arXiv:astro-ph/9705220v1 27 May 1997 2 1 UA n. ne oprtv gemn ihteNtoa Scienc National the Op with National agreement Observatory, cooperative National under Inc., Peak AURA, Kitt astronomer, Visiting rsn drs:MTCne o pc eerh ulig37-66 Building Research, Space for Center MIT address: Present h bevtre fteCrei nttto fWashingto of Institution Carnegie the of Observatories The eateto hsc n srnm,TeUiest fNwM New of University The Astronomy, and Physics of Department eateto hsc n srnm,TeUiest fAlab of University The Astronomy, and Physics of Department 26 h n trfrigrgosaesrnl concentr strongly galaxies are these regions of star-forming and appearance I lopsided H the the for 2276, wh origin gas, ionized tidal sta and a stars old young The of medium. structures asymmetric external the an through motion t from than pressure rather companions, with interactions gravitational interac spira for medium. These evidence diffuse 2276. previous (hot) NGC some surrounding and showing 1961 as NGC selected of were imaging continuum high–re and and I spectroscopy, H and imaging optical servations, rvttoa neatosi orGlx Groups Galaxy Poor in Interactions Gravitational u eut ao otapcso hs aaisa en sha being as galaxies these of aspects most favor results Our erpr h eut ftesailaayi fdeep of analysis spatial the of results the report We ai .Davis S. David arcaA Henning A. Patricia luuru,N 87131 NM Albuquerque, onS Mulchaey S. John ucloaA 35487 AL Tuscaloosa ABSTRACT A91101-1292 CA 1 n ila .Keel C. William and and 1 B abig,M 02139-4307 MA Cambridge, 2B, Foundation. e ia srnm bevtre,oeae by operated Observatories, Astronomy tical ,83SnaBraaS. Pasadena St., Barbara Santa 813 n, xc,80Yl ld,NE, Blvd., Yale 800 exico, m,26Glae Hall, Gallalee 206 ama, 2 ROSAT easymmetric he ouinVLA solution c suggests ich inwt a with tion nNGC In . tdalong ated sfollow rs R ob- HRI e by ped ls the western edge of the disk. In this case, the ROSAT HRI detects the brightest star-forming regions as well as the diffuse disk emission, the most distant galaxy with such a detection. An asymmetric ionization gradient in the H II regions suggests radial movement of gas, which might have occurred in either tidal or wind scenarios. The X-ray emission from NGC 1961 is dominated by a point source near the nucleus of the galaxy but extended emission is seen out to a radius of ∼ 0.′8. Previous studies of the enrichment of the intragroup medium in the NGC 2300 group indicates that stripping may be important in this sys- tem, but the density of the IGM is much too tenuous to effectively strip the gas from the galaxy. However, we propose that gravitational in- teractions in the group environment may enhance stripping. During a gravitational encounter the disk of the spiral galaxy may be warped, making ram pressure stripping more efficient than in a quiescent disk.

Subject headings: interstellar medium - X-rays: galaxies - galaxies: in- dividual NGC 1961 - galaxies: individual NGC 2276

2 1. Introduction fuse medium. Confirmation of this process would be important in suggesting a new arena The discovery of hot gas in clusters of for environmental influences outside of rich galaxies presented the possibility that inter- clusters. actions between a galaxy and its environment We report here new multi-wavelength ob- could influence its evolution. A galaxy’s mo- servations of two of the strongest candidates tion through the cluster gas may play a role for such interactions, NGC 1961 (Arp 184) in the segregation of elliptical and S0 galax- and NGC 2276 (Arp 25 and 114), analyzed ies from gas rich spirals by stripping of the with the aim of distinguishing the gravita- interstellar medium. Evidence that stripping tional effects of surrounding galaxies from hy- does occur in clusters is strongest in the case drodynamic effects of surrounding gas. We of Virgo, where the spiral galaxies near the trace the young and old stellar components as cluster center show a pronounced HI deficit well as ionized, neutral, and X-ray gas, to dis- associated with truncation and asymmetry of tinguish the response to tidal perturbations, the H I disks (Cayatte et al. 1990) which is at- which is to first order similar for stars and tributed to the hot cluster medium stripping gas, from the effects of a gaseous disturbance, HI gas from the galaxies. An extreme case inconsequential for old stars but manifest in may be found in NGC 5291 on the outskirts low-density gas tracers. of the IC 4329 cluster, where a large H I mass is found outside the optical galaxy and sys- These galaxies were selected as showing the tematically offset outwards from the cluster most likely signatures of sweeping by a hot ex- core, with intense star formation seen along ternal medium based on radio and optical ev- the “upstream” sides of H I clumps (Long- idence. Radio studies of bright spiral galaxies more et al. 1979, Malphrus et al. 1995). (Condon 1983; Condon et al. 1990) identi- There is evidence that this process also oc- fied a population of spiral galaxies with ex- curs for the hot gas seen in elliptical galaxies. traordinarily strong disk emission at centime- The X-ray plume from M86 is thought to be ter wavelengths. Optical work shows that this due to stripping as the elliptical plunges into is invariably linked to strong disk star for- the hot cluster gas (Forman Jones & Tucker mation, and often to interactions. The link 1985; Irwin & Sarizan 1996). The discovery to galaxy interactions is hardly unusual given of diffuse gas in poor groups (e.g. Mulchaey the abundant evidence that tidal forces can et al. 1993; Ponman & Bertram 1993) raised induce star formation, but some of the most the possibility that gas stripping could also striking examples have no plausible nearby occur in environments much less dense than companion. In two of the best studied spi- that of clusters. Since the group environment rals of this kind (NGC 1961 and NGC 2276), is less dense that the cluster environment and there is no obviously interacting nearby com- the velocity dispersions of groups are also less, panion and yet a strong asymmetry appears the effects of stripping should be more subtle in the optical and radio maps, suggesting than those seen in clusters. Nonetheless, op- that these galaxies may be interacting with tical and radio work has suggested that some the intragroup medium rather than another spirals in rather sparse environments show ev- galaxy. However, the galaxy environments of idence of interaction with a surrounding dif- such groups have made the interpretation am-

3 biguous in each case; there is usually a bright last 40 years, three of which were in the ap- galaxy close enough in projection to poten- parent “leading edge” and close to but not tially act as a tidal perturber, perhaps several necessarily directly associated with bright H crossing times ago so that the interaction is II regions (Iskudarian & Shakhbazian 1967, not immediately apparent. Shakhbazian 1968, Iskudarian 1968, Treffers NGC 1961 is also notable for its exception- 1993). While the elliptical NGC 2300 and ally large linear size among spirals (Rubin, the spiral NGC 2276 form a relatively iso- Roberts & Ford 1979 ; Romanishin 1983). lated pair according to position-driven algo- Rubin et al. find that the optical rotation rithms, they are rather far apart (≈ 150 kpc −1 −1 curve implies an encircled mass above 1012 for H0 = 50 km s Mpc ), which at first M⊙, and mention a significant role for non- glance makes a tidal origin for the asymmet- circular motions. This large mass is confirmed ric disk unlikely. The galaxy gained height- by Shostak et al. (1982) who map the galaxy ened interest with the discovery of a diffuse using 21 cm spectral imaging. The rotation X-ray medium in its surroundings, the NGC curve derived from the channel maps is well 2300 group (Mulchaey et al. 1993). organized and implies a mass of ∼ 1.5 × 1012 The theme of our analysis is to ask whether M⊙. The integrated H I map shows that the we can distinguish the effects of gravitational bulk of the neutral hydrogen emission is coin- interactions involving neighboring galaxies from cident with the optical galaxy, but the H I is those of ram pressure driven by a surround- also extended to the NW. This extension con- ing diffuse medium, and if so whether there tains ≈10% of the total H I mass. Faint spiral is any strong evidence that ram pressure is arms are seen in the optical image coincident responsible for the peculiar features of these with the H I extension. Opposite the H I ex- galaxies. Both galaxies are members of poor tension, on the SW side of the galaxy, the H groups (Maia, da Costa & Latham 1986) so I distribution is sharply truncated. This H I that their distorted morphologies might be in- morphology leads Shostak et al. (1982) to in- terpreted as evidence for a recent interaction terpret this as stripping of the H I by the hot with another galaxy in each group. intergalactic medium. They noted rough co- incidence of diffuse X-ray structure in an Ein- 2. Observations stein IPC image with an extended H I feature, and interpreted this as evidence of the hot 2.1. ROSAT X-ray Observations stripping medium. However, ROSAT PSPC NGC 1961 and NGC 2276 were observed observations do not confirm the presence of by the ROSAT PSPC and the HRI. The such a hot cloud (Mulchaey et al. 1996). combination of these two instruments allows NGC 2276 is a very luminous Sc I spiral, the spectral and spatial properties of the X- in the NGC 2300 poor group. This is the ray emission for the galaxies in this group most Hα-luminous galaxy in the Kennicutt to be determined. The useful HRI exposure and Kent (1983) survey, and is remarkable for NGC 1961 is almost 83 ksec and that for not only for its disturbed morphology but for NGC 2276 is about 74 ksec. A log of obser- a star formation rate high enough to have vations is given in Table 1. generated 4 observed supernovae within the The ROSAT PSPC observed NGC 1961

4 for 14.8 ksec in March of 1993. The galaxy levels for a single interesting parameter. The was 10.′4 off-axis. The total exposure time for abundance is constrained to be <3.4 solar at NGC 2276 is ∼23ks and is divided between the 90% confidence level. The best fit has three different exposures. While the galaxy is χ2 =2.38 for 5 degrees of freedom. The flux within the central 20′ ring of the PSPC, no from this galaxy is 1.79×10−13 erg s−1 cm−2. useful spatial information was derived from At the assumed distance for this galaxy (79.7 these observations. However, the PSPC ob- Mpc) the soft X-ray luminosity is 1.36×1041 servations of NGC 1961 and NGC 2276 allow erg s−1. us to fit a spectral model to the X-ray emis- NGC 2276 is in three PSPC fields and at a sion from these galaxies and thus accurately different position in the PSPC in each expo- determine the X-ray flux from these galaxies. sure. We extracted the spectrum from each of the fields as described above and fit the result- 2.1.1. PSPC Spectral Results ing spectra simultaneously with XSPEC. For To determine the flux from an object de- the initial observation of NGC 2276 (rp900161) tected with the ROSAT HRI some informa- rebinning the spectrum results in 5 useful tion about the spectral characteristics of the channels. For the two remaining observations object must be known. The HRI provides this rebinning results in 9 channels between only very crude (∼ two channel) spectral in- 0.4 and 2.0 keV with sufficient counts to jus- formation. However, with the addition of tify spectral analysis. The three fields yield PSPC data a spectral model can be deter- a total of 371 counts for NGC 2276 which mined which can then be used to derive an corresponds to a count rate of 0.021 counts −1 accurate conversion from HRI counts to flux. s . We attempted to fit a Raymond-Smith plasma with Galactic absorption to the spec- The spectra for NGC 1961 and NGC 2276 trum but the fit yields unphysical tempera- were extracted from each field with the back- tures, kT >5 keV and Nh ∼ 0. Since the ground determined locally. The galaxy spec- ′ Raymond-Smith model gave unacceptable re- trum was extracted using a 3 circle centered sults we fit a simple power law model with on the galaxy position. The background was Galactic absorption to the data. The photon taken from an annulus with an inner radius of 2 ′ ′ index of the best fit power law (χ = 0.47 for 3 and an outer radius of 6 . NGC 1961 has ν −1 17 dof) is -1.42. The X-ray flux in the 0.4 a total of 215 net counts (0.146 counts s −12 −2 −1 ′ – 2.0 keV band is 7.26×10 erg cm s , 0.5 - 2.0 keV) in a 3 circle centered on the which for the assumed distance to this group galaxy. The spectrum is rebinned so that each −1 −1 of 45.7 Mpc (H0 = 50 km s Mpc ) yields a channel has a minimum of 25 counts and the luminosity of 1.82×1042 erg s−1. The Galac- poorly calibrated channels (Snowden 1994) tic column has been fixed at 3.2×1020 cm−2, are dropped from the spectral fit. This results the value given by Stark et al. (1992). in a spectrum with 8 channels. We fit the resulting spectrum with a Raymond-Smith 2.1.2. HRI Spatial Results plasma model with the Galactic absorption fixed at 9.13×1020atoms cm−2 (Starke et al. To determine any X-ray structure in these +0.19 1992). The best fit temperature is 0.67−0.24 two galaxies the ROSAT HRI data must keV, where the errors are the 90% confidence be corrected for known spatial irregularities.

5 This is accomplished by flat fielding the im- 0.305×1011 counts cm2 erg−1. Assuming that ages using the procedures outlined in Snow- this also holds true for the diffuse component den et al. (1994). The exposure maps are alone, we obtain a flux of 1.09×10−13 erg s−1 generated using information of the aspect and cm−2 which at the assumed distance of the wobble of the satellite along with a map of galaxy corresponds to an X-ray luminosity of the known detector irregularities. Using this 8.31×1040 erg s−1. information a map can be generated which accurately reproduces the spatial variations 2.3. NGC 2276 for each observation. The data and resulting ′′ The HRI data for NGC 2276 have been re- exposure maps are binned in 5 pixels. The ′′ sampled into 5 bins and then smoothed with data are then divided by the exposure map to ′′ a Gaussian with σ=10 . Figure 3 shows the generate the flat fielded image, which is used B band image of NGC 2276 with the X-ray in the spatial analysis below. contours from the smoothed image overlaying the optical image. Two prominent regions of 2.2. NGC 1961 X-ray emission can be seen; the strongest is The flat-fielded HRI data, with 5′′ pix- to the northwest of the nucleus and is a 12 σ els and smoothed with a Gaussian (σ=10′′ ) peak. The nucleus of the galaxy is also an X- , for NGC 1961 are shown overlaying a dig- ray source and is at 7h 27m21s +85◦ 45′ 13.9′′. itized Sky Survey (blue-light) image in fig- X-ray emission from the disk of the galaxy ure 1. The centroid of the X-ray emission is can been seen around the nuclear region and at 5h42m04s.3 +69◦22′46′′.3, in excellent agree- along the western edge of the disk. ment with the optical position of the galaxy The total counts from the galaxy were de- given in the RC3. The counts for this source termined using a circle with radius 90′′ cen- are extracted using a circle with a radius of tered on the nuclear X-ray emission from the ′ 6.5 centered at the peak of the X-ray emis- galaxy. The background is determined using sion. The net number of HRI counts in the an annular ring with an inner radius of 90′′ galaxy is 447.7±60.43 counts, which corre- and an outer radius of 180′′ from the center −3 sponds to a count rate of 5.46×10 ± 0.74 of the galaxy and all point sources were re- −3 −1 ×10 counts s . Analysis of the X-ray pro- moved from this annulus. The background file of NGC 1961 shows that the X-ray emis- subtracted HRI count rate for this galaxy is sion is extended. Figure 2 shows the profile 1.33×10−2± 0.12 ×10−2 counts s−1. Using of NGC 1961 and the profile of a point source the spectral fit from above we can then de- in the same field. The point source is only rive the conversion between HRI counts and ′ ∼2.5 from the position of NGC 1961 and has flux as 0.203×1011 counts cm2 erg−1. Using been scaled to match the counts in the in- an elliptical aperature (22′′ x 37′′ ) centered ′ ner 0.2. An excess of counts can been seen on the peak of the emission from the north- ′ ′ between 0.25 and 0.8 from the center, and west quadrant of the galaxy, we determine the −3 −1 is 272 counts (3.32×10 counts s ). Using count rate is 5.58×10−3± 0.73×10−3 counts the flux determined from the PSPC observa- s−1, which corresponds to an X-ray luminos- tion of this galaxy we determine the conver- ity of 7.26×1041 erg s−1. The count rate from −1 sion factor between counts s and flux to be the nuclear region is 2.58×10−3± 0.32 ×10−3

6 counts s−1 which corresponds to an X-ray lu- patterns which sometimes plagued such data. minosity of 3.2×1041 erg s−1 in a 22′′ circular Thus, the fluxes should be reliable at the 15% aperature. This implies that the X-ray lumi- level for all but the faintest H II regions; is- nosity from the disk of this galaxy is at least sues of blending and size definition are more 7.9 ×1041 erg s−1. important error sources than known instru- mental effects. 3. Optical Data 3.1. NGC 1961 Images in a variety of passbands and spec- tral data were acquired for both galaxies to The optical morphology of NGC 1961 is trace both young and old stellar components. somewhat confused by inclination issues. The outer regions alone would indicate a substan- Narrow-band images, with filters(of typical ◦ bandwidth 60 A˚ ) isolating Hβ, [O III] λ5007, tially inclined spiral pattern, seen perhaps 45 Hα+[N II] λ6583, and [S II] λλ6717 + 6731 from the plane. However, the inner isophotes at each galaxy’s redshift, along with adja- are almost circular in regions devoid of obvi- cent continuum bands at 5125 and 6694 A˚ ous extinction; the usually flattened bulge ge- for continuum subtraction, were obtained at ometry of spirals would suggest that this area the KPNO 2.1m telescope with the video- is seen nearly face-on. The dust lanes cut this camera system in October 1983. The field bulge strongly, obviously at a significant angle of view was 140 arcseconds, well matched to to the sky plane. One might view the system the size of NGC 2276 and requiring two point- as being strongly warped, with the bulge and ings for NGC 1961 (so that only the central inner spiral features north of the nucleus seen β close to the plane of the sky and the outer part was observed in the H , [O III], and [S ◦ II] passbands). Additional broad-band im- arms plus inner dust features seen about 40 ages were obtained with CCDs for NGC 2276 from edge-on. The very disturbed nature of (in B at the KPNO 2.1m and BVRI at the the disk makes normal morphological classifi- Lowell 1.1m) and NGC 1961 (BRI and nar- cation (as well as inclination estimates) more rowband for Hα). The Video Camera data guesswork than we might like. The apparent used a combination of internal quartz and pitch angles of the arms to the north and east night-sky flat fields, with geometric distor- are inconsistent with the dust geometry just tions produced by the image-tube chain cor- south of the nucleus, and the more open arms rected after flat-field division. Since this cor- to the west, for any simple coplanar geometry. rects to constant surface-brightness (rather The disk must be far from the plane of the than point-source) response, the H II region sky in order to give the very large rotation fluxes required a modest correction (reaching velocities measured by Rubin et al. (1979), 25% only at the extreme corners) to recover where the observed velocities before any incli- accurate integrated fluxes. The correction nation correction are among the largest ever (which amounts to the Jacobean of the local seen in a spiral disk. However, beyond this, coordinate transformation) was derived from analysis of the geometry is limited; in the observations of photometric standard stars in words of Rubin et al., “It is difficult to know NGC 2419. The device was quite stable for just how much symmetry we can force on these observations, with none of the Moire NGC 1961”.

7 For NGC 1961, aperture spectra of the nu- by blind offsets from the 8th-magnitude star cleus and several of the brightest complexes of SAO 001148 (only 2.4 arcminutes from NGC H II regions were obtained using the KPNO 2276), since even the galaxy nucleus could not 2.1m and Intensified Image-Dissector Scanner be seen through the guiding eyepiece. Using (IIDS) with 6.1′′ circular apertures and the these data to set the absolute intensity scales Mount Lemmon 1.5m telescope and similar for the line images, and assuming that the [N scanner (4.7′′ apertures), in late 1983. The II]/Hα ratios in these areas are typical, allows line properties from the aperture spectropho- measurement of the intensities of all four lines tometry of both galaxies are listed in Ta- for any desired location from the images. A ble 2. Numerous additional regions could be lower-resolution slit spectrum, obtained with measured in at least Hα after calibrating the the KPNO 4-meter telescope and Cryogenic narrow-band images via the aperture mea- Camera, was obtained with an east-west slit surements (typically with 5-arcsecond aper- position crossing the nucleus, to get spatially tures). Properties of these additional regions continuous coverage of the [O III]/Hβ ratio for both NGC 1961 and NGC 2276 are given and measure the [N II]/Hα ratio with higher in Table 3. The intent was to select the confidence. ones bright enough for meaningful line ratios, The broad-band images are useful in dis- rather than a sample complete in Hα flux. tinguishing the young stellar population (Hα The table includes as well equivalent widths of and blue light) from the older population ′′ Hα emission, measured in the same 5 aper- which should be unaffected by purely gas- tures, as a guide to the emission-line contrast dynamical processes (dominating the I-band of each association. light). In NGC 2276, all show the same char- A more complete listing of H II regions in acteristic truncation at the western edge (Fig- NGC 1961 is provided; these are relatively ure 3), which in itself suggests that gravita- better separated and physically larger (typ- tional effects produce the asymmetry (as dis- ical Hα FWHM 4′′ ≈ 1 kpc) than we see in cussed by Gruendl et al. 1993). The eastern NGC 2276, so at the distance of NGC 1961 part of the disk, with a much lower mean star- it is more appropriate to speak of H II com- formation rate as measured from Hα, appears plexes rather than individual H II regions. normal, with a smooth intensity profile and no such truncation. 3.2. NGC 2276 Figure 4 shows the distribution of Hα emis- The line images for NGC 2276 were cali- sion. Hα imaging traces the distribution of brated by means of aperture spectrophotom- OB stars and hence recent star formation. etry of the brightest two H II regions, ob- Figure 4 shows that the Hα flux from this tained using 8′′ apertures with the Intensi- galaxy is very asymmetric, with the majority fied Reticon Scanner (IRS) at what was at of Hα flux originating from the western side of the time (January 1984) the #1 0.9m tele- the galaxy. The truncated side of the galaxy scope at KPNO. Use of such a small telescope disk is lined by HII regions while the eastern was driven by the inability to use the 2.1m half of the galaxy has only a handful of Hα with IIDS spectrograph within 5◦ of the ce- knots. The Hα flux from the nuclear region −13 −1 −2 lestial pole. The H II regions were acquired is 1.96×10 erg s cm . The integrated

8 Hα flux from the galaxy is 3.39×10−12 erg s−1 the high-inclination models by Howard et al. cm−2. (1993). Line-ratio measurements show another as- The optical data for NGC 2276 has been pect of this asymmetry - not only is the dis- re-sampled and smoothed so that the reso- tribution of H II regions different on the two lution matches that of the X-ray data and sides of the galaxy, so is their ionization. then fluxes were extracted from regions which While both sides of the galaxy show the famil- match the X-ray regions. Table 4 lists the flux iar abundance-linked gradient in [O III]/Hβ, and luminosity determined for the different this gradient is twice as steep on the west- regions discussed in §4.2. These values allow ern (“upstream”) side of the disk. Such a sit- us to compare the stellar populations as re- uation might be found if, for example, disk vealed through their X-ray sources as well as gas on this side had been swept inward from the direct starlight and recombination radia- its original position by whatever process. A tion. similar azimuthal structure to the ionization gradient has been reported for M101 (Ken- 4. Radio Data nicutt & Garnett 1996), at a lower level. It 3 may be relevant that the optical disk of M101 VLA synthesis maps at 1.4 and 5 GHz is also rather asymmetric, at a level difficult were obtained using data from the A & B ar- to reconcile with the faintness of its immedi- ray. These data were combined and cleaned using APCLN to obtain the final images. The ate companion NGC 5474. We are not aware ′′ of any detailed modelling of the chemical re- final resolution is ∼4 . Both these galaxies sults of gas sweeping within a disk, so only exhibit strong and small radio sources associ- schematic considerations can be applied. The ated with luminous H II regions, as noted by diffuse gas (represented observationally by H Condon (1983). I) will be driven inward on the “upstream” 4.1. NGC 1961 side, perhaps with material dropping out of the radial flow as some gas condenses to a The radio morphology of NGC 1961 at much denser molecular form with correspond- 20cm (figure 5) is quite similar to that seen ingly smaller cross-section for pressure-driven in the B-band image. Diffuse emission can be acceleration. To first order, the abundance seen around the nucleus and a ridge of emis- gradient will by the ratio of initial and fi- sion can be seen about 1′ to the south which nal galactocentric distances, whether this dis- corresponds to a spiral arm seen in the op- tance is the total radial flow or the motion to tical image. The 20cm flux from NGC 1961 dropout from radial flow. Such an externally- within 60′′ is 150.6 mJy. The strongest in- driven change in the abundance gradient will dividual source in this galaxy is the nucleus persist for an only orbital time or so, since at 10.61 mJy. Several pointlike sources can clouds initially at similar radii may have dif- be seen to the west of the nuclear region and ferent radial velocities and thus radial diffu- sion driven by their different orbits will set in. 3The National Radio Astronomy Observatory is a fa- This may be seen from typical n-body models cility of the National Science Foundation operated un- der cooperative agreement by Associated Universities, for perturbed disks, for example shown well in Inc.

9 these are labeled in figure 5. Knot 1 has a flux cal value of 8′′ (2 kpc). That is, if the parti- of 4.26 mJy; knot 2’s flux is 2.61 mJy; knot 3 cles giving rise to the synchrotron emission at has a flux of 0.94 mJy. Subtracting the flux centimeter wavelengths are mostly injected by from these sources from the total flux from supernovae close to the locations of present H the galaxy yields a flux of 132.2 mJy for the II regions, they travel typical distances along diffuse component. the large-scale magnetic fields of 2 kpc. Several luminous H II regions in both NGC 4.2. NGC 2276 1961 and NGC 2276 contain strong nonther- The 20cm radio data shown in figure 6 re- mal radio sources, compact on 1-arcsecond flects the morphology of the optical data dis- scales. The most prominent is the bright ′′ ′′ cussed above. The southwest edge of NGC 2276 H II region 40 W and 19 N of the nu- appears to end abruptly and there are at least cleus in NGC 2276, which is brighter than five radio bright sources within 15′′ of the the nuclear source at 20 cm. The reason for truncated edge of the radio disk. To the east this exceptional emission is unclear. Super- no sharp boundary is seen and the radio emis- nova remnants (SNR) in dense environments sion gradually is lost in the noise. The extent might be temporarily very bright, although of the diffuse radio emission on the eastern we did not detect any enhanced [S II] emission side of the galaxy is approximately twice that from shocks in these regions. Quantitatively, of the truncated side. However from figure SNR show line ratios [S II] λλ6717, 6731/Hα 6 it is clear that the majority of the diffuse of 0.4 and larger (D’Odorico 1978, Dopita et radio emission from this galaxy is from the al. 1984). This criterion has proven effec- truncated side of the galaxy. The total radio tive in identifying SNR in galaxies of the Lo- emission from this galaxy is 282.5 mJy. The cal Group (Long et al. 1990), although at nuclear region is comma shaped with the tail larger distances blending with neighboring H of the comma appearing to lead into a spiral II regions and the diffuse ISM reduces the pu- arm. One strong point source can be seen to rity of the samples found in this way (Blair & the northwest of the nucleus along the edge Long 1997). By this criterion, a single emis- of the radio emission. The nuclear region is sion region (number 10 in our listing) stands the strongest point-like source and using a 6′′ out both in [S II]/Hα and [O III]/Hβ in the aperture we find that the emission from the direction expected for shocked gas in a SNR. nucleus is 14.41 mJy. The point source on This is, however, not a detected 20-cm source; the northwest edge of the radio emission has a all the radio-bright regions have typical val- flux of 6.27 mJy. Subtracting the point source ues of [S II]/Hα in the range 0.18–0.28. If emission from the total gives a flux in the dif- number 10 is a single SNR, it falls in the class fuse component of 261.8 mJy. of extraordinarily luminous remnants so far populated only by the remnant in NGC 4449 The morphological relation between Hα (Blair et al. 1983), with comparable emission- and radio continuum emission can be quan- line luminosity but considerably lower ioniza- tified through a “smearing relation”. Pieces tion (since the NGC 4449 remnant was not of the Hα image were convolved with various detected in [S II] by Blair et al.). Any sub- assumed streaming lengths, locally along the stantial population of SNR accounting for the arm pitch. The best match occurs for a typi-

10 radio emission is either masked by the sur- The western, or truncated, side has enhanced rounding emission of normal H II regions or X-ray, optical, and radio emission when com- obscured by dust (at levels somewhat lower pared to the eastern half. The optical emis- than the NGC 4449 SNR, so that our limits sion lines of [O III] and [S II] are restricted to are mildly interesting but not as compelling the sharp boundary along the western limb of as one might like). the galaxy. The diffuse X-ray emission can be The 21-cm HI line data, obtained with the seen along the truncated side of the galaxy in VLA in C array, also reflect the asymmetric figure 3, along with the X-ray bright region morphology seen in the other wavebands. The to the northwest of the nuclear region. The emission is sharply peaked along the western X-ray contours give an impression of being edge of the galaxy (Fig 7.) The integrated swept back in the same direction as the opti- HI flux (16 Jy km/s) lies within the range of cal and HI images. values reported for single-dish observations of NGC 2276 (Huchtmeier and Richter 1989). 6. Discussion

5. Summary of Observations The goal of these observations is to deter- mine if stripping of the ISM is occurring in The data for NGC 1961 show that the op- these two groups. By using a combination tical broad band and line emission from this of X-ray, optical, and radio imaging, along galaxy are fairly symmetric in the inner re- with spectroscopy, we find that the effects of gions. The X-ray and radio images are also the group is at least as complex as the analo- fairly regular. The only exception to this gous process in clusters (Moore et al. 1996). is that in the radio image the southern spi- Ram pressure stripping or gravitational inter- ral arm appears to have enhanced emission actions alone are insufficient to explain the when compared with the northern spiral arm. data. We find evidence that stripping of the The X-ray emission is peaked on the optical ISM and gravitational interactions are affect- and radio nucleus of the galaxy and is mostly ing the galaxies. point-like, but an extended component can be seen. The extended X-rays appear to be elon- 6.1. NGC 1961 gated in the east-west direction, as is the op- The HI distribution in NGC 1961 is strongly tical image (see figure 1). At the lowest con- suggestive that stripping of the neutral gas is tour level, a weak tail-like feature can be seen occurring (Shostak et al. 1982). However, the extending to the southeast of the nuclear re- PSPC data for this group do not show the gion, which might indicate that some of the presence of hot intragroup gas and the upper hot ISM from the galaxy is being stripped. limit for X-ray emission is <1.5×1041 erg s−1 However, since this contour is only 3 σ above (Mulchaey et al. 1996). Thus, any gas which the background it should be interpreted with is present in the group to strip the HI must be caution, and it is the opposite direction of the too cool to be detected using the PSPC data neutral hydrogen tail seen by Shostak et al. or too diffuse to be separated from the X-ray (1982). background. In contrast to NGC 1961, NGC 2276 ap- Tracers of star formation and supernova pears highly asymmetric in all wavebands.

11 remnants show that in the inner regions of In this context, we note that the Ein- NGC 1961 the star formation is symmetri- stein IPC X-ray luminosity (0.2 - 4.0 keV) cally distributed about the nuclear region. of NGC 1961 is 2.40×1041 erg s−1 which is Low resolution radio maps show that the HI not unusual for the blue luminosity of the (Shostak et al. 1982) and the 20cm emission galaxy (Fabbiano, Kim & Trinchieri 1992) is compressed along the northern edge and and thus the X-ray luminosity does not seem much flatter along the southern edge, again strongly enhanced. The X-ray emission from suggesting that stripping may be occurring NGC 1961 is mostly point-like in the HRI in this galaxy. However, the high resolution data and the peak emission is spatially coinci- 20cm map (fig 5) shows that this may be the dent with the nucleus of the galaxy. This im- result of the southern spiral arm being dis- plies that most of the X-rays are from a region torted and pulled further (at least in projec- less than ∼5.8 kpc across. Using the total tion) from the nuclear region. The distorted luminosity of the galaxy and the luminosity spiral arms seen in the radio map (fig 5) are of the extended component we determine the also evident in the B-band image. The stellar point source has a luminosity of 7.25×1040erg and the gaseous component are distorted in s−1. the same manner, which is not plausible with Since interactions are also known to en- ram pressure stripping. hance star formation and Condon, Frayer & As has been found in the optical and far- Broderick (1991) classify this galaxy as a star- infrared bands, there is reason to expect in- burst based on the ratio of infrared to radio teractions to enhance the X-ray luminosity flux, we compute the star formation rate. The of galaxies. This would happen through en- star formation rate for this galaxy can be es- hancement of star formation, with the X- timated from its far-infrared luminosity. The ray emission driven by massive stars and su- total far-infrared luminosity from 43 to 123 5 2 pernovae, and through producing a global µm is given by LIR ≈6×10 D (2.58f60µm+f100µm) hot (expanding) medium if the star-formation (Lonsdale et al. 1985;Thronson & Telesco rate is high enough to produce an unbound, 1986) with D being the distance to the galaxy outflowing gas. Detailed studies of individual in Mpc. Using the measured IR 60µm flux of objects show that these processes can be seen, 6.60 mJy and the 100µm flux of 22.07 mJy, 11 but there is yet no statistical study of the we find LIR = 1.49×10 M⊙. From this we −1 overall situation. Observations of the well- find the SFR = 31 M⊙ yr for the high mass known interacting pair NGC 4038/9 with the stars alone (Thronson & Telesco 1986), which −1 ROSAT PSPC (Read et al. 1995) and ASCA translates to about 150 M⊙/yr for all stars (Sansom et al. 1996) show that emission is assuming a Salpeter IMF. While NGC 1961 seen both from the giant H II regions and is a very luminous galaxy ( listed as the most galaxy nuclei, and from a global, approxi- luminous galaxy of any type in the Revised mately bipolar gas interpreted as an outflow, Shapley-Ames Catalog, with MB = −23.7), as has been seen in some more powerful IR- so that an extensive measure of SFR such bright or merging systems. At this point, it as this would be unusually high even with- seems likely but unproven that interactions out a burst, such a high SFR requires a burst increase the X-ray luminosity of galaxies. even for a galaxy this large and bright. To

12 get an intensive measure of recent SFR be- the stars and gas are initially bent out of the havior, we derived an integrated Hα+[N II] plane of the galaxy together, this can expose strength via integration in the narrow-band a larger area of the gas disk to the intragroup images, which yields an equivalenth width of medium and this would make ram pressure the sum of these lines of 35.5 A˚ , and flux stripping much more likely. Also H I disks are of 6.3 × 10−12 erg cm−2 s in Hα alone, using often substantially more extended than stel- the spectroscopic value for [N II]/Hα, giving lar disks, so if the group does contain gas, a luminosity of L(Hα=7×1042 erg s−1 includ- it would take much less gas to strip the HI ing a somewhat uncertain correction for fore- gas in the outer disturbed portion of the disk ground extinction. While the Hα luminosity than the gas farther in, where the starlight is −1 corresponds to a large SFR (62 M⊙/yr us- easily detected. Thus, the group gas might ing a Salpeter IMF), as implied by the to- remain diffuse enough to remain undetected tal FIR strength, the equivalent width cor- in the ROSAT PSPC data and still gener- responds to a rather mild or very protracted ate the swept-back appearance of the HI data. burst (following, for example, Kennicutt et al. So given the plethora of data on this galaxy 1987). This may thus be a starburst galaxy, it seems that the most likely scenario is that though the overall SFR would be high in any NGC 1961 has undergone a gravitational en- case since this is such a luminous (presumably counter, which has distorted the stellar and massive) galaxy. The high SFR might be due gaseous components of the disk, disturbed the to an interaction, especially since the mor- spiral arms, slightly enhanced the star forma- phology suggests disturbances in the outer tion rate, and allowed ram-pressure stripping spiral pattern. While the star formation is to remove the outer part of the original H I quite widespread, this cannot argue strongly disk. either for or against an enhancement from in- teractions, since the distribution of H II re- 6.2. NGC 2276 gions in clearly interacting spirals span a wide This spiral galaxy was initially selected for range in radial and azimuthal distributions study because of its unusual optical morphol- (Keel et al. 1993). ogy. The optical continuum and emission line The similarity of the distortions seen in the morphology of this galaxy is very asymmet- gas and stars cannot be explained using ram- ric, with the bulk of the line emission from pressure stripping. The coincidence of the the western side of the galaxy. This is also re- gaseous and stellar components argues that flected in the radio maps of this galaxy where ram-pressure stripping plays little if any role the brightest 20cm emission is confined to the in affecting the morphology of this galaxy in western half of the galaxy. the inner regions. However, the HI distribu- The X-ray morphology of NGC 2276 fol- tion is indicative of stripping, thus it may be lows the optical and radio morphology of the that both processes are occurring. galaxy. In contrast to NGC 1961, the nu- If a gravitational encounter has distorted clear source in NGC 2276 is not the strongest the disk of the galaxy, then the gas distribu- source; the disk emission along the western tion can become complex forming tails and edge of this galaxy is just over twice the lu- ring structures (Moore et al. 1996). Even if minosity of the nuclear region. The total

13 log(Lx/Lopt) is -1.82 while the average Lx/Lopt by Forbes & Thomson, and detect as well a for late type spirals (T=4-10) is -6.90±0.27 nearly symmetric “bowtie” structure within (Fabbiano, Gioia & Trinchieri 1988). The about 15′′ of the nucleus (the edges of which log(Lx/LF IR) is -2.36, close to -3 as expected appear in their residual image as 4θ residu- for normal galaxies or LINERS (Green, An- als). All of these features are evidence for derson & Ward 1992) but not typical of star- various levels of gravitational interaction. burst galaxies which have log(Lx/LF IR) ≈ −4 Davis et al. (1996) and Mulchaey et al. (Heckman, Armus & Miley 1990). (1993) show that the density of the intragroup The star formation rate can be estimated medium itself is too small to significantly per- from the Hα luminosity, which for NGC 2276 turb the ISM of the galaxy. Gruendl et al. is 3.39×1041 erg s−1. Using SFR=7.07×10−42 (1993), using Fabry-Perot observations of the −1 −1 η LHα M⊙ yr (Hunter et al. 1986) and η= Hα velocity field, conclude that a tidal in- 0.5, we estimate a star formation rate of ∼ 5 teraction with NGC 2300 is the most likely −1 M⊙ yr . Using the measured IR 60µm flux explanation for the truncated stellar disk and of 11.97 mJy and the 100µm flux of 28.96 mJy, Hα velocity field. Given the large projected −1 we find that the SFR=15M⊙ yr . This is in separation between the galaxies, about 150 reasonable agreement with the SFR derived kpc, if the transverse relative velocity is in from the Hα flux given that these estimates the range expected from velocity dispersions are likely to be accurate only to within a fac- in small groups < 400 km s−1, the encounter tor of two. As in the case of NGC 1961, it is would have been nearly parabolic and taken useful to consider an intensive quantity such place more than 4 × 108 years ago. This is as Hα equivalent width to assess the history a long time for immediate interaction effects of the SFR. The integrated spectral data pre- to play out, suggesting that perhaps internal sented by Kennicutt (1992) give an Hα equiv- disk modes excited by the interaction con- alent width of 59 A˚ , to be compared with the tinue to enhance the star formation even well 32 A˚ from aperture photometry by Kennicutt after the perturber has departed. et al. (1987). This falls well above the range From these lines of evidence, it appears populated by smoothly declining SFR models that the morphology of NGC 2276 is primar- (as shown in Kennicutt et al. 1987), putting ily being affected by a gravitational encounter this in the global-burst category. which has disturbed the stellar and gaseous The unusual morphology of this galaxy has component, enhanced the star formation rate been attributed to either ram pressure strip- along the truncated side of the galaxy, and ping or from a gravitational interaction with again pulled the HI out of the plane of the NGC 2300, an elliptical with signs of a recent galaxy so that stripping is much more likely. merger (Forbes & Thomson 1992, who find The vigorous star formation along the trun- evidence that it hosts a cooling flow). We cated side of the galaxy may be enhanced by have examined this object further, via analy- ram pressure but it cannot be the dominate sis of a Lowell I-band image, to confirm the force shaping the morphology. The interac- reported shell-like morphology. By subtract- tion might be important in making some of ing the best overall r1/4 model, we confirm the the disk H I much more vulnerable to strip- tidal extension and western “shell” reported ping than it would have been originally, so

14 that the combination of two effects is not com- NGC 1961 also shows evidence for ram plete coincidence. pressure stripping with an HI extension to the northeast (Shostak et al. 1982). The dis- 7. Conclusions torted spiral arms indicate that this galaxy has also had a gravitational encounter; how- We have used the ROSAT HRI to study ever, no likely companion is apparent. Since, the X-ray emission from the disk of two spi- the X-ray observations do not reveal the pres- ral galaxies in small groups, NGC 1961 and ence of a dense intragroup medium that could NGC 2276. To the best of our knowledge strip the quiescent HI gas we believe that this these are the most distant spiral galaxies to may be another example where stripping is have their X-ray disk emission resolved. The made more efficient after a gravitational en- X-ray structure of NGC 1961 consists of a counter. nuclear point-like source and diffuse emission We began this study in the hope that se- from the disk. The nuclear source has an X- lecting galaxies with the extremes of asym- ray luminosity consistent with that of a low metry and star formation expected for in- level AGN. The X-ray luminosity of the disk teraction with a surrounding diffuse medium is 8.31×1040 erg s−1 and is consistent with would furnish the best evidence of gas strip- that seen from other normal spiral galaxies. ping in a relatively simple environment. As it The disk emission from NGC 2276 is clumpy happened, even in these carefully picked in- and is strongly correlated with the bright star stances, evidence for stripping is subtle, and forming regions seen on the western side of the the group environments are rich enough that galaxy. The X-ray luminosity of the disk com- 41 −1 gravitational interactions with other mem- ponent is 7.9 ×10 erg s , again not unusual bers still dominate the morphology and star- for spiral galaxies of this blue magnitude. The forming properties of these spirals. The case nuclear region is also detected with a lumi- 41 −1 of NGC 2276 suggests that seeing a combina- nosity of 3.2×10 erg s indicating that this tion of these two effects may not be entirely galaxy may also have an active nucleus. fortuitous – the tidal disturbance may render Tidal interactions have been shown to en- an H I disk more vulnerable to external hy- hance the star formation in galaxies on both drodynamical forces than it would otherwise nuclear and global scales (Keel et al. 1985, be. The difficulty of separating these effects Bushouse 1987, Kennicutt et al. 1987; also in nearby galaxies in rather simple environ- see references in Keel 1991) to an extent which ments may serve as a cautionary tale for un- is consistent with the Hα luminosity and star tangling such effects in clusters and their role formation rates derived above. So from the in galaxy evolution. evidence we have presented above it appears that the strong star formation and distorted This research made use of the HEASARC, morphologies of these two spiral galaxies is NED, and SkyView databases. We acknowl- most likely due to a tidal interaction with a edge support from NASA ROSAT grant NAG5- companion galaxy and not due to ram pres- 2703. Some of the VLA data were obtained sure effects. However, the swept back appear- in collaboration with Jim Condon, who also ance of the HI gas indicates that ram pressure reduced the radio-continuum data. stripping may be occurring.

15 REFERENCES Howard, S.A., Keel, W.C., Byrd, G.G., & Burkey, J.M. 1993, ApJ, 417, 502 Blair, W.P., Kirshner, R.P., & Winkler, P.F., Jr. 1983, ApJ 272, 84 Huchtmeier, & Richter, O. G. 1989, A Gen- eral Catalog of HI Observations of Galax- Blair, W.P. & Long, K.S. 1997, ApJS, 108, ies, (Springer, Berlin) 261 Hunter, D.A., Gillett, F.C., Gallagher. G.S., Bushouse, H.A. 1987, ApJ, 320, 49 III, Rice, W.L. & Low, F.J. 1986, 303, 171 Cayatte, V., Balkowski, C., Van Gorkom, Irwin, J. A. & Sarizan, C. L. 1996, ApJ 471, J.M. & Kotanyi, C. 1990, AJ 100, 604 683 Condon, J.J., Frayer, D.T. & Broderick, J.J. Iskudarian, S.G., 1968, Astr. Ts., 480 1991 AJ, 101, 362 Iskudarian, S.G. & Shakhbazian, R.K. 1967, Condon, J.J., Helou, G., Sanders, D.B. & Astrofizika, 3, 133 Soifer, B.T. 1990, ApJS, 73, 359 Keel, W.C., 1991, in Dynamics of Galaxies Condon, J.J. 1983, ApJS, 53, 459 and Their Molecular Cloud Distributions, Dahlem, M., Hartner, G.D. & Junkes, N. ed. F. Combes and F. Casoli, IAU Sym. 1994, ApJ, 432, 598 146 (Kluwer), 243 Davis, D.S., Mushotzky, R.F., Mulchaey, J.S., Keel, W.C., Frattare, L.M., & Laurikainen, Worrall, D., Birkinshaw, M. & Burstein, D. E. 1993, BAAS, 25, 1356 1996, ApJ, 460, 601 Keel, W.C., Kennicutt, R.C., Jr., van der D’Odorico, S. 1978, Mem Soc Astr Ital 49, Hulst, J.M., & Hummel, E. 1985, AJ, 90, 563 708 Dopita, M., D’Odorico, S., & Benvenuti, P. Kennicutt, R.C., Jr. 1992, ApJS, 79, 255 1984, ApJ, 236, 628 Kennicutt, R.C., Jr., & Garnett, D. R. 1996, Fabbiano, G., Kim, D.-W. & Trinchieri G., ApJ, 456, 504 1992, ApJS, 80, 531 Kennicutt, R.C., Jr., Keel, W.C., van der Fabbiano, G., Gioia, I.M. & Trinchieri G., Hulst, J.M., Hummel, E., & Roettiger, K., 1988, ApJ, 324, 749 1987, AJ, 97, 1022 Forbes, D. A. & Thomson, R. C. 1992, MN- Kennicutt, R. C., Jr. & Kent, S. M. 1983 AJ, RAS, 254, 723 88, 1094 Forman, W., Jones, C. & Tucker W. 1985, Long, K.S., Blair, W.P., Kirshner, R.P., & ApJ 293, 102 Winkler, P.F., Jr., 1990, ApJS 61, 73 Green, P.J., Anderson, S.F. & Ward, M.J. Longmore, A.J., Hawarden, T.G., Cannon, 1992, MNRAS, 254, 30 R.D., Allen, D.A., Mebold, U., Goss, W.M. Gruendl, R.A., Vogel, S.N., Davis, D.S., & & Reif, K. 1979, MNRAS, 188, 285 Mulchaey, J.S. 1993, ApJL, 413, L81 Lonsdale, C. J., Helou, G., Good, J. C. & Heckman, T.M, Armus, L. & Miley, G.K. Rice, W. 1985, Cataloged Galaxies and 1990, ApJS, 74, 833

16 Quasars Observed in the IRAS Survey (Jet Treffers, R. 1993, IAUC 5850 Propulsion Laboratory, Pasadena) Maia, M. A. G., da Costa, L. N.& Latham, D. W., 1989, ApJS, 69, 809 Malphrus, B.K., Simpson, C.E., Gottesman, S.T., & Hawardem, T.G. 1995, BAAS, 27, 1361 Moore, B., Katz, N., Lake, G., Dressler, A., Oemler, Jr., A. 1996 Nature, 379, 613 Mulchaey, J. S., Davis, D. S., Mushotzky, R. F.& Burstein, D. 1993, ApJ, 404, L9 Mulchaey, J. S., Davis, D. S., Mushotzky, R. F.& Burstein, D. 1996, ApJ, 456, 80 Ponman, T. J. & Bertram, D. 1993, Nature, 363, 51 Thronson H. A. & Telesco C. M. 1986, ApJ, 311, 98 Read, A. M., Ponman, T. J., & Wolstencroft, R. D. 1995, MNRAS, 277, 397 Rubin, V.C., Roberts, M.S., & Ford, W.K., Jr. 1979, ApJ, 230, 35 Romanishin, W. 1983, MNRAS, 204, 909 Sansom, A. E., Dotani, T., Okada, K., Ya- mashita, A., & Fabbiano, G. 1996, MN- RAS, 281, 48 Shakhbazian, R.K. 1968, Astrofizika, 4, 319 Snowden, S. L., McCammon, D., Burrows, D. N. & Mendenhall, J. A. 1994, ApJ, 424, 714 Shostak, G.S., van der Kruit, P.C., Hummel, E., Shaver, P.A., & van der Hulst, J.M. 1982, A&A 115, 293 Stark, A. A., Gammie, C. F., Wilson, R. W., Bally, J., Linke, R. A., Heiles, C. & Hur- witz, M. 1992, Ap. J. Supp., 79, 77 Thronson, H.A., Jr., Telesco, C.M. 1986, ApJ, 311, 98 This 2-column preprint was prepared with the AAS LATEX macros v4.0.

17 Figure Captions Fig. 1–The HRI data (contours) is overlaying the optical image of NGC 1961 from the dig- itized sky survey. The X-ray data have been smoothed with a Gaussian with σ=10′′ and the contour levels are ( 6.66, 7.77, 8.88, 9.98, 11.09, 12.20, 13.31, 14.42, 15.53, 16.64) ×10−2 counts s−1 arcmin−2. Fig. 2–The azimuthally averaged X-ray pro- file of NGC 1961 (triangles) along with the azimuthally average profile of a stellar object from the same image (circles). NGC 1961 has excess emission from 0.′25 to ∼0.′7. Fig. 3–The HRI data (contours) is overlay- ing the optical image of NGC 2276 from the 2-meter telescope at Kitt Peak. The X-ray data have been smoothed with a Gaussian with σ=10′′ and the contour levels are (1.08, 1.21, 1.34, 1.46, 1.59, 1.72, 1.85, 1.98, 2.10, 2.23, 2.36) ×10−2 counts s−1 arcmin−2. Fig. 4–The Hα image of NGC 2276. North is up and east is to the left. The FOV is 2′ across. Fig. 5–The 20 cm radio image of NGC 1961. Fig. 6–The 20 radio image of NGC 2276. Note the sharp edge of the radio disk to the west of the nucleus. Fig. 7.–The 21-cm HI line image of NGC 2276. The maps have been smoothed to a resolution of 30′′ x 18′′ (twice the original resolution) with long axis at position angle -10 degrees. Contours are the integrated HI emission with levels at 12, 18, 24, 30, ... 90 mJy/beam. The greyscale represents the ve- locity field.

18

Table RO S AT data

Galaxy Sequence RA Dec Exp osure Observation

Number J J seconds Date

NGC

h m s  0 00

RP Mar

h m s  0 00

RH Mar

NGC

h m s  0 00

RP Apr

h m s  0 00

RP Aug

h m s  0 00

RP Aug

h m s  0 00

RH Mar

Table Ap erture sp ectrophotometry of H I I regions

Region H H O I I I  S I I Notes

NGC

NGC

NGC nucleus

NGC O I I at

NGC O I I at

Notes

1 15 2 1

A as observed no reddening correction Fluxes in units of erg cm s

Typical N I IHalpha ratios p er galaxy were used to correct to net H

Table H I I region measures from narrowband images

Galaxy ID lo cation FH N I I O I I IH H H S I IH EWH A

NGC nuc

N W

S W

N W

N W

S W <

S E

N E

N E

N E

S W

S E <

N W

N W

N E

S W

N W

S W

NGC nuc

W N

W N

W N

W N

E N

E N

E N

E N

E N

E N

E S

E S

W S

W S

W N

W S

W S

W S

W S

W S

W S

E S

E S

E S

E S

E S

E S

E S

E S

Table Flux Luminosity data

Galaxy Band ux Luminosity

1 2 1

erg s cm erg s

NGC

12 41

Xray  

NGC

Total

10 43

B  

12 41

H  

12 42

Xray  

Western Source

11 42

B  

13 40

H  

12 41

Xray  

Southern Source

11 42

B  

13 40

H  

13 41

Xray  

SE Source

11 43

B  

13 40

H  

12 41

Xray  

Plot file version 4 created 23-NOV-1996 12:23:24 GREY: N-2276 IPOL 2276.XMOM1.1 CONT: N-2276 IPOL 2536.2 KM/S CONVL19-39.SQASH.1 2350 2400 2450 2500

85 47 00

46 30

00

45 30

00 DECLINATION (J2000)

44 30

00

43 30

07 29 00 28 30 00 27 30 00 26 30 00 RIGHT ASCENSION (J2000) Grey scale flux range= 2345.6 2515.2 Kilo METR/SEC Peak contour flux = 9.0528E-02 JY/BEAM Levs = 6.0200E-03 * ( -5.00, -4.00, -3.00, -2.00, 2.000, 3.000, 4.000, 5.000, 6.000, 7.000, 8.000, 9.000, 10.00, 11.00, 12.00, 13.00, 14.00, 15.00, 16.00)