Understanding the Evolution of the Magellanic System A Survey of the Ionized Gas in the using WHAM Barger, K. A. and Haffner, L. M. Department of , University of Wisconsin-Madison, USA

The Magellanic System exhibits some of the nearest examples of interactions affecting galaxy evolution with extended gaseous structures strewn throughout the halos of the and the . Models shown that tidal forces can partly – if not dominantly – produce this stripped material as the Clouds encounter each other and the Milky Way. Interaction with halo gas and exposure to radiation from all three galaxies shapes their extended structures, their evolution, and their ability to form . In this work, we use the Wisconsin Hα Mapper to study the faint, warm ionized gas in the Magellanic Bridge, a structure between the Clouds. H i studies have extensively mapped the neutral gas in this region, but the amount and extent of the ionized gas remains uncertain. A census of the material allows us to ask questions addressing the evolution of this system: How much gas have the galaxies lost to this structure? How does the gas loss affect future formation? Is this environment conducive to star formation?

Introduction: 350 Dominant Hα Galaxy interactions have influenced the evolution of ] -30 s component

many galaxies. Understanding how these interactions ] e 300

0.50 -1 e -32 agrees with the

disturb their gas is essential to unraveling the evolution r

of these galaxies. Unfortunately the great distances to g fainter H i peak

e 0.40 -34 250 most galaxies makes investigating this gas difficult. The D LMC [ nearby Magellanic Clouds offer a unique opportunity to -36 0.30 e

study the evolution of two richly interacting galaxies. d 200 u

t -38 velocity center [km s

i 0.20 _ t These interactions can greatly alter the morphology of H 150 Intensity [R]

a -40

galaxies. In the case of the Magellanic System, the Small L

0.10

Magellanic Cloud has a gravitational binding force ten c -42 100 i

times less than the . This makes t 0.00 100 150 200 250 300 350 -1 the much more susceptible to c -44 HI velocity center [km s ] a SMC ram pressure and to tidal stripping, especially at the l a -46 halo and the outer arms regions with lower gravitational G 302 297 292 287 282 Figure 4: The dominant Hα and H i peak position. force and column density.[1] Most studies suggest that Galactic Longitude [Degrees] recent tidal interactions from a nearby encounter The large separation in Hα and H i peak position at the between the Milky Way and the Magellanic Clouds red crosses stems from a complex multi-peak distribution, as seen in Figure 1, where the dominant created the Magellanic Bridge and other prominent Figure 2: This Hα Magellanic Bridge map features emission over the +100 [e.g.,2] Hα component agrees with the fainter H peak. The tidal features, although there are other studies that to +350 km s–1 local standard of rest velocity range. These observation were i suggest alternate formation mechanisms.[e.g., 3] taken over three months span and total 24 hours of observations. The ratios of the Hα and H i emission vary considerably. atmosphere was subtracted using the atmospheric template shown as black The intricate two and three peak distribution exists Though this interaction has caused a burst of star line in panel (b) of Figure 3. The contour lines trace the 1019 cm–2 H i [4,5] throughout the Bridge, including at the interfaces with the Magellanic Clouds. formation at the tidally disturbed Small Magellanic column density at increments of 10, 20, 35, and 50. The brightest Hα Cloud tail – the portion of the Magellanic Bridge that emission follows the high density H i gas in the Small Magellanic Cloud Tail interfaces with the Small Magellanic Cloud – the gas (lower left) and the Large Magellanic Cloud (upper right). We fully sampled The interaction with the Large Magellanic Cloud loss will ultimately cause a reduction in star formation. this region with a half-beam step pattern to completely observe the large scale and the Milky Way has caused the Small Magellanic The Hα and the Hi emission from the Bridge traces the structure of the ionized gas. Cloud to lose gas. This loss affects the star forming changing star formation rate and gas density of the potential of this galaxy. The Magellanic Bridge Small Magellanic Cloud. This study explores the warm 4 accounts for much of this mass loss. This study (a) ionized gas in the Magellanic Bridge to gain insight on 3 (i) shows that ionized gas exists throughout the Bridge. how this system is evolving. (ii) 2 Quantifying how much ionized and neutral gas 1 exists within the Bridge is necessary to interpreting 0 how the Small Magellanic Cloud, and similarly 0.15 (iii) Hα H i (b) interacting galaxies, will evolve. 0.10 ]

3 15 3 -1 (a) 0.05 SMC Tail (d) MB On 4 2 10 2 0.00 The MB has a complex distribution with multi-peak 2 1 5 1 -0.05 (iv) spectral features. Often the dominant Hα peak 0 0 0 0 0.15 corresponds with a weaker Hi peak, suggesting that (c) -1 -5 -1 -2 0.10

Intensity [R (km/s) some of these components have a higher ionization 3 15 3 9 )] 0.05 (b) (e) MB Off 4 -1 13 )] SMC Tail 6 10 12 fraction. These regions could indicate active local -1 2 10 2 1 8 11 14 15

/(km 0.00 4

-2 2 3 5 7 star formation, more exposure of the gas to 1 5 1 2

cm -0.05 18 [mR/(km 0 0 radiation from the galaxies, or perhaps an interface _ 0 0 H 0.05 (v) (d) -1 -5 HI [10 between the neutral portion of the Bridge and hot -1 -2 0.00 20 30 3 -0.05 (c) SMC Tail 25 (f) LMC Interface 4 halo gas. 15 2 20 0 100 200 300 2 10 15 1 Geocentric Velocity [km/s] 10 The Magellanic Bridge has a lower metallicity than 5 0 0 5 both the Magellanic Clouds. This bridge gas could 0 0 -1 -2 125 150 175 200 225 250 100 125 150 175 200 225 250 -1 Figure 3: The average Hα emission, towards two directions faint in Hα, -1 originate from the lower metallicity halo and outer vLSR [km s ] vLSR [km s ] established a model used to construct a synthetic atmospheric template. This disk regions – regions also more susceptible to tidal template was used to remove the atmospheric emission in the Bridge forces – of the Small Magellanic Cloud. Figure 1: Hα (red) and H i[4,5] (black) emission. Most of the observations. This averaged spectra consist of more than 4.5 hours of Understanding the complex velocity distribution of observations taken over 10 days towards (l,b) = (60.0, –67.0) and (89.0, –71.0). Magellanic Bridge sightlines have a complex, multi-peaked, the Magellanic Bridge is necessary to unraveling The (a) and (b) panels show this average spectra as a dotted line and the velocity distribution. In many regions, the bright Hα where this gas originates and the fate of this gas. corresponding fit in gray. The (b) panel emphasizes the faint emission in panel feature corresponds to the fainter H i feature. Panel(e) (a) and displays the constructed atmospheric template as a black solid line. represents a typical observation off the Bridge; this flat Panel (c) illustrates the faint atmospheric lines in dark gray against the fit for spectra indicates that most of the atmospheric emission has the averaged spectra. Panel (d) displays the residuals between the average [1] Bregman, J. N. 2004, in Astrophysics and Space Science Library, Vol. 312, been adequately removed. The region highlighted in gray High Velocity Clouds, ed. H. van Woerden, B. P. Wakker, U. J. Schwarz, & K. spectra and the total fit. The (i) marker indicates the geocoronal line at –2.3 S. de Boer, 341–369 represents the location that the bright OH line, marked as km s−1 and the (ii) marker denotes a bright OH line at 272.44 km s−1 in the [2]Gardiner, L. T., & Noguchi, M. 1996, MNRAS, 278, 191 (ii) in panel (a) of Figure 3, was removed; unfortunately, [3] Besla, G., et al. 2010, ApJ, 721, L97 this study is insensitive to faint Hα emission in this gray geocentric velocity frame. Galactic emission is labeled by markers (iii) and (iv). [4] Hartmann, D. & Burton, W. B., ed. 1997, Atlas of Galactic Neutral The (v) marker indicates residuals from the OH line subtraction caused by a [5] Kalberla, P. M. W., Burton, W. B., Hartmann, D., Arnal, E. M., Bajaja, E., region because this emission could easily be subtracted Morras, R., & Pőppel, W. G. L. 2005, A&A, 440, 775 during the OH line removal. slight mismatch in instrument profile.

[email protected]  www.astro.wisc.edu/~kbarger The National Science Foundation supported the Wisconsin Hα Mapper through grants AST 0204973 and AST 0607512