arXiv subjects

Apurba Bera

Publications and source records attributed to Apurba Bera.

23 records · Page 2Linked to original sources

arXiv2019

Atomic hydrogen in star-forming galaxies at intermediate redshifts

We have used the upgraded Giant Metrewave Radio Telescope to carry out a deep (117 on-source hours) L-band observation of the Extended Groth Strip, to measure the average neutral hydrogen (HI) mass and median star formation rate (SFR) of star-forming galaxies, as well as the cosmic HI mass density, at $0.2 < z < 0.4$. This was done by stacking the HI 21cm emission and the rest-frame 1.4 GHz radio continuum from 445 blue star-forming galaxies with $\rm M_B \leq -17$ at $z_{\rm mean} \approx 0.34$. The stacked HI 21cm emission signal is detected at $\approx 7σ$ significance, implying an average HI mass of $\rm \langle M_{HI} \rangle = (4.93 \pm 0.70) \times 10^9 \: M_{\odot}$. We also stacked the rest-frame 1.4 GHz radio continuum emission of the same galaxies, to obtain a median SFR of $(0.54 \pm 0.06) \: {\rm M}_\odot$ yr$^{-1}$; this implies an average atomic gas depletion time scale of $\rm \langle Δt_{HI}\rangle \approx$ 9 Gyr, consistent with values in star-forming galaxies in the local Universe. This indicates that the star-formation efficiency does not change significantly over the redshift range $0 - 0.4$. We used the detection of the stacked HI 21cm emission signal to infer the normalized cosmic HI mass density $(\rm ρ_{HI}/ρ_{c,0})$ in star-forming galaxies at $z \approx 0.34$. Assuming the local relation between HI mass and absolute B-magnitude, we obtain $\rm ρ_{HI}/ρ_{c,0} = (4.81 \pm 0.75) \times 10^{-4}$, implying no significant evolution in $\rm ρ_{HI}/ρ_{c,0}$ from $z \approx 0.4$ to the present epoch.

astro-ph.GA↗

arXiv2018

Probing star formation in galaxies at $z \approx 1$ via a Giant Metrewave Radio Telescope stacking analysis

We have used the Giant Metrewave Radio Telescope (GMRT) to carry out deep 610 MHz continuum imaging of four sub-fields of the DEEP2 Galaxy Redshift Survey. We stacked the radio emission in the GMRT images from a near-complete (absolute blue magnitude ${\rm M_B} \leq -21$) sample of 3698 blue star-forming galaxies with redshifts $0.7 \lesssim z \lesssim 1.45$ to detect (at $\approx 17σ$ 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 $\rm L_{1.4 \; GHz} = (4.13 \pm 0.24) \times 10^{22}$ 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 $\rm (24.4 \pm 1.4)\; M_\odot$ yr$^{-1}$ for blue star-forming galaxies with $\rm M_B \leq -21$ at $0.7 \lesssim z \lesssim 1.45$. We detect the main-sequence relation between SFR and stellar mass, $\rm M_\star$, obtaining $\rm SFR = (13.4 \pm 1.8) \times [(M_{\star}/(10^{10} \;M_\odot)]^{0.73 \pm 0.09} \; M_\odot \; yr^{-1}$; the power-law index shows no change over $z \approx 0.7 - 1.45$. 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 sub-sample of the DEEP2 galaxies at $z \approx 1.3$; neutral atomic gas thus appears to be a transient phase in high-$z$ star-forming galaxies.

astro-ph.GA↗

arXiv2017

FRB event rate predictions for the Ooty Wide Field Array

We developed a generic formalism to estimate the event rate and the redshift distribution of Fast Radio Bursts (FRBs) in our previous publication (Bera et al. 2016), considering FRBs are of an extragalactic origin. In this paper we present (a) the predicted pulse widths of FRBs by considering two different scattering models, (b) the minimum total energy required to detect events, (c) the redshift distribution and (d) the detection rates of FRBs for the Ooty Wide Field Array (OWFA). The energy spectrum of FRBs is modelled as a power law with an exponent $-α$ and our analysis spans a range $-3\leq α\leq 5$. We find that OWFA will be capable of detecting FRBs with $α\geq 0$. The redshift distribution and the event rates of FRBs are estimated by assuming two different energy distribution functions; a Delta function and a Schechter luminosity function with an exponent $-2\le γ\le 2$. We consider an empirical scattering model based on pulsar observations (model I) as well as a theoretical model (model II) expected for the intergalactic medium. The redshift distributions peak at a particular redshift $z_p$ for a fixed value of $α$, which lie in the range $0.3\leq z_p \leq 1$ for the scattering model I and remain flat and extend up to high redshifts ($z\lesssim 5$) for the scattering model II.

astro-ph.HE↗

arXiv2016

On modelling the Fast Radio Burst (FRB) population and event rate predictions

Assuming that Fast Radio Bursts (FRBs) are of extragalactic origin, we have developed a formalism to predict the FRB detection rate and the redshift distribution of the detected events for a telescope with given parameters. We have adopted FRB 110220, for which the emitted pulse energy is estimated to be $E_0 = 5.4 \times 10^{33}J$, as the reference event. The formalism requires us to assume models for (1) pulse broadening due to scattering in the ionized inter-galactic medium - we consider two different models for this, (2) the frequency spectrum of the emitted pulse - we consider a power law model $E_ν \propto ν^{-α}$ with $-5 \leq α\leq 5$, and (3) the comoving number density of the FRB occurrence rate $n(E,w_i,z)$ - we ignore the z dependence and assume a fixed intrinsic pulse width $w_i = 1$ms for all the FRBs. The distribution of the emitted pulse energy $E$ is modelled through (a) a delta-function where all the FRBs have the same energy $E = E_0$ , and (b) a Schechter luminosity function where the energies have a spread around $E_0$. The models are all normalized using the 4 FRBs detected by Thornton et al. (2013). Our model predictions for the Parkes telescope are all consistent with the inferred redshift distribution of the fourteen FRBs detected there to date. We also find that scattering places an upper limit on the redshift of the FRBs detectable by a given telescope; for the Parkes telescope this is $z \sim 2$. Considering the upcoming Ooty Wide Field Array, we predict a FRB detection rate of $\sim 0.01$ to $\sim 10^3$ per day.

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arXiv2015

The effect of non-Gaussianity on error predictions for the Epoch of Reionization (EoR) 21-cm power spectrum

The Epoch of Reionization (EoR) 21-cm signal is expected to become increasingly non-Gaussian as reionization proceeds. We have used semi-numerical simulations to study how this affects the error predictions for the EoR 21-cm power spectrum. We expect $SNR=\sqrt{N_k}$ for a Gaussian random field where $N_k$ is the number of Fourier modes in each $k$ bin. We find that non-Gaussianity is important at high $SNR$ where it imposes an upper limit $[SNR]_l$. For a fixed volume $V$, it is not possible to achieve $SNR > [SNR]_l$ even if $N_k$ is increased. The value of $[SNR]_l$ falls as reionization proceeds, dropping from $\sim 500$ at $\bar{x}_{HI} = 0.8-0.9$ to $\sim 10$ at $\bar{x}_{HI} = 0.15 $ for a $[150.08\, {\rm Mpc}]^3$ simulation. We show that it is possible to interpret $[SNR]_l$ in terms of the trispectrum, and we expect $[SNR]_l \propto \sqrt{V}$ if the volume is increased. For $SNR \ll [SNR]_l$ we find $SNR = \sqrt{N_k}/A $ with $A \sim 0.95 - 1.75$, roughly consistent with the Gaussian prediction. We present a fitting formula for the $SNR$ as a function of $N_k$, with two parameters $A$ and $[SNR]_l$ that have to be determined using simulations. Our results are relevant for predicting the sensitivity of different instruments to measure the EoR 21-cm power spectrum, which till date have been largely based on the Gaussian assumption.

astro-ph.CO↗