arXiv subjects

S. Ramya

Publications and source records attributed to S. Ramya.

At least 19 recordsLinked to original sources

arXiv2022

ALMaQUEST -- VII: Star Formation Scaling Relations of Green Valley Galaxies

We utilize the ALMA-MaNGA QUEnch and STar formation (ALMaQUEST) survey to investigate the kpc-scale scaling relations, presented as the resolved star forming main sequence (rSFMS: $\Sigma_{\rm SFR}$ vs. $\Sigma_{*}$), the resolved Schmidt-Kennicutt relation (rSK: $\Sigma_{\rm SFR}$ vs. $\Sigma_{\rm H_{2}}$), and the resolved molecular gas main sequence (rMGMS: $\Sigma_{\rm H_{2}}$ vs. $\Sigma_{*}$), for 11478 star-forming and 1414 retired spaxels (oversampled by a factor of $\sim20$) located in 22 green valley (GV) and 12 main sequence (MS) galaxies. For a given galaxy type (MS or GV), the retired spaxels are found to be offset from the sequences formed by the star-forming spaxels on the rSFMS, rSK, and rMGMS planes, toward lower absolute values of sSFR, SFE, and $f_{\rm H_{2}}$ by $\sim$ 1.1, 0.6, and 0.5 dex. The scaling relations for GV galaxies are found to be distinct from that of the MS galaxies, even if the analyses are restricted to the star-forming spaxels only. It is found that for star-forming spaxels, sSFR, SFE, and $f_{\rm H_{2}}$ in GV galaxies are reduced by $\sim$0.36, 0.14, and 0.21 dex, respectively, compared to those in MS galaxies. Therefore, the suppressed sSFR/SFE/$f_{\rm H_{2}}$ in GV galaxies are associated with not only an increased proportion of retired regions in GV galaxies but also a depletion of these quantities in star-forming regions. Finally, the reduction of SFE and $f_{\rm H_{2}}$ in GV galaxies relative to MS galaxies is seen in both bulge and disk regions (albeit with larger uncertainties), suggesting that statistically, quenching in the GV population may persist from the inner to the outer regions.

astro-ph.GA↗

arXiv2019

The ALMaQUEST Survey: The molecular gas main sequence and the origin of the star forming main sequence

The origin of the star forming main sequence ( i.e., the relation between star formation rate and stellar mass, globally or on kpc-scales; hereafter SFMS) remains a hotly debated topic in galaxy evolution. Using the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we show that for star forming spaxels in the main sequence galaxies, the three local quantities, star-formation rate surface density (\sigsfr), stellar mass surface density (\sigsm), and the \h2~mass surface density (\sigh2), are strongly correlated with one another and form a 3D linear (in log) relation with dispersion. In addition to the two well known scaling relations, the resolved SFMS (\sigsfr~ vs. \sigsm) and the Schmidt-Kennicutt relation (\sigsfr~ vs. \sigh2; SK relation), there is a third scaling relation between \sigh2~ and \sigsm, which we refer to as the `molecular gas main sequence' (MGMS). The latter indicates that either the local gas mass traces the gravitational potential set by the local stellar mass or both quantities follow the underlying total mass distributions. The scatter of the resolved SFMS ($σ\sim 0.25$ dex) is the largest compared to those of the SK and MGMS relations ($σ\sim$ 0.2 dex). A Pearson correlation test also indicates that the SK and MGMS relations are more strongly correlated than the resolved SFMS. Our result suggests a scenario in which the resolved SFMS is the least physically fundamental and is the consequence of the combination of the SK and the MGMS relations.

astro-ph.GA↗

arXiv2019

Properties of radio-loud quasars in Sloan Digital Sky Survey

We present a study of a sample of 223 radio-loud quasars (up to redshift $<$0.3) in order to investigate their spectral properties. Twenty-six of these radio-loud quasars are identified as Flat Spectrum Radio Quasars (FSRQs) and fifty-four are identified as Steep Spectrum Radio Quasars (SSRQs) based on their radio spectral index. We study the [O III] line properties of these quasars to investigate the origin and properties of blue wings (shift of the profile towards lower wavelengths) and blue outliers (shift of the whole spectroscopic feature). Most of the quasars show blue wings with velocities up to 420 km $s^{-1}$. We find that around 17\% of the quasars show outliers with velocities spanning 419 to -315 km $s^{-1}$. Finally, we revisit the $\it M_{\rm BH} - σ$ relation of our sample using [S II]$λ$6716, 6731 and [O III] linewidths as surrogates for stellar velocity dispersions, $σ$, to investigate their location on the $\it M_{\rm BH} - σ$ relation for quiescent galaxies. Due to the strong blending of [S II] with $\rm H_α$, we could estimate $σ_{[\rm SII]}$ of only 123 quasars. We find that the radio-loud quasars do not show a relationship between $\it M_{\rm BH}$ and $σ_{\rm [SII]/[OIII]}$ up to a redshift of 0.3, although they cluster around the local relation. We find an overall offset of 0.12$\pm$0.05 dex of our sample of radio-loud quasars from the $\it M_{\rm BH} - σ$ relation of quiescent galaxies. Quasars in our highest redshift bin (z=0.25-0.3) show a deviation of $\sim$0.33 $\pm$ 0.06 dex with respect to the local relation.

astro-ph.HE↗

arXiv2018

Design of Automated Dual Band 4G Jammer using MATLAB Simulink

This paper presents a design of efficient smart jammer to jam the 4G signals, specifically band 3 and band 40 which can be predominantly used in India. The MATLAB Simulink tool was used for the analysis of the circuit design. Simulink, an advanced tool gives accurate results comparable to real-time analysis. The DSP toolbox of the Simulink library has been largely used to construct and view the results of the model. The main objective of this paper is to receive the LTE signals, filter band 3 and band 40, add noise and increase the amplitude of the signal. The uniqueness of this design is the use of full wave rectifier in the circuit and the trigger enabled blocks. Full wave rectifier with added circuitry acts as a trigger to the jammer which contains the noise block and the gain block. Sine wave generators were used to replicate real-time signals and additional signals were added as noise. The advantage of the design is when none of the bands are detected, the output will not be generated, thus saving power. The incorporation of the detector circuit and trigger circuit ensures that power is not wasted by the jamming circuit when the signal is not detected. These jammers can be used in examination halls, conference halls and in secure location where telecommunication signals are unwanted.

eess.SP↗

arXiv2017

Arp 202: a TDG formed in a parent's extended dark matter halo?

We report on H$α$ + [NII] imaging of the Arp 202 interacting pair and its tidal dwarf galaxy (TDG) candidate as well as a GMOS long slit spectrum from the TDG candidate, observed with the Gemini North telescope. Our H$α$ + [NII] imaging reveals the TDG to have an elongated structure, $\sim$ 1.9 kpc in length with the two principal star forming knots at either end. Our observations also show the TDG candidate has a recessional V$_{Hα}$ $\sim$ 3032 km s$^{-1}$, within 100 km s$^{-1}$ of the parent pair's mean velocity and an oxygen abundance of 12+log(O/H) = 8.10$\pm$0.41. The TDG's oxygen abundance is in good agreement with that of a star forming region in NGC 2719A, one of the parent galaxies, which has an estimated oxygen abundance of 12+log(O/H) = 8.05$\pm$0.41. The TDG's V$_{Hα}$ and oxygen abundance confirm previous results validating the candidate as a TDG. The absence of detectable emission from the TDG in $\textit{Spitzer}$ 3.6 $μ$m and 4.5 $μ$m images together with the lack of absorption lines and weak continuum in the spectrum is consistent with absence of an old population ($\gtrsim$ 0.5 Gyr). The location of the TDG within the interaction debris and the absence of indicators of an old stellar population in the TDG are consistent with a scenario in which the TDG is formed from HI stripped from the parent galaxies and within the extended dark matter halo of one of the parents as proposed by (Bournaud et al. 2003; Duc et al. 2004).

astro-ph.GA↗

arXiv2017

Resolved star formation and molecular gas properties of green valley galaxies: a first look with ALMA and MaNGA

We study the role of cold gas in quenching star formation in the green valley by analysing ALMA $^{12}$CO (1-0) observations of three galaxies with resolved optical spectroscopy from the MaNGA survey. We present resolution-matched maps of the star formation rate and molecular gas mass. These data are used to calculate the star formation efficiency (SFE) and gas fraction ($f_{\rm~gas}$) for these galaxies separately in the central `bulge' regions and outer disks. We find that, for the two galaxies whose global specific star formation rate (sSFR) deviates most from the star formation main sequence, the gas fraction in the bulges is significantly lower than that in their disks, supporting an `inside-out' model of galaxy quenching. For the two galaxies where SFE can be reliably determined in the central regions, the bulges and disks share similar SFEs. This suggests that a decline in $f_{\rm~gas}$ is the main driver of lowered sSFR in bulges compared to disks in green valley galaxies. Within the disks, there exist common correlations between the sSFR and SFE and between sSFR and $f_{\rm~gas}$ on kpc scales -- the local SFE or $f_{\rm~gas}$ in the disks declines with local sSFR. Our results support a picture in which the sSFR in bulges is primarily controlled by $f_{\rm~gas}$, whereas both SFE and $f_{\rm~gas}$ play a role in lowering the sSFR in disks. A larger sample is required to confirm if the trend established in this work is representative of green valley as a whole.

astro-ph.GA↗

arXiv2015

Detection of Molecular Gas in Void Galaxies : Implications for Star Formation in Isolated Environments

We present the detection of molecular gas from galaxies located in nearby voids using the CO line emission as a tracer. The observations were done using the 45m Nobeyama Radio Telescope. Void galaxies lie in the most under dense parts of our universe and a significant fraction of them are gas rich, late type spiral galaxies. Although isolated, they have ongoing star formation but appear to be slowly evolving compared to galaxies in denser environments. Not much is known about their star formation properties or cold gas content. In this study we searched for molecular gas in five void galaxies. The galaxies were selected based on their relatively high IRAS fluxes or Ha line luminosities, both of which signify ongoing star formation. All five galaxies appear to be isolated and two lie within the Bootes void. We detected CO line emission from four of the five galaxies in our sample and the molecular gas masses lie between 10^8 to 10^9 Msolar. We did follow-up Ha imaging observations of three detected galaxies using the Himalayan Chandra Telescope and determined their star formation rates (SFRs). The SFR varies from 0.2 to 1 Msolar/yr, which is similar to that observed in local galaxies. Our study indicates that although void galaxies reside in under dense regions, their disks contain molecular gas and have star formation rates similar to galaxies in denser environments.

astro-ph.GA↗

arXiv2015

The WiFeS S7 AGN survey: Current status and recent results on NGC 6300

The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) is a targeted survey probing the narrow-line regions (NLRs) of a representative sample of ~140 nearby (z<0.02) Seyfert galaxies by means of optical integral field spectroscopy. The survey is based on a homogeneous data set observed using the Wide Field Spectrograph WiFeS. The data provide a 25x38 arcsec$^2$ field-of-view around the galaxy centre at typically ~1.5 arcsec spatial resolution and cover a wavelength range between ~3400 - 7100 $Å$ at spectral resolutions of ~100 km s$^{-1}$ and ~50 km s$^{-1}$ in the blue and red parts, respectively. The survey is primarily designed to study gas excitation and star formation around AGN, with a special focus on the shape of the AGN ionising continuum, the interaction between radio jets and the NLR gas, and the nature of nuclear LINER emission. We provide an overview of the current status of S7-based results and present new results for NGC 6300.

astro-ph.GA↗

arXiv2015

Awakening of The High Redshift Blazar CGRaBS J0809+5341

CGRaBS J0809+5341, a high redshift blazar at z = 2.144, underwent a giant optical outburst on 2014 April 19 when it brightened by $\sim$5 mag and reached an unfiltered apparent magnitude of 15.7 mag. This implies an absolute magnitude of -30.5 mag, making it one of the brightest quasars in the Universe. This optical flaring triggered us to carry out observations during the decaying part of the flare covering a wide energy range using the {\it Nuclear Spectroscopic Telescope Array}, {\it Swift}, and ground based optical facilities. For the first time, the source is detected in $γ$-rays by the Large Area Telescope onboard the {\it Fermi Gamma-Ray Space Telescope}. A high optical polarization of $\sim$10\% is also observed. Using the Sloan Digital Sky Survey spectrum, accretion disk luminosity and black hole mass are estimated as $1.5 \times 10^{45}$ erg s$^{-1}$ and $10^{8.4}~M_{\odot}$ respectively. Using a single zone leptonic emission model, we reproduce the spectral energy distribution of the source during the flaring activity. This analysis suggests that the emission region is probably located outside the broad line region, and the jet becomes radiatively efficient. We also show that the overall properties of CGRaBS J0809+5341 seems not to be in agreement with the general properties observed in high redshift blazars up to now.

astro-ph.HE↗

arXiv2015

Probing the Physics of Narrow Line Regions in Active Galaxies III: Accretion and Cocoon Shocks in the LINER NGC1052

We present Wide Field Spectrograph (WiFeS) integral field spectroscopy and HST FOS spectroscopy for the LINER galaxy NGC 1052. We infer the presence of a turbulent accretion flow forming a small-scale accretion disk. We find a large-scale outflow and ionisation cone along the minor axis of the galaxy. Part of this outflow region is photoionised by the AGN, and shares properties with the ENLR of Seyfert galaxies, but the inner ($R \lesssim 1.0$~arcsec) accretion disk and the region around the radio jet appear shock excited. The emission line properties can be modelled by a "double shock" model in which the accretion flow first passes through an accretion shock in the presence of a hard X-ray radiation, and the accretion disk is then processed through a cocoon shock driven by the overpressure of the radio jets. This model explains the observation of two distinct densities ($\sim10^4$ and $\sim10^6$ cm$^{-3}$), and provides a good fit to the observed emission line spectrum. We derive estimates for the velocities of the two shock components and their mixing fractions, the black hole mass, the accretion rate needed to sustain the LINER emission and derive an estimate for the jet power. Our emission line model is remarkably robust against variation of input parameters, and so offers a generic explanation for the excitation of LINER galaxies, including those of spiral type such as NGC 3031 (M81).

astro-ph.GA↗

arXiv2015

Probing the Physics of Narrow Line Regions in Active Galaxies II: The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7)

Here we describe the \emph{Siding Spring Southern Seyfert Spectroscopic Snapshot Survey} (S7) and present results on 64 galaxies drawn from the first data release. The S7 uses the Wide Field Spectrograph (WiFeS) mounted on the ANU 2.3m telescope located at the Siding Spring Observatory to deliver an integral field of $38\times25$~ arcsec at a spectral resolution of $R=7000$ in the red ($530-710$nm), and $R=3000$ in the blue ($340-560$nm). {From these data cubes we have extracted the Narrow Line Region (NLR) spectra from a 4 arc sec aperture centred on the nucleus. We also determine the H$β$ and [OIII]~$λ$5007 fluxes in the narrow lines, the nuclear reddening, the reddening-corrected relative intensities of the observed emission lines, and the H$β$ and \lOIII\ luminosities {determined from spectra for which the stellar continuum has been removed.} We present a set of images of the galaxies in [OIII]~$λ$5007, [NII]~$λ$6584 and H$α$ which serve to delineate the spatial extent of the extended narrow line region (ENLR) and {\bf also to} reveal the structure and morphology of the surrounding \HII\ regions. Finally, we provide a preliminary discussion of those Seyfert~1 and Seyfert~2 galaxies which display coronal emission lines in order to explore the origin of these lines.

astro-ph.GA↗

arXiv2014

Molecular Gas and Star Formation in Voids

We present the detection of molecular gas using CO(1-0) line emission and follow up Halpha imaging observations of galaxies located in nearby voids. The CO(1-0) observations were done using the 45m telescope of the Nobeyama Radio Observatory (NRO) and the optical observations were done using the Himalayan Chandra Telescope (HCT). Although void galaxies lie in the most under dense parts of our universe, a significant fraction of them are gas rich, spiral galaxies that show signatures of ongoing star formation. Not much is known about their cold gas content or star formation properties. In this study we searched for molecular gas in five void galaxies using the NRO. The galaxies were selected based on their relatively higher IRAS fluxes or Halpha line luminosities. CO(1--0) emission was detected in four galaxies and the derived molecular gas masses lie between (1 - 8)E+9 Msun. The H$α$ imaging observations of three galaxies detected in CO emission indicates ongoing star formation and the derived star formation rates vary between from 0.2 - 1.0 Msun/yr, which is similar to that observed in local galaxies. Our study shows that although void galaxies reside in under dense regions, their disks may contain molecular gas and have star formation rates similar to galaxies in denser environments.

astro-ph.GA↗

arXiv2014

S7 : Probing the physics of Seyfert Galaxies through their ENLR & HII Regions

Here we present the first results from the Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) which aims to investigate the physics of ~140 radio-detected southern active Galaxies with z<0.02 through Integral Field Spectroscopy using the Wide Field Spectrograph (WiFeS). This instrument provides data cubes of the central 38 x 25 arc sec. of the target galaxies in the waveband 340-710nm with the unusually high resolution of R=7000 in the red (530-710nm), and R=3000 in the blue (340-560nm). These data provide the morphology, kinematics and the excitation structure of the extended narrow-line region, probe relationships with the black hole characteristics and the host galaxy, measures host galaxy abundance gradients and the determination of nuclear abundances from the HII regions. From photoionisation modelling, we may determine the shape of the ionising spectrum of the AGN, discover whether AGN metallicities differ from nuclear abundances determined from HII regions, and probe grain destruction in the vicinity of the AGN. Here we present some preliminary results and modelling of both Seyfert galaxies observed as part of the survey.

astro-ph.GA↗

arXiv2012

Star formation in bulgeless late type galaxies: clues to their evolution

We present GMRT 1280 MHz radio continuum observations and follow-up optical studies of the disk and nuclear star formation in a sample of low luminosity bulgeless galaxies. The main aim is to understand bulge formation and overall disk evolution in these late type galaxies. We detected radio continuum from five of the twelve galaxies in our sample; the emission is mainly associated with disk star formation. Only two of the detected galaxies had extended radio emission; the others had patchy disk emission. In the former two galaxies, NGC3445 and NGC4027, the radio continuum is associated with star formation triggered by tidal interactions with nearby companion galaxies. We did follow-up Halpha imaging and nuclear spectroscopy of both galaxies using the Himalayan Chandra Telescope (HCT). The Halpha emission is mainly associated with the strong spiral arms. The nuclear spectra indicate ongoing nuclear star formation in NGC3445 and NGC4027 which maybe associated with nuclear star clusters. No obvious signs of AGN activity were detected. Although nearly bulgeless, both galaxies appear to have central oval distortions in the R band images; these could represent pseudobulges that may later evolve into large bulges. We thus conclude that tidal interactions are an important means of bulge formation and disk evolution in bulgeless galaxies; without such triggers these galaxies appear to be low in star formation and overall disk evolution.

astro-ph.CO↗

arXiv2011

AGN Activity and Black Hole Masses in Low Surface Brightness Galaxies

We present medium resolution optical spectroscopy of a sample of nine Low Surface Brightness (LSB) galaxies. For those that show clear signatures of AGN emission, we have disentangled the AGN component from stellar light and any Fe I and Fe II contribution. We have decomposed the H_alpha line into narrow and broad components and determined the velocities of the broad components; typical values lie between 900--2500 km/s. Of the galaxies in our study, UGC 6614, UGC 1922, UGC 6968 and LSBC F568-6 (Malin~2) show clear signatures of AGN activity. We have calculated the approximate black hole masses for these galaxies from the H_alpha line emission using the virial approximation. The black hole masses are ~3x10^{5} M_sun for three galaxies and lie in the intermediate mass black holes domain rather than the supermassive range. UGC 6614 harbors a BH of mass 3.8x10^{6} M_sun; it also shows an interesting feature blueward of H_alpha and H_beta implying outflow of gas or a one-sided jet streaming towards us. We have also measured the bulge stellar velocity dispersions using the Ca II Triplet lines and plotted these galaxies on the M-sigma plot. We find that all the three galaxies UGC 6614, UGC 6968 and F568-6 lie below the M-sigma relation for nearby galaxies. Thus we find that although the bulges of LSB galaxies may be well evolved, their nuclear black hole masses are lower than those found in bright galaxies and lie offset from the M-sigma correlation.

astro-ph.CO↗

arXiv2010

Radio Continuum and HI study of Blue Compact Dwarf Galaxies

The multifrequency radio continuum and 21cm HI observations of five blue compact dwarf (BCD) galaxies, Mrk 104, Mrk 108, Mrk 1039, Mrk 1069 and I Zw 97 using the Giant Meterwave Radio Telescope (GMRT) are presented here. Radio continuum emission at 610 MHz and 325 MHz is detected from all the observed galaxies whereas only a few are detected at 240 MHz. In our sample, three galaxies are members of groups and two galaxies (Mrk 1069 and I Zw 97) are isolated galaxies. The radio emission from Mrk 104 and Mrk 108 is seen to encompass the entire optical galaxy whereas the radio emission from Mrk 1039, Mrk 1069, I Zw 97 is confined to massive HII regions. This, we suggest, indicates that the star formation in the latter group of galaxies has recently been triggered and that the environment in which the galaxy is evolving plays a role. Star formation rates (SFR) calculated from 610 MHz emission is in the range 0.01-0.1 M_sun/yr; this is similar to the SFR obtained for individual star forming regions in BCDs. The integrated radio spectra of four galaxies are modelled over the frequency range where data is available. We find that two of the galaxies Mrk 1069 and Mrk 1039, show a turnover at low frequencies which is well fitted by free-free absorption whereas the other two galaxies, Mrk 104 and Mrk 108, show a power law at the lowest GMRT frequencies. The flatter spectrum, localized star formation and radio continuum in isolated galaxies lend support to stochastic self-propagating star formation (SSPSF). The HI observations of four galaxies Mrk 104, Mrk 108, Mrk 1039 and Mrk 1069 show extended disks as large as ~1.1-6 times the optical size. All the observed BCDs (except Mrk 104) show rotating disk with a half power width of ~50-124 km/s. Solid body rotation is common in our sample. We note that the tidal dwarf (TD) origin is possible for two of the BCDs in our sample.

astro-ph.CO↗

arXiv2007

GMRT Observations of the 2006 outburst of the Nova RS Ophiuchi: First detection of emission at radio frequencies < 1.4 GHz

The first low radio frequency (<1.4 GHz) detection of the outburst of the recurrent nova RS Ophiuchi is presented in this letter. Radio emission was detected at 0.61 GHz on day 20 with a flux density of ~48 mJy and at 0.325 GHz on day 38 with a flux density of ~ 44 mJy. This is in contrast with the 1985 outburst when it was not detected at 0.327 GHz even on day 66. The emission at low radio frequencies is clearly non-thermal and is well-explained by a synchrotron spectrum of index alpha ~ -0.8 (S propto nu^alpha) suffering foreground absorption due to the pre-existing, ionized, warm, clumpy red giant wind. The absence of low frequency radio emission in 1985 and the earlier turn-on of the radio flux in the current outburst are interpreted as being due to higher foreground absorption in 1985 compared to that in 2006, suggesting that the overlying wind densities in 2006 are only ~30% of those in 1985.

astro-ph↗

arXiv2007

Study of star formation in NGC 1084

We present UBVRI broad band, H$α$ narrow band photometry of the star forming complexes in the infra-red bright galaxy NGC 1084. Results of medium resolution spectroscopy of some of the brighter complexes are also discussed. Spectroscopic data is used to better estimate the internal reddening within the galaxy which is found to be highly variable and to calculate metallicity which is close to the solar value. Diagnostic diagram identifies the shocked regions within this galaxy. The narrow band H$α$ flux and its equivalent width are used to determine the star formation rates of the complexes and the distribution of ages. Star formation rates for a few of the complexes are found to be as high as 0.5 $M_{\odot}$/year. The star forming complexes lie in the age range 3 Myr to 6.5 Myr. U-B vs V-I colour-colour mixed population model created using the Starburst99 model colours is used to estimate the ages of the stellar populations present within these regions. Using this technique, it is found that the star formation in NGC 1084 has taken place in a series of short bursts over the last 40 Myr or so. It is proposed that the likely trigger for enhanced star formation is merger with a gas rich dwarf galaxy.

astro-ph↗