ABSTRACT
We present low-frequency observations at 315 and 745 MHz of the edge-on, nearby galaxy NGC 4631 with upgraded Giant Metrewave Radio Telescope. We compare the observed surface brightness ...profiles along the minor axis of the galaxy with those obtained from hydrodynamical simulations of galactic outflows. These are 3D simulations that replicate star formation in a Milky-Way-sized galaxy and follow magnetized outflows emerging from the disc. We detect a plateau-like feature in the observed emission at a height of 2–3 kpc from the mid-plane of the galaxy, in qualitative agreement with that expected from simulations. This feature is believed to be due to the compression of magnetic fields behind the outer shocks of galactic outflows. We model the observed surface brightness profiles by assuming an exponential as well as a Gaussian fitting model. Using χ2 statistics, we find that the exponential model fits the profiles better and we use it to determine the scale heights. We estimate the scale height for the synchrotron radio emission to be ∼1 kpc. The time-scales for advection due to outflows and diffusion of cosmic ray electrons are ≳5 and ∼160 Myr, respectively. Because advection acts on a time-scale much shorter than diffusion, we conclude that in NGC 4631 advection, rather than diffusion, plays the dominant role in the formation of radio halo. The spectral index image with regions of flatter radio spectral index in the halo appears to indicate possible effects of gas outflow from the plane of the galaxy.
Abstract
We derive the oblateness parameter
q
of the dark matter halo of a sample of gas-rich, face-on disk galaxies. We have assumed that the halos are triaxial in shape but their axes in the disk ...plane (
a
and
b
) are equal, so that
q
=
c
/
a
measures the halo flattening. We have used the H
i
velocity dispersion, derived from the stacked H
i
emission lines and the disk surface density, determined from the H
i
flux distribution, to determine the disk potential and the halo shape at the
R
25
and 1.5
R
25
radii. We have applied our model to 20 nearby galaxies, of which six are large disk galaxies with
M
(stellar) > 10
10
, eight have moderate stellar masses, and six are low-surface-brightness dwarf galaxies. Our most important result is that gas-rich galaxies that have
M
(gas)/
M
(baryons) > 0.5 have oblate halos (
q
< 0.55), whereas stellar-dominated galaxies have a range of
q
values from 0.21 ± 0.07 in NGC4190 to 1.27 ± 0.61 in NGC5194. Our results also suggest a positive correlation between the stellar mass and the halo oblateness
q
, which indicates that galaxies with massive stellar disks have a higher probability of having halos that are spherical or slightly prolate, whereas low-mass galaxies have oblate halos (
q
< 0.55).
ABSTRACT
We present radio observations of ultraluminous infrared galaxies (ULIRGs) using the Giant Metrewave Radio Telescope (GMRT) and combine them with archival multifrequency observations to ...understand whether ULIRGs are the progenitors of the powerful radio loud galaxies in the local Universe. ULIRGs are characterized by large infrared luminosities (LIR > 1012 L⊙), large dust masses (∼108 M⊙), and vigorous star formation (star formation rates ∼10–100 M⊙ yr−1). Studies show that they represent the end stages of mergers of gas-rich spiral galaxies. Their luminosity can be due to both starburst activity and active galactic nuclei (AGNs). We study a sample of 13 ULIRGs that have optically identified AGN characteristics with 1.28 GHz GMRT observations. Our aim is to resolve any core-jet structures or nuclear extensions and hence examine whether the ULIRGs are evolving into radio loud ellipticals. Our deep, low frequency observations show marginal extension for only one source. However, the integrated radio spectra of 9 ULIRGs show characteristics that are similar to that of GPS/CSS/CSO/young radio sources. The estimated spectral ages are 0.4–20 Myr and indicate that they are young radio sources and possible progenitors of radio galaxies. Hence, we conclude that although most ULIRGs do not show kpc scale extended radio emission associated with nuclear activity, their radio spectral energy distributions do show signatures of young radio galaxies.
We report deep Giant Metrewave Radio Telescope (GMRT) 610 MHz continuum imaging of four subfields of the DEEP2 Galaxy Redshift Survey. We stacked the radio emission in the GMRT images from a ...near-complete (absolute blue magnitude MB ≤ −21) sample of 3698 blue star-forming galaxies with redshifts 0.7 z 1.45 to detect (at 17 significance) the median rest-frame 1.4 GHz radio continuum emission of the sample galaxies. The stacked emission is unresolved, with a rest-frame 1.4 GHz luminosity of L1.4GHz = (4.13 0.24) × 1022 W Hz−1. We used the local relation between total star formation rate (SFR) and 1.4 GHz luminosity to infer a median total SFR of (24.4 1.4) M yr−1 for blue star-forming galaxies with MB ≤ −21 at 0.7 z 1.45. We detect the main-sequence relation between SFR and stellar mass, M , obtaining SFR = (13.4 1.8) × (M /1010 M )0.73 0.09 M yr−1; the power-law index shows no change over z 0.7-1.45. We find that the nebular line emission suffers less extinction than the stellar continuum, contrary to the situation in the local universe; the ratio of nebular extinction to stellar extinction increases with decreasing redshift. We obtain an upper limit of 0.87 Gyr to the atomic gas depletion time of a subsample of DEEP2 galaxies at z 1.3; neutral atomic gas thus appears to be a transient phase in high-z star-forming galaxies.
We present Murchison Widefield Array observations of the supernova remnant (SNR) 1987A between 72 and 230 MHz, representing the lowest frequency observations of the source to date. This large lever ...arm in frequency space constrains the properties of the circumstellar medium created by the progenitor of SNR 1987A when it was in its red supergiant phase. As of late 2013, the radio spectrum of SNR 1987A between 72 MHz and 8.64 GHz does not show any deviation from a non-thermal power law with a spectral index of −0.74 ± 0.02. This spectral index is consistent with that derived at higher frequencies, beneath 100 GHz, and with a shock in its adiabatic phase. A spectral turnover due to free–free absorption by the circumstellar medium has to occur below 72 MHz, which places upper limits on the optical depth of ≤0.1 at a reference frequency of 72 MHz, emission measure of ≲13 000 cm−6 pc, and an electron density of ≲110 cm−3. This upper limit on the electron density is consistent with the detection of prompt radio emission and models of the X-ray emission from the supernova. The electron density upper limit implies that some hydrodynamic simulations derived a red supergiant mass-loss rate that is too high, or a wind velocity that is too low. The mass-loss rate of ∼5 × 10−6 M⊙ yr−1 and wind velocity of 10 km s−1 obtained from optical observations are consistent with our upper limits, predicting a current turnover frequency due to free–free absorption between 5 and 60 MHz.
Abstract
We have acquired radio-continuum data between 70 MHz and 48 GHz for a sample of 19 southern starburst galaxies at moderate redshifts (0.067 < z < 0.227) with the aim of separating ...synchrotron and free–free emission components. Using a Bayesian framework, we find the radio continuum is rarely characterized well by a single power law, instead often exhibiting low-frequency turnovers below 500 MHz, steepening at mid to high frequencies, and a flattening at high frequencies where free–free emission begins to dominate over the synchrotron emission. These higher order curvature components may be attributed to free–free absorption across multiple regions of star formation with varying optical depths. The decomposed synchrotron and free–free emission components in our sample of galaxies form strong correlations with the total-infrared bolometric luminosities. Finally, we find that without accounting for free–free absorption with turnovers between 90 and 500 MHz the radio continuum at low frequency (ν < 200 MHz) could be overestimated by upwards of a factor of 12 if a simple power-law extrapolation is used from higher frequencies. The mean synchrotron spectral index of our sample is constrained to be α = −1.06, which is steeper than the canonical value of −0.8 for normal galaxies. We suggest this may be caused by an intrinsically steeper cosmic ray distribution.
We present a multi-frequency study of radio relics associated with the galaxy clusters A4038, A1664, and A786. Radio images, integrated spectra, spectral index maps, and fits to the integrated ...spectra in the framework of the adiabatic compression model are presented. Images of the relic in A4038 at 150, 240, and 606 MHz with the Giant Meterwave Radio Telescope have revealed extended ultra-steep spectrum ( Delta *a ~ --1.8 to --2.7) emission of extent 210 X 80 kpc2. The model of passively evolving radio lobes compressed by a shock fits the integrated spectrum best. The relic with a circular morphology at the outskirts of the cluster A1664 has an integrated spectral index of ~ -- 1.10 ? 0.06 and is best fit by the model of radio lobes lurking for ~4 X 107 yr. The relic near A786 has a curved spectrum and is best fit by a model of radio lobes lurking for ~3 X 107 yr. At 4.7 GHz, a compact radio source, possibly the progenitor of the A786 relic, is detected near the center of the radio relic. The A786 radio relic is thus likely a lurking radio galaxy rather than a site of cosmological shock as has been considered in earlier studies.
We present a search for transient and highly variable sources at low radio frequencies (150-200 MHz) that explores long time-scales of 1-3 yr. We conducted this search by comparing the TIFR GMRT Sky ...Survey Alternative Data Release 1 (TGSS ADR1) and the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey catalogues. To account for the different completeness thresholds in the individual surveys, we searched for compact GLEAM sources above a flux density limit of 100 mJy that were not present in the TGSS ADR1; and also for compact TGSS ADR1 sources above a flux density limit of 200 mJy that had no counterpart in GLEAM. From a total sample of 234 333 GLEAM sources and 275 612 TGSS ADR1 sources in the overlap region between the two surveys, there were 99 658 GLEAM sources and 38 978 TGSS ADR sources that passed our flux density cut-off and compactness criteria. Analysis of these sources resulted in three candidate transient sources. Further analysis ruled out two candidates as imaging artefacts. We analyse the third candidate and show it is likely to be real, with a flux density of 182 plus or minus 26 mJy at 147.5 MHz. This gives a transient surface density of p = (6.2 plus or minus 6) x 10 super( -5) deg super( -2). We present initial follow-up observations and discuss possible causes for this candidate. The small number of spurious sources from this search demonstrates the high reliability of these two new low-frequency radio catalogues.
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