Dwarf Galaxies

The Large and Small Magellanic clouds, orbiting the Milky Way (copyright RSAA/ANU).

Below is a description of some of the work I have done and some of the projects I have participated in concerning dwarf galaxies. Unfortunately, lack of time prevents me from putting on the web everything I am involved in, but my publications list (which is up-to-date) should alleviate this problem somewhat.

My interest in dwarf galaxies centers around so-called dwarf irregular and blue compact dwarf (BCD) galaxies. I am particularly interested in their dark matter content and distribution, (self-propagating) star formation, and environmental influences, mainly through HI observations.

ESO 364-G 029 HoII NGC 2915 FCC 35

ESO 364-G 029:

Together with Thijs Kouwenhoven (University of Sheffield), I have carried out a multi-wavelength study of the dwarf Magellanic irregular galaxy ESO364-G029, part of an effort to enlarge the number of well-studied Magellanic-type galaxies. We obtained both broadband optical imaging and neutral hydrogen radio synthesis observations. The optical morphology characteristically shows a bar-like main body with a one-sided spiral arm, an approximately exponential light distribution, and offset photometric and kinematic centers. The HI distribution is mildly asymmetric and, although slightly offset from the photometric center, roughly follows the optical brightness distribution, extending to over 1.2 Holmberg radii. In particular, the highest HI column densities closely follow the bar, one-arm spiral, and a third optical extension. The rotation is solid-body in the inner parts but flattens outside of the optical extent. The total HI flux F_HI = 23.1 Jy km/s, yielding a total HI mass M_HI = 6.4x10^8 M_sun and a total HI mass-to-blue-luminosity ratio M_HI/L_B = 0.96 M_sun/L_Bsun. The HI data suggest a very complex small-scale HI structure, with evidence of large shells and/or holes, but deeper observations are required for a detailed study. Follow-up observations are also desirable for a proper comparison with the Large Magellanic Cloud, where despite an optical morphology very similar to ESO364-G029 the HI bears little resemblance to the optical.

Left: Low resolution total HI map of ESO364-G029 (contours; ATCA data) superposed on a Digitized Sky Survey image (greyscale). Right: High resolution total HI map (contours and grayscale). Note that, contrary to the Large Magellanic Cloud, the HI follows the optical stellar component rather well.

A paper describing this work has appeared in Kouwenhoven, M. B. N., Bureau, M., Kim, S., & de Zeeuw, P. T. 2007, A&A, 470, 123. You can also download it here in preprint format.


Over a period covering my undergraduate, graduate, and postdoctoral years, I worked on meutral hydrogen VLA D-array observations of the dwarf irregular galaxy Holmberg II in collaboration with Prof. Claude Carignan (Université de Montréal). HoII is a prototype galaxy for studies of shell formation and the data were extracted from the multi-configuration dataset of Puche et al. (1992). We detected HI to radii over 16' or 4 R_25, almost a factor of two better than previous studies. The total HI mass is 6.44x10^8 Solar masses. The integrated HI map has a comet-like appearance, with a large but faint component extending to the northwest and the HI appearing compressed on the opposite side. This suggests that HoII is affected by ram pressure from an intragroup medium (IGM). The velocity field shows a clear rotating disk pattern and a rotation curve corrected for asymmetric drift was derived. However, the gas at large radii may not be in equilibrium. Puche et al. (1992) multi-configuration data were also reanalyzed and we showed that they overestimated their fluxes by over 20%.The rotation curve derived for HoII is well defined for radii up to 10 kpc, but for 10-18 kpc the velocities are only defined on the approaching side, such that this part of the rotation curve should be used with caution. An analysis of the mass distribution, using the whole extent of the rotation curve, yields a total mass of 6.3x10^9 Solar masses, of which about 80% is dark. Similarly to what is seen in many dwarfs, there is more luminous mass in HI than in stars. One the other hand, luminous matter dominates within the optical body of the galaxy and dark matter only in the outer parts, analogous to what is seen in massive spirals rather than dwarfs.

Total HI map of HoII (contours; VLA D-array data) superposed on a Digitized Sky Survey image (greyscale). The observed HI morphology is reminiscent of ram pressure.

HoII lies northeast of the M81 group's core, along with Kar52 (M81 Dwarf A) and UGC4483. No signs of interaction are observed, however, and we argued that HoII is part of the NGC2403 subgroup, infalling towards M81. This suggests ram pressure stripping and the presence of an IGM in the M81 group. Stripping of the outer parts of the disk would require an IGM density greater than 4.0x10^-6 atoms cm^-3 at the location of HoII. This corresponds to about 1% of the virial mass of the group uniformly distributed over a volume just enclosing HoII and it is consistent with the known X-ray properties of small groups. The HI tail is also consistent with additional turbulent viscous stripping and evaporation, at least for low IGM temperatures. We argued that existing observations of HoII do not support self-propagating star formation scenarios, whereby the HI holes and shells are created by supernova explosions and stellar winds. Many HI holes are located in low surface density regions of the disk, where no star formation is expected or observed. We discussed various alternative mechanisms and suggest that ram pressure can help. Ram pressure has the capacity to enlarge preexisting holes and lower their creation energies, helping to bridge the gap between the observed star formation rate and that required to create the holes.

A paper describing this work has appeared in Bureau, M., Carignan, C. 2002, AJ, 123, 1316. You can also download it here in preprint format.


During my PhD thesis at MSSSO, I became interested in the unique blue compact dwarf (BCD) galaxy NGC2915. I studied it in detail along with Prof. Ken Freeman (MSSSO), Dr. David Pfitzner (then at MSSSO) and Dr. Gerhardt Meurer (Johns Hopkins University). NGC 2915 is a blue compact dwarf galaxy with a very extended H I disk. This disk shows a short central bar and extended spiral arms, both reaching far beyond the optical component. We used Tremaine & Weinberg's method to measure the pattern speed of the bar and spiral arms from the HI radio synthesis data, the first time this was achieved in a late-type gas-rich system. Our measurements yield a pattern speed of 0.21+/-0.06 km s^-1 arcsec^-1 (8.0+/-2.4 km s^-1 kpc^-1 for D=5.3 kpc), in disagreement with the general view that corotation in barred disks lies just outside the end of the bar, but consistent with recent models of barred galaxies with dense dark matter halos. Our adopted bar semilength, r_b=180 arcsec, puts corotation at more than 1.7 r_b. The existence of the pattern is also problematic. Because NGC2915 is isolated, gravitational interactions can not account for the structure observed in the HI disk. We also argued that the low surface density observed in the disk and the location of the pseudorings make it unlikely that swing amplification or bar-driven spiral arms can explain the bar and spiral pattern. Based on the similarity of the dark matter and HI surface density profiles, we discussed the possibility of dark matter distributed in a disk and following closely the HI distribution. The disk then becomes gravitationally unstable and can naturally form a bar and spiral pattern. This explanation is however difficult to reconcile with other properties of NGC2915. We also considered the effect of a massive and extended triaxial dark matter halo with a rotating figure. The existence of such halos is supported by cold dark matter simulations showing strongly triaxial dark halos with slow figure rotation. The observed structure of the HI disk can then arise through forcing by the rotating triaxial figure. We thus associate the measured pattern speed in NGC2915 with the figure rotation of its dark halo. If our interpretation is right, objects like NGC2915 offer new opportunities to probe the structure and dynamics of dark halos.

Total HI map of NGC2915 (blue; ATCA data) superposed on a optical image (red). The HI disk shows a barred spiral morphology to large radii, without an optical counterpart.

A paper describing this work has appeared in Bureau, M., Freeman, K. C., Pfitzner, D. W., & Meurer, G. R. 1999, AJ, 118, 2158. You can also download it here in preprint format.

Following this work, Frederic Masset ( CE-Saclay, UNAM) and I carried out further numerical modeling to discriminate between the triaxial halo and heavy disk hypotheses. Hydrodynamic simulations were run for each case and compared to observations using customized column density and kinematic constraints. The spiral structure can be accounted for by both an unseen bar or triaxial halo, but the large bar mass or halo pattern frequency required make this unlikely. In particular, the spin parameter is much higher than predicted by typical cold dark matter structure formation scenarios. The massive disk models show that when the observed gas surface density is scaled up by a factor about 10, the disk develops a spiral structure resembling closely the observed one. This is consistent with more limited studies in other galaxies and suggests that the disk of NGC2915 contains much more mass than is visible, tightly linked to the neutral hydrogen. A classic (quasi-)spherical halo is nevertheless still required, as increasing the disk mass further to fit the circular velocity curve would make the disk violently unstable. Scaling the observed surface density profile by an order of magnitude brings the disk and halo masses to comparable values within the disk radius, but the surface density remains under Kennicutt's star formation threshold for a gaseous disk and no stars are expected to form, as required by observations.

Simulated HI column density maps for triaxial halos with varying pattern speeds.

A paper describing this work has appeared in Masset, F., Bureau, M. 2003, ApJ, 586, 152. You can also download it here in preprint format.


During my PhD thesis at MSSSO, I followed up on intriguing observations of the galaxy FCC35 obtained during my study of the Fornax cluster. This work was in collaboration with Dr. Mary Putman ( University of Michigan), Jeremy Mould ( NOAO), Dr. Lister Staveley-Smith (ATNF), and Prof. Ken Freeman (MSSSO). The Fornax cluster galaxy FCC35 shows an unusual multiply peaked integrated HI profile. We thus observed FCC35 with the ATCA (ATNF) and found a compact HI source of 2.2x10^8 Solar masses and a spatially overlapping complex of HI gas with the same mass. By combining optical observations with HI data, it is clear that FCC35 is a young compact source of star formation with associated HI and a nearby intergalactic HI cloud devoid of stars. FCC35 is thus a blue compact dwarf (BCD) or HII galaxy, having large amounts of neutral hydrogen, very blue colors (U - V=0.1), and a low-metallicity spectrum with strong narrow emission lines. Together with the presence of the HI cloud, this suggests that FCC35 and the high star formation rate within it may be the result of a recent interaction within the Fornax cluster.

Left: Total HI map of FCC35 (contours; ATCA data) superposed on a Digitized Sky Survey image (greyscale). Left: Same for the HI cloud devoid of stars (different velocity range).

A paper describing this work has appeared in Putman, M. E., Bureau, M., Mould, J. R., Staveley-Smith, L., & Freeman, K. C., AJ, 115, 2345. You can also download it here in preprint format.

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This page was last modified on August 1 2007