A&A 384, 24-32 (2002)
DOI: 10.1051/0004-6361:20011803

Large-scale star formation in galaxies

II. The spirals NGC 3377A, NGC 3507 and NGC 4394.
Young star groupings in spirals

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


1 Introduction

Global star formation in a spiral galaxy is influenced by several factors. Different authors have investigated the dependence of the star formation rate and efficiency on environment. Star formation in interacting galaxies is generally enhanced, while its dependence upon galaxy morphological type is not well assessed (Young et al. 1996 and references therein). Some recent papers have shown the importance of the determination of empirical correlations between large-scale star forming regions and parent galaxy properties in understanding the star formation process. In this framework Elmegreen et al. (1994, 1996) and Elmegreen & Salzer (1999), analyzing different samples of spirals, have found a clear correlation between the size, the total B magnitude of a galaxy and the size of its largest stellar complex. These authors suggest that these scaling laws can be related to the properties of the interstellar medium and of the instability process itself. However, we note that Selman & Melnick (2000) have recently explained that the correlation between the size of the largest superassociation and the luminosity of the parent galaxy, found by Elmegreen et al. (1994), is most likely due to a size-of-sample effect.

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.

2 Observations and data reduction

All data were obtained with the SPIcam camera at the 3.5 m telescope of the Apache Point Observatory (New Mexico, USA)[*], in four different broadbands: U, B, V and R. SPIcam is a backside illuminated thinned SITe CCD, with a size of $1024\times1024$ pixels, with an angular scale of $0.28\hbox{$^{\prime\prime}$ }\!$/pixel and a field of view of about $4.8 \hbox{$^\prime$ }\times4.8\hbox{$^\prime$ }$.

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.


 

 
Table 1: Observational log.
  Filter Date Exp. time FWHM
    (d/m/y) (s) (arcsec)
NGC 3377A U< 21/11/98 3$\times$400 1.18
  B 21/11/98 3$\times$200 1.18
  V 21/11/98 3$\times$200 1.40
  R 21/11/98 3$\times$200 1.18
NGC 3507 U 21/12/98 3$\times$400 1.04
  B 21/12/98 3$\times$200 1.12
  V 21/12/98 3$\times$200 0.92
  R 21/12/98 3$\times$200 0.84
NGC 4394 U 17/01/99 2$\times$800,1$\times$400 1.12
  B 18/01/99 3$\times$200 1.06
  V 18/01/99 3$\times$200 0.90
  R 18/01/99 3$\times$200 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 $7\hbox{$^{\prime\prime}$ }$, 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.

3 Results

3.1 NGC 3377A

NGC 3377A is an almost face-on dwarf spiral galaxy, located about $7 \hbox{$^\prime$ }$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 $M_{\rm HII}$/$L_{\rm B}$ ratio ($\sim$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 $M_{\odot}$/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, $N_{\rm lim}$, 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 $N_{\rm lim}$ pixels is less than 10$\%$. The computed $N_{\rm lim}$ 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''.


 

 
Table 2: YSGs of NGC 3377A. N is the identification number, X and Y are the coordinates in arcsecs. R is the galactocentric distance (in kpc), corrected for the inclination of the galaxy. B is the total blue magnitudes. $\Sigma _{B}$ is the surface brightness (in mag/arcsec2). Magnitudes are corrected by the extinction, from RC3. Dxy is the linear dimension (in pc).
N X Y R B $\Sigma _{B}$ Dxy N X Y R B $\Sigma _{B}$ 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 ${\rm d}N\propto D_{xy}^\alpha {\rm d}D_{xy}$we find $\alpha=-2.3\pm0.3$ (correlation coefficient r=0.93). The shaded strips shown in Fig. 4 are the Dxy intervals directly affected by the introduction of the $N_{\rm lim}$ threshold for each galaxy. Such shaded areas are therefore certainly incomplete.


  \begin{figure} \par\includegraphics[width=8.8cm,clip]{ms1623f1.ps}\end{figure} 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.
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Knezek et al. (1999) give a multi-color map of NGC 3377A where several H$\alpha$ "knots'' are evident over the whole optical disk. A comparison of our YSG map with the H$\alpha$ knots shows that a large percentage of the latter (about $80\%$) correspond to YSGs. Two out of the six H$\alpha$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$\alpha$ 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$\alpha$ emission, but are well characterized in our work.

3.2 NGC 3507

The spiral NGC 3507 is a typical barred galaxy (SBb type), with two long regular symmetric arms circling around the nucleus and the bar. The inner parts of each arm are more regular than the outer parts. The isophotal major axis is $D_{25}=3.4 \hbox{$^\prime$ }$ (RC3), the inclination is 26$^\circ $ (Schulman et al. 1997) while the position angle is about 90$^\circ $(de Jong & van der Kruit 1994). According to Garcia et al. (1993), NGC 3507 is the brightest member of a small group of galaxies. From the radial velocity of 906 km s-1 (RC3) and H0=75 km s-1/Mpc, a distance of 12.1 Mpc is obtained that corresponds to a scale of 16.4 pc/pixel. Schulman et al. (1997) studied the HI distribution and dynamics in a small sample of galaxies, including NGC 3507. This paper concludes that in NGC 3507 there are no High Velocity Clouds (HVCs, i.e. HI regions with peculiar motion). Since HVCs are thought to be star formation indicators (probably originating in Supernova explosions) the lack of HVCs in NGC 3507 suggests a low star formation rate in this galaxy. Figure 2 shows the map of the 90 YSGs detected: most of the objects are located along the inner part of the arms. In Table 3 we give the main properties of the YSGs. Galactic and internal B-absorptions are taken to be 0.00 mag and 0.10 mag, respectively, from RC3. The YSGs average size is 121 pc, with a standard deviation of 147 pc and the slope of the fitted power law to the size distribution is the smallest one with $\alpha=-1.6$ $\pm$ 0.3 (r=0.80).


   
Table 3: YSGs of NGC 3507. See caption of Table 2.
N X Y R B $\Sigma _{B}$ Dxy N X Y R B $\Sigma _{B}$ 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


  \begin{figure} \par\includegraphics[width=8.8cm,clip]{ms1623f2.ps}\end{figure} 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$^\circ $. Coordinates are in arcseconds and the offset is relative to the galactic centre.
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3.3 NGC 4394

NGC 4394 is a barred and ringed spiral galaxy, with integrated B magnitude 11.53 mag and with $D_{25}=3.6 \hbox{$^\prime$ }$ (RC3). Its proximity to M 85, just $6\hbox{$^\prime$ }$ East, and their similar radial velocities suggest that NGC 4394 may belong to the M 85 subgroup of the Virgo Cluster. Thus we assume as the distance of this galaxy that deduced by Ferrarese et al. (2000) (about 16 Mpc) for the M 85 subcluster based on Cepheid indicators. The inclination and the position angle are taken from Chapelon et al. (1999) and are 25$^\circ $ and 103$^\circ $, respectively.

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, $\sigma _B$, 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 $\alpha$ of the power law is $-2.7\pm0.3$ (r=0.92).


   
Table 4: YSGs of NGC 4394. See caption of Table 2.
N X Y R B $\Sigma _{B}$ Dxy N X Y R B $\Sigma _{B}$ Dxy N X Y R B $\Sigma _{B}$ 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              


  \begin{figure} \par\includegraphics[width=8.8cm,clip]{ms1623f3.ps}\end{figure} 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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  \begin{figure} \par\includegraphics[width=8.8cm,clip]{ms1623f4.ps}\end{figure} 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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  \begin{figure} \par\includegraphics[width=8.8cm,clip]{1623f5.eps}\end{figure} Figure 5: The surface luminosity density ($\sigma _B$, in $10^6~L_{B,\odot}$ kpc-2) of YSGs averaged over concentric circular annuli as a function of the galactrocentric radius ( R, in kpc) in NGC 3377A, NGC 3507 and NGC 4394. The YSG positions were corrected for the galaxy inclination.
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  \begin{figure} \par\includegraphics[width=8.8cm,clip]{ms1623f6.ps}\end{figure} 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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4 Discussion and conclusions

With our semi-automatic identification algorithm we have determined the population and main characteristics of Young Star Groupings in the spiral galaxies NGC 3377A, NGC 3507 and NGC 4394. The data collected constitute not only an enlargement of the YSG data-base but also a direct source of astrophysical information about the properties of the regions of star formation in spirals.

The analysis of the YSG differential B luminosity functions (Fig. 6) shows that their high luminosity tails can be well fitted with power laws ${\rm d}N\propto L_B^\beta {\rm d}L_B$. We find $\beta=-1.79\pm0.10$, $-1.31\pm0.08$ and $-1.90\pm0.18$, 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 $N_{\rm lim}$ 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 $N_{\rm lim}$. 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).

  \begin{figure} \par\includegraphics[width=8.8cm,clip]{ms1623f7.ps}\end{figure} 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 ( $N_{\rm YSG}$) vs. MB. Panel c) Total area of the YSG ( $A_{\rm YSG}$) vs. MB. Panel d) Blue magnitude of the brightest YSG ( $M_{B,{\rm max}}$) vs. the diameter of the galaxy (D25) taken from RC3.
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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 ( ${\rm d}N \propto D_{xy}^{-4.2} {\rm d}D_{xy}$). 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.

References

 

Copyright ESO 2002