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We present statistical analysis of a fluence-limited sample of over 1100 giant pulses from the Crab pulsar, with fluence > 130 Jy ms at ∼1330 MHz. These were detected in ∼260 h of observation with the National Centre for Radio Astrophysics (NCRA) 15 m radio telescope. We find that the pulse-energy distribution follows a power law with index |$\rm \alpha \approx -3$| at least up to a fluence of ∼5 Jy s. The power-law index agrees well with that found for lower-energy pulses in the range 3–30 Jy ms. The fluence distribution of the Crab pulsar hence appears to follow a single power law over ∼3 orders of magnitude in fluence. We do not see any evidence for the flattening at high fluences reported by earlier studies. We also find that, at these fluence levels, the rate of giant-pulse emission varies by as much as a factor of ∼5 on time-scales of a few days, although the power-law index of the pulse-energy distribution remains unchanged. The slope of the fluence distribution for Crab giant pulses is similar to that recently determined for the repeating FRB 121102. We also find an anti-correlation between the pulse fluence and the pulse width, so that more energetic pulses are preferentially shorter. [ABSTRACT FROM AUTHOR]

Carbon radio recombination lines (CRRLs) trace the interface between molecular and atomic gas. We present Giant Metrewave Radio Telescope observations of CRRLs, Cα244–Cα250 towards Cassiopeia A. We use a novel technique of stacking the emission lines in the visibility domain to obtain, for the first time, sub-pc resolution optical depth maps of these CRRLs. The emission shows a wide range of spatial and velocity structures, some of which are unresolved within our synthesis beam of 0.29 pc and velocity channel of 0.55 km s−1. These variations in the emission optical depth and line width are indicative of inhomogeneity and fragmentation in the intervening Perseus Arm gas. We compare the distribution of the CRRL emission with that of diffuse and dense molecular gas using existing CO and H2CO studies. We find that the CO emission in the −47 km s−1 Perseus Arm component is primarily concentrated along an elongated structure detected in our CRRL maps, to the south of which lies the high-density molecular clumps traced by H2CO. This spatial distribution of CRRL and molecular tracers is similar to what one would observe for a photodissociation region (PDR). In the other Perseus Arm component centred at −37 km s−1, there is evidence for high column density (⁠|$N_{\rm H_2}\sim 10^{22}$|  cm−2) molecular clumps embedded in diffuse CO and CRRL emission towards the centre of Cassiopeia A. We propose that the CRRL emissions coincident with molecular tracers originate from the line of sight integrated component of the C+ envelope of the molecular gas. [ABSTRACT FROM AUTHOR]

We present high resolution H  i 21 cm Giant Meterwave Radio Telescope observations of the superthin galaxy FGC 1540 with a spatial resolution of 10 arcsec × 8 arcsec and a spectral resolution of 1.73 km s−1 and an rms noise of 0.9 mJy per beam. We obtain its rotation curve as well as deprojected radial H  i surface density profile by fitting a 3D tilted-ring model directly to the H  i data cubes using the publicly available software, Fully Automated tirrific. We also present the rotation curve of FGC 1540 derived from its optical spectroscopy study using the 6-m BTA telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. We use the rotation curve, the H  i surface density profile together with Spitzer 3.6  $$\mu$$ m, and the SDSS i -band data to construct the mass models for FGC 1540. We find that both the Pseudo-isothermal () as well as Navarro–Frenk–White dark matter (DM) haloes, fit the observed rotation curve equally well. The PIS model indicates a compact DM halo (R C/ R D < 2), with the best-fitting core radius (R C) approximately half the exponential stellar disc scale length (R D), which is in agreement with the mass models of superthin galaxies studied earlier in the literature. Since the vertical thickness of the galactic stellar disc is determined by a balance between the net gravitational field and the velocity dispersion in the vertical direction, the compact DM halo may be primarily responsible in regulating the superthin vertical structure of the stellar disc in FGC 1540 as was found in case of the superthin galaxy UGC7321. [ABSTRACT FROM AUTHOR]

We report a deep Giant Metrewave Radio Telescope search for Galactic H  i 21-cm absorption towards the quasar B0438−436, yielding the detection of wide, weak H  i 21-cm absorption, with a velocity-integrated H  i 21-cm optical depth of 0.0188 ± 0.0036 km s−1. Comparing this with the H  i column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of 3760 ± 365 K, one of the highest measured in the Galaxy. This is consistent with most of the H  i along the sightline arising in the stable warm neutral medium. The low-peak H  i 21-cm optical depth towards B0438−436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the H  i 21-cm absorption and emission spectra. This yields a gas kinetic temperature of $$T_{\rm k} \le (4910 \pm \lt t\gt 1900)$$ K, and a spin temperature of $$T_{\rm s} = (1000 \pm 345)$$ K for the gas that gives rise to the H  i 21-cm absorption. Our data are consistent with the H  i 21-cm absorption arising from either the stable WNM, with $$T_{\rm s} \ll T_{\rm k}$$, $$\rm T_k \approx 5000$$ K, and little penetration of the background Lyman-α radiation field into the neutral hydrogen, or the unstable neutral medium, with $$T_{\rm s} \approx T_{\rm k} \approx 1000\,{\rm K}$$. [ABSTRACT FROM AUTHOR]

The cosmos is strange and beautiful, full of unanticipated objects. The only way to understand it is to observe it-in as many different ways as possible. In this article, we take a look at the wonders revealed upon observing radio emissions from celestial objects through the Giant Metre-wave Radio Telescope (GMRT), near Pune, India. [ABSTRACT FROM AUTHOR]

Foreground removal is the most important step in detecting the large-scale redshifted HI 21-cm signal. Modelling foreground spectra is challenging and is further complicated by the chromatic response of the telescope. We present a multifrequency angular power spectrum (MAPS) estimator for use in a survey for redshifted HI 21-cm emission from z ∼ 3.35 and demonstrate its ability to accurately characterize the foregrounds. This survey will be carried out with the two wide-field interferometer modes of the upgraded Ooty Radio Telescope, called the OotyWide Field Array (OWFA), at 326.5 MHz.We have tailored the two-visibility correlation for OWFA to estimate the MAPS and test it with simulated foregrounds. In the process, we describe a software model that encodes the geometry and the details of the telescope and simulates a realistic model for the bright radio sky. This article presents simulations that include the full chromatic response of the telescope in addition to the frequency dependence intrinsic to the foregrounds.We find that the visibility correlation MAPS estimator recovers the input angular power spectrum accurately and that the instrument response to the foregrounds dominates the systematic errors in the recovered foreground power spectra. [ABSTRACT FROM AUTHOR]

ABSTRACT Foreground removal is a major challenge for detecting the redshifted 21 cm neutral hydrogen (H i) signal from the Epoch of Reionization. We have used 150 MHz Giant Metrewave Radio Telescope observations to characterize the statistical properties of the foregrounds in four different fields of view. The measured multifrequency angular power spectrum Cℓ(Δν) is found to have values in the range 104-2 × 104 mK2 across 700 ≤ ℓ ≤ 2 × 104 and Δν ≤ 2.5 MHz, which is consistent with model predictions where point sources are the most dominant foreground component. The measured Cℓ(Δν) does not show a smooth Δν dependence, which poses a severe difficulty for foreground removal using polynomial fitting. The observational data were used to assess point source subtraction. Considering the brightest source (∼1 Jy) in each field, we find that the residual artefacts are less than 1.5 per cent in the most sensitive field (FIELD I). Considering all the sources in the fields, we find that the bulk of the image is free of artefacts, the artefacts being localized to the vicinity of the brightest sources. We have used FIELD I, which has an rms noise of 1.3 mJy beam−1, to study the properties of the radio source population to a limiting flux of 9 mJy. The differential source count is well fitted with a single power law of slope −1.6. We find there is no evidence for flattening of the source counts towards lower flux densities which suggests that source population is dominated by the classical radio-loud active galactic nucleus. The diffuse Galactic emission is revealed after the point sources are subtracted out from FIELD I. We find Cℓ ∝ ℓ−2.34 for 253 ≤ ℓ ≤ 800 which is characteristic of the Galactic synchrotron radiation measured at higher frequencies and larger angular scales. We estimate the fluctuations in the Galactic synchrotron emission to be [ABSTRACT FROM AUTHOR]

ABSTRACT Foreground removal is a challenge for 21-cm tomography of the high-redshift Universe. We use archival Giant Metrewave Radio Telescope (GMRT) data (obtained for completely different astronomical goals) to estimate the foregrounds at a redshift of ∼1. The statistic we use is the cross power spectrum between two frequencies separated by Δν at the angular multipole ℓ, or equivalently the multi-frequency angular power spectrum Cℓ(Δν). An earlier measurement of Cℓ(Δν) using these data had revealed the presence of oscillatory patterns along Δν, which turned out to be a severe impediment for foreground removal. Using the same data, in this paper we show that it is possible to considerably reduce these oscillations by suppressing the sidelobe response of the primary antenna elements. The suppression works best at the angular multipoles ℓ for which there is a dense sampling of the u− v plane. For three angular multipoles ℓ= 1405, 1602 and 1876, this sidelobe suppression along with a low order polynomial fitting completely results in residuals of (≤ 0.02 mK2), consistent with the noise at the 3σ level. Since the polynomial fitting is done after estimation of the power spectrum it can be ensured that the estimation of the H i signal is not biased. The corresponding 99 per cent upper limit on the H i signal is , where is the mean neutral fraction and b is the bias. [ABSTRACT FROM AUTHOR]

The redshifted 21-cm signal from neutral hydrogen (H ) is an important future probe of the high-redshift Universe. We have analysed Giant Metrewave Radio Telescope (GMRT) observations towards detecting this signal from . The multi-frequency angular power spectrum is used to characterize the statistical properties of the background radiation across angular scales arcsec to 10 arcmin, and a frequency bandwidth of with resolution . The measured which ranges from 7 to is dominated by foregrounds, the expected H signal is several orders of magnitude smaller and detecting this is a big challenge. The foregrounds, believed to originate from continuum sources, is expected to vary smoothly with whereas the H signal decorrelates within , and this holds the promise of separating the two. For each ℓ, we use the interval to fit a fourth-order polynomial which is subtracted from the measured to remove any smoothly varying component across the entire bandwidth . The residual , we find, has an oscillatory pattern with amplitude and period, respectively, and at the smallest ℓ value of 1476, and the amplitude and period decreasing with increasing ℓ. Applying a suitably chosen high pass filter, we are able to remove the residual oscillatory pattern for where the residual is now consistent with zero at the 3σ noise level. Based on this we conclude that we have successfully removed the foregrounds at and the residuals are consistent with noise. We use this to place an upper limit on the H signal whose amplitude is determined by , where and b are the H neutral fraction and the H bias, respectively. A value of greater than 7.95 would have been detected in our observation, and is therefore ruled out at the 3σ level. For comparison, studies of quasar absorption spectra indicate which is times smaller than our upper limit. We have not succeeded in completely removing the residual oscillatory pattern, whose cause is presently unknown to us, for the larger ℓ values. [ABSTRACT FROM AUTHOR]

The OH molecule, found abundantly in the Milky Way, has four transitions at the ground-state rotational level ( ) at cm wavelengths. These are E1 transitions between the and hyperfine levels of the Λ doublet of the state. There are also forbidden M1 transitions between the hyperfine levels within each of the doublet states, occurring at frequencies of 53.171 and 55.128 MHz. These are extremely weak and hence difficult to detect. However, there is a possibility that the level populations giving rise to these lines are inverted under special conditions, in which case it may be possible to detect them through their maser emission. We describe the observational diagnostics for determining when the hyperfine levels are inverted and identify a region around W44 where these conditions are satisfied. A high-velocity-resolution search for these hyperfine OH lines using the low-frequency feeds on four antennas of the Giant Metrewave Radio Telescope (GMRT) and the new GMRT Software Backend was performed on a target surrounding W44. We place a 3σ upper limit of ∼17.3 Jy (at 1 km s−1 velocity resolution) for the 55-MHz line from this region. This corresponds to an upper limit of 3 ×108 for the amplification of the Galactic synchrotron emission providing the background. [ABSTRACT FROM AUTHOR]

We present Giant Metrewave Radio Telescope H i observations of two extremely metal-deficient (XMD) galaxies, UM 133 and SDSS J 093546.4, with no known nearby companions. Earlier H i observations of XMD galaxies have shown that disturbed H i morphologies and kinematics, generally related to tidal interaction with a companion, are common in them. However, some of these galaxies were already known to be in pairs before the H i observations. The two galaxies studied here were specifically selected because they do not have any known nearby companions. None the less, we find that both the galaxies have highly disturbed H i morphologies and velocity fields. These could have arisen from interactions with more distant companions than considered in our isolation criteria or merger with extremely gas-rich dwarf companions. It is also possible that distorted H i distributions of these XMD galaxies are the result of cold gas accretion from the intergalactic medium. Our observations provide further support to the idea that inflow of metal-poor gas from the outskirts of a galaxy to the central star-forming regions (in the case of interaction), or cold gas accretion is the probable cause for the observed low emission-line metallicities of XMD galaxies. [ABSTRACT FROM AUTHOR]

We used observations from the Giant Metrewave Radio Telescope to measure the atomic hydrogen gas content of 324 galaxies around the galaxy cluster Abell 370 at a redshift of (a look-back time of ∼4 billion years). The H i 21 cm emission from these galaxies was measured by co-adding their signals using precise optical redshifts obtained with the Anglo-Australian Telescope. The average H i mass measured for all 324 galaxies is , while the average H i mass measured for the 105 optically blue galaxies is . The significant quantities of gas found around Abell 370 suggest that there has been substantial evolution in the gas content of galaxy clusters since redshift . The total amount of atomic hydrogen gas found around Abell 370 is up to approximately eight times more than that seen around the Coma cluster, a nearby galaxy cluster of similar size. Despite this higher gas content, Abell 370 shows the same trend as nearby clusters that galaxies close to the cluster core have lower H i gas content than galaxies further away where the galaxy density is lower. The optically blue galaxies contain the majority of the H i gas surrounding the cluster. However, there is evidence that the optically red galaxies contain appreciable quantities of H i gas within their central regions. The Abell 370 galaxies have H i mass-to-optical-light ratios similar to local galaxy samples and have the same correlation between their star formation rate and H i mass as found in the nearby galaxies. The average star formation rate derived from [O ii] emission and from deredshifted 1.4 GHz radio continuum for the Abell 370 galaxies also follows the correlation found in the local Universe. The large amounts of H i gas found around the cluster can easily be consumed entirely by the observed star formation rate in the galaxies over ∼4 billion years (from ) to the present day. Abell 370 appears set to evolve into a gas-poor system similar to galaxy clusters observed in the local Universe. [ABSTRACT FROM AUTHOR]

Although our understanding of the evolution of the stellar population of galaxies has improved dramatically in the recent past, our understanding of the evolution of the neutral atomic gas content in star forming galaxies remains limited. This is largely because of the sensitivity limit of the current generation of radio telescopes. The scenario is changing however, both because of new instrumentation and deeper integrations with the existing telescopes. In this article, I review the current status of, and future prospects for, low radio frequency observations of gas in high redshift galaxies. The indications are that the community is moving from an era of fasting to an epoch of feasting. [ABSTRACT FROM AUTHOR]

We study the relationship between the gas column density and the star formation rate (SFR) surface density for a sample of 23 extremely faint dwarf irregular galaxies drawn from the Faint Irregular Galaxy GMRT (Giant Metrewave Radio Telescope) Survey (FIGGS). Our sample galaxies have a median H i mass of and a median blue magnitude . is derived from Galaxy Evolution Explorer ( GALEX) data, while is derived from the GMRT-based FIGGS H i 21-cm survey data. We find that averaged over the star-forming region of the disc lies below most estimates of the ‘threshold density’ for star formation, and that the average is also lower than would be expected from the ‘Kennicutt–Schmidt’ law. This deviation is indicative of an environmental dependence of star formation efficiencies, since the Kennicutt relation was derived from data on the central regions of large spiral and starburst galaxies. The star formation efficiency in small galaxies may be more relevant to modelling of star formation in gas-rich, low-metallicity systems in the early universe. We also use our data to look for small-scale (400 and 200 pc) correlations between and . For 18 of our 23 galaxies, we find that can be parametrized as having a power-law dependence on . The power-law relation holds until one reaches the sensitivity limit of the GALEX data, i.e. we find no evidence for a ‘threshold density’ below which star formation is completely cut off. The power-law slopes and coefficients however vary substantially from galaxy to galaxy, and are in general steeper than the value of ∼1.4 derived for large galaxies by Kennicutt. Further, as for the globally averaged quantities, the at 400 pc resolution is in general lower than that predicted by the Kennicutt relation, with the deviation decreasing with increasing . Our computation of uses a calibration that assumes solar metallicity and a standard Salpeter initial mass function (IMF), similarly, the we use is not corrected for the molecular gas density. Incorporating corrections for molecular gas and low metallicity will increase the deviation from the Kennicutt relation. Conversely, truncating the IMF at the high-mass end would decrease the deviation from the Kennicutt relation. For the five galaxies for which a power law does not provide a good parametrization of the relation, there are substantial offsets between the ultraviolet (UV) bright regions and the H i high column density maps. Four of these five galaxies have H i masses near the lower end of our sample distribution, while the remaining galaxy has a large central H i hole. We have 200 pc resolution images for 10 of our galaxies. At this resolution, the offsets between the peaks in the H i and UV images are more pronounced, and a power-law parametrization is possible for only five of the 10 galaxies. [ABSTRACT FROM AUTHOR]

We present Giant Metrewave Radio Telescope, H i 21 cm observations of SBS 0335–052E and SBS 0335–052W, a close pair of dwarf galaxies, which are further unusual in being the most metal-poor star-forming galaxies known. We present images at several angular resolutions, ranging from ∼40 to 4 arcsec. These images show that SBS 0335–052 is a strongly interacting system, with a faint diffuse H i bridge seen at low resolution, and elongated tails seen at the higher resolutions. The overall morphology suggests that the pair represents a major (as both galaxies have similar H i masses) merger of extremely gas-rich galaxies, which is currently past the first close encounter. The low-resolution velocity field is dominated by the velocity difference between the two galaxies and the velocity gradient along the tidal features. However, for SBS 0335–052W at least, at high angular resolution, one sees a central velocity field that could be associated with the spin of the original undisturbed disc. The two galaxies have very similar H i masses, but very different optical properties and current star formation rates. A possible reason for this is the differing amounts of tidally induced star formation, because of the different spin orientations of these interacting galaxies. The highest angular resolution H i images show that the ionized superbubble, identified by Thuan, Izotov & Lipovetsky, in the Hubble Space Telescope images of SBS 0335–052E, is extended along one of the diffuse tidal features, and that there is a high-density H i clump at the other end of the superbubble. The star formation in SBS 0335–052E occurs mainly in a group of superstar clusters (SSCs) with a clear age gradient; the age decreases as one approaches the dense H i clump. We suggest that this propagating star formation is driven by the superbubble expanding into a medium with a tidally produced density gradient. The high pressures associated with the compressed material would also naturally explain why current star formation is mainly concentrated in SSCs. [ABSTRACT FROM AUTHOR]

The Faint Irregular Galaxies GMRT Survey (FIGGS) is a Giant Metrewave Radio Telescope (GMRT) based H i imaging survey of a systematically selected sample of extremely faint nearby dwarf irregular galaxies. The primary goal of FIGGS is to provide a comprehensive and statistically robust characterization of the neutral interstellar medium properties of faint, gas-rich dwarf galaxies. The FIGGS galaxies represent the extremely low mass end of the dwarf irregular galaxies population, with a median and median H i mass of , extending the baseline in mass and luminosity space for a comparative study of galaxy properties. The H i data are supplemented with observations at other wavelengths. In addition, distances accurate to ∼10 per cent are available for most of the sample galaxies. This paper gives an introduction to FIGGS, describes the GMRT observations and presents the first results from the H i observations. From the FIGGS data, we confirm the trend of increasing H i to optical diameter ratio with decreasing optical luminosity; the median ratio of for the FIGGS sample is 2.4. Further, on comparing our data with aperture synthesis surveys of bright spirals, we find at best marginal evidence for a decrease in average surface density with decreasing H i mass. To a good approximation, the discs of gas-rich galaxies, ranging over three orders of magnitude in H i mass, can be described as being drawn from a family with fixed H i average surface density. [ABSTRACT FROM AUTHOR]

Foreground subtraction is the biggest challenge for future redshifted 21-cm observations to probe reionization. We use a short Giant Meter Wave Radio Telescope (GMRT) observation at 153 MHz to characterize the statistical properties of the background radiation across ∼1° to subarcmin angular scales, and across a frequency band of 5 MHz with 62.5 kHz resolution. The statistic we use is the visibility correlation function, or equivalently the angular power spectrum . We present the results obtained from using relatively unsophisticated, conventional data calibration procedures. We find that even fairly simple-minded calibration allows one to estimate the visibility correlation function at a given frequency . From our observations, we find that is consistent with foreground model predictions at all angular scales except the largest ones probed by our observations where the model predictions are somewhat in excess. On the other hand, the visibility correlation between different frequencies seems to be much more sensitive to calibration errors. We find a rapid decline in , in contrast with the prediction of less than 1 per cent variation across 2.5 MHz. In this case, however, it seems likely that a substantial part of the discrepancy may be due to limitations of data reduction procedures. [ABSTRACT FROM AUTHOR]

We use observations from the Giant Metrewave Radio Telescope (GMRT) to measure the atomic hydrogen gas content of star-forming galaxies at (i.e. a look-back time of ∼3 Gyr). The sample of galaxies studied were selected from Hα-emitting field galaxies detected in a narrow-band imaging survey with the Subaru Telescope. The Anglo-Australian Telescope was used to obtain precise optical redshifts for these galaxies. We then co-added the H i 21-cm emission signal for all the galaxies within the GMRT spectral line data cube. From the co-added signal of 121 galaxies, we measure an average atomic hydrogen gas mass of . We translate this H i signal into a cosmic density of neutral gas at of . This is the current highest redshift at which Ωgas has been constrained from 21-cm emission and our value is consistent with that estimated from damped Lyα systems around this redshift. We also find that the correlations between the Hα luminosity and the radio continuum luminosity and between the star formation rate (SFR) and the H i gas content in star-forming galaxies at are consistent with the correlations found at . These two results suggest that the star formation mechanisms in field galaxies ∼3 Gyr ago were not substantially different from the present, even though the SFR is three times higher. [ABSTRACT FROM AUTHOR]

We present the results of Giant Metrewave Radio Telescope H i 21-cm line observations of the extremely metal deficient (XMD) blue compact galaxy (BCG) HS 0822+3542. HS 0822+3542 is the smallest known XMD galaxy; from Hubble Space Telescope ( HST) imaging, it has been suggested that it actually consists of two still smaller (∼100 pc sized) ultra-compact dwarfs that are in the process of merging. The brighter of these two putative ultra-compact dwarfs has an ocular appearance, similar to that seen in galaxies that have suffered a penetrating encounter with a smaller companion. From our H i imaging, we find that the gas distribution and kinematics in this object are similar to that of other low-mass galaxies, albeit with some evidence for tidal disturbance. On the other hand, the H i emission has an angular size ∼25 times larger than that of the putative ultra-compact dwarfs. The optical emission is also offset from the centre of the H i emission. HS 0822+3542 is located in the nearby Lynx–Cancer void, but has a nearby companion low surface brightness dwarf galaxy SAO 0822+3545. In light of all this, we also consider a scenario where the optical emission from HS 0822+3542 comes not from two merging ultra-compact dwarfs but from multiple star-forming regions in a tidally disturbed galaxy. In this model, the ocular appearance of the brighter star-forming region could be the result of triggered star formation. [ABSTRACT FROM AUTHOR]

We present results of deep, high velocity resolution (∼1.6 km s-1) Giant Metrewave Radio Telescope (GMRT) HI 21 cm-line observations of a sample of extremely faint (MB > -12.5) dwarf irregular galaxies. Unlike previous studies, we find large-scale velocity gradients in even the faintest galaxies of our sample i.e. their velocity fields are not "chaotic". For some of our sample galaxies, where the velocity fields are in fact completely consistent with ordered rotation, we derive rotation curves and determine the structure of the dark matter halos of the galaxies from mass modelling, using the "asymmetric drift" corrected rotation velocities. Finally, we compile from literature a sample of galaxies with known dark matter distribution, estimated from HI synthesis observations, and discuss the results and their implications for hierarchical galaxy formation models. [ABSTRACT FROM AUTHOR]

We present a deep Giant Metrewave Radio Telescope (GMRT) search for H i 21-cm emission from three dwarf galaxies, viz. POX 186, SC 24 and KKR 25. Based, in part, on previous single-dish H i observations, these galaxies have been classified as a blue compact dwarf (BCD), a dwarf irregular and a transition galaxy, respectively. However, in conflict with previous single-dish detections, we do not detect H i in SC 24 or KKR 25. We suggest that the previous single-dish measurements were probably confused with the local Galactic emission. In the case of POX 186, we confirm the previous non-detection of H i but with substantially improved limits on its H i mass. Our derived upper limits on the H i mass of SC 24 and KKR 25 are similar to the typical H i mass limit for dwarf spheroidal (dSph) galaxies, whereas in the case of POX 186, we find that its gas content is somewhat smaller than is typical of BCD galaxies. [ABSTRACT FROM AUTHOR]