arXiv subjects

Jayaram N. Chengalur

Publications and source records attributed to Jayaram N. Chengalur.

At least 91 records · Page 5Linked to original sources

arXiv2007

Gas rich galaxies from the FIGGS survey

The FIGGS (Faint Irregular Galaxy GMRT Survey) is aimed at creating a multi-wavelength observational data base for a volume limited sample of the faintest gas rich galaxies. In this paper we discuss two very gas rich galaxies that were observed as part of the FIGGS survey, viz. NGC 3741 and And IV. These galaxies are unusual in that they have extremely extended gas disks and very high ratios of dark to luminous matter. The very extended HI disks provide an unique opportunity to trace the extended distribution of dark matter around faint galaxies. We compare the baryon fraction of these galaxies with a sample of galaxies with well measured rotation curves and discuss whether extremely gas rich dwarf galaxies have abnormally small baryon fractions.

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arXiv2007

FIGGS: Faint Irregular Galaxies GMRT Survey

The Faint Irregular Galaxies GMRT Survey (FIGGS) is a large program aimed at providing a comprehensive and statistically robust characterisation of the neutral ISM properties of faint (M_B > -14.5), nearby, gas rich, dwarf irregular galaxies using the Giant Metrewave Radio Telescope (GMRT). Here we briefly describe the survey and discuss some of the science that we anticipate can be done with this data set.

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arXiv2007

Life in the last lane: Star formation and chemical evolution in an extremely gas-rich dwarf

We present an analysis of HI, H-alpha, and oxygen abundance data for NGC 3741. This galaxy has a very extended gas disk (~8.8 times the Holmberg radius), and a dark to luminous (i.e. stellar) mass ratio of ~149, which makes it one of the ``darkest'' dwarf irregular galaxies known. However its ratio of baryons (i.e. gas + stellar) mass to dark mass is typical of that in galaxies. Our new high-resolution HI images of the galaxy show evidence for a large scale (purely gaseous) spiral arm and central bar. Despite the gaseous spiral feature and the on-going star formation, we find that the global gas density in NGC 3741 is significantly lower than the Toomre instability criterion. We also find that the star formation rate is consistent with that expected from the observed correlations between HI mass and SFR and the global Kennicutt-Schmidt law respectively. We measure the oxygen abundance to be 12+ log(O/H)=7.66$\pm$0.10, which is consistent with that expected from the metallicity-luminosity relation, despite its extreme gas mass ratio. We also examine the issue of chemical evolution of NGC 3741 in the context of closed-box model of chemical evolution. The effective oxygen yield of NGC 3741 is consistent with recent model estimates of closed-box yields, provided one assumes that the gas has been efficiently mixed all the way to edge of the HI disk (i.e. >8 times the optical radius). This seems a priori unlikely. On the other hand, using a sample of galaxies with both interferometric HI maps and chemical abundance measurements, we find that the effective yield is anti-correlated with the total dynamical mass, as expected in leaky box models (slightly abridged).

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arXiv2007

HI power spectrum of the spiral galaxy NGC628

We have measured the HI power spectrum of the nearly face-on spiral galaxy NGC628 (M74) using a visibility based estimator. The power spectrum is well fitted by a power law $P(U)=AU^α$, with $α=- 1.6\pm0.2$ over the length scale $800 {\rm pc} {\rm to} 8 {\rm kpc}$. The slope is found to be independent of the width of the velocity channel. This value of the slope is a little more than one in excess of what has been seen at considerably smaller length scales in the Milky-Way, Small Magellanic Cloud (LMC), Large Magellanic Cloud (SMC) and the dwarf galaxy DDO210. We interpret this difference as indicating a transition from three dimensional turbulence at small scales to two dimensional turbulence in the plane of the galaxy's disk at length scales larger than galaxy's HI scale height. The slope measured here is similar to that found at large scales in the LMC. Our analysis also places an upper limit to the galaxy's scale height at $800\ {\rm pc}$ .

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arXiv2007

Outflowing atomic and molecular gas at $z \sim 0.67$ towards 1504+377

We report the detection of OH 1667 MHz and wide HI 21cm absorption at $z \sim 0.67$ towards the red quasar 1504+377, with the Green Bank Telescope and the Giant Metrewave Radio Telescope. The HI 21cm absorption extends over a velocity range of $\sim 600$ km/s blueward of the quasar redshift ($z=0.674$), with the new OH 1667 MHz absorption component at $\sim -430$ \kms, nearly coincident with earlier detections of mm-wave absorption at $z \sim 0.6715$. The atomic and molecular absorption appear to arise from a fast gas outflow from the quasar, with a mass outflow rate ${\dot M} \sim 12 M_\odot$ yr$^{-1}$ and a molecular hydrogen fraction $f_{\rm H_2} \equiv (N_{\rm H_2}/N_{\rm HI}) \sim 0.2$. The radio structure of 1504+377 is consistent with the outflow arising due to a jet-cloud interaction, followed by rapid cooling of the cloud material. The observed ratio of HCO$^+$ to OH column densities is $\sim 20$ times higher than typical values in Galactic and high-$z$ absorbers. This could arise due to small-scale structure in the outflowing gas on sub-parsec scales, which would also explain the observed variability in the HI 21cm line.

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arXiv2007

The HI content of star-forming galaxies at z = 0.24

We use observations from the Giant Metrewave Radio Telescope (GMRT) to measure the atomic hydrogen gas content of star-forming galaxies at z = 0.24 (i.e. a look-backtime of ~3 Gyr). The sample of galaxies studied were selected from Halpha-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 coadded the HI 21 cm emission signal for all the galaxies within the GMRT spectral line data cube. From the coadded signal of 121 galaxies, we measure an average atomic hydrogen gas mass of (2.26 +- 0.90)*10^9 solar masses. We translate this HI signal into a cosmic density of neutral gas at z = 0.24 of Omega_gas = (0.91 +- 0.42)*10^-3. This is the current highest redshift at which Omega_gas has been constrained from 21 cm emission and our value is consistent with that estimated from damped Lyman-alpha systems around this redshift. We also find that the correlations between the Halpha luminosity and the radio continuum luminosity and between the star formation rate and the HI gas content in star-forming galaxies at z = 0.24 are consistent with the correlations found at z = 0. 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 star formation rate is 3 times higher.

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arXiv2006

HI studies of eXtremely Metal Deficient galaxies - II: GMRT observations of SBS 1129+576

We present GMRT HI observations of an eXtremely Metal Deficient (XMD) galaxy SBS 1129+576 (12 + log(O/H) = 7.41). Our HI observations show that the galaxy is strongly interacting with a companion galaxy, SBS 1129+577. A third, smaller galaxy, SDSS J113227.68+572142.3, is also present in the data cube. Our low-resolution map shows a bridge of emission connecting the two larger galaxies and a large one-armed spiral distortion of the disc of SBS 1129+577. Assuming that most of the bridge gas originally came from SBS 1129+576, ~1/3 of its original gas mass has been stripped off. The third smaller galaxy does not show any sign of interaction with the other two galaxies. The higher resolution maps show that SBS 1129+577 has a central bar and a ring surrounding the bar; there is also a hint of an integral-shaped warp in SBS 1129+576. All these features are very likely to have been induced by the tidal interaction. In both SBS 1129+576 and SBS 1129+577, there is, in general, a good correspondence between regions with high HI column density and those with ongoing star formation. The inclination-corrected HI column density in the two brightest HII regions in SBS 1129+576 and near the HII regions in SBS 1129+577 is generally above the threshold density for star formation observed in other blue compact galaxies. (abridged)

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arXiv2006

HI in XMD Galaxies III. GMRT observations of BCG HS0822+3542

We present Giant Metrewave Radio Telescope (GMRT) HI 21-cm line data for the smallest known eXtremely Metal Deficient (XMD) blue compact galaxy (BCG) HS0822+3542. From HST imaging it has been suggested that HS0822+3542 actually consists of two still smaller (~ 100pc 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 HI 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 HI 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 HI emission. HS0822+3542 is located in the Lynx-Cancer void, but has a nearby companion LSB dwarf galaxy SAO0822+3545. In light of all this we also consider a scenario where the optical emission from HS0822+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.

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arXiv2006

Power spectrum of HI intensity fluctuations in DDO 210

We measure the power spectrum of HI intensity fluctuations in the extremely faint (M_B ~ -10.9) dwarf galaxy DDO 210 using a visibility based estimator that is well suited in very low signal to noise ratio regimes. DDO 210's HI power spectrum is well fit by a power law $P_HI(U)= A U^α$ with $α=-2.75 \pm0.45$ over the length-scales 80 pc to 500 pc. We also find that the power spectrum does not change with an increase in the velocity channel width, indicating that the measured fluctuations correspond mainly to density fluctuations. However, Kolmogorov turbulence (with a velocity structure function spectral slope of 2/3) cannot be ruled out from the present observations. The value of the slope $α$ is similar to that obtained in the Milkyway. In contrast to the Milkyway, DDO 210 has three orders of magnitude less HI, no spiral arms, and also no measurable ongoing star formation. The fact that the power spectrum slope is nonetheless similar in these two galaxies (and also similar to the values measured for the LMC and SMC) suggests that there is some universal, star formation independent, phenomenon responsible for producing fine scale structure in the gas.

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arXiv2005

A multiwavelength investigation of the temperature of the cold neutral medium

We present measurements of the H I spin temperatures (T_s) of the Cold Neutral Medium (CNM) towards radio sources that are closely aligned with stars for which published H_2 ortho-para temperatures (T_{01}) are available from UV observations. Our sample consists of 18 radio sources close to 16 nearby stars. The transverse separation of the lines of sight of corresponding the UV and radio observations varies from 0.1 to 12.0 pc at the distance of the star. The ultraviolet (UV) measurements do not have velocity information, so we use the velocities of low ionization species (e.g Na I, K I, C I) observed towards these same stars to make a plausible identification of the CNM corresponding to the H_2 absorption. We then find that T_{01} and T_s match within observational uncertainties for lines-of-sight with H_2 column density above 10^{15.8} cm^{-2}, but deviate from each other below this threshold. This is consistent with the expectation that in the CNM T_s tracks the kinetic temperature due to collisions and that T_{01} is driven towards the kinetic temperature by proton exchange reactions.

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arXiv2005

A search for HI in some peculiar faint dwarf galaxies

We present a deep Giant Metrewave Radio Telescope (GMRT) search for HI 21 cm emission from three dwarf galaxies, viz. POX 186, SC 24 and KKR 25. Based, in part, on previous single dish HI observations, these galaxies have been classified as a BCD, a dwarf irregular and a transition galaxy respectively. However, in conflict with previous single dish detections, we do not detect HI 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 HI but with substantially improved limits on its HI mass. Our derived upper limits on the HI mass of SC 24 and KKR 25 are similar to the typical HI mass limit for dwarf spheroidal galaxies, whereas in the case of POX 186, we find that its gas content is somewhat smaller than is typical of BCD galaxies.

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arXiv2005

Resolving the mystery of the dwarf galaxy HIZSS003

The nearby galaxy HIZSS003 was recently discovered during a blind HI survey of the zone of avoidance (Henning et al. 2000). Follow up VLA as well as optical and near-IR imaging and spectroscopy (Massey et al. 2003; Silva et al. 2005) confirm that it is a low metallicity dwarf irregular galaxy. However there were two puzzling aspects of the observations, (i) current star formation, as traced by H$α$ emission, is confined to a small region at the edge of the VLA HI image and (ii) the metallicity of the older RGB stars is higher than that of the gas in HII region. We present high spatial and velocity resolution Giant Meterwave Radio Telescope (GMRT) observations that resolve these puzzles by showing that HIZSS003 is actually a galaxy pair and that the HII region lies at the center of a much smaller companion galaxy (HIZSS003B) to the main galaxy (HIZSS003A). The HI emission from these two galaxies overlaps in projection, but can be separated in velocity space. HIZSS003B has an HI mass of 2.6 X 10^6 M$_\odot$, and a highly disturbed velocity field. Since the velocity field is disturbed, an accurate rotation curve cannot be derived, however, the indicative dynamical mass is ~5 X 10^7 M$_\odot$. For the bigger galaxy HIZSS003A we derive an HI mass of 1.4 X 10^7 M$_\odot$. The velocity field of this galaxy is quite regular and from its rotation curve we derive a total dynamical mass of ~6.5 X 10^8 M$_\odot$.

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arXiv2005

A dwarf galaxy with a giant HI disk

We present Giant Meterwave Radio Telescope (GMRT) HI 21cm images of a nearby dwarf irregular galaxy NGC 3741 (M_B ~ -13.13 mag) which show it to have a gas disk that extends to ~ 8.3 times its Holmberg radius. This makes it probably the most extended gas disk known. Our observations allow us to derive the rotation curve (which is flat in the outer regions) out to ~ 38 optical scale lengths. NGC 3741 has a dynamical mass to light ratio of ~ 107 and is one of the ``darkest'' irregular galaxies known. However, the bulk of the baryonic mass in NGC 3741 is in the form of gas and the ratio of the dynamic mass to the baryonic mass (~ 8), falls within the range that is typical for galaxies. Thus the dark matter halo of NGC 3741 has acquired its fair share of baryons, but for some reason, these baryons have been unable to collapse to form stars. A comparison of NGC 3741's dark halo properties with those of a sample of galaxies with well measured rotation curves suggests that if one has to reconcile the observations with the expectation that low mass galaxies suffer fractionally greater baryon loss then baryon loss from halos occurs in such a way that, in the net, the remaining baryons occupy a fractionally smaller volume of the total halo.

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arXiv2004

The strange case of a sub-DLA with very little HI

We report a deep search for HI 21cm emission from the $z= 0.00632$ sub-DLA toward PG1216+069 with the Giant Metrewave Radio Telescope. No emission was detected and our $5σ$ upper limit on the mass of any associated galaxy is $M_{\rm HI} \lesssim 10^7 M_\odot$, nearly 3 orders of magnitude less than M$_{\rm HI}^\star$. The $z \sim 0.006$ absorber is thus the most extreme known deviation from the standard paradigm in which high column density quasar absorption lines arise in the disks of gas-rich galaxies.

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arXiv2004

The GMRT study of extremely faint dwarf irregular galaxies

We present results of deep, high velocity resolution ($\sim$1.6 kms$^{-1}$) Giant Meterwave Radio Telescope (GMRT) HI 21 cm-line observations of a sample of extremely faint (M$_B>-$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 modeling, 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.

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arXiv2004

Conjugate 18cm OH Satellite Lines at a Cosmological Distance

We have detected the two 18cm OH satellite lines from the $z \sim 0.247$ source PKS1413+135, the 1720 MHz line in emission and the 1612 MHz line in absorption. The 1720 MHz luminosity is $L_{\rm OH} \sim 354 L_\odot$, more than an order of magnitude larger than that of any other known 1720 MHz maser. The profiles of the two satellite lines are conjugate, implying that they arise in the same gas. This allows us to test for any changes in the values of fundamental constants, without being affected by systematic uncertainties arising from relative motions between the gas clouds in which the different lines arise. Our data constrain changes in $G \equiv g_p [α^2/y]^{1.849}$, where $ y \equiv m_e/m_p$; we find $ΔG/G = 2.2 \pm 3.8 \times 10^{-5}$, consistent with no changes in $α$, $g_p$ and $y$.

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arXiv2004

Kinematics of two dwarf galaxies in the NGC 6946 group

We present high velocity resolution (~ 1.6 km/s) Giant Meterwave Radio Telescope (GMRT) HI 21 cm synthesis images of the dwarf galaxies KK98 250 and KK98 251, as well as optical broad band images of KK98 250. We find that, despite being faint (M_B <-14), both galaxies have regular velocity fields. In both cases the velocity fields are consistent with rigid body rotation. We derive rotation curves for the galaxies and fit mass models to them. For both galaxies, we find acceptable fits using isothermal halos while NFW halos provide a poor fit. Finally, we compile from literature a sample of galaxies with HI synthesis observations and I band magnitudes. From this sample we find, in agreement with earlier studies (which used single dish HI data), that dwarf galaxies tend to lie below the I band Tully-Fisher relation defined by brighter galaxies.

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arXiv2003

The temperature of the WNM in the Milky Way

We report high spectral resolution Australia Telescope Compact Array HI 21 cm observations resulting in the detection of the warm neutral medium (WNM) of the Galaxy in absorption against two extragalactic radio sources, PKS 1814-637 and PKS 0407-658. The two lines of sight were selected on the basis of the simplicity of their absorption profiles and the strength of the background sources; the high velocity resolution of the spectra then enabled us to estimate the kinetic temperatures of the absorbing gas by fitting multiple Gaussians to the absorption profiles. Four separate WNM components were detected toward the two sources, with peak optical depths $τ_{max} = (1.0 \pm 0.08) \times 10^{-2}$, $(1.4 \pm 0.2) \times 10^{-3}$, $(2.2 \pm 0.5) \times 10^{-3}$ and $(3.4 \pm 0.5) \times 10^{-3}$ and kinetic temperatures $T_{k} = 3127 \pm 300$ K, $3694 \pm 1595$ K, $3500 \pm 1354$ K and $2165 \pm 608$ K respectively. All four components were thus found to have temperatures in the thermally unstable range $500 < T_{k} < 5000$ K; this suggests that thermal equilibrium has not been reached throughout the WNM.

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