 29 | -34 | 3.59 | 21.48 | 22.61 | 195 | 86 | -37 | 0 | 3.2 | 23.13 | 22.75 | 108 | 148 | -3 | 36 | 2.82 | 22.2 | 22.74 | 130 |
25 | 38 | -34 | 4.19 | 22.93 | 23.03 | 108 | 87 | -37 | 1 | 3.17 | 21.99 | 22.72 | 195 | 149 | 3 | 35 | 2.82 | 23.37 | 22.99 | 65 |
26 | 48 | -34 | 4.84 | 21.56 | 22.92 | 173 | 88 | 41 | 2 | 3.57 | 19.5 | 22.44 | 412 | 150 | 4 | 35 | 2.82 | 23.49 | 22.84 | 65 |
27 | -20 | -32 | 3.12 | 20.68 | 22.81 | 304 | 89 | -71 | 0 | 6.1 | 22.59 | 22.9 | 108 | 151 | -34 | 36 | 3.96 | 23.53 | 23.03 | 87 |
28 | -51 | -33 | 5.18 | 23.96 | 23.31 | 65 | 90 | -41 | 1 | 3.5 | 21.83 | 22.42 | 152 | 152 | -20 | 36 | 3.27 | 23.24 | 22.86 | 108 |
29 | 10 | -33 | 2.71 | 22.7 | 22.72 | 108 | 91 | 37 | 2 | 3.23 | 23.24 | 22.98 | 87 | 153 | -31 | 36 | 3.87 | 21.6 | 22.79 | 195 |
30 | -24 | -29 | 3.19 | 19.42 | 22.6 | 477 | 92 | -42 | 3 | 3.6 | 21.07 | 22.73 | 260 | 154 | 0 | 36 | 2.86 | 23.51 | 22.86 | 65 |
31 | 19 | -32 | 2.98 | 23.02 | 22.76 | 108 | 93 | -37 | 3 | 3.17 | 21.88 | 22.61 | 173 | 155 | -17 | 36 | 3.18 | 23.29 | 22.91 | 87 |
32 | 33 | -33 | 3.73 | 23.11 | 22.73 | 87 | 94 | -47 | 4 | 4 | 22.91 | 23.01 | 108 | 156 | -13 | 36 | 3.06 | 23.22 | 22.84 | 65 |
33 | 12 | -32 | 2.7 | 23.15 | 22.5 | 65 | 95 | -36 | 5 | 3.16 | 21.72 | 22.57 | 173 | 157 | -45 | 37 | 4.76 | 22.93 | 22.95 | 108 |
34 | -12 | -31 | 2.74 | 22.27 | 23.04 | 173 | 96 | 46 | 5 | 4.02 | 23.66 | 23.15 | 87 | 158 | -24 | 37 | 3.51 | 22.32 | 22.5 | 108 |
35 | 17 | -31 | 2.83 | 22.97 | 22.47 | 87 | 97 | -50 | 6 | 4.32 | 22.66 | 22.9 | 108 | 159 | -8 | 37 | 2.99 | 22.92 | 22.95 | 108 |
36 | 14 | -31 | 2.69 | 22.63 | 22.36 | 87 | 98 | 52 | 8 | 4.51 | 21.41 | 22.89 | 217 | 160 | -23 | 37 | 3.48 | 22.59 | 22.76 | 108 |
37 | 18 | -30 | 2.81 | 21.96 | 22.39 | 130 | 99 | -41 | 8 | 3.58 | 23.48 | 22.83 | 65 | 161 | -3 | 37 | 2.9 | 23.48 | 22.97 | 87 |
38 | 22 | -30 | 2.95 | 21.67 | 22.31 | 152 | 100 | -36 | 11 | 3.18 | 22.04 | 22.47 | 152 | 162 | -6 | 37 | 2.97 | 22.68 | 22.93 | 130 |
39 | 11 | -30 | 2.5 | 22.96 | 22.58 | 87 | 101 | -42 | 11 | 3.71 | 23.19 | 22.81 | 65 | 163 | 0 | 37 | 2.94 | 23.71 | 23.06 | 87 |
40 | 13 | -29 | 2.56 | 23.25 | 22.59 | 87 | 102 | 29 | 12 | 2.73 | 23.57 | 23.06 | 87 | 164 | 0 | 40 | 3.12 | 20 | 22.78 | 456 |
41 | 9 | -29 | 2.39 | 23.45 | 22.8 | 65 | 103 | -37 | 12 | 3.28 | 22.86 | 22.48 | 65 | 165 | -33 | 38 | 4.06 | 22.12 | 23.05 | 152 |
42 | 18 | -28 | 2.69 | 21.83 | 22.47 | 152 | 104 | 30 | 13 | 2.83 | 22.68 | 22.92 | 108 | 166 | -26 | 38 | 3.71 | 22.26 | 22.85 | 152 |
43 | 21 | -28 | 2.83 | 22.91 | 22.54 | 87 | 105 | 42 | 16 | 3.93 | 21.18 | 23.04 | 282 | 167 | -5 | 38 | 3.03 | 23.03 | 23.05 | 108 |
44 | 7 | -29 | 2.33 | 23.44 | 22.79 | 87 | 106 | -42 | 16 | 3.79 | 20.79 | 22.7 | 304 | 168 | 21 | 39 | 3.61 | 23.19 | 23.03 | 87 |
45 | 37 | -29 | 3.82 | 23.2 | 22.7 | 65 | 107 | -38 | 16 | 3.47 | 23.14 | 22.49 | 87 | 169 | 8 | 41 | 3.33 | 21.18 | 22.69 | 173 |
46 | 41 | -27 | 4.07 | 22.83 | 23.01 | 108 | 108 | 36 | 18 | 3.49 | 20.72 | 22.88 | 304 | 170 | -20 | 41 | 3.61 | 23.67 | 23.16 | 87 |
47 | -53 | -26 | 5.06 | 21.9 | 22.93 | 152 | 109 | -35 | 17 | 3.23 | 23.08 | 22.57 | 87 | 171 | -7 | 43 | 3.39 | 22.78 | 22.89 | 108 |
48 | 8 | -26 | 2.2 | 21.69 | 22.42 | 152 | 110 | 45 | 18 | 4.21 | 22.46 | 23.05 | 108 | 172 | -1 | 43 | 3.38 | 23.61 | 22.96 | 87 |
49 | -17 | -25 | 2.53 | 21.3 | 22.76 | 195 | 111 | 48 | 18 | 4.43 | 22.38 | 22.87 | 108 | 173 | 0 | 43 | 3.4 | 22.86 | 22.8 | 87 |
50 | -34 | -25 | 3.62 | 20.78 | 22.7 | 239 | 112 | -37 | 19 | 3.48 | 23.11 | 22.46 | 65 | 174 | -10 | 45 | 3.61 | 20.89 | 22.8 | 260 |
51 | 12 | -25 | 2.2 | 22.16 | 22.18 | 87 | 113 | -40 | 21 | 3.77 | 20.25 | 22.58 | 325 | 175 | -33 | 46 | 4.53 | 23.73 | 23.08 | 65 |
52 | -20 | -24 | 2.65 | 22.78 | 22.96 | 130 | 114 | 67 | 20 | 6.03 | 21.5 | 22.6 | 152 | 176 | 64 | 47 | 6.78 | 22.35 | 22.99 | 130 |
53 | 37 | -24 | 3.64 | 21.13 | 22.73 | 217 | 115 | -32 | 22 | 3.22 | 22.95 | 22.3 | 87 | 177 | -71 | 50 | 7.12 | 22.89 | 23.14 | 87 |
54 | 41 | -25 | 3.98 | 22.6 | 22.85 | 108 | 116 | -48 | 23 | 4.46 | 23.57 | 23.19 | 87 | 178 | -24 | 57 | 4.84 | 22.7 | 23.24 | 108 |
55 | -33 | -23 | 3.45 | 23.35 | 22.98 | 87 | 117 | -32 | 23 | 3.24 | 22.96 | 22.31 | 87 | 179 | -64 | 59 | 7.03 | 23.45 | 23.19 | 65 |
56 | -69 | -22 | 6.21 | 22.56 | 23 | 108 | 118 | -38 | 24 | 3.69 | 22.89 | 22.73 | 87 | 180 | -48 | 59 | 6.09 | 23.03 | 23.13 | 108 |
57 | -42 | -21 | 4.08 | 22.05 | 21.54 | 87 | 119 | -101 | 25 | 8.79 | 21.44 | 22.3 | 152 | 181 | -49 | 68 | 6.65 | 21.08 | 22.66 | 195 |
58 | -37 | -19 | 3.61 | 20.93 | 22.79 | 260 | 120 | 12 | 24 | 2.23 | 23.46 | 23.08 | 108 | 182 | -26 | 69 | 5.75 | 22.81 | 23.12 | 108 |
59 | 40 | -20 | 3.77 | 23.49 | 22.84 | 65 | 121 | -40 | 25 | 3.92 | 21.96 | 22.89 | 173 | 183 | -99 | 76 | 10.13 | 22.75 | 22.68 | 108 |
60 | 44 | -18 | 3.99 | 21.24 | 22.56 | 195 | 122 | 10 | 26 | 2.27 | 21.9 | 22.83 | 173 | 184 | -97 | 78 | 10.1 | 23.38 | 23.01 | 87 |
61 | 40 | -18 | 3.65 | 23.03 | 22.52 | 87 | 123 | 32 | 26 | 3.56 | 22.43 | 23.02 | 130 | 185 | 11 | 93 | 7.42 | 23.55 | 23.17 | 65 |
62 | -29 | -17 | 2.91 | 21.01 | 22.71 | 239 | 124 | -15 | 28 | 2.48 | 20.63 | 22.33 | 325 |
![]() |
Figure 3: Panel a) B band image of NGC 4394. Panel b) map of the identified YSGs. North is left and East is down. Coordinates are in arcseconds and the offset is relative to the galactic centre. |
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![]() |
Figure 4: Size distribution of the YSGs. The size (in pc) is defined as the average of the x and y extents of the YSG. Shaded areas are the incompleteness strips as described in the text. |
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![]() |
Figure 5:
The surface luminosity density
(![]() ![]() |
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![]() |
Figure 6: The differential luminosity function of the YSGs in B-band. Crosses, circles and triangles refer to NGC 3377A, NGC 3507 and NGC 4394, respectively. |
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The analysis of the YSG differential B luminosity functions (Fig. 6)
shows that their high luminosity tails can be well fitted with power laws
.
We find
,
and
,
for NGC 3377A, NGC 3507 and NGC 4394,
respectively (the correlation coefficients r are equal to 0.94, 0.99 and 0.99).
These values are in good agreement with those found by Elmegreen & Salzer (1999) for the star forming
complexes in a sample of 11 galaxies.
Again, the introduction of the
threshold described in Sect. 3.1 implies a corresponding detection
limit in the YSG luminosity. We evaluated such detection limits for each galaxy by plotting the
luminosity as a function of number of pixels of the YSGs and then determining the value of the luminosity
corresponding to
.
The detection limits, the dotted lines in Fig. 6,
are close to the peaks of the luminosity functions, implying statistical incompleteness in those regions.
The sample of six spirals studied so far (three in Paper I and three in this paper) allows
us a preliminary investigation of the existence of correlations among the YSG and parent galaxy properties.
In Fig. 7 we show some evidence of positive correlations among the number, dimension,
integrated B magnitude and the total area of the YSG populations with global
characteristics of the galaxies (integrated B magnitude and dimension).
![]() |
Figure 7:
Correlations between YSG and parent galaxy properties. Panel a)
Logarithm of the size of the largest YSG vs. the integrated B magnitude of the galaxy (MB); the straight line is the
Elmegreen et al. (1994) relation (see text). Panel b) Number of YSGs
(
![]() ![]() ![]() |
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Incidentally, we note that in Fig. 7 the two ringed galaxies, NGC 7217 and
NGC 4394, are close to each other and, at least in panels b and d,
they seem to behave differently from the rest of the sample. Of course,
a larger number of ringed galaxies is necessary to check how real such a
behaviour is. In particular, the presence of these two galaxies
in our small sample results in a shallower slope of
the correlation between the size of the largest YSG
in a galaxy and the galaxy MB with respect to the one obtained by
Elmegreen et al. (1994). They claimed that their correlation closely matches
the expected variation in the characteristic length of the gaseous gravitational instability
with MB.
Such a correlation has been suggested by Selman & Melnick (2000) to be the result of a size-of-sample
effect acting on a universal size distribution (
). In order to check
this suggestion, we can compare the -4.2 slope with those of the fitted power laws
to the size distributions of the YSGs in NGC 3377A,
NGC 3507 and NGC 4394, limited to the statistically
reliable size ranges. The slopes found (-2.3,-1.6,-2.7 for NGC 3377A, NGC 3507, and NGC 4394,
respectively) are quite close to the value, -2, for a star forming molecular cloud size distribution
(see Solomon et al. 1987; Larson 1981). They are always too shallow to explain the Elmegreen et al. (1994)
relation by means of the size-of-sample effect. We stress, however, that our six points in Fig. 7a
suggest a slope not as steep as Elmegreen et al.'s relation.
However, we remark that our sample is still too small to draw firm conclusions about this question.