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A Search for Ionized Gas in the Draco and Ursa Minor Dwarf Spheroidal Galaxies

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A Search for Ionized Gas in the Draco and Ursa Minor Dwarf Spheroidal Galaxies View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server A Search for Ionized Gas in the Draco and Ursa Minor Dwarf Spheroidal Galaxies J.S. Gallagher, G. J. Madsen, R. J. Reynolds Department of Astronomy, University of Wisconsin–Madison, 475 North Charter Street, Madison, WI 53706-1582; [email protected], [email protected], [email protected] E. K. Grebel Max-Planck Institute for Astronomy, K¨onigstuhl 17, D-69117 Heidelberg, Germany; [email protected] T. A. Smecker-Hane Department of Physics & Astronomy, 4129 Frederick Reines Hall, University of California, Irvine, CA 92697-4575; [email protected] ABSTRACT The Wisconsin Hα Mapper has been used to set the first deep upper limits on the intensity of diffuse Hα emission from warm ionized gas in the Draco and Ursa Minor dwarf spheroidal 1 galaxies. Assuming a velocity dispersion of 15 km s− for the ionized gas, we set limits of IHα 0:024R and IHα 0:021R for the Draco and Ursa Minor dSphs, respectively, averaged ≤ ≤ over our 1◦ circular beam. Adopting a simple model for the ionized interstellar medium, these limits translate to upper bounds on the mass of ionized gas of .10% of the stellar mass, or 10 times the upper limits for the mass of neutral hydrogen. Note that the Draco and Ursa Minor∼ dSphs could contain substantial amounts of interstellar gas, equivalent to all of the gas injected by dying stars since the end of their main star forming episodes &8 Gyr in the past, without violating these limits on the mass of ionized gas. 1. Introduction (1999, 2000) yielded upper limits on any diffuse 18 2 neutral gas at column densities of < 10 cm− , Among the unusual characteristics of dwarf implying that any distributed gas would have low spheroidal (dSph) galaxies is the apparent lack mean volume density and therefore could be ion- of any interstellar gas, even in systems such as ized. Bowen et al. (1997) searched for a hot ion- Fornax or Carina, where star formation occurred ized ISM in the outskirts of the Leo I dSph by in the last 1 to 3 Gyr (Skillman & Bender 1995; seeking interstellar ultraviolet lines with no detec- also see reviews by Gallagher & Wyse 1994; Mateo tion, thereby setting ionized gas column density 1998; Grebel 1999). Stetson, Hesser, & Smecker- limits along two sight lines to background QSOs Hane (1998) find a sequence of stars in color- similar to those of the H i surveys. Gizis, Mould magnitude diagram of the Fornax dSph that ap- and Djorgovsky (1993) studied the Fornax dSph 8 pears to be only a few times 10 yrs old. The ma- with ROSAT and found no extended component jor exception to this trend may be the Sculptor that could be associated with very hot gas and set dSph, where an H i cloud could be associated with a limit of 105 M . the galaxy (Carignan et al. 1998; Carignan 1999). ≤ The limits on interstellar gas column densities Recent sensitive 21 cm H i observations by Young from the ultraviolet and H i observations corre- 1 4 spond to gas masses of < 10 M or .1% of the spectra by faint atmospheric lines (Madsen et al. estimated stellar mass (see Grebel, Gallagher, & 2001; Hausen et al. 2002), the observations were Harbeck 2003). As noted by Young (2000), even if alternated between the galaxies (ONs), and a cou- gas were initially removed from a dSph at the time ple of directions (OFFs) a few degrees away from that its star formation ceased, mass loss from stars the ONs. The OFF directions were checked to en- should have replenished the interstellar medium to sure that no potential systematic contaminants, detectable levels. Evidently, dSphs either lose es- i.e. bright stars or high velocity H i gas, were 1 sentially all their interstellar gas or their interstel- present within the 200 km s− spectral window. lar gas lies in a form that has yet to be detected. Each individual observation was 120 s, and the to- For example, their gas could reside in dense, cold tal integration time toward each ON direction was clumps or be nearly fully ionized but at a moder- 78 min. The average of the OFF spectra was sub- ate temperature 104 K that can be sustained tracted from the average of the corresponding ON by photoionization.∼ In either case, substantial spectra to yield spectra of the galaxies which min- amounts of gas could escape detection by previous imized the contamination by atmospheric lines. H i, x-ray, and ultraviolet absorption line studies. Figures 1 and 2 show the resultant Hα spectra In this paper we report results from observations toward the Draco and Ursa Minor dSphs, respec- with the highly-sensitive Wisconsin Hα Mapper tively. The spectra were corrected for atmospheric (WHAM) with the goal of detecting Hα emission and internal transmission effects, and their inten- from ionized interstellar matter in the Draco and sities calibrated using synoptic observations of a Ursa Minor dSphs. No emission is found to sen- portion of the North America Nebula, which has a sitive limits, and we discuss the implications of an intensity IHα = 800 R (Scherb 1981). The these results. velocity scales were calibrated using a bright at- 1 mospheric OH line near -420 km s− with respect 2. Observations to Hα. The x-axis of each plot is heliocentric ve- locity, and the y-axis is milli-Rayleighs per km s 1. We have used WHAM to search for the Hα emis- − sion from the Draco and Ursa Minor dSphs. 3. Results The WHAM instrument is a spectrometer lo- cated at the Kitt Peak National Observatory and 3.1. Upper Limits on Hα Intensities is remotely operated from Madison, Wisconsin. WHAM consists of a 0.6 m siderostat coupled to The spectra in Figures 1 and 2 show no sta- a 15 cm dual-etalon Fabry-Perot system (Tufte tistically significant emission lines, providing only 1997; Reynolds et al. 1998). It produces an upper limits to the Hα emission from these galax- ies. To estimate these upper limits, we assume an optical spectrum integrated over its circular, 1◦ 1 emission line exists at the heliocentric velocities of diameter field of view within a 200 km s− wide 1 spectral window, centered on any wavelength be- each of the dSph systems, -293 km s− for Draco 1 tween 4800 A˚ and 7400 A,˚ with a resolution of and -248 km s− for Ursa Minor (Armandroff, Ol- 1 szewski, & Pryor 1995). In order to set a limit on 12 km s− . WHAM was designed to detect very faint emission lines from the diffuse interstellar the expected width of an Hα line, we assume that medium, and is capable of detecting Hα emission the gas would have a velocity dispersion close to 6 that of the stars, which have line-of-sight disper- down to an emission measure of 0:02 cm− pc. 1 It’s capabilities are well-matched≈ to surveys for sions of about 10 km s− (e.g., Olszewski, Aaron- faint diffuse Hα emission from extended sources, son, & Hill 1995; Armandroff, Olszewski, & Pryor such as the Galactic satellite dSph galaxies. 1995; Hargreaves et al 1996; Kleyna et al. 2002). However, it is likely that the velocity dispersion of We chose to study the Draco and Ursa Minor the ionized gas would be larger than that of the dSphs since they are the closest northern exam- stars, as the gas receives additional energy input ples of these systems. Our WHAM observations from the host stars as they evolve. Furthermore, if were all taken on the night of 2002 May 6, and we impose the condition that a significant amount employed an ’ON minus OFF’ technique. In or- der to accurately remove the contamination of the a 106 2 1 1 1R= 4π photons cm− sec− sr − 2 of ionized gas is actually present in these galaxies, Galactic extinction at Hα toward the Draco (bII = II we cannot use a dispersion that is larger than is al- 86◦) and Ursa Minor (b =45◦) dSphs is less lowed in order to keep the gas bound in the system than 10%. Furthermore, the variation in Galactic 1 (Vesc & 15 km s− ). We therefore adopt a disper- extinction across the projected angular extent of 1 sion of 15 km s− for the Hα emission line that we the Draco dSph is negligible (Odenkirchen et al. attribute to ionized gas bound to the Draco and 2001) We do not make any correction for Galactic Ursa Minor dSph galaxies. extinction. Adopting the above heliocentric velocities and Another possible systematic error could be pho- 1 15 km s− dispersion, we use the observed spectra tospheric Hα absorption by the stars in the host to constrain the maximum area (intensity) of an galaxies themselves. This dip in the background undetected Hα line with these parameters. In par- stellar continuum would tend to reduce the mea- ticular, we increase the area of the imposed spectra sured intensity of any Hα emission feature from line until the average value of the observed spec- the ionized gas. To estimate the magnitude of this tra within the FWHM of the line is more than effect, we consider a completely saturated Hα ab- 3σ below the peak of the imposed line (See Fig- sorption line at the velocity of the host galaxy. ures 1 and 2). In this sense, we consider the lim- The strength of such a saturated line, as observed its to the strengths of the imposed emission lines by WHAM, is at most equal to the surface bright- to be 3σ upper limits.
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