Mon. Not. R. Astron. Soc. 304, 8±26 (1999)
Late-type dwarf irregular galaxies in the Virgo cluster ± I. Ha and red
continuum data
Ana Heller,1 Elchanan Almoznino1 and Noah Brosch1;2 *
1
The Wise Observatory and the School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv
69978, Israel
2
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
Accepted 1998 October 22. Received 1998 October 19; in original form 1998 April 22
A B S T R AC T
We present Ha and red continuum observations for a sample of late-type low surface
brightness (LSB) dwarf irregular galaxies, consisting of all the ImIV and V galaxies with
mB # 17:2 in the Virgo cluster, and compare them with similar data for a representative sample
of high surface brightness (HSB) dwarf irregular galaxies, also in the Virgo cluster. Line ¯uxes
and equivalent widths are listed for individual H ii regions, and total Ha emission is measured
for the entire galaxy. Although signi®cant line emission originates in the H ii regions that we
have identi®ed, it does not make up the entire Ha output of all galaxies.
For those objects in the LSB sample with Ha emission, we ®nd typical star formation rates
(SFRs) from 6:9 ´ 10 3 to as high as 4:3 ´ 10 2 M( yr 1. This is, on average, one order of
magnitude weaker than for HSB objects, although the SFRs overlap. On average, ,2 H ii regions
are detected per LSB galaxy, for a total of 38 H ii regions among 17 galaxies with Ha emission.
The H ii regions are smaller and fainter than in HSB galaxies in the same Virgo cluster
environment, have Ha line equivalent widths about 50 per cent of those in HSBs, and cover
similar fractions of the galaxies. When more than one H ii region is present in a galaxy, we observe
a strong intensity difference between the brightest and the second brightest H ii regions. The lineemitting regions of LSB galaxies are preferentially located at the periphery of the galaxy, while in
HSBs they tend to be central. The Ha line strength of an H ii region is correlated with the red
continuum light underneath the region; this holds for both LSBs and HSBs.
We do not identify fundamental differences in the star formation properties of the LSB and
HSB dwarf galaxies that we have studied, and we infer that these galaxies must be similar, with
the difference being the intensity of the present star formation burst.
Key words: stars: formation ± H ii regions ± galaxies: clusters: individual: Virgo ± galaxies:
irregular ± galaxies: stellar content.
1
INTRODUCTION
The surface brightness of a galaxy is primarily determined by the
stellar luminosity function and by the projected density of stars, and
these depend on the past and ongoing star formation (SF). Much of
our current understanding of the SF phenomenon on all scales is
biased by the high surface brightness (HSB) objects that de®ne the
classical Hubble sequence. However, irregular galaxies in general,
and dwarfs in particular, offer unique tests of SF models, since
they lack the more complex SF-triggering mechanisms, such as
prominent spiral arms with the associated density waves and
differentially rotating discs.
* On sabbatical leave from the Wise Observatory and the School of Physics
and Astronomy, Tel Aviv University.
Low surface brightness (LSB) galaxies are de®ned as objects
with central blue surface brightnesses m…B†0 > 23 mag arcsec 2 .
Recent studies of LSB spiral (Sc and later) galaxies (McGaugh et al.
1995a, b; de Blok, van der Hulst & Bothun 1995) indicate that these
are slowly evolving systems. The indication of a slow evolution but
not full quiescence is based on the blue colours, presence of H ii
regions, low metallicites, extended gas discs, and high mass-to-light
ratios M…H i†=L. The relative isolation of these systems and the low
density of their gas component may be the reasons for the smallamplitude SF bursts, and therefore the low overall star formation
rates (SFRs).
The SF history of a quiescent galaxy with a suitable gas reservoir
in a cluster environment may be in¯uenced by external factors such
as the temperature of the hot intracluster medium (ICM) and the
galaxy's interaction with other cluster galaxies and with the ICM.
Therefore it is important to choose carefully the environment of a
q 1999 RAS
Late-type dwarf irregulars in Virgo ± I
sample of galaxies studied to learn about SF processes. Recent work
on HSB dwarf galaxies (DGs) in the Virgo cluster [blue compact
dwarf (BCD) or ImIII/BCD: Almoznino 1996; Almoznino &
Brosch 1998, hereafter AB98] points to a possible evolutionary
connection between their rather blue underlying stellar component
and LSB DGs. We approach these questions here by analysing
optical narrow- and broad-band CCD observations of lower luminosity objects (ImIV±V) in the same environment, the Virgo cluster
(VC). The two samples (HSB and LSB) comprise altogether 47 latetype irregular galaxies.
In this paper we concentrate on the morphology of H ii regions in
our sample of LSB galaxies, and on the SFRs derived from galaxywide Ha ¯uxes as well as from individual H ii regions for the
objects in the LSB sample. We compare these with similar observables from the HSB sample of dwarf galaxies presented in AB98.
The Ha radiation is directly coupled to the radiation at wavelengths
Ê and originates from OB stars of lifetime
shorter than 912 A
.5 ´ 106 yr, indicating on-going massive star formation, while
optical broad-band colours provide information on older stellar
populations. The surface photometry of the galaxies in our sample
through UBVRI ®lters, with calibrated ¯uxes, radial surface brightness, contour colour maps and pro®les, central surface brightness,
and scalelengths, will be presented in forthcoming papers along
with a full analysis of the entire data set.
The plan of the paper is as follows. We ®rst describe the sample in
Section 2, then the observations that we performed and their
reduction in Section 3. The results are discussed in Section 4 and
our ®ndings are summarized in Section 5.
2
THE SAMPLE
The selection of LSB galaxies is based primarily on the morphological classi®cation of DGs in the Virgo Cluster Catalog (Binggeli,
Sandage & Tammann 1985, hereafter VCC). We selected galaxies
listed in VCC as Magellanic irregulars of subtypes ImIV or ImV
with mB # 17:2 and members in the VC. The faintest ImV galaxies
listed in VCC have mB ˆ 17:5, thus our selection is safely distant
from the limit of the VCC cut-off. All our galaxies have single-dish
H i measurements with detected non-zero total ¯ux from the
Arecibo survey of VCC late-type dwarf galaxies of Hoffman et al.
(1987, 1989a, b). We imposed a recession velocity limit of 3000 km
s 1 to prevent the inclusion of background objects.
We adopt here a uniform sample distance of 18 Mpc for which
1 arcsec ˆ 87 pc. This assumption is justi®ed as being the representative distance to galaxies within the Virgo cluster (e.g. FouqueÂ
et al. 1990). Other distance estimates are in the same range:
16.0 61.5 Mpc (van den Bergh 1996), 17.2±17.4 Mpc (Gonzalez
& Faber 1997), and 20.7 6 2.4 Mpc (Federspiel, Tammann &
Sandage 1998). Note, though, that the VC has signi®cant depth,
thus the assumption that all galaxies are at the same distance is
probably true only at the 10 per cent level. As most of our
conclusions are drawn from ratios of quantities measured in the
Table 1. LSB dwarf sample.
VCC
mB
m67
a
e
f
F (Ha)
EW (Ha)
17
168
169
217
260
328
329
350
367
381
477
530
565
584
826
963
1455
1465
1468
1585
1753
1784
1816
1822
1952
1992
2034
15.20
17.10
16.50
15.50
15.70
16.90
16.80
17.05
17.20
16.50
16.96
15.80
15.70
15.80
15.00
17.20
16.80
15.00
15.00
15.45
16.81
15.84
16.20
15.60
16.00
15.50
15.82
15.95 6 0.06
16.03 6 0.07
16.78 6 0.10
16.60 6 0.06
16.49 6 0.06
16.24 6 0.06
18.00 6 0.10
17.97 6 0.07
16.84 6 0.06
16.42 6 0.27
16.96 6 0.15
15.43 6 0.12
16.46 6 0.08
15.92 6 0.05
15.23 6 0.05
17.00 6 0.05
16.29 6 0.07
15.88 6 0.07
15.87 6 0.09
14.85 6 0.10
18.04 6 0.06
16.10 6 0.10
16.16 6 0.15
15.97 6 0.22
16.63 6 0.10
15.32 6 0.11
INDEF
46
22
20
56
27
33
8
19
22
(24)
27
41
19
28
59
37
23
25
28
41
16
26
28
36
22
26
26
5
3
3
7
2
6
0
7
2
(7)
0
2
2
1
4
1
4
4
4
5
2
6
6
6
5
2
±
24
0
6
38
9
45
0
30
0
0
6
6
0
0
22
85
75
49
25
44
81
0
0
0
45
34
0
14.78 6 0.68
2.94 6 1.20
0.44 6 0.29
2.26 6 0.15
0.28 6 0.11
2.12 6 0.10
0.37 6 0.12
0.34 6 0.04
0.03 6 0.66
0.16 6 0.43
0.33 6 0.14
0.77 6 0.31
1.67 6 0.80
0.08 6 0.50
2.59 6 0.29
1.73 6 0.16
1.70 6 0.13
4.39 6 0.20
3.50 6 0.31
8.22 6 0.76
0.68 6 0.05
0.23 6 0.21
1.87 6 0.60
0.09 6 0.30
2.39 6 0.70
6.12 6 0.40
1.54 6 0.50
105 6 15
0
765
29 6 3
361
20 6 1
17 6 7
16 6 2
0
265
763
361
19 6 13
164
962
28 6 5
17 6 2
29 6 2
23 6 3
21 6 3
32 6 4
262
16 6 7
0
32 6 16
24 6 3
261
log (SFR)
1.36
0
3.49
2.17
3.09
2.21
2.96
3.00
0
3.33
2.96
2.65
2.31
3.63
2.12
2.30
2.30
1.89
1.99
1.62
2.70
3.17
2.26
0
2.15
1.75
2.35
Note. The total blue magnitude is the estimate from the VCC and m67 is the monochromatic magnitude of the continuum near
Ê . The blue magnitudes of VCC 1585, 1753 and 1784 are from a private communication by Bruno Binggeli and are
6700 A
different from the values listed in the VCC. The semi-major axis a is in arcsec and is derived from an ellipse ®tting to the 25 mag
arcsec 2 isophote. F(Ha) is the Ha ¯ux in units of 10 14 erg cm 2 s 1 . The ellipticity values are given as 10 ´ e and are rounded
off to single digits. The covering factor f is given as a percentage of the galaxy area in red continuum light. The SFR is in M( yr 1
and is obtained by assuming a uniform distance of 18 Mpc to all galaxies.
q 1999 RAS, MNRAS 304, 8±26
9
10
A. Heller, E. Almoznino and N. Brosch
Figure 1. Distribution of LSB dwarfs studied here and HSB dwarfs from
AB98 projected on the Virgo cluster. The different subclusterings identi®ed
by Hoffman et al. are indicated schematically. BCD galaxies are represented
as ®lled circles and LSBs are empty circles. The size of the symbols is
proportional to the blue magnitude of each galaxy in the VCC.
same galaxy, an even larger error in the distance would have no
effect. The ¯uxes and ¯ux densities would, however, change.
The sample, as de®ned here, is complete and is representative of
LSB dwarf galaxies, although not of the lowest surface brightness
ones. The reason is that galaxies with even lower surface brightness
exist, in the same neighbourhood, and to study them as done here for
ImIV±Vobjects is dif®cult and requires extensive observing time at
major telescopes.
The 27 galaxies in the LSB sample are listed in Table 1. We
identify them by their number in VCC and list their total blue
magnitude (which is by-eye estimate from VCC), the total red
magnitude (measured here, as explained below, and presented with
a measurement error), the measured semi-major axis a of the optical
image in arcsec and ellipticity e ˆ 1 …b=a† derived from an ellipse
®t to the 25 mag arcsec 2 isophote, and the Ha results. The latter are
given as total ¯ux, total equivalent width, and fractional coverage of
a galaxy by H ii regions ( f ). The projected distribution of the
objects in the VC is shown in Fig. 1, where each object is marked as
a circle with a diameter proportional to its apparent magnitude.
We have added to the ®gure the galaxies from the HSB sample
(®lled circles, N ˆ 16) discussed in AB98, and indicated the
various subclusterings in the VC identi®ed in Hoffman et al.
(1989b). The HSB sample will be compared at various points in
this paper with the LSB objects, and some information that has
appeared already in AB98 will be repeated here. This HSB
comparison sample is composed of irregular galaxies, and the
objects were selected to belong to the BCD morphological class.
3
O B S E RVAT I O N S A N D R E D U C T I O N
Most LSB images analysed here were obtained with the Wise
Observatory (WO) 1.0-m telescope using a TEK 1024 ´ 1024
pixel thinned and back-illuminated CCD. This chip has a very
low readout noise (6.5 e 1 ) and its quantum ef®ciency in the regions
Ê ). The plate scale for the
of interest is high (,80 per cent at 6500 A
1
WO images is 0.7 arcsec pixel . One galaxy studied here was
observed by EA in 1992 at the WO using an RCA CCD with 0.9arcsec pixels (see AB98). A few exposures were obtained by NB at
the f =4 prime focus of the Russian Academy of Sciences Special
Astrophysical Observatory 6.0-m (BTA) telescope. The CCD used
for those observations was a 512 ´ 512 array, with a projected pixel
size of 0.154 ´ 0.205 arcsec2 . The readout noise of the device used
on the BTA was 30 e and the conversion from data numbers (DNs)
was 13.8 e ˆ 1 DN. The BTA images were binned off-line by
4 ´ 3 pixels, which gives 0.615-arcsec square pixels, comparable to
the plate scale of the WO images.
The image sets analysed here consist of line and continuum
images for each galaxy obtained from 1995 to 1997. We collected
three 1800-s (WO) or two 600-s (BTA) exposures through the line
®lter and an equal number of continuum exposures per object. The
line ®lters were chosen to match, as far as possible, the rest-frame
Ha emission of each galaxy, and are described in AB98. We used a
Ê ®lter centred on 6700 A
Ê to measure the continuum con,53-A
tribution. The reduction was done with iraf and the ¯ux calibration
used observations of HZ 44, which was measured several times each
night through different airmasses with the same ®lters as used for
the galaxies. Note that although this standard star shows Ha and
He i absorptions, these do not affect the calibration because the
®lters used here are redshifted with respect to these features in the
standard.
The images were debiased and ¯at-®elded with twilight frames,
and cosmic rays and hot pixels were removed with standard iraf
tasks. The images through the same ®lter were co-added after being
registered by cross-correlation. The sky background was subtracted
as a tilted plane ®tted to the ®nal combined image in each band,
after the galaxy and the bright stars were masked off. The standard
deviation of the ®tted plane from the actual background light
distribution was used to estimate the noise introduced by sky
subtraction.
The images were calibrated in ¯ux using the observations of HZ
44 and were convolved with a 2±4 arcsec FWHM Gaussian,
according to the seeing. We then performed aperture photometry
on stars in the ®eld, normalizing the results of the line and
continuum bands. In this way we also corrected for any relative
change of continuum sloping in the two bands. The scaling between
the on-line and off-line images was applied to the full-resolution
images, and the net ¯uxed Ha images were derived by subtracting
off-line from on-line ¯uxed images. As expected, the stars vanished
from the net image, with the exception of cases where imperfect
point spread function (PSF) matching caused small distortions near
very bright stars. This method is very similar to that used successfully by e.g. Hodge (1969), Hunter & Gallagher (1986) and recently
Gavazzi et al. (1998).
The photometry and the derivation of line and continuum ¯uxes
are based on measurements of the HZ 44 spectrophotometric
standard and ¯ux scaling from it to the galaxy, after correcting for
differential extinction. We estimate the photometric accuracy from
the extinction coef®cients obtained for the on- and off-bands; these
are both 0.1960.12 and are consistent with the long-term average of
the R-band extinction coef®cient at WO (0.21: Brosch 1992). As the
typical airmass differences between observations of the standard
and of the target galaxies were always # 0.3, this indicates that the
error that could be attributed to extinction is at most ,0.06 mag.
The off-line continuum images were smoothed to 1-arcsec
resolution and iso-intensity contours were traced around each
galaxy. Polygonal apertures were de®ned by the contour of the
25 mag arcsec 2 isophote. Generally, this isophote level corresponded to a signal-to-noise ratio ˆ 1 per pixel in the ®nal images
q 1999 RAS, MNRAS 304, 8±26
Late-type dwarf irregulars in Virgo ± I
11
Table 2. HSB dwarf sample.
VCC
mB
mR
a
e
f
F (Ha)
EW (Ha)
10
22
24
144
172
324
410
459
513
562
985
1179
1374
1725
1791
2033
16.20 6 0.09
16.83 6 0.03
16.12 6 0.03
15.29 6 0.03
15.38 6 0.03
14.58 6 0.03
17.76 6 0.04
15.29 6 0.04
15.84 6 0.03
16.59 6 0.03
16.62 6 0.04
15.98 6 0.05
15.00 6 0.02
15.18 6 0.04
14.85 6 0.04
15.61 6 0.04
14.88 6 0.10
15.82 6 0.05
14.89 6 0.07
14.61 6 0.03
14.53 6 0.03
13.62 6 0.02
16.89 6 0.04
14.40 6 0.07
14.56 6 0.02
15.86 6 0.05
15.61 6 0.07
15.06 6 0.04
14.19 6 0.02
14.48 6 0.03
14.09 6 0.03
14.54 6 0.03
24
5
22
13
27
29
9
22
13
11
13
18
32
27
36
8
5
2
3
3
6
2
2
2
2
3
5
5
6
6
7
1
98
0
0
100
0
74
100
38
52
48
23
36
59
30
29
0
9.30 6 1.90
0.5 6 2.0
1.50 6 4.90
70.5 6 7.6
0.37 6 0.60
63.1 6 4.3
0.50 6 0.05
29.7 6 3.3
21.7 6 2.0
13.4 6 0.9
3.1 6 1.8
6.0 6 2.4
29.7 6 1.3
24.2 6 2.9
40.3 6 1.9
2.1 6 1.5
31 6 7
0
0
235 6 25
162
210 6 14
1.7 6 0.2
99 6 11
72 6 7
45 6 3
10 6 6
20 6 8
99 6 4
81 6 10
134 6 6
765
log (SFR)
1.56
0
0
0.68
0
0.73
2.83
1.06
1.20
1.41
2.04
1.75
1.06
1.14
0.93
2.21
Note. The total blue magnitude is the estimate from the VCC. The semi-major axis a is in arcsec and is derived from an ellipse
®tting to the 25 mag arcsec 2 isophote. The Ha ¯ux is in units of 10 14 erg cm 2 s 1 . The ellipticity values are given as 10 ´ e and
are rounded off to single digits. The covering factor f is given as a percentage of the galaxy area in red continuum light. The SFR is
in M( yr 1 and is obtained by assuming a uniform distance of 18 Mpc to all galaxies.
Table 3. H ii region statistics in LSB and HSB samples.
Parameter
H ii regions present
Single H ii region
Multiple H ii regions
Covering factor of
`continuum' galaxy
BCD
SB
12/16 (75%)
[8/12 (67%) pure BCD]
9/12 (75%)
3/12 (25%)
67% (pure BCD)
57% (all galaxies)
17/27 (63%)
7/17 (42%)
10/17 (49%)
37%
Note. The covering factor listed in the last row is obtained by dividing the
summed area of all H ii regions by the area of the galaxy, as de®ned by the 25
mag arcsec 2 isophote on the red continuum image.
for all galaxies. The total Ha ¯ux from the galaxies was obtained by
measuring within the polygonal aperture the averaged fl in the net
emission images, and multiplying the results by the FWHM of the
Ha line ®lter. We did not compensate for the contribution of [N ii]
lines to the derived Ha ¯uxes, as the ratio [N ii]/Ha in dwarf
irregular galaxies is expected to be small owing to their generally
low metal abundances (Gallagher & Hunter 1989; Skillman, Kennicutt & Hodge 1989).
The red continuum emission from each galaxy, f(off), in ¯ux
density units, was also measured within the same polygonal
aperture. The total ¯ux was transformed into monochromatic
Ê (m67 ), which are listed in Table 1. The
magnitudes at ,6700 A
ratio between the net Ha emission and the red continuum ¯ux
measured from the off-line image is the emission-line equivalent
width (EW). Although it is not yet clear which stellar population is
responsible for this red emission, i.e. whether it is produced by the
stars formed in the present SF event or by a pre-existing stellar
population, we list these EW values along with the line ¯uxes in
Ê is not
Table 1. The `monochromatic' magnitude at l . 6700 A
equivalent to the R-band magnitude; a transformation of the ¯ux
density for a zeroth-magnitude star in Bessel (1979) for the Rc band
into the monochromatic magnitude system yields m67 . 0:44.
Therefore the use of this system introduces some zero-point
shifts. The errors, in both the line and continuum measurements,
q 1999 RAS, MNRAS 304, 8±26
are derived from the rms combination of the errors of the polygonal
photometry of the galaxy and the calibration of the spectrophotometric standard star.
We could not measure the dimensions of the red continuum
image for VCC 381 because of its extreme faintness. The numbers
quoted in parentheses in Table 1 are from NED (NASA/IPAC
Extragalactic Database). In the case of VCC 2034 a star is projected
against the galaxy; we could not accurately subtract its contribution
to enable a reliable determination of m67 . Similar data for the HSB
sample are presented in Table 2. This is done here only for the
purpose of completeness and comparison; the full data set for the
HSB galaxies is part of AB98 and is discussed there. We note that
the entire sample consists of `dwarf' galaxies, as they all have
MB > 18. The two samples studied here are thus representative of
the late-type dwarf irregular kind, and they come in two `¯avours'
of high and low surface brightness.
Table 2 lists the mB and mR magnitudes for the HSB galaxies;
these are the values actually measured through the broad-band B
and R ®lters (where the latter contains Ha and [N ii] contributions).
The level of line contamination of the R magnitude can be estimated
from the equivalent width of Ha. In contrast, Table 1 lists the mB
estimated in VCC for the LSBs, and gives as m67 the measured
monochromatic magnitude of the continuum, which is free of line
contamination.
Table 3 presents the statistics of detections of H ii regions in our
two samples (HSB from AB98 and LSB discussed here), and the
last columns of Tables 1 and 2 list the on-going global average SFR
of each galaxy, derived from the relation of Kennicutt, Tamblyn &
Congdon (1994) scaled to the 18-Mpc adopted distance of the VC:
SFR ˆ 2:93 ´ 1011 F (Ha), where F(Ha) is the total line ¯ux in erg
s 1 cm 2 and the SFR is in M( yr 1.
Finally, we also measured the ¯uxes from the individual H ii
regions which were identi®ed in the Ha images by integrating the
net ¯ux collected within a circular aperture around each H ii region.
The size of this aperture was determined by ®rst visually inspecting
the net line image using the imexam task from iraf, followed by
phot with a number of apertures. This aperture size, taken here to
represent the size of the H ii region, is that for which an increase in
12
A. Heller, E. Almoznino and N. Brosch
Table 4. Properties of individual H ii regions in LSB galaxies.
Galaxy (VCC)
17
17
17
17
17
17
169
169
169
217
217
217
260
328
350
477
530
530
826
826
826
826
963
1455
1465
1465
1468
1585
1585
1585
1585
1753
1753
1753
1952
1952
1992
1992
H ii region ID
F (Ha)
f (off)
EW (Ha)
a
b
c
d
e
f
a
b
c
a
b
c
a
a
a
a
a
b
a
b
c
d
a
a
a
b
a
a
b
c
d
a
b
c
a
b
a
b
6.92 6 0.15
0.46 6 0.04
0.54 6 0.05
0.68 6 0.05
0.48 6 0.04
1.49 6 0.09
0.37 6 0.04
0.15 6 0.03
0.16 6 0.03
0.73 6 0.06
0.73 6 0.06
0.20 6 0.03
0.18 6 0.03
2.15 6 0.14
0.22 6 0.05
0.27 6 0.04
0.26 6 0.03
0.18 6 0.03
1.79 6 0.06
0.11 6 0.02
0.40 6 0.03
0.31 6 0.03
1.65 6 0.08
1.65 6 0.09
3.04 6 0.06
0.99 6 0.04
3.17 6 0.08
3.94 6 0.14
1.74 6 0.10
0.49 6 0.07
0.38 6 0.05
0.32 6 0.06
0.20 6 0.04
0.13 6 0.03
1.31 6 0.09
0.80 6 0.08
2.45 6 0.10
2.53 6 0.11
15 6 1.4
6.0 6 0.7
7.0 6 0.8
6.0 6 0.7
2.0 6 0.5
3.0 6 2.0
0.8 6 0.4
4.0 6 0.7
0.8 6 0.5
4.0 6 0.1
10 6 1
4.0 6 0.8
2.7 6 0.5
114 6 3
3.0 6 0.6
5.0 6 0.8
19 6 1
18 6 1
60 6 2
30 6 1
10 6 3
3.0 6 0.3
51 6 2
90 6 3
50 6 1
30 6 1
36 6 1
76 6 3
74 6 3
50 6 3
10 6 1
10 6 1
10 6 1
561
30 6 2
70 6 3
40 6 2
90 6 3
461
77
77
113
240
497
462
38
200
183
73
50
67
19
73
54
14
10
30
4
40
103
32
18
61
33
88
52
24
10
38
32
20
26
44
11
61
28
log (SFR)
1.70
2.87
2.80
2.70
2.85
2.37
2.96
3.36
3.33
2.67
2.67
3.23
3.29
2.20
3.19
3.10
3.12
3.28
2.28
3.49
2.93
3.04
2.32
2.31
2.05
2.53
2.03
1.93
2.29
2.84
2.95
3.02
3.23
3.42
2.41
2.63
2.14
2.13
Ê 1 . The SFR calculation
Note. F(Ha) is in 10 14 erg cm 2 s 1 and f (off) is in 10 17 erg cm 2 s 1 A
is explained in the text. The equivalent width of the line is obtained by EW ˆ F…Ha†=f …off†. The H ii
region labelled f in VCC 17 appears on a particularly faint part of the galaxy; we could not measure
the underlying continuum accurately enough to provide a good-quality Ha equivalent width.
size did not add more ¯ux. The individual H ii region ¯uxes are
listed in Table 4. We also list there the SFR derived for each H ii
region, as well as the ¯ux in the red continuum underneath it along
with the line EW, as explained above. The errors listed for the H ii
regions are only photometric. For completeness, we list in Table 5
similar parameters for the H ii regions in the HSB sample. The H ii
regions are identi®ed by the name of the galaxy in column (1), and
by the label given to the H ii region in the speci®c ®gure in column
(2). The HSB data equivalent to these presented in Table 1 for the
LSB galaxies have been published in AB98.
4
R E S U LT S
Figs 2±7 present mosaic images of the LSB galaxies observed at the
WO. We show three to ®ve galaxies per ®gure in rows. Each row
contains the image through the line ®lter at the left, the image
through the continuum ®lter in the middle, and the net Ha image at
the right. Each top-right image on a page of LSB galaxies has a scale
mark near its top-right corner; the mark is 10 arcsec long (ˆ 870 pc
at the adopted distance of the VC) and indicates the scale of all the
images on the page. Fig. 8 shows the two galaxies observed at the
BTA telescope and the single galaxy from the LSB sample observed
at WO by EA with a different CCD. Figs 9±11 show the BCD
galaxies from AB98 in the same format as used here for the LSB
objects, for comparison and completeness. These images have not
been presented elsewhere. Figs 8 to 11 have individual 10-arcsec
bars for each object. For each galaxy we have marked and labelled
the individual H ii regions that we recognized in the net Ha image.
The underlying LSB galaxies, measured on the red continuum
images, have an average major axis diameter of 58 arcsec < 5 kpc.
For comparison, the average major axis of the HSB sample is 40
arcsec < 3.5 kpc.The semi-major axes of the LSB galaxies are listed
in Table 1, along with the ellipticity parameter e. These were
derived from the ®t of an ellipse to the red continuum light at the
25 mag arcsec 2 isophote. We also list there f , the fractional
coverage of a galaxy by H ii regions. This is obtained by dividing
q 1999 RAS, MNRAS 304, 8±26
Late-type dwarf irregulars in Virgo ± I
13
Table 5. Properties of individual H ii regions in HSB galaxies.
Galaxy (VCC)
10
144
324
410
459
513
562
985
1179
1374
1374
1374
1725
1725
1791
1791
1791
1791
H ii region ID
F (Ha)
f (off)
EW (Ha)
a
a
a
a
a
a
a
a
a
a
b
c
a
b
a
b
c
d
8.57 6 1.76
71.00 6 3.23
60.9 6 4.21
4.16 6 0.38
20.5 6 2.36
20.7 6 1.91
12.7 6 0.88
1.44 6 0.23
5.77 6 2.26
21.3 6 0.98
3.86 6 0.18
1.78 6 0.08
10.13 6 1.17
5.86 6 0.68
7.25 6 0.33
5.89 6 0.27
11.1 6 0.51
2.19 6 0.10
0.23 6 0.05
0.42 6 0.07
0.59 6 0.10
0.05 6 0.01
0.23 6 0.04
0.31 6 0.06
0.08 6 0.01
0.05 6 0.02
0.13 6 0.05
0.25 6 0.05
0.18 6 0.08
0.06 6 0.06
0.10 6 0.02
0.09 6 0.02
0.05 6 0.01
0.05 6 0.01
0.15 6 0.03
0.07 6 0.03
37
169
104
90
87
68
154
30
46
84
22
32
98
63
135
129
76
30
log (SFR)
1.60
0.68
0.75
1.91
1.22
1.22
1.43
2.37
1.77
1.20
1.95
2.29
1.53
1.76
1.67
1.76
1.49
2.19
Note. F(Ha) and f (off) are in 10 14 erg cm 2 s 1 . The SFR calculation is explained in the text. The
equivalent width of the line is obtained by EW ˆ F…Ha†=f …off†.
the sum of all aperture areas with which we measured the individual
H ii regions by the area of the `continuum' galaxy, de®ned by the
measuring polygon described above.
We identi®ed 38 H ii regions, single or in complexes, in 17
galaxies of the LSB sample. Owing to the limiting spatial resolution
of this study (,300 pc at the VC), and the limiting surface brightness, we were not able in some cases to separate individual small
H ii regions from their nearest neighbours, or to detect the faintest
H ii regions. Therefore our survey was sensitive to H ii `complexes'
of typical sizes larger than the 300-pc resolution limit, or to giant
H ii regions. About half of the galaxies with Ha emission have a
single H ii region and the rest have two or more regions. In two cases
(VCC 17 and 826), and possibly in VCC 328 as well, the H ii regions
seem to be arranged on the circumference of a circle or an ellipse. In
VCC 17 it seems that the centre of this distribution shows an H ii
region as well; in VCC 826 the centre seems empty. We emphasize
that the localization of the H ii regions on the circumference of an
ellipse is a subjective result; we have not attempted to ®t such an
ellipse to the distribution of the H ii regions, and indeed such a ®t
may not succeed in a number of cases because of the strong intensity
of one H ii region compared with others in the same galaxy.
In contrast with the LSB galaxies, the HSB objects show, in most
cases, strong Ha emission originating from a single H ii region. The
line-emitting region is similar in size to the entire galaxy. The global
Ha emission, in terms of the line equivalent widths, shows that the
LSB sample has more low EW values and the global line emission
Ê , whereas in the HSB sample ,one-third of the
hardly reaches 100 A
H ii regions reach or exceed this value. The entire issue of the
morphology of SF in irregular galaxies is discussed in another paper
(Brosch, Heller & Almoznino 1998a).
The data in Table 3 indicate that the difference between HSB and
LSB galaxies is more pronounced when considering H ii regions
than for entire galaxies. The difference in median SFRs for
individual SF regions is more than one order of magnitude; the
HSBs have much more intense SF than LSBs. We compare in Fig. 12
the SFRs found for individual H ii regions in the sample of HSB
galaxies from AB98 and in the sample of LSBs studied here. We
include all the listed Ha values, not only those higher than the 1.5j
q 1999 RAS, MNRAS 304, 8±26
level. In general, H ii regions in HSB galaxies show more intense
SFR, by 0.5 dex on average relative to H ii regions in LSBs.
However, there are H ii regions in objects classi®ed as blue compact
galaxies that show SFR levels as low as those in some LSBs. This is
not exceptional, as in both groups of galaxies there are cases where
no Ha emission is observed. In addition, we note that not only is the
Ha emission from H ii regions in HSBs more intense than in LSBs,
but also the line equivalent width is higher (the median Ha EW in
Ê versus 80 A
Ê in HSBs).
LSBs is 47 A
5
DISCUSSION
The properties of SF, derived from the surface brightness of Ha,
have been discussed by Brosch, Heller & Almoznino (1998b).
Brie¯y, in dwarf irregulars the average Ha surface brightness
correlates reasonably well with the mean blue surface brightness
(j B ). If the SF process is regulated on a local scale, the correlation
should be tighter if we consider only the H ii regions, and not the
entire galaxies. We therefore test here the correlation of Ha
emission against the red continuum surface brightness for individual H ii regions. This is preferred to jB , because it is determined
here independently as part of the derivation of the Ha brightness.
We show in Fig. 13 the correlation between the Ha ¯ux of an H ii
region and the red continuum emission underlying the region, as
measured on the off-line image with the same aperture as used for
the Ha measurement of the LSB sample (stars). Although a trend
for more intense line emission when the red continuum is more
intense seems to be present, it is by no means tight. A formal linear
regression indicates a correlation coef®cient of 0.62 (F ˆ 23).1 A
similar regression for the fewer H ii regions in BCDs (®lled squares)
shows a tighter relation (correlation coef®cient 0.82, F ˆ 32). Both
correlations are signi®cant and indicate the regulation of the SF
activity by the local population of stars. In particular, note that the
1
F is the ratio between the mean square deviation due to the regression and
the mean square deviation due to the residual variation. For a linear
regression, which is the present situation, F ˆ t 2 and this is the equivalent
of a t-test. For more details see Draper & Smith (1981).
14
A. Heller, E. Almoznino and N. Brosch
Figure 2. Mosaic of Ha line (left), continuum (centre) and net Ha (right) images of four galaxies in the LSB sample. North is up and east is to the left. The scale is
indicated by a 10 arcsec long bar in the top-right corner. The galaxies shown are (top to bottom) VCC 17, 169, 217 and 260.
q 1999 RAS, MNRAS 304, 8±26
Late-type dwarf irregulars in Virgo ± I
Figure 3. As Fig. 2. The galaxies shown are VCC 328, 329, 350 and 367.
q 1999 RAS, MNRAS 304, 8±26
15
16
A. Heller, E. Almoznino and N. Brosch
Figure 4. As Fig. 2. The galaxies shown are VCC 381, 477, 530 and 565.
q 1999 RAS, MNRAS 304, 8±26
Late-type dwarf irregulars in Virgo ± I
Figure 5. As Fig. 2. The galaxies shown are VCC 584, 826, 1455 and 1465.
q 1999 RAS, MNRAS 304, 8±26
17
18
A. Heller, E. Almoznino and N. Brosch
Figure 6. As Fig. 2. The galaxies shown are VCC 1585, 1753, 1784 and 1816.
q 1999 RAS, MNRAS 304, 8±26
Late-type dwarf irregulars in Virgo ± I
Figure 7. As Fig. 2. The galaxies shown are VCC 1822, 1952, 1992 and 2034.
q 1999 RAS, MNRAS 304, 8±26
19
20
A. Heller, E. Almoznino and N. Brosch
Figure 8. As Fig. 2. The galaxies shown are VCC 168, 963 and 1468.
q 1999 RAS, MNRAS 304, 8±26
Late-type dwarf irregulars in Virgo ± I
21
Figure 9. Mosaic of line (left), continuum (centre) and net Ha (right) images of four galaxies in the HSB sample. North is up and east is to the left. The scale is
indicated by a 10 arcsec long bar in the top-right corner for each object. The galaxies shown are (top to bottom) VCC 10, 144, 172 and 324.
q 1999 RAS, MNRAS 304, 8±26
22
A. Heller, E. Almoznino and N. Brosch
Figure 10. As Fig. 9. The galaxies shown are VCC 410, 459, 513, 562 and 985.
q 1999 RAS, MNRAS 304, 8±26
Late-type dwarf irregulars in Virgo ± I
Figure 11. As Fig. 9. The galaxies shown are VCC 1179, 1374, 1725 and 1791.
q 1999 RAS, MNRAS 304, 8±26
23
24
A. Heller, E. Almoznino and N. Brosch
Figure 12. Distribution of star formation rates for individual H ii regions in
the HSB and LSB galaxy samples.
Figure 13. Correlation between the logarithms of the Ha emission and of the
red continuum light for individual H ii regions in LSB galaxies. The data for
H ii regions of BCDs are plotted as squares and those for the H ii regions of
LSBs are represented by stars. The units are 10 14 erg cm2 s1 for the line ¯ux
Ê 1 for the continuum ¯ux density.
and 10 14 erg cm2 s 1 A
two samples `line up' nicely on the plot, indicating that the
connection between Ha line strength and underlying line continuum brightness transcends the morphological type of a galaxy.
An inspection of Table 4 and of Figs 2 to 7 indicates that in most
cases of multiple H ii regions in LSB galaxies, one is much brighter
than the others. This indicates either that the SF does not occur
simultaneously within the galaxy, or that, if it does, not all starforming sites reach the same strength of SF at the same time. The
fraction of objects with more than one H ii region among the HSB
sample is much smaller, but the cases of VCC 1374, and to a lesser
degree of VCC 1725 and 1791, are similar to the LSBs. We can
quantify this difference between H ii regions in the same galaxy
with the data in Table 4: the Ha ¯ux ratio between the brightest H ii
region and the second brightest is ,3 (median 2.3) and the similar
ratio of equivalent widths is ,2.
Not all Ha emission from an irregular galaxy originates in the
bright H ii regions. We have compared the summed Ha line ¯ux
from the H ii regions of one galaxy listed in Tables 3 and 4 with the
integrated line emission from the same galaxy (Tables 1 and 2), and
found that a signi®cant fraction of the Ha ¯ux is not recovered. In
particular, for VCC 17, 260 and 530 the H ii regions that we
identi®ed produced less than 70 per cent of the total galaxy line
output (but more than 50 per cent). In a number of objects where we
did not detect individual H ii regions, we nevertheless measured a
signi®cant Ha ¯ux (VCC 329, 1816 and 2034: all detected in the
line at the 3j level and all from the LSB sample). However, in eight
objects it seems that virtually the entire Ha output originates in the
individual H ii regions. It is clear that the detection of an H ii region
results from a combination of compactness, signal strength and
contrast against the background of this region.
We ®nd that the fractional coverage of a galaxy by H ii regions
(the ratio between the summed areas of the apertures with which we
measured the H ii regions and the total area of a galaxy on the red
continuum image) is amazingly similar for LSB and HSB objects,
and in both cases it peaks near 50 per cent, as Fig. 14 shows. The
average value of f in HSB objects is a factor of ,2 larger than in
LSB objects, but the median values are similar: 38 per cent for the
LSBs and 43 per cent for the HSBs. As is often the case with small
samples, the median values are more representative of the `typical'
behaviour than the means, which are driven by outlier values.
However, it seems that the HSB galaxies lack the faint H ii regions
which are detected in the LSB sample.
The location of H ii regions within the red continuum envelope,
which presumably traces the distribution of old stars or of red
supergiants, shows an interesting trend: in the large majority of
cases where a galaxy has H ii regions, these appear preferentially at
the periphery of the red envelope. Only two cases of possibly central
(`nuclear') H ii regions have been identi®ed: VCC 963 and 1455. In
three other cases the H ii regions were indiscriminately distributed
over the red surface of the galaxies, with no special preference for
either centre or periphery.
The trend for SF activity to be relegated to the periphery of
irregular galaxies is discussed in more detail in Brosch et al. (1998a)
and could, in principle, indicate a coupling between the interstellar
matter in a galaxy and the gas in the surroundings of the galaxy. This
has been suggested as a possible explanation of lopsided SF activity
in the Large Magellanic Cloud (de Boer et al. 1998). In this case, the
SF activity could be triggered by some sort of shear or pressure
which compresses the ISM in the galaxy and starts making stars,
which then move away from the location of formation and age. The
difference between the brightest H ii region and the other H ii
regions of the same LSB galaxy could, in this case, be the result
q 1999 RAS, MNRAS 304, 8±26
Late-type dwarf irregulars in Virgo ± I
25
tions), thus its line EW decreases sharply. We can test this using the
data presented here.
We have calculated the `contrast' in line emission and in underlying continuum emission for those objects that show multiple H ii
regions. Speci®cally, we have calculated the ratio between the Ha
¯uxes of the most intense H ii region and the least intense one in a
galaxy, and plotted this in Fig. 15 against the ratio of the red
continuum ¯ux densities of the same two H ii regions. The distribution of points shows that the stronger Ha emission is related to the
more intense red continuum, an opposite trend (to ®rst order) to that
expected from an assumption of SF activation by interaction with
external gas. The correlation is low (0.51, F ˆ 4) but indicative and
the trend, being opposite to what was expected, allows us to reject
the hypothesis. On the other hand, a regulation of the SF by the local
stellar density would make a plausible possibility and would ®t well
with our ®nding that the SFR per unit solid angle correlates with the
blue surface brightness (Brosch et al. 1998b).
6
CONCLUSIONS
We have studied a sample of very low surface brightness irregular
galaxies in the Virgo cluster, and found that the majority show
indications of current star formation. We summarize our ®ndings as
follows.
Figure 14. Distribution of the coverage factor of a galaxy by H ii regions.
(1) The LSB galaxies have a modest total SFR, typically of
,7 ´ 10 3 M( yr 1, while HSB galaxies have a typical SFR of
,6 ´ 10 2 M( yr 1.
(2) The typical global Ha equivalent width of LSB galaxies is a
factor of ,2 lower than in BCDs.
(3) The H ii regions of BCDs tend to be centred on the red
continuum images of the galaxies.
(4) LSB galaxies in which we detected multiple H ii regions
have one very bright region with others much fainter, mostly located
at the edges of the galaxies.
(5) On average, there are ,2 H ii regions per LSB galaxy. They
are smaller and fainter than those found in the BCDs of the Virgo
cluster.
(6) The fractional coverage of a galaxy by H ii regions is similar
in LSBs and in BCDs, and so is the equivalent width of the line
produced by individual H ii regions.
(7) The H ii regions that we detected produce more than half of
the total Ha line emission of an LSB galaxy, and in many cases the
entire galaxy line emission.
(8) The strength of the Ha emission from an H ii region is
correlated with the intensity of the red continuum underneath the
H ii region in both (HSB and LSB) samples.
Our ®ndings do not indicate that there is a fundamental and basic
difference between LSB and HSB irregular galaxies in their starforming properties as re¯ected in their Ha emission and its relation
to the underlying red continuum light, but support the possibility
that these two types of objects represent similar phenomena, which
differ only in the intensity of the star formation process.
Figure 15. Ha line contrast versus off-line continuum contrast between the
brightest and the faintest H ii regions that we detected in galaxies with
multiple H ii regions.
of ageing. The prediction from this scenario is that as an H ii region
gets older its total line emission ¯ux decreases, while the underlying
red continuum emission increases (because of the contribution from
the ageing stars on top of that from any previous stellar populaq 1999 RAS, MNRAS 304, 8±26
AC K N O W L E D G M E N T S
EA is supported by a special grant from the Ministry of Science and
the Arts to develop Tel Aviv University Ultaviolet Explorer
(TAUVEX), a UV space imaging experiment. AH acknowledges
support from the US±Israel Binational Science Foundation and
travel grants from the Sackler Institute of Astronomy. NB is grateful
for the continued support of the Austrian Friends of Tel Aviv
26
A. Heller, E. Almoznino and N. Brosch
University. The Telescope Allocation Committee of the Russian
Academy of Sciences 6.0-m BTA is thanked for generous allocation
of telescope time over the years. Astronomical research at Tel Aviv
University is partly supported by a Center of Excellence award from
the Israel Science Foundation. We acknowledge Bruno Binggeli for
an updated catalogue of the Virgo cluster, G. Lyle Hoffman for
additional H i information on Virgo galaxies, and Liese Van Zee for
copies of images from her samples of galaxies. Rebecca Koopman
kindly set up electronic access to her thesis, which furnished
comparison images of spiral galaxies. We are grateful for constructive remarks from Sara Beck, Lyle Hoffman, Simon Pustilnik,
Gotthard Richter, Liese van Zee, and an anonymous referee. This
research made use of the NASA/IPAC Extragalactic Database
(NED), which is operated by the Jet Propulsion Laboratory,
California Institute of Technology, under contract with the National
Aeronautics and Space Administration.
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This paper has been typeset from a TE X=LA TE X ®le prepared by the author.
q 1999 RAS, MNRAS 304, 8±26