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Home , Dwarf spheroidal galaxy, Sculptor Dwarf Galaxy

arXiv:astro-ph/9908012v1 2 Aug 1999 Abstract. 1999 mmdd, accepted 1999; mmdd, Received 2 1 faGlci ap(i 96 bt aomv1998). Razoumov & Ibata 1996, (Lin enhancement Previous warp or Galactic creation a mass, of dwarf’s the on depending .B Burton B. W. Spheroidal Dwarf Sagittarius the Towards Observations HI temo tr oeigsvrlhnrd fsur de- square a of as hundreds several appears around covering grees galaxy spheroidal of stream dwarf Sagittarius The Introduction 1. il rdc nteMlyWy nldn itninof distension plau- including might Way, to encounter H Milky reasonable the the an seems on such It produce which system. al. sibly proba- this effects et is for for (Ibata encounter first search This center the 1998). Galactic not al. the bly et from Mateo the 1997; kpc from 1994, kpc 17 25 some and process Way the Milky Sun in the is through apparently plunging and of plane Galactic the to ular wr aay h aasonhr ii h H the the limit with here shown interaction data to The that galaxy. assigned gas dwarf disk unambiguously Way Milky be the in can disturbance any for dence h eoiyo h aitru wr to dwarf Sagittarius the of velocity the the with H velocity the and do position Nor both Cloud. Magellanic in as Large Stream associated Magellanic is the to it analogous consider be might no show H data of The evidence galaxy. spheroidal dwarf Sagittarius the nitrsigecpin(ainne l 98 Carignan 1998, al. is nearby et 1999). of (Carignan galaxy exception typical dSph interesting system (Korib- the an this dwarf although making Sgr spheroidals, 1994), the dwarf al. of center et the alski near positions three H ino h g wr nasac o eta a associated gas neutral for re- search a central in the dwarf includes Sgr the which of area gion large a over hydrogen oe1 qaedgesbtenGlci latitudes Galactic between degrees square 18 some and edopitrqet to requests offprint Send 10.11 10.;1.91 g wr peodl 11.09 Spheroidal; Dwarf Sgr 11.09.1: 11.04.2; 11(09.11.1; are: codes thesaurus later) Your hand by inserted be (will no. manuscript A&A ainlRdoAtooyOsraoy ..Bx2 re Ban Green 2, Box [email protected] P.O. Observatory, Nethe Radio The National Leiden, RA 2300 [email protected] 9513, Box P.O. Leiden, Sterrewacht i ehv esrdthe measured have We vrmr hn5 qaedgesi h ieto of direction the in degrees square 50 than more over − 18 i ae ntedrcino h wr aay and, galaxy, dwarf the of direction the in layer ◦ . 5. λ 1c esrmnshv eetdn H no detected have measurements cm 21 ehv esrdthe measured have We l i 5 = soitdwt h wr peodlwhich spheroidal dwarf the with associated 1 n ei .Lockman J. Felix and ◦ , b = ..Bro rFJ Lockman F.J. or Burton W.B. : − λ 15 1c msinln fneutral of line emission cm 21 ◦ ti lnae perpendic- elongated is It . λ 1c ieo Galactic of line cm 21 i < aaso evi- show data 2 00M 7000 i asat mass ⊙ − over i 13 at ◦ 4 measured. − ereintervals. degree rawti bu 10 the cover about which within (1980), Burton area & Liszt of observations the aiueoe h einbuddb 4 by bounded region the over latitude rmteLie/wneo uvywihcvr h sky the covers declination which sensitivity, of lower survey north at Leiden/Dwingeloo taken the albeit from region, extended more a aatcH Galactic Observations 2. is it Cloud. as a as Large Magallenic the the such en- to with phenomena an analogue associated other of an or phases for tail first evidence tidal the for from perturbation or a resulting counter, for gas galaxy, Galactic dwarf the in by entrained or with nGenBn,W,wihhsa nua eouinof resolution angular an has NRAO which the of WV, 21 Telescope Bank, Foot Green 140 in the using galaxy dwarf 0k s km 20 wr peodl hsnielvlalw 3 a M allows 200 level noise this Spheroidal, Dwarf oesaot10 ms km 1000 about covers pcr ontdvaeb oeta 0 rmtecali- the from the H 10% Leiden/Dwingeloo that than the show of more checks scale by intensity Spot deviate brated not diode. do mea- noise spectra laboratory a using of calibrated The surements band. was signal scale the above temperature MHz +5 frequency reference a . ms km 2.0 tznt a bu 0K yia m os ee na in level col- a noise to equivalent rms is uncertainty typical noise density This a umn mK. 30 K; about 20 temperature is spectrum about system was The zenith (1997). at Burton & Hartmann of ms km ◦ . V294 USA, 24944, WV k 4 rlands, 0 ′ ◦ . eas xmndH examined also We pcr eetkneey0 every taken were Spectra tte2c aeegho h H the of wavelength 21cm the at 0 ≤ − ≤ 2 11.09.4) .2; ⊙ 1 l − ihrsett h S.Tecanlsaigis spacing channel The LSR. the to respect with b − fH of 1 ≤ 1 − ≤ ad tte2 p itneo h Sagittarius the of distance kpc 25 the At band. h aawr bandb wthn against switching by obtained were data The . i 6 ◦ . i pcr eetkntwr h Sagittarius the toward taken were spectra .I l,mr hn20H 250 than more all, In 5. 15 nasnl 4 otpointing. Foot 140 single a in ◦ . ,wt netninto extension an with 5, − ◦ 30 fteGlci etr loa half– at also center, Galactic the of ◦ i − σ thl–eresaig n from and spacing, half–degree at 1 pcr htwr vial over available were that spectra ( N etrda eoiyo +50 of velocity a at centered HI ◦ . nGlci ogtd and longitude Galactic in 5 1 = ) ASTROPHYSICS ASTRONOMY i . 8 ie ahspectrum Each line. × ◦ . 0 AND 10 b ≤ i 17 σ = pcr were spectra l cm eeto of detection − ≤ 18 − i 8 2 ◦ . ◦ . survey over 5 vra over and 0 2 W. B. Burton and Felix J. Lockman: Sagittarius

7 8 1300 13 1200 5 11 2 125011001150 6 4 1050 -5 1517 3 -5 950 1000 900 850 800

1 750

0.5 700

650

-10 -10 600 550 500 latitude latitude 450

0.5 550 1 500 450 -15 -15

400

8 6 4 8 6 4 longitude longitude

Fig. 1. Contours and grey scale of neutral hydrogen Fig. 2. Contours and grey scale of neutral hydrogen in- integrated over +130 kms−1to +170 kms−1 over the tegrated over the range −400 kms−1 to +500 kms−1. region of our observations. The H i column descreases The H i column descreases smoothly from the Galactic smoothly from the , and there is no indi- plane. There is no indication of significant H i associated cation of significant H i associated with the Sagittarius with the Sagittarius over this ex- dwarf spheroidal galaxy in these data. The contours are tended velocity range. The contours are labelled in units labelled in units of K km s−1, each equivalent to 1.8×1018 of K kms−1, each equivalent to 1.8 × 1018 cm−2. cm−2. H i that is related to the Sagittarius dwarf would be ex- 3. Expected Location of HI Emission pected to be either at the velocity of the dwarf (in the Frame) or to be spatially extended in re- The brightest parts of the Sagittarius dwarf spheroidal lation to the dwarf. The former possibility can be tested galaxy are near l = +5◦,b = −15◦; stars associated with simply, because there is not expected to be Galactic H i at this system have been found from latitudes −4◦ to perhaps the velocity of the dwarf; the later possibility requires com- −40◦, and its true extent is still uncertain (Alard 1996, parison of the Galactic H i toward the dwarf with the sit- Alcock et al. 1997, Mateo et al. 1998). The mean stellar uation in directions away from it. velocity over the central region is within a few km s−1 of +170 km s−1 with respect to the Galactic Standard 4. Results of Rest, which, at the center of the dwarf, corresponds to +150 km s−1 with respect to the Local Standard of Figure 1 shows the H i intensity integrated over +130 Rest (Ibata et al. 1997). At such a low Galactic longitude, to +170 kms−1 (LSR); this velocity range extends ±2σ conventional Galactic rotation is almost entirely perpen- about the mean stellar velocity of the Sagittarius dwarf dicular to the line of sight, so unperturbed spheroidal (Ibata et al. 1997). The amount of Galactic disk gas at the location of the dwarf galaxy would have H i decreases smoothly with latitude over this area; there an LSR velocity of order −10km s−1. The longitude of the is no evidence for a concentration at the latitude of the Sgr dwarf spheroidal passes, however, close to the Galactic central part of the Sagittarius dwarf within this velocity nucleus where gas in the bulge regions displays both high range. A comparable view of the H i is given in Figure positive– and negative–velocity motions within about 6◦ 2, which shows contours of H i intensity integrated over of the Galactic plane (e.g. Liszt and Burton 1980). Any all observed velocities. Again, there is no significant con- W. B. Burton and Felix J. Lockman: Sagittarius Dwarf Galaxy 3

J5 -5

-10 latitude

-15 SDS

-400 -200 0 200 400 v(km/s)

Fig. 3. Latitude–velocity diagram showing contours of HI brightness temperature averaged at each latitude over longitudes 4◦ to 8◦. The LSR velocity and position of the brightest stellar component of the Sagittarius Dwarf Spheroidal galaxy is marked by the initials SDS. The width spanned by the initials is plus and minus twice the velocity dispersion of the stellar component; the height of the initials corresponds approximately to the angular resolution of the 140 Foot Telescope. The H i cloud marked J5 (Cohen 1974) is likely associated with gas patterns associated with the region (Liszt & Burton 1980). Contours are drawn at brightness–temperature emission levels of 0.015, 0.03, 0.05, 0.07, 0.1, 0.2, 0.5, 1.0, 2.0, 4.0, 6.0, 8.0, and 10.0 K. centration of H i at the location of the Sagittarius dwarf The feature labeled J5 in Figure 3 is a cloud found by spheroidal within that extended velocity range. Cohen (1975) that has previously been interpreted as one of the observational vagaries stemming from the projec- Figure 3 shows a latitude–velocity map of the HI made tion of kinematics associated with the barred potential of after spectra at constant latitude have been averaged over the inner few kpc of the Galaxy (see Burton & Liszt 1978, the longitude range of our observations. The centroid of Liszt & Burton 1980, Burton & Liszt 1983). J5 has a ve- the Sagittarius dwarf spheroidal galaxy is marked by the locity with respect to the Galactic center of +140 km s−1, letters ‘SDS’. while at its latitude the Sagittarius dwarf spheroidal has For comparison, Figure 4 shows a similar display of a velocity of about +170 kms−1 in the same coordinate data for directions above the Galactic plane from the system. Because of the velocity difference and lack of any Leiden/Dwingeloo survey of Hartmann & Burton (1997). evident spatial coincidence, and because feature J5 fits Note that the positive–latitude figure is plotted with the naturally into models of the Galactic barred potential, we latitude scale inverted to facilitate comparison with the believe that an association of this H i feature with the data toward the dwarf galaxy. Sagittarius dwarf spheroidal galaxy is unlikely. The Liszt & Burton model of the Galactic core also accounts for the These figures show again that there is no bright H i fea- small H i feature at b ∼ +5◦, v ∼−75 kms−1 seen in Fig- ture at the location and velocity of the dwarf galaxy. More- ure 4 at the location and velocity mirror–symmetric to the over, at the latitude of the Sagittarius dwarf spheroidal J5 feature. Feature J5 and its mirror–symmetric counter- the appearance of the H i contours at all velocities is quite part are a single pair of more than the dozen apparently similar above and below the Galactic plane, showing no in- anomalous paired H i features lying within the Galactic dication of a disturbance in the Milky Was disk gas stem- core discussed by Burton & Liszt (1978). All important ge- ming from the proximity of the dwarf. 4 W. B. Burton and Felix J. Lockman: Sagittarius Dwarf Galaxy

5

10 latitude

15

-400 -200 0 200 v(km/s)

Fig. 4. Latitude–velocity diagram showing contours of HI brightness temperature averaged at constant latitudes over longitudes 4◦ to 8◦ for regions above the Galactic plane. The data are from the Leiden/Dwingeloo survey. For easier comparison with Figure 3 the latitude scale has been inverted. Contours are the same as for Figure 3, but the lowest two contours have been omitted because the noise level of the data is higher here. ometrical aspects of the barlike model of the inner–Galaxy tem in the region covered by the current observations was gas distribution proposed by Liszt & Burton (1980) whose derived by averaging all H i spectra with b ≤ −13◦ and consequences subsume the individual anomalous H i fea- 4◦.0 ≤ l ≤ 8◦.0. The average H i spectrum thus obtained tures, have subsequently been confirmed by the direct ev- shows some emission above the noise level over the veloc- idence for a stellar bar in the Milky Way shown by the ity range +140

There is no indication of a train of gas analogous to the Koribalski B., Johnson S., Otrupcek R., 1994, MNRAS 270, Magellanic Stream (see Mathewson et al. 1974) which is L43 focussed at the position and velocity of the Large Mag- Lin D., 1996, in Gravitational Dynamics, eds. O. Lahav, E. ellanic Cloud. The absence of H i in the dwarf spheriodal Terlevich, R. Terlevich; Cambridge Contemporary Astro- is consistant with earlier, more limited, H i measurement physics, Cambridge Liszt H.S., Burton, W.B., 1980, ApJ 236, 779 (Koribalski et al 1994) and with observations of similar Mateo M., Olszewski E.W., Morrison H.L., 1998, ApJ Letters systems. In general, the dwarf spheroidal companions of 508, L55 the Galaxy and of M31 do not have a detectable amount of Mathewson D.S., Cleary M.N., Murray J.D., 1974, ApJ 190, HI. The one exception is the , which 291 4 has > 3 × 10 M⊙ of H i located in two clouds each about Saraber M.J.M., Shane W.W., 1974, A&A 30, 365 500 pc from the center of that galaxy (Carignan et al 1998, Carignan 1999). These clouds have peak column densities in excess of a few times 1019 cm−2, an order of magnitude greater than any feature in our data at the position and velocity of the Sagittarius dwarf spheroidal galaxy (Figure 1). The brightest part of the Sagittarius dwarf spheriodal galaxy is now about 5 kpc below the plane of the Milky Way, and some stars associated with it may be within 2 kpc of the disk. It appears that it will strike the Milky Way at a location near the line of nodes of the Galactic warp, where the disk H i is expected to be distributed approximately symmetrically about z = 0. The H i at a galactocentric distance of 17 kpc has, on average, a scale- height of about 400 pc (Burton 1992). The main part of the Sagittarius dwarf is thus unlikely to be encountering the Milky Way gaseous disk gas as of yet, consistent with our finding that the Sagittarius dwarf has not yet affected the Milky Way H i in any significant way.

Acknowledgements. The National Radio Astronomy Observa- tory is operated by Associated Universities, Inc., under a coop- erature agreement with the U. S. National Science Foundation.

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