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Mon. Not. R. Astron. Soc. 000, 000-000 (0000)

Printed 17 May 2006

A (MN L TEX style file v2.2)

Gas distribution, kinematics and star formation in faint dwarf g a la x ie s
Ayesha Begum1 , Jayaram. N. Chengalur1, I. D. Karachentsev2, S. S. Kaisin and M. E. Sharina2
1 2

2

arXiv:astro-ph/0511253 v1 9 Nov 2005

National Centre for Radio Astrophysics, Post Bag 3, Ganeshkhind, Pune 411 007, India Special Astrophysical Observatory, Nizhnii Arkhys 369167, Russia

ABSTRACT

We compare the gas distribution, kinematics and the current star formation in a sample of 10 very faint (-13.37 < MB < -9.55) dwarf galaxies. For 5 of these galaxies we present fresh, high sensitivity, GMRT HI 21cm observations. We find that the large scale HI distribution in the galaxies is typically irregular and clumpy, with the peak gas density rarely occurring at the geometric center. We also find that the velocity fields for all the galaxies have an ordered component, although in general, the patterns seen do not fit that expected from a rotating disk. For all our galaxies we construct maps of the HI column density at a constant linear resolution of 300 pc; this forms an excellent data set to check for the presence of a threshold column density for star formation. We find that while current star formation (as traced by H emission) is confined to regions with relatively large (NHI > (0.4 - 1.7) × 1021 cm-2 ) HI column density, the morphology of the H emission is in general not correlated with that of the high HI column density gas. Thus, while high column density gas may be necessary for star formation, in this sample at least, it is not sufficient to ensure that star formation does in fact occur. We examine the line profiles of the HI emission, but do not find a simple relation between regions with complex line profiles and those with on-going star formation. Our sample includes examples of regions where there is on-going star formation, but the profiles are well fit by a single Gaussian, as well as regions where there is no star formation but the line profiles are complex. Finally, we examine the very fine scale ( 20 - 100 pc) distribution of the HI gas, and find that at these scales the emission exhibits a variety of shell like, clumpy and filamentary features. The H emission is sometimes associated with high density HI clumps, sometimes the H emission lies inside a high density shell, and sometimes there is no correspondence between the H emission and the HI clumps. In summary, the interplay between star formation and gas density in these galaxy does not seem to show the simple large scale patterns observed in brighter galaxies. Key words: galaxies: dwarf - galaxies: individual: UGC 4459 galaxies: individual: UGC 7298 galaxies: individual: KDG 52 galaxies: individual: CGCG 269-049 radio lines: galaxies

1 INTRODUCTION In the currently popular hierarchical models of galaxy formation, star formation starts in small objects; these in turn later merge to form larger galaxies. In such a model, extremely small nearby galaxies are likely candidate "primeval galaxies", in the sense that they may represent the earliest units of star formation in the universe. There is some observational support for these models, even in the very local universe, viz. (i) the Milkyway itself appears to be

still growing via the accretion of small companions like the Sagittarius dwarf galaxy (see e.g. Majewski et al.(2003)), and (ii) nearby dwarf galaxies have stellar populations that are at least as old as the oldest stars in the Milkyway (see Grebel (2005) for a recent review). In detail, however, the star formation history of nearby dwarf galaxies appears to be extremely varied. At the two extreme ends, dwarf spheroidals have little gas or ongoing star formation while the relatively rare dwarf irregulars are gas rich and also generally have measurable ongoing star formation. Their past star formation histories also appear to have been different - at a given luminosity



E-mail:ayesha@ncra.tifr.res.in


2
dwarf spheroidals are more metal rich than dwarf irregulars, indicative of rapid chemical enrichment in dwarf spheroidals in the past (Grebel (2004)). Why is it that dwarf irregulars, despite having a substantial reservoir of gas have resisted converting it into stars? What keeps the gas in dwarf irregulars from collapse? It is widely believed that the smallest dwarf irregular galaxies have chaotic gas velocity fields (e.g. Lo et al.(1993)), in this case the crucial question then becomes, what sustains these chaotic gas motions? In this context, it is interesting to note that for galaxies which have been observed with sufficient sensitivity and velocity resolution, the velocity field has invariably turned out to have a measurable ordered component, (Begum et al.(2003), Young et al.(2003), Begum & Chengalur (2004)). Does this generally hold for extreme dwarf irregulars, or do some of them genuinely have no ordered components in their velocity fields? Irrespective of the exact nature of the velocity fields, the question of why dwarf irregulars have been unable to convert their gas into stars remains. In spiral galaxies, the current star formation rate appears to depend on at most two parameters (i) the gas surface density and (ii) some measure of the dynamical time. In practice, models which depend only on the gas surface density, such as the Schmidt star formation law, or those which depend on both these parameters, such as the Toomre's instability criterion (Toomre (1964)) appear to provide an equally good fit to the observations (Kennicutt (1998)). For irregular galaxies, Skillman (1987) has proposed that star formation occurs only above a threshold column density, and that this threshold may be related to a critical amount of dust shielding required for molecular gas formation. Are any of these models extrapolatable to the faintest dwarf irregulars? We present here deep, high velocity resolution ( 1.6 km s-1 ) Giant Metrewave Radio Telescope (GMRT) HI observations, as well as H observations of a sample of faint (MB > - 13.0 mag) galaxies, aimed at addressing the above issues. The rest of the paper is divided as follows. The dwarf galaxy sample is presented in Sect. 2, the GMRT observations are detailed in Sect. 3, while the results are presented in Sect. 4 and discussed in Sect. 5. the 6-meter SAO telescope using a 2048×2048 pixel CCD camera. The scale was 0.36 /pixel, and the total area imaged was 6×6 . The H + [NII] emission line fluxes were obtained by observing œ each galaxy through two filters: a narrow (75 A) interference filœ œ ter centered on 6567 A, and a middle-width filter ( = 6063 A, œ = 167 A) to determine the nearby continuum level. The integration times were 2×300 sec in the middle-width filter and 2×600 sec in H. Because the range of radial velocities was small, we used the same H filter for all objects. The images were bias subtracted and flat fielded following standard procedures. After flat fielding, the next step was to subtract the sky emission from both continuum and narrow band filter images. The continumm filter images were then scaled relative to the narrow band images using 5-10 unsaturated stars, and then subtracted from the narrow band filter images. The continuum-subtracted H images were flux calibrated using observations from the same night of two or more of Feige's photometric standards. Corrections for the Galactic extinction were made assuming A(H)= 2.32 E(B-V) using the data from Schlegel et al. (1998). The star formation rate for these galaxies were calculated from the derived H luminosities, using the conversion factor from Kennicutt (1998a) SFR = 7.9 × 10-
42

L(H) M yr-

1

(1)

2 DWARF GALAXY SAMPLE The optical properties of our sample of ten galaxies are given in Table 1. Fresh HI observations for five galaxies in the sample viz. KDG 52, UGC 4459, CGCG 269-049, UGC 7298 and KK 230 are presented in this paper. GMRT HI data for KK 44 (Camelopardalis B), GR 8 and DDO 210 were presented in our previous papers (Begum et al.(2003), Begum & Chengalur (2003,2004)), although we include here fresh maps and measurements at angular scales that are relevant to the issues discussed in this paper. GR 8 and DDO 210 were also observed with the VLA; the VLA data for these galaxies are presented in Young et al.(2003). VLA HI data for Sag DIG and Leo A were obtained from the VLA archive. These observations have been discussed earlier by Young & Lo (1996) (Leo A) and Young et al.(1997) (Sag DIG); once again we present here only those maps and measurements that are relevant to this paper.

The calculated SFR for our sample galaxies are given in Table 5. In case of KDG 52 and KK 230, no H emission was detected; the derived limits on the SFR for these galaxies is also listed in Table 5. For Leo A, Sag DIG and GR 8, H images were downloaded from NED. Details of these images can be found in Hunter & Elmegreen (2004). The H image of UGC 4459 was kindly provided by U. Hopp; details can be found in Schulte-Ladbeck & Hopp (1998). For DDO 210, van Zee (2000) detected a single source of H emission in the galaxy; however follow up observations suggested that it does not arise in a normal HII region, but probably comes from dense outflowing material from an evolved star. In all the figures of DDO 210 in this paper, we show the location of this emission by a star, but caution the reader that it may not actually represent a star forming region. Except for KK 230, broadband optical observations of all our sample galaxies were available in the literature. For KK 230, V and I band HST ACS images were used to obtain the total magnitude of the galaxy. The derived magnitude is I(R < 40 ) = 15m 6+0m 15, . . and the integrated (V - I) colour inside the same radius is 0.90. Assuming a typical color (B - V) = 0.50 for KK 230, we estimated its integrated blue magnitude to be B = 17m 0 + 0m 25. . . 3.2 HI observations and data analysis HI 21cm observations of KDG 52, UGC 4459, CGCG 269-049, UGC 7298 and KK 230 were conducted with the GMRT (Swarup et al. (1991)) between Nov. 2001 and Nov. 2002. KK 44, GR 8 and DDO 210 were also observed with the GMRT; details can be found in Begum et al.(2003), Begum & Chengalur (2003,2004). Data for Sag DIG and Leo A were obtained from the VLA archive. These observations are also discussed in Young & Lo (1996,1997). Here we briefly describe only the fresh GMRT observations. For all galaxies, the observing bandwidth of 1 MHz was divided into 128 spectral channels, yielding a spectral resolution of 7.81 kHz (velocity resolution of 1.65 km s-1 ). The setup for the observations is given in Table 2. The flux and bandpass calibration were done using 3C48, 3C147 and 3C286. The phase calibration was done once in every 30 min by observing the VLA calibrator

3 OBSERVATIONS AND ANALYSIS 3.1 Optical observations and data analysis H observations of some of our sample galaxies, viz. KK 44, KDG 52, CGCG 269-049, UGC 7298 & KK 230 were carried out at


Faint dwarf galaxies
Table 1. Optical parameters of the sample galaxies

3

Galaxy KK 44 KDG 52 UGC 4459 Leo A CGCG 269-049 UGC 7298 GR 8 KK 230 Sag DIG DDO 210

RA(J2000) 04h 08h 08h 09h 12h 12h 12h 14h 19h 20h 53m 23m 34m 59m 15m 16m 58m 07m 29m 46m 06. 56. 06. 26. 46. 28. 40. 10. 59. 51. 9s 0s 5s 4s 7s 6s 4s 7s 0s 8s

Dec(J2000) +67 05 57 +71 01 46 +66 10 45 +30 44 47 +52 23 15 +52 13 38 +14 13 03 +35 03 37 -17 40 41 -12 50 53

MB -11.85 -11.49 -13.37 -11.36 -12.46 -12.27 -12.11 -9.55 -11.49 -11.09

D (Mpc) 3.34 3.55 3.56 0.69 3.4 4.21 2.10 1.9 1.1 1.0

B-V 0.8 0.24 0.45 0.15 0.29 0.32 0.40 0.3 0.24

RHo ( ) 0.7 0.65 0.80 3.5 0.60 0.55 0.95 0.87 1.8 1.8

iopt ( ) 65 24 30 54 77 58 25 35 45 62

references 1,3 3 3,7 3,4,8 3 3,7 2,3 3 3,6 3,5

References: 1-Begum et al. (2003), 2-de Vaucouleurs & Moss (1983), 3-Karachentsev et al.(2004), 4- Karachentseva & Sharina (1988) 5-Lee et al.(1999), 6Lee & Kim (2000), 7-Makarova (1999), 8-Tolstoy et al. (1998) Table 2. Parameters of the GMRT observations

Galaxy

Date of observations

Velocity coverage (km s-1 ) 10 - 220 -60 - 130 65 - 275 65 - 275 -40- 170

Time on source (hours) 18 14 16 16 18

synthesised beam (arcsec2 ) 42 × 39 ,26 16 × 15 , 6 45 × 38 ,29 18 × 16 ,3 42 × 39 ,28 18 × 17 ,4 42 × 37 ,26 16 × 15 ,4 48 × 45 ,34 26 × 24 ,4 × 23 × 6 × 27 × 3 × 24 × 3 × 24 × 4 × 31 × 3

synthesised beam (pc2 ) 723×671, 447×396 275×258, 103×103 777×656,500×466 310×276,52×52 692×642, 461×396 297 × 280, 66×50 857×755, 530 ×490 326 ×306, 82×82 442× 415,313×286 240×221, 37×28

Noise (mJy) 1. 1. 1. 1. 2. 1. 2. 1. 1. 1. 7, 3, 9, 4, 0, 7, 0, 6, 6, 2, 1. 0. 1. 1. 1. 1. 1. 1. 1. 0. 5 9 6 2 8 2 8 1 4 8

Continuum Noise (3) (26 × 22 ),(3 × 3 ) (mJy) 0.9, 0.42 1.0, 0.45 0.5, 0.3 0.5, 0.3 0.4, 0.2

KDG 52 UGC 4459 CGCG 269-049 UGC 7298 KK 230

21-23, 27, Jun 2002 15, 23, 24, Nov 2002 23-25, Nov 2002 23-25, Nov 2002 6 Jun, 8 May, 26 Nov 2001

sources 0831+557 (UGC 4459), 1216+487 (UGC 7298), 1216+487 (CGCG 269-049), 3C286 (KK 230) and 0834+555 (KDG 52). The galaxies UGC 7298 and CGCG 269-049 are close in space ( 12 ) as well as in velocity, hence both were included in a single GMRT pointing (the field of view of the GMRT 24 ). The data were reduced in the usual way using standard tasks in classic AIPS. For each run, bad visibility points were edited out, after which the data were calibrated. The GMRT does not do online doppler tracking - any required doppler shifts have to be applied during the offline analysis. For UGC 7298 and CGCG 269049, the differential doppler shift over our observing interval was much less than the channel width, hence, there was no need to apply any offline correction. On the other hand, the differential shifts for UGC 4459, KK 230 and KDG 52 were significant, hence, for each of these galaxies, the calibrated (u,v) data set for each day was shifted in the frequency space to the heliocentric velocity of the galaxy, using the task CVEL in AIPS. For each galaxy, data for all the runs were then combined using the AIPS task DBCON. The GMRT has a hybrid configuration (Swarup et al. (1991)) which simultaneously provides both high angular resolution ( 3 , if one uses baselines between the arm antennas) as well as sensitivity to extended emission (from baselines between the antennas in the central array). Data cubes were therefore made using various (u,v) cutoffs to get the images of HI emission at various spatial resolutions (see Table 2 for details). Except for the highest resolution HI data cubes for each galaxy, all the data cubes were decon-

volved using the AIPS task IMAGR. For the highest resolution data cubes in each galaxy, the signal to noise ratio (SNR) was too low for CLEAN to work reliably. Despite this, the low SNR of the images implies that the inability to deconvolve does not greatly degrade the dynamic range or fidelity of these images. The morphology of the emission in these galaxies should hence be accurately traced, apart from an uncertainty in the scaling factor (this essentially arises because the main effect of deconvolving weak emission at about the noise level corresponds to multiplying by a scale factor; see e.g. Jorsater & van Moorsel (1995), Rupen (1999)). ÅÅ Continuum images were also made for all the galaxies by av eraging the line free channels. No extended (26 × 22 ) or compact (3 × 3 ) emission was detected from any of the galaxies. The 3 limits for each galaxy are given in Table 2. Moment maps were made from the data cubes using the AIPS task MOMNT. Maps of the velocity field and velocity dispersion were also made in GIPSY using single Gaussian fits to the individual profiles. The velocity field produced by Gaussian fitting is in reasonable agreement with that obtained from moment analysis. The velocity dispersion (obs ), as estimated by fitting single Gaussian component to the line profiles is given in Table 3. In all cases, no measurable variation of velocity dispersion was seen (within the errorbars) across each galaxy. This lack of substantial variation of across each galaxy is typical of such faint dwarf irregular galaxies (e.g. Begum & Chengalur (2004), Begum et al. (2003), Skillman et al. (1988)). As discussed in more detail in section 5.3, single Gaus-


4
UGC4459 UGC7298 CGCG269 KK230 KDG52

Figure 1. The HI global profile for our sample galaxies derived from our coarsest resolution HI distribution. The channel separation is 1.65 km s dotted line shows a Gaussian fit to the line profiles.

-1

. The

sian profiles are not necessarily a good fit throughout the galaxy; there are regions where the emission profile is skewed or is otherwise more complex than a single Gaussian.

1

2

4 RESULTS 4.1 Large scale HI distribution and kinematics The global HI profiles for our sample galaxies, obtained from the coarsest resolution data cubes (see Table 2) are shown in Fig. 1. Column (2-7) in Table 3 lists the parameters derived from the global HI profiles. Col. (1) gives the galaxy name, (2) the integrated HI flux along with the errorbars, (3) the velocity width at 50% of the peak (V50 ), along with the errorbars, (4) the central heliocentric velocity (V ) and its errorbars, (5) the HI mass along with its errorbars, (6) the HI mass-to-light ratio (MHI /LB ), (7) the ratio of the GMRT flux to the single dish flux (FI/FISD ). The single dish fluxes for all the galaxies are taken from the tabulation in Karachentsev et al.(2004). In the case of CGCG 269-049, single dish data is not available. The parameters measured from the GMRT HI profiles are in good agreement with those values obtained from the single dish observations, in particular the HI flux measured at the GMRT agrees with the single dish flux for all the galaxies. This indicates that no flux was missed because of the missing short spacings in our interferometric observations. Col.(8) shows the velocity dispersion, along with error bars, as measured from a single Gaussian fit to the line profiles, (9) represents the HI radius at a column density of 1019 atoms cm-2 , (10) the inclination as measured from the HI moment 0 maps, (11) the ratio of the HI diameter to the Holmberg diameter. For all the galaxies the HI emission extends to 2- 3 times the optical diameter, a typical ratio for dwarf irregular galaxies. The integrated HI emission of our sample galaxies overlayed on the optical Digitized Sky Survey (DSS) images are shown in Fig.3[A]- 7[A]. The HI distribution in CGCG 269-049 and KK 230 are dominated by a single clump of high column density, while the HI in UGC 4459 and UGC 7298 are concentrated in two highcolumn-density regions, separated by a low-column density region in the center. In the case of KDG 52, the HI is distributed in a clumpy, incomplete ring. Inclinations (iHI ) of our sample galaxies (except for KDG 52) were estimated from the HI moment 0 maps by fitting elliptical annuli to the two lowest resolution images. For KDG 52, only the lowest resolution HI distribution is sufficiently smooth to be used for ellipse fitting. For all other galaxies, the inclination derived from these two resolution images match within the errorbars. The estimated inclination for each galaxy (assuming an intrinsic thickness qo = 0.2) is tabulated in Table 3. Comparing this value to the op3

1 ---CGCG 269-049 2 ---UGC 4459 3 ---UGC 7298 4 ---GR 8 5 ---DDO 210 6 ---KK 44 7 ---Leo A 8 ---KK 230 9 ---Sag DIG 10---KDG 52

4 5 6 7 9 10 8

Figure 2. The deprojected gas surface mass density (SMD) distribution for our sample galaxies. For all galaxies (except DDO 210, Sag DIG and Leo A), the SMD was computed for a linear resolution of 500 pc. For DDO 210, Sag DIG and Leo A the linear resolution is 300 pc. The actual angular resolutions are 29 × 27 (UGC 4459), 26 × 24 (UGC 7298), 28 × 24 (CGCG 269-049), 48 × 45 (KK 230), 26 × 23 (KDG 52),41 × 39 (GR 8), 61 × 56 (DDO 210), 31 × 29 (KK 44), 67 × 65 (Sag DIG) and 78 × 72 (Leo A). The gas SMD is obtained by scaling the HI SMD profile by 1.4 to account for primordial He.

tical inclination (Table 1), shows that the two inclinations are in agreement for UGC 4459 and KDG 52, whereas for the rest of the sample galaxies the optical inclination is either found to be much higher (UGC 7298 and CGCG 269-049) or lower (KK 230) than the inclinations derived from the HI morphology. Using the derived HI inclination, the deprojected HI radial surface mass density profiles (SMD) for each galaxy were obtained by averaging the HI distribution over elliptical annuli in the plane of the galaxy. The derived SMD profiles for each galaxy are given in Fig. 2. We next discuss in detail the large scale HI distribution and kinematics for the five galaxies for which fresh GMRT observations are presented in the current paper. For similar details on the other galaxies in our sample, the reader is referred to Begum et al.(2003), Begum & Chengalur (2003,2004) and Young & Lo (1996,1997).


Faint dwarf galaxies
Table 3. Results from GMRT observations

5

Galaxy

FI (Jy km s

-1

)

V50 (km s-1 ) 21. 18. 20. 29. 26. 21. 26. 17. 19. 19. 4(1. 8(0. 6(1. 6(1. 6(2. 4(1. 0(1. 0(2. 4(0. 1(1. 0) 7) 7) 8) 2) 7) 2) 0) 8) 0)

V (km s-1 ) 77.5(1.0) 21.7(0.7) 116.0(1.9) 19.2(2.3) 159.0(3.4) 174.0 (2.0) 217.0(2.2) 63.3(1.8) -78.5(1.0) -139.5(2.0)

MHI (106 M ) 12.2(1.2) 4.7(0.4) 10.8(1.1) 64.2(6.5) 12.7(1.3) 21.6(2.1) 10.38(1.0) 1.9(0.2) 5.4(0.2) 2.8(0.3)

M

HI

/LB

FI/FIS

D

obs (km s-1 ) 7. 9. 9. 9. 9. 8. 9. 7. 7. 6. 3(0. 5(1. 0(1. 0(1. 5(1. 5(1. 0(0. 5(0. 5(1. 5(1. 8) 3) 0) 6) 0) 3) 8) 5) 7) 0)

RHI ( ) 1. 8. 1. 2. 1. 1. 2. 1. 2. 2. 6 0 8 2 3 8 1 5 1 4

iHI ( ) 65 62 23 31 43 28 28 51 33 27

RHI /R

Ho

KK 44 Leo A KDG 52 UGC 4459 CGCG 269-049 UGC 7298 GR 8 KK 230 Sag DIG DDO 210

4.6(0.4) 42.0(4.0) 3.8(0.4) 21.5(2.2) 4.7(0.5) 5.2(0.5) 9.0(0.9) 2.2(0.2) 23.0(1.0) 12.1(1.2)

1.4 1.02 1.8 1.4 0.9 1.7 1.02 1.9 1.02 1.00

1.02 0.88 0.85 1.01 - 1.06 1.03 0.86 0.92 1.05

2. 2. 2. 2. 2. 3. 2. 3. 2. 1.

3 3 7 8 3 1 3 3 3 3

4600

4800

5000

114

116

118

120

71 03 30

[A]

71 03 30

[B]

00
00

02 30

02 30

00

00

01 30

01 30

00

00

00 30

00 30

00 08 24 15 10 05 00 23 55 50 45 RIGHT ASCENSION (J2000) 40 35

00 08 24 15 10 05 00 23 55 50 45 RIGHT ASCENSION (J2000) 40 35

Figure 3. [A] The B band optical DSS image of KDG 52 (greyscales) with the GMRT 26 × 23 resolution integrated HI emission map (contours) overlayed. The contour levels are 0.25, 1.00, 1.75, 2.49, 3.10 & 3.67 ×1020 atoms cm-2 . [B] The velocity field of the galaxy at 26 × 23 resolution. -1 and range from 113.0 km s-1 to The contours are in the steps of 1.0 km s 118.0 km s-1 .

result of beam smearing. To check for this possibility, the individ ual channel maps in the 42 × 39 data cube were inspected. In the channel maps, the peak of the diffuse emission in the central as well as in the northern region in the galaxy occurs at the same heliocentric velocity as that of nearby HI clumps, suggesting that they may arise due to beam smearing. As a further check, the clean com ponents from the 42 × 39 resolution data cube were convolved with a smaller restoring beam of 30 × 30 , to generate a new data cube. The diffuse emission which was visible in 42 × 39 data cube is not seen in the channel maps in this cube, i.e. no clean components were found in the region of diffuse emission. Finally, the HI flux measured from a genuine 30 × 30 resolution data cube (i.e. made from the visibility data by applying the appropriate UV range and taper) is the same as that measured from the 42 × 39 data cube. All these indicate that the diffuse emission in 42 × 39 is entirely due to beam smearing.

DECLINATION (J2000)

DECLINATION (J2000)

4.2 Notes on individual galaxies 4.2.1 KDG 52 KDG 52 (also called M81DwA) was discovered by Karachentseva(1968) and was later detected in HI by Lo & Sargent (1979). The neutral hydrogen in this galaxy is distributed in a clumpy, broken ring surrounding the optical emission (Fig. 3[A]). The central HI hole has a diameter of 40 ( 688 pc); similar central HI holes are seen in other faint dwarf galaxies (e.g. Sag DIG; Young & Lo (1997), DDO 88; Simpson et al. (2005)). The HI hole is not exactly centered on the optical emission; the HI column density at the eastern side of the optical emission is NHI 4 × 1020 atoms cm-2 , while the rest of the optical emission lies inside the HI hole. Prior to this work, there have been two HI interferometric studies of KDG 52. It was observed with the WSRT by Sargent et al.(1983) with a velocity resolution of 8 km s-1 and later re-observed with a high velocity resolution in the C array of the VLA (Westpfahl et al. (1999)). The overall morphology of the earlier images compares well with that of our image. Our coarsest resolution HI distribution and velocity field (not shown) shows faint emission in the center and in the northern region of the galaxy, a feature that is not visible at the higher resolutions. One may suspect that this HI emission is not real but is the

The velocity field obtained from 26 × 23 resolution data cube is given in Fig. 3[B]. The velocity field shows a large scale gradient across the galaxy with a magnitude of 1.7 km s-1 kpc-1 . However the velocity field is clearly not consistent with pure rotation. One can still crudely estimate the maximum possible circular velocity in the following way; the velocity difference from one edge of the galaxy to the other is 6 km s-1 , this implies that the magnitude of any circular velocity component must be limited to Vrot sin(i ) 3 km s-1 . Puche & Westpfahl (1994) have tried to model this velocity field, and find that a combination of rotation (with a magnitude of 7 km s-1 ) and expansion (with a magnitude of 5 km s-1 ) provides a reasonable fit. A similar combination of rotation and expansion was found to provide a good fit to the kinematics of another of our sample galaxies, viz. GR 8 (Begum & Chengalur (2003)).





KDG 52 is a member of M81 group of galaxies. Bureau et al.(2004), have suggested that this galaxy is probably a tidal dwarf, formed through gravitational collapse of the tidal debris from the previous interactions of Holmberg II with UGC 4483. In Sec. 5.1, we estimate the dynamical mass of this galaxy from the virial theorem; this mass estimate implies that the galaxy has a significant amount of dark matter. This would argue against a tidal dwarf origin for KDG 52, since tidal dwarfs are generally not expected to be dark matter dominated (e.g. Braine et al.(2002)).


6
15
4.5 5.0 5.5 6.0

20

25

30

155

160

165

4

5

6

7

8

66 12 30

[A]

66 13 00 12 30

[B]
DECLINATION (J2000)

52 24 30

[A]
DECLINATION (J2000)

52 25 00

[B]

24 30

00

00
11 30

00
00

DECLINATION (J2000)

DECLINATION (J2000)

11 30 00 10 30 00 09 30

23 30

23 30

00

00

00

10 30

22 30

22 30
00

00

00
09 30

00 08 30 00

21 30 12 16 00 15 55 50 45 40 RIGHT ASCENSION (J2000) 35

00

12 16 00

15 55

50 45 40 RIGHT ASCENSION (J2000)

35

08 34 25

20

15

10 05 00 33 55 RIGHT ASCENSION (J2000)

50

08 34 30

15 00 RIGHT ASCENSION (J2000)

33 45

Figure 4. [A] The B band optical DSS image of with the GMRT 29 × 27 resolution integrated tours) overlayed. The contour levels are 0.22, 3.33, 18.91, 22.00, 24.12, 28.25 and 31.36 ×1020 atom locity field for galaxy at 29 × 27 resolution. The of 2.0 km s-1 and range from 13.0 km s-1 to 33.0

UGC 4459 (greyscales) HI emission map (con6.45, 9.56, 12.67, 15.79, s cm-2 . [B] The HI vecontours are in the steps km s-1 .

Figure 5. [A] The B band optical DSS image of CGCG 269-049 (greyscales) with the GMRT 28 × 24 resolution integrated HI emission map (contours) overlayed. The contour levels are 0.08, 1.12, 2.18, 3.23, 4.28, 5.33, 6.39, 7.44, 8.49 and 9.55 ×1020 atoms cm-2 . [B] The HI velocity field for galaxy at 28 × 24 resolution. The contours are in the steps of 2.0 km s-1 and range from 151.0 km s-1 to 165.0 km s-1 .

4.2.2 UGC 4459 UGC 4459 is a member of M81 group of galaxies. It is relatively metal poor, with 12+log(O/H) 7.62 (Kunth & Ostlin (2000)). Å The optical appearance of UGC 4459 is dominated by bright blue clumps, which emit copious amounts of H (Fig. 4[A], 9 & 12). The two high density peaks seen in the integrated HI map coincide with these star forming regions (Fig. 4[A]). The velocity field of UGC 4459 (Fig. 4[B]) shows a large scale gradient (aligned along the line connecting the two star forming regions) across the galaxy. The magnitude of the average velocity gradient across the whole HI disk is 4.5 km s-1 kpc-1 . However we note that the gradient is not uniform across the galaxy. The receding (southeastern) half of the galaxy shows a rapid change in velocity with galacto-centric distance, while the approaching (northwestern) half of the galaxy shows a much more gentle gradient. UGC 4459 is a fairly isolated dwarf galaxy with its nearest neighbor UGC 4483 at a projected distance of 3.6 ( 223 kpc) and at a velocity difference of 135 km s-1 . Being a member of the M81 group, it is possible that interaction with intra-group gas could produce such disturbed kinematics. To check for this possibility, we estimated the ram pressure required to strip gas from this galaxy. The threshold condition for ram pressure stripping is given by (Gunn & Gott (1972))
IGM

v

2

2 G g

(2)

where, IGM is the density of the intra-group medium (IGM) and v is the relative velocity of the galaxy moving through the IGM. and g are stellar and gas surface density respectively. Taking v 190 km s-1 , typical for M81 group (Bureau & Carignan (2002)), and values for and g from the location in the galaxy where the velocity field begins to look perturbed, we find that the IGM volume density required to strip the ISM from UGC 4459 is nIGM 8 × 10-5 cm-3 . UGC 4459 is located at a projected separation of 8.4 (520 kpc) to the South-West of M81 (which we can take to be the center of the M81 group). The nIGM required for ram pressure stripping of UGC 4459 is much higher than nIGM expected at this location ( 1.4 × 10-6 cm-3 ; assuming that 1% of the virial mass of the group is dispersed uniformly in a hot IGM within a sphere just enclosing UGC 4459; Bureau & Carignan (2002)).

Hence, it seems unlikely that the peculiar kinematics of the galaxy is due to IGM ram pressure. Given the kine