Download Searching for stars in high-velocity clouds

Mon. Not. R. Astron. Soc. 336, 155–158 (2002)
Searching for stars in high-velocity clouds
J. Davies, S. Sabatini, L. Davies, S. Linder, S. Roberts, R. Smith and Rh. Evans
University of Cardiff, Department of Physics and Astronomy, PO Box 913, Cardiff CF24 3YB
Accepted 2002 May 27. Received 2002 May 21; in original form 2002 March 28
ABSTRACT
It has been suggested that some high-velocity hydrogen clouds, rather than being associated
with the Galaxy, may be dispersed throughout the Local Group. If true these hydrogen clouds
may be the ‘debris’ left over from the formation of the Local Group predicted by numerical
simulations. Some of these clouds have measured H I column densities higher than that normally
assumed to be the threshold for star formation, and they are metal-enriched. We have carried
out V- and I-band CCD observations of three compact high-velocity hydrogen clouds from
the sample of Braun & Burton in an attempt to detect resolved giant branch tip stars and/or
diffuse emission. We detect no stars that can be associated with the hydrogen clouds down to a
magnitude limit of m V = 23.3 and m I = 22.0. We find no diffuse emission down to a central
surface brightness limit of ≈25.0µV .
Key words: galaxies: dwarf – galaxies: luminosity function, mass function – dark matter.
1
INTRODUCT ION
Over the last few years a number of authors have re-opened the
discussion on the nature of the high-velocity clouds (HVCs) detected
in 21-cm surveys of the sky. In particular they have suggested that
some of these clouds, rather than being local to the Galaxy, are
the ‘debris’ left over after the formation of the Local Group. These
HVCs could then be the numerous low-mass objects expected from
simulations of the formation of galaxy groups (Blitz et al. 1999;
Braun & Burton 1999, 2000; Blitz 2001; Braun & Burton 2001).
A large number of HVCs (velocities greater than about ±100 km
s−1 compared with the local standard of rest) have now been detected
in extensive surveys of the sky (Hartman & Burton 1997; Wakker &
van Woerden 1991; Putman et al. 2002). The nature of these clouds
has been debated a number of times over the years (see for example
Oort 1966). The two general competing theories are that HVCs are
associated with the Galaxy (infalling, blown out or tidally stripped
gas), in which case they are rather nearby and of relatively low mass,
or that they are extra-Galactic, associated with the Local Group and
having relatively high mass of the order of 107 M (see Blitz 2001
and references therein).
The current measured faint-end slope of the galaxy luminosity
function of the Local Group is ≈−1.1 (Mateo 1998). Converting
this to a mass function and including an extragalactic population
of HVCs would steepen the faint-end slope considerably, putting it
in better agreement with the initial results from models of galaxy
formation [see, for example, Stoehr et al. (2002) and references
therein].
E-mail: [email protected]
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2002 RAS
The true nature of the HVCs remains, however, a puzzle, but
could be decided if good estimates of their distances could be obtained. This has proved to be very difficult. Distances have been
estimated by using gas absorption features in stars presumed to
be behind and in front of the clouds (Keenan et al. 1995; van
Woerden et al. 1999; Wakker 2001), by using absorption features in
distant quasars (Sembach et al. 2002) and by measuring the Hα flux
arising from the presumed ionization by hot Galactic stars (BlandHawthorn et al. 1998; Bland-Hawthorn & Maloney 1999). None
of these estimates has placed strong limits on the distances to a
significant number of HVCs.
Metal-line absorption features in the spectra of stars or quasars
projected behind some clouds also imply enrichment in some way
by stars (Bowen, Blades & Pettini 1995; Bowen & Blades 1993; van
Woerden et al. 1999; Sembach et al. 2002). For example, Sembach
et al. (2002) have measured a metallicity of ≈0.5 solar for one
compact HVC. They say that such a low metallicity is inconsistent
with an origin for the gas inside the Milky Way. They suggest that
it is associated with the Magellanic Clouds or Stream. If stars are
associated with some HVCs then there is also the possibility of
making much better estimates of their distances.
Braun & Burton (1999) have proposed that a particular class of
HVC are more likely to be ‘extra-Galactic’, the group know as
compact HVCs. These HVCs are relatively isolated and are compact
in the sense that they do not appear to be the result of tidal interaction
with the Galaxy. Braun & Burton (1999) say that this group of
HVCs have positional and kinematic properties similar to those of
Local Group galaxies. They give positions (good to approximately
0.5 arcmin), peak H I column densities (of the order of 1020 atom
cm−2 ) and sizes for 66 compact HVCs. The central H I column
densities of these objects are of the order of the critical column
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J. Davies et al.
Table 1. Compact high-velocity clouds observed, and offset fields.
Name
RA (2000)
Dec. (2000)
Dec. (2000)
(offset field)
HVC 125 + 41-207
HVC 070 + 51-146
HVC 018 + 47-145
12h 22m 0s
15h 48m 48s
15h 39m 42s
75◦ 32’30
43◦ 53’0
10◦ 18’0
78◦ 32’30
46◦ 53’0
13◦ 18’0
density assumed necessary for star formation (van der Hulst et al.
1987).
In this paper we describe CCD imaging of the high column density
region of three compact HVCs in an attempt to detect stars.
2
T H E DATA
In 2001 May we used the imaging CCD camera on the 1.0-m Jacobus
Kapteyn Telescope to image the central regions of three compact
HVCs from the list of Braun & Burton (1999). A single 10-arcmin
field was observed for each HVC in two colours (V and I). The field
of view is somewhat smaller than the typical H I FWHM sizes of
≈40 arcmin (Braun & Burton 2000), but they are approximately
centred at the peak H I column density. We also imaged fields, in
an identical way, that were offset by 3◦ from our targets so that we
could compare on- and off-source data (see Table 1).
We obtained total exposure times of 2.5 and 2.0 h for each field
and offset in the V and I bands respectively. The data were reduced
in the standard way using bias frames and twilight flats. Calibration
18
was carried out using standards and by comparison with fields taken
on two excellent photometric nights. We estimate a maximum calibration error of 0.1 mag in both the V and I bands. Galactic extinction
in the V band for each object is of the order of 0.1 mag (Schlegel,
Finkbeiner & Davis 1998). Mean sky brightnesses for the fields obtained were disappointingly brighter than expected: 20.5 in V and
18.8 in I. Using the measured sky noise from the frames, we calculate limiting magnitudes (signal-to-noise ratio of 10) of m V = 23.3
and m I = 22.0. Detections fainter than these limits were rejected.
The SEXTRACTOR package (Bertin & Arnouts 1996) was used to
detect objects and distinguish stars from faint galaxies. Only objects with a class index greater than 0.5 in both V and I were included in the stellar sample. Stellar magnitudes were obtained from
the SEXTRACTOR ‘best flux’ estimator (obtained by fitting a profile
to each star).
3
DATA A N A LY S I S
Our original intention was to look for individual faint stars that
might be associated with each compact HVC. Visual inspection of
the fields showed no clustering of stars about the HVC field centres.
Armandroff, Jacoby & Davies (1999a), using similar data, showed
that they could identify stars at the tip of the giant branch (and
hence estimate a distance) for dwarf companion galaxies of M31.
Although our data do not go quite as faint as those of Armandroff
et al., we should also be able to identify stars at the tip of the giant
branch if the HVCs have similar stellar populations and they reside
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HVC125+41-207
19
19
20
20
21
21
22
HVC125+41-207
Off-set field
22
0
1
2
3
0
1
(V-I)
18
2
3
2
3
2
3
(V-I)
18
HVC070+51-146
19
19
20
20
21
21
22
HVC070+51-146
Off-set field
22
0
1
2
3
0
1
(V-I)
18
(V-I)
18
HVC018+47-145
19
19
20
20
21
21
22
HVC018+47-145
Off-set field
22
0
1
2
(V-I)
3
0
1
(V-I)
Figure 1. Colour–magnitude diagrams for the three fields centred on HVCs and their offset fields. In each case the dashed line is the selection boundary and
the rectangular box indicates the region where giant branch stars would be expected if the HVCs were at the distance of M31.
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Searching for stars in high-velocity cloud
Figure 2. A simulated galaxy has been added to the centre of the frame and
then convolved with a template. Although the central intensity of the galaxy
is only a small fraction of the sky value, it is still easily detectable.
at similar distances to that of M31. In Fig. 1 we show the colour–
magnitude plot for the three HVC fields and offsets. For a similar
distance to M31 we would expect a cluster of stars (tip of red giant
branch) around m I = 21–22 with colours of (V − I ) = 1.2–1.8 (box
in Fig. 1). None of the colour–magnitude diagrams shows such a
feature. We conclude that (i) the compact HVCs contain no stars; or
(ii) they are at greater distances than M31; or (iii) they have much
lower surface brightnesses than the dwarf companions of M31 found
by Armandroff et al. (the density of stars is very low); or (iv) the
stars are not coincident with the H I peak column density; or (v)
the compact HVCs have similar surface brightnesses to the M31
companions, but they do not have the bright giant branch stars of
the M31 companions.
With these data it is not possible to pursue (i), (ii), (iii) and
(iv) above, but we can investigate (v) further. The mean integrated
central surface brightnesses of the four M31 dwarfs described by
Armandroff, Jacoby & Davies (1999b) are about 25µV and they
have scalesizes of the order of an arcminute. We have convolved
our data with a matched template to try to highlight possible diffuse emission from a population of faint unresolved stars. To test
the filtering we have ‘patched out’ bright stars and then added simulated dwarf galaxies to our data frames. The simulated galaxies
have properties typical of the M31 dwarf galaxies described by
Armandroff et al. (1999a,b). In Fig. 2 we show the result for a
simulated galaxy of exponential scalesize 30 arcsec and central intensity 0.5σ (25.3µV ), where σ is the standard deviation of the
mean sky value. Even though the surface brightness of the simulated galaxy is very low, the integrated signal is large (total apparent V magnitude of about 14). We conclude that a filtering technique like this is able easily to reveal diffuse emission at surface
brightness levels similar to the M31 dwarf companion galaxies as
long as it is ‘concentrated’ and not uniformly distributed across our
fields.
In Figs 3 , 4 and 5 we show the convolved fields centred on our
three HVCs. For HVC 125 + 41-207 (Fig. 3) there is a brightening
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Figure 3. HVC 125 + 41-207 field convolved with an exponential filter.
Figure 4. HVC 70 + 51-146 field convolved with an exponential filter.
at the centre of the field, but there is a bright star close to the centre
that has been ‘patched out’. The residuals from this have probably
caused the brightening. This conclusion is supported by the image
of HVC 70 + 51-146 (Fig. 4). The two bright regions at the top and
left both correspond to bright stars in the original image. Although
the star is ‘patched out’, a residual bright halo remains. Again in
Fig. 5 the brightest regions in the convolved images correspond to
regions where stars have been removed. We find no evidence for
diffuse emission that could be associated with the HVCs.
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The question remains over the nature of the mass function of the
Local Group – where have all the small galaxies predicted by the
simulations gone?
REFERENCES
Figure 5. HVC 018 + 47-145 field convolved with an exponential filter.
4
CONCLUSIONS
Understanding the luminosity function of the Local Group is fundamental to our understanding of the luminosity function of galaxies.
The Local Group offers us the opportunity of finding and measuring
the faintest and lowest surface brightness objects that contribute to
the luminosity function. It is hard to believe that the Local Group is
a special environment, and so it is an important indicator of the form
of the field galaxy luminosity function. A lack of stars within HVCs
does not rule out the scenario proposed by Braun & Burton (1999)
and Blitz et al. (1999), but it does rule out HVCs being of the same
class of object as the other known dwarf galaxies. If extragalactic
then for some reason these H I clouds, unlike other dwarf galaxies,
have not formed many (if any) stars. Although these observations do
not support either interpretation of the nature of HVCs, one might
question, with a little more conviction, their extragalactic nature.
Recent H I observations of nearby groups have also failed to detect
galactic-sized H I clouds (Zwaan 2001), again suggesting that the
HVCs are low in mass and associated with the Galaxy in some way.
Armandroff T., Jacoby H., Davies J., 1999a, AJ, 118, 1220
Armandroff T., Jacoby H., Davies J., 1999b, in Davies J., Impey C., Phillipps
S., eds, ASP Conf. Ser. Vol. 170, The Low Surface Brightness Universe.
Astron. Soc. Pac., San Francisco, p. 111
Bertin E., Arnouts S., 1996, A&AS, 117, 393
Bland-Hawthorn J., Maloney P., 1999, ApJ, 510, L33
Bland-Hawthorn J., Veilleux S., Cecil G., Putman M., Gibson B., Maloney
P., 1998, MNRAS, 299, 611
Blanton et al., 2001, AJ, 121, 2358
Blitz L., 2001, in Hibbard J. E., Rupen M., van Gorkom J. H., eds, ASP
Conf. Ser. Vol. 240, Gas and Galaxy Evolution. Astron. Soc. Pac., San
Francisco, p. 469
Blitz L., Spergal D., Teulen P., Hartman D., Burton W., 1999, ApJ, 514, 818
Bowen D., Blades J., 1993, ApJ, 403, L55
Bowen D., Blades J., Pettini M., 1995, ApJ, 448, 662
Braun R., Burton W., 1999, A&A, 341, 437
Braun R., Burton W., 2000, A&A, 354, 853
Braun R., Burton W., 2001, A&A, 375, 219
Hartman D., Burton W., 1997, Atlas of Galactic Neutral Hydrogen. Cambridge Univ. Press, Cambridge
Keenan F., Shaw C., Bates B., Dufton P. L., Kemp S., 1995, MNRAS, 272,
599
Mateo M., 1998, in Richtler T., Braun J. M., eds, Proc. Bonn/BochumGraduiertenkolleg Workshop, The Magellanic Clouds and Other Dwarf
Galaxies. Shaker Verlag, Aachen, p. 53
Oort K., 1966, Bull. Astron. Inst. Neth., 18, 421
Putman M. et al., 2002, AJ, 123, 873
Schlegel D., Finkbeiner D., Davis M., 1998, ApJ, 500, 525
Sembach K., Gibson B., Fenner Y., Putman M., 2002, astro-ph/0203217
Stoehr F., White S., Tormen G., Springel V., 2002, astro-ph/0203342
van der Hulst T., Skillman E., Kennicutt R., Bothun G., 1987, A&A, 177,
63
van Woerden H., Schwarz U., Peletier R., Wakker B., Kalberla P., 1999, Nat,
400, L138
Wakker B., 2001, ApJS, 136, 463
Wakker B., van Woerden H., 1991, A&A, 250, 509
Zwaan M., 2001, MNRAS, 325, 1142
This paper has been typeset from a TEX/LATEX file prepared by the author.
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