A&A 386, 134-139 (2002)
DOI: 10.1051/0004-6361:20020216

H$\alpha $ surface photometry of galaxies in the Virgo cluster
III. Observations with INT and NOT 2.5 m telescopes[*], [*]

A. Boselli1 - J. Iglesias-Páramo 1 - J. M. Vílchez 2 - G. Gavazzi 3


1 - Laboratoire d'Astronomie Spatiale, Traverse du Siphon, 13376 Marseille Cedex 12, France
2 - Instituto de Astrofísica de Andalucía (CSIC), Apdo. 3004, 18080, Granada, Spain
3 - Università degli Studi di Milano-Bicocca, Piazza delle scienze 3, 20126 Milano, Italy

Received 3 December 2001 / Accepted 5 February 2002

Abstract
We present H$\alpha $ line imaging observations of 30 galaxies obtained at the 2.5 m INT and NOT telescopes. The observed galaxies are mostly BCD Virgo cluster galaxies. H$\alpha $+[NII] fluxes and equivalent widths, as well as images of all the detected targets are presented. With these observations, H$\alpha $ data are available for $\simeq$50% of the BCD galaxies listed in the VCC.

Key words: galaxies: photometry - galaxies: fundamental parameters


1 Introduction

This paper is the third of a series devoted to H$\alpha $+[NII] imaging observations of Virgo cluster galaxies: in Gavazzi et al. (2002; Paper I), we present H$\alpha $ imaging data obtained with the 2.1 m of San Pedro Martir, mostly of low luminosity spirals and irregulars. In Boselli & Gavazzi (2002; Paper II) we present data obtained with the 1.2 m telescopes at the Observatoire de Haute Provence and at Calar Alto for the brightest galaxies in the cluster. This paper presents H$\alpha $ imaging data obtained at the 2.5 m INT and NOT telescopes for blue compact dwarfs (BCDs) galaxies in Virgo. These data are aimed at completing the H$\alpha $ survey of BCD galaxies of Almoznino et al. (1998) and Heller et al. (1999) aimed at studying the star formation activity in low-mass, dwarf galaxies. These objects are particulary useful to test whether the scaling relations of giant spirals, as derived by Boselli et al. (2001), hold true at the low end of the luminosity function. Given the lack of spiral patterns in BCDs, star formation cannot be triggered by compression from gravitational density waves and rotational shear, as in giant spirals, but is probably governed by random collision of interstellar clouds (Hunter et al. 1998). These characteristics make BCDs and dwarf irregulars a unique class of objects for studying the physical conditions behind the star formation.

Data for other bright galaxies, serendipitously observed in the Wide Field Camera (WFC) at the INT are also presented.

The analysis based on these data will be discussed in future communications.

2 The sample


   
Table 1: The target galaxies.

VCC		 NGC/IC UGC RA(1950) Dec a b Vel Clust Dist $\theta$ Type $m_{\rm pg}$ $B_{\rm T}^0$ $H_{\rm T}$
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

24		 - - 120802.5 120220.0 1.00 0.37 1289 M 32 4.99 BCD 14.95 15.79 12.68
144 - - 121245.0 60220.0 0.63 0.32 2014 W 32 7.66 BCD 15.31 15.18 12.95
213 3094 7305 121423.4 135413.0 0.93 0.71 -162 N 17 3.60 dS/BCD 14.26 14.74 11.52
324 - 7354 121636.5 40758.0 1.35 1.15 1524 S 17 9.01 BCD 14.78 14.30 11.83
404 - 7387 121743.8 42844.0 1.71 0.21 1733 S 17 8.60 Scd 15.00 13.90 -
428 - - 121807.9 140959.0 0.39 0.11 794 A 17 2.89 BCD 17.50 - -
459 - - 121839.6 175457.0 0.84 0.36 2108 A 17 5.74 BCD 14.95 - 12.72
513 - - 121924.0 23718.0 0.73 0.61 1832 S 17 10.28 BCD 15.10 - 12.33
655 4344 7468 122105.8 174906.0 1.55 1.55 1147 A 17 5.44 Spec/BCD 13.21 13.58 10.66
787 4376 7498 122245.1 60104.0 1.84 1.07 1136 B 23 6.79 Scd 13.69 13.38 11.08
793 - - 122249.8 132060.0 0.47 0.34 1906 A 17 1.50 Im 16.74 17.26 14.85
802 - - 122257.0 134624.0 0.64 0.21 -215 A 17 1.71 BCD 17.40 17.61 14.64
810 - - 122301.8 133006.0 0.77 0.77 -340 A 17 1.53 dE 16.95 17.06 14.04
815 - - 122305.4 132512.0 0.66 0.53 -700 A 17 1.47 dE 16.10 16.59 -
846 - - 122318.0 132818.0 0.89 0.80 -730 A 17 1.46 dE 16.20 16.58 -
848 - - 122319.8 60506.0 1.16 0.98 1537 B 23 6.70 Im/BCD 14.72 15.18 13.35
873 4402 7528 122335.3 132320.0 3.95 1.16 234 A 17 1.36 Sc 12.56 11.74 8.58
1297 - - 122800.0 124600.0 0.51 0.45 1486 A 17 0.12 E 14.33 14.42 10.25
1313 - - 122816.6 121916.0 0.45 0.20 1254 A 17 0.35 BCD 17.15 17.30 15.60
1316 4486 7654 122817.6 124002.0 11.00 11.00 1292 A 17 0.00 E 9.58 9.82 6.21
1369 - - 122901.2 122024.0 0.50 0.20 1022 A 17 0.37 dE 17.30 - -
1437 - - 123001.2 92658.0 0.59 0.45 1160 S 17 3.25 BCD 15.12 - 12.21
1488 3487 - 123041.4 94030.0 1.26 0.53 1157 S 17 3.05 E 14.76 - -
1725 - - 123509.4 85001.0 1.55 0.97 1068 S 17 4.19 Sm/BCD 14.51 14.61 12.13
1804 - - 123708.4 94024.0 0.75 0.20 1898 E 17 3.70 Im/BCD 15.63 16.30 13.31
1955 4641 7889 124036.5 121928.0 1.36 1.07 2012 E 17 3.02 Spec/BCD 14.32 - 11.16
2007 3716 - 124215.6 82254.0 0.78 0.41 1857 E 17 5.49 Im/BCD 15.20 - 13.33
2033 - - 124333.0 84454.0 0.73 0.73 1486 E 17 5.42 BCD 14.65 15.60 13.06
2034 - - 124336.6 102612.0 0.78 0.52 1500 E 17 4.36 Im 15.82 16.24 13.23
2037 - - 124343.8 102848.0 0.88 0.38 1142 E 17 4.37 Im/BCD 15.92 16.20 12.55
2045 - - 124424.0 102724.0 1.29 0.43 - E 17 4.52 dE 16.33 - -

Galaxies observed in this work have been selected from the Virgo Cluster Catalogue (VCC) of Binggeli et al. (1985), which is complete to the optical B magnitude $m_{\rm pg} = 18.0$. We selected all BCD galaxies (BCD, Im/BCD, Sm/BCD) classified as cluster members, possible members or belonging to the W, W', M clouds or to the southern extension (Binggeli et al. 1985, 1993).

Among the 67 BCD Virgo cluster members matching these criteria, 34 objects (51%) either included in the present work or in Paper I have an H$\alpha $ measurement. If limited to the ISO sample described in Boselli et al. (1997), 16 out 18 BCD galaxies (89%) have H$\alpha $ data.

Given the large field of view of the WFC at the INT, some galaxies not matching the selection criteria were serendipitously observed in the fields of other targets.

The target galaxies, as well as the serendipitously observed objects, are listed in Table 1, arranged as follow:

3 Observations


 

 
Table 2: Logbook of the observations.

Telescope		 Date Nights (ass./used) CCD Pixel size

2.5 m INT		 17-20/2/1999 4/3.5 WFC 0.333
2.56 m NOT 29/4-1/5/2000 3/0.5 Loral/Lesser $2048 \times 2048$ 0.188
2.56 m NOT 27-29/4/2001 3/1.5 Loral/Lesser $2048 \times 2048$ 0.188


Narrow band imaging in the H$\alpha $ emission line ($\lambda$ = 6562.8 Å) of galaxies was obtained in 1999 at the 2.5 m INT and in 2000 and 2001 at the 2.56 m NOT telescope at El Roque de los Muchachos, La Palma, Canary Islands. The INT observations were carried out with the Wide Field Camera (WFC) attached at the prime focus of the telescope. The WFC is composed by a science array of four thinned AR coated EEV 4 K$\times$2 K CCDs, plus a fifth acting as autoguider. The pixel scale at the detectors is 0.33 arcsecpixel-1, which gives a total field of view of about $34\times 34$ arcmin2. Given the particular arrangement of the detectors, a squared area of about $11 \times 11$ arcmin2 is lost at the top right corner of the field. The top left corner of detector #3 is also lost because of vignetting of the filters.

The f/11 2.56 m NOT telescope is equipped with a Loral/Lesser 2048$\times$2048 pixelCCD detector. The pixel size is 0.19 arcsec. At the adopted gain, the electron/adu conversion is 1 e-/adu, with a readout noise of 6 e-. A total of 4 nights at the INT and 6 at NOT were allocated to this project. Of these, 5.5 were totally or partly useful due to technical problems or weather limitations, as reported in Table 2 (logbook of the observations and CCD technical data). One galaxy (VCC 24) was observed at the INT in service mode.
Each galaxy was observed through one narrow band interferometric filter (see Table 3) centered at the redshifted H$\alpha $ line (ON), while the red continuum near H$\alpha $ (OFF) was generally taken in a broad band filter. The filters given in Table 3 are used as ON or OFF band for each target as specified in Cols. 2 and 3 of Table 4. The flux from the [NII] emission lines at $\lambda$6548 Å and $\lambda$6584 Å is included in the ON band observations. The typical integration time was of $3\times 1800$-1500 min ON- and $3\times 600$-200 min OFF-band for the INT observations, and $3\times 1500$-900 min ON- and $3\times 300$-900 min for the broad and narrow OFF-band observations at the NOT. NOT observations were obtained during good seeing conditions ($\sim$1 arcsec), fairly good at the INT (1.5-2 arcsec).

The observations were calibrated using the standard stars Feige 56, 66, 67 and BD 33+2642 from the catalogue of Massey et al. (1988) or from the IRAF compilation. Observations of the standard stars were repeated every 2 hours, with an integration time of 3-30 s. At least once per night a calibration star was observed on each CCD of the WFC to allow cross calibration of the different chips. The zero point of each galaxy is determined assuming an extinction law of slope 0.06 for the 1999 run, 0.1 for the other runs. Repeated measurements gave <10% differences, which we assume as the typical uncertainty of the photometry given in this work. All frames except one were obtained in photometric conditions. The determination of the H$\alpha $ equivalent width can however be achieved also in non photometric conditions using field stars to normalize the ON and OFF band images, as described in Paper I.

4 Image analysis

The data reduction of the CCD images follows a procedure identical to the one described in previous papers of this series (Gavazzi et al. 1998; Papers I and II), based on IRAF/ STSDAS[*] data reduction packages. To remove the detector response each image is bias subtracted and divided by the mean of several flat field exposures obtained on the twilight sky. In some cases images were fitted with a 2-D polynomial function to remove second order structures in the image. A median combination of the realigned images allows removal of cosmic rays. Subtraction of contaminating objects, such as nearby stars and galaxies, is done by direct editing of the frames. The sky background is determined in each frame in concentric object-free annuli around the object. The typical uncertainty on the mean background is estimated 10% of the rms in the individual pixels. This represents the dominant source of error in low S/N regions.

Total counts in the two frames have been obtained by integrating the pixel counts over the area covered by each galaxy, as derived by the optical major and minor diameters. Fluxes and equivalent widths and errors are computed using Eqs. (1) and (2) and (6) and (7) of Paper I, assuming K=1.

We corrected for the contamination of the H$\alpha $+[NII] line emission in the broad band filter (OFF-band) using the following relations:

\begin{displaymath}{F({\rm H}\alpha+[{\rm NII}])_{\rm c}} {=F({\rm H}\alpha+[{\r... ...m d}\lambda}{\int R_{\rm OFF}(\lambda){\rm d}\lambda}}\right)} \end{displaymath} (1)

and

\begin{displaymath}{{\rm H}\alpha+[{\rm NII}]EW_{\rm c}} \end{displaymath}


\begin{displaymath}{={\rm H}\alpha+[{\rm NII}]EW_{\rm o} \left(1+{\frac{{\rm H}\alpha+[{\rm NII}]{EW}_{\rm o}}{\int R_{\rm OFF}}}\right)} \end{displaymath}


\begin{displaymath}{\times \left(1+{\frac{\int R_{\rm ON}(\lambda){\rm d}\lambda}{\int R_{\rm OFF}(\lambda){\rm d}\lambda}}\right)} \end{displaymath} (2)

where $F({\rm H}\alpha+[{\rm NII}])_{\rm o}$ and ${\rm H}\alpha+[{\rm NII}]EW_{\rm o}$ are the observed values (from Eqs. (1) and (2) in Paper I), $F({\rm H}\alpha+[{\rm NII}])_{\rm c}$ and ${\rm H}\alpha+[{\rm NII}]EW_{\rm c}$ the corrected ones and  $R_{\rm ON}$ and  $R_{\rm OFF}$ the trasmissivity of the filters given in Col. 4 of Table 3.


 

 
Table 3: The characteristics of the adopted filters

Filter		 $\lambda$ $\Delta \lambda$ $R(\lambda)$ $\int R(\lambda){\rm d}\lambda$ Telescope

6568		 6574 95 89 83.95 INT
R 6380 1520 83 1307.73 INT
6562 6562 33 66 24.68 NOT
6564 6564 46 74 39.18 NOT
6610 6610 50 76 43.74 NOT
6883 6883 65 77 50.50 NOT
r 6800 1020 86 1249.41 NOT


5 Results


 

 
Table 4: The results of the observations.

This work Literature
VCC ON OFF Tel Year $T_{\rm ON}$ $T_{\rm OFF}$ R(Ha) $n_{\frac{{\rm ON}}{{\rm OFF}}}$ H $\alpha+[{\rm NII}]EW$ $F({\rm H}\alpha+[{\rm NII}])$ Phot Morph Notes ${\rm H}\alpha+[{\rm NII}]EW$ $F({\rm H}\alpha+[{\rm NII}])$ Ref. Notes
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)  

24		 6568 r INT 2001 33 10 0.86 0.30 $3 \pm 1$ $-14.03\pm0.04 $ C C   -1 - A *
144 6610 r NOT 2001 45 15 0.75 0.18 $161 \pm 1$ $-12.23\pm0.04 $ P C   $159 \pm22$ -12.72 A  
213 6564 r NOT 2000 40 10 0.61 0.13 $24 \pm 2$ $-12.67\pm0.05$ P El          
324 6568 R INT 1999 75 25 0.85 0.20 $65 \pm 8$ $-12.29\pm0.06$ P C   $56 \pm 4$ -12.20 A *
404 6568 R INT 1999 75 25 0.83 0.21 $8 \pm 1$ $-13.22\pm0.07$ P D          
428 6562 r NOT 2000 60 30 0.53 0.09 $251 \pm19$ $-12.97\pm0.04$ P C          
459 6610 r NOT 2001 45 15 0.75 0.19 $48 \pm 1$ $-12.66\pm0.04$ P E   $56 \pm 7$ -12.53 A *
513 6610 r NOT 2001 45 15 0.73 0.19 $49 \pm 1$ $-12.72\pm0.04$ P C   $53 \pm 5$ -12.66 A  
655 6568 R INT 1999 77 18 0.87 0.23 $7 \pm 1$ $-12.75\pm0.07$ P El   $3 \pm 1$   Hi *
787 6568 R INT 1999 47 10 0.87 0.46 $22 \pm 1$ $-12.48\pm0.04$ P C+E          
793 6568 R INT 1999 75 20 0.73 0.23 $- \pm 4$ - P            
802 6568 R INT 1999 75 20 0.86 0.24 $38 \pm 4$ $-13.54\pm0.06$ P E          
810 6568 R INT 1999 75 20 0.85 0.23 $-1 \pm 6$ - P            
815 6568 R INT 1999 75 20 0.84 0.23 $-1 \pm 7$ - P            
846 6568 R INT 1999 75 20 0.84 0.23 $-1 \pm 5$ - P            
848 6568 R INT 1999 47 10 0.83 0.47 $28 \pm 5$ $-13.02\pm0.08$ P E   14   GS  
873 6568 R INT 1999 75 20 0.88 0.23 $12 \pm 1$ $-12.09\pm0.05 $ P D   $10 \pm 2$ -12.18 II *
1297 6568 R INT 1999 75 5 0.85 1.00 $-1 \pm 1$ - P            
1313 6568 R INT 1999 75 25 0.86 0.20 $351 \pm10$ $-12.84\pm0.04$ P C          
1316 6568 R INT 1999 75 5 0.86 1.00 $ 2 \pm 3 $ $-12.05\pm0.63 $ P FIL   $1 \pm 3$ -11.73 M *
1369 6568 R INT 1999 75 25 0.87 0.20 $-1 \pm 8$ - P            
1437 6568 R INT 1999 90 23 0.87 0.20 $ 17 \pm 2 $ $-13.16\pm0.07$ P C          
1488 6568 R INT 1999 90 23 0.87 0.20 $ -1 \pm 3 $ - P   *        
1725 6568 R INT 1999 75 7 0.87 0.25 $43 \pm 9 $ $-12.66\pm0.08$ P C+E   $38 \pm 5$ -12.62 A *
1804 6610 6883 NOT 2001 60 45 0.73 1.47 $3 \pm 1$ $-14.37\pm0.19$ P D          
1955 6610 r NOT 2000 20 5 0.74 0.25 $ 9 \pm 4$ $-13.10\pm0.17$ P C          
2007 6610 6883 NOT 2001 60 45 0.73 1.45 $ 10 \pm 1$ $-13.77\pm0.04 $ P E          
2033 6568 R INT 1999 75 25 0.85 0.22 $ 13 \pm 3$ $ -13.32\pm0.10$ P C   $ 5 \pm 3 $ -13.68 A *
2034 6568 R INT 1999 75 10 0.85 0.50 $ 2 \pm 2$ $-14.29\pm0.44$ P L   $2 \pm 1$ -13.81 H *
2037 6568 R INT 1999 75 10 0.87 0.50 $17 \pm 6$ $-13.51\pm0.16$ P C   $16 \pm1$   Hi  
2045 6568 R INT 1999 75 10 0.87 0.50 $-1 \pm 4 $ - P   *        


Notes: long slit spectra were taken by Gavazzi et al., in prep., for VCC 24 ( ${\rm H}\alpha+[{\rm NII}]EW=19$ Å), VCC 324 ( ${\rm H}\alpha+[{\rm NII}]EW=313$ Å), VCC 459 ( ${\rm H}\alpha+[{\rm NII}]EW=67$ Å), VCC 655 ( ${\rm H}\alpha+[{\rm NII}]EW=22$ Å), VCC 873 ( ${\rm H}\alpha+[{\rm NII}]EW=14$ Å), VCC 1725 ( ${\rm H}\alpha+[{\rm NII}]EW=77$ Å) and VCC 2033 ( ${\rm H}\alpha+[{\rm NII}]EW=33$ Å).

VCC 1316: our H$\alpha $ data are already discussed in Gavazzi et al. (2000b).

VCC 1488: possible vignetting.

VCC 2034: the flux might be contaminated by a nearby bright star.

VCC 2045: no redshift available; assumed at the average velocity of the cluster, ${\rm vel} =1200$ kms-1

References:

A: Almoznino & Brosch (1998); H: Heller et al. (1999); Hi: Hippelein et al. in prep; M: Macchetto et al. (1996); II: Paper II. GS: Gavazzi et al., in preparation (spectroscopic survey drifting the telescope over the entire disc of the galaxy).


The results of the present observations are listed in Table 4, arranged as follow:

The red continuum images of all the observed galaxies are shown as contours plots superposed to the H$\alpha $+[NII] net image (grey levels) in Fig. 1. In most BCD galaxies (53% of the targets) the H$\alpha $ emission is dominated by a central sturburst region, as in VCC 1313, or by giant HII regions at the edges of the disc, as in VCC 459 (24%). In few objects, such as VCC 655, the emission is along a ring-like structure, or it is diffuse and/or homogeneously distributed along the disc (VCC 2034).
  \begin{figure} \par\includegraphics[width=6.7cm,clip]{MS2163f2.eps} \end{figure} Figure 2: Comparison with data in the literature: open dots are for A, filled dots for Hi, open squares for M, filled squares for H, + for I and X for G (see Table 4).
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5.1 Comparison with the literature

Fluxes and equivalent widths given in this paper are in general consistent with available measurements, as shown in Fig. 2. The average value of the difference between our measurements and those in the literature is: ${\rm H}\alpha+[{\rm NII}] EW_{\rm TW}- {\rm H}\alpha+[{\rm NII}] EW_{\rm L}= 1.9 \pm 4.5$ Å and $-\log F({\rm H}\alpha+[{\rm NII}])_{\rm TW}+\log F({\rm H}\alpha+[{\rm NII}])_{\rm L} = 0.02 \pm 0.29~\rm erg~cm^{-2}\,s^{-1}$. This has been estimated excluding long-slit spectroscopic data from (Gavazzi et al. in prep.), which, in the case of BCD, could be non-representative of the whole galaxy but biased towards high surface brightness HII regions, as in the case of VCC 24, 324, 655 and 2033. The significant difference between our H$\alpha $+[NII] flux estimate and that of Heller et al. (1999) for VCC 2034 might be due to the contamination by a nearby star.

6 Summary and conclusion

We present H$\alpha $+[NII] imaging data (fluxes and equivalent widths) of 30 Virgo galaxies obtained at the 2.5 m INT and at the 2.56 m NOT telescopes at El Roque de los Muchachos, La Palma. The present observations of BCD galaxies in the Virgo cluster are aimed at completing at low luminosity a large project of multifrequency observations of galaxies spanning a large range in morphological type and luminosity and belonging to different environments (cluster-isolated) for the purpose of constructing a large data-set suitable for statistical studies.

Acknowledgements

We wish to thank the night operators for their assistance during the observations. This project is partly financed by the French PNG. The data presented here have been taken using ALFOSC, which is owned by the Instituto de Astrofisica de Andalucia (IAA) and operated at the Nordic Optical Telescope under agreement between IAA and the NBIfAFG of the Astronomical Observatory of Copenhagen. We thank L. Cortese for helping us in cros-calibrating photometric observations with spectroscopic data.

References

 

Online Material


 \begin{figure}\par\par\includegraphics[width=7.7cm,clip]{MS2163f1a.eps}\includeg... ...ip]{MS2163f1e.eps}\includegraphics[width=7.7cm,clip]{MS2163f1f.eps} \end{figure} Figure 1: H$\alpha $+[NII] (grey) and red continuum (contour) (both in arbitrary scales) images of the Virgo cluster galaxies.
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 \begin{figure}\par\includegraphics[width=7.7cm,clip]{MS2163f1g.eps}\includegraph... ...ip]{MS2163f1m.eps}\includegraphics[width=7.7cm,clip]{MS2163f1n.eps} \end{figure} Figure 1: continued.
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 \begin{figure}\par\includegraphics[width=7.7cm,clip]{MS2163f1o.eps}\includegraph... ...ip]{MS2163f1s.eps}\includegraphics[width=7.7cm,clip]{MS2163f1t.eps} \end{figure} Figure 1: continued.
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 \begin{figure}\par\includegraphics[width=7.7cm,clip]{MS2163f1v.eps}\includegraph... ...\par\hspace*{4cm} \includegraphics[width=7.7cm,clip]{MS2163f1z.eps} \end{figure} Figure 1: continued.
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