arXiv subjects

Luis C. Ho

Publications and source records attributed to Luis C. Ho.

At least 289 records · Page 16Linked to original sources

arXiv2021

The Disk Veiling Effect of the Black Hole Low-Mass X-ray Binary A0620-00

The optical light curves of quiescent black hole low-mass X-ray binaries often exhibit significant non-ellipsoidal variabilities, showing the photospheric radiation of the companion star is veiled by other source of optical emission. Assessing this "veiling" effect is critical to the black hole mass measurement. Here in this work, we carry out a strictly simultaneous spectroscopic and photometric campaign on the prototype of black hole low-mass X-ray binary A0620-00. We find that for each observation epoch, the extra optical flux beyond a pure ellipsoidal modulation is positively correlated with the fraction of veiling emission, indicating the accretion disk contributes most of the non-ellipsoidal variations. Meanwhile, we also obtain a K2V spectral classification of the companion, as well as the measurements of the companion's rotational velocity $v \sin i = 83.8\pm1.9$ km s$^{-1}$ and the mass ratio between the companion and the black hole $q=0.063\pm0.004$.

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arXiv2021

Metallicity in Quasar Broad Line Regions at Redshift $\sim$ 6

Broad line regions (BLRs) in high-redshift quasars provide crucial information of chemical enrichment in the early universe. Here we present a study of BLR metallicities in 33 quasars at redshift $5.7<z<6.4$. Using the near-IR spectra of the quasars obtained from the Gemini telescope, we measure their rest-frame UV emission line flux and calculate flux ratios. We then estimate BLR metallicities with empirical calibrations based on photoionization models. The inferred median metallicity of our sample is a few times the solar value, indicating that the BLR gas had been highly metal-enriched at $z\sim6$. We compare our sample with a low-redshift quasar sample with similar luminosities and find no evidence of redshift evolution in quasar BLR metallicities. This is consistent with previous studies. The Fe II$/$Mg II flux ratio, a proxy for the Fe$/α$ element abundance ratio, shows no redshift evolution as well, further supporting rapid nuclear star formation at $z\sim6$. We also find that the black hole mass-BLR metallicity relation at $z\sim6$ is consistent with the relation measured at $2<z<5$, suggesting that our results are not biased by a selection effect due to this relation.

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arXiv2021

A New Iron Emission Template for Active Galactic Nuclei. I. Optical Template for the H$β$ region

We present a new empirical template for iron emission in active galactic nuclei (AGN) covering the $4000-5600$ A range. The new template is based on a spectrum of the narrow-line Seyfert 1 galaxy Mrk 493 obtained with the Hubble Space Telescope. In comparison with the canonical iron template object I Zw 1, Mrk 493 has narrower broad-line widths, lower reddening, and a less extreme Eddington ratio, making it a superior choice for template construction. We carried out a multicomponent spectral decomposition to produce a template incorporating all permitted and forbidden lines of Fe II identified in the Mrk 493 spectrum over this wavelength range, as well as lines from Ti II, Ni II, and Cr II. We tested the template by fitting it to AGN spectra spanning a broad range of iron emission properties, and we present a detailed comparison with fits using other widely used monolithic and multi-component iron emission templates. The new template generally provides the best fit (lowest $χ^2$) compared to other widely used monolithic empirical templates. In addition, the new template yields more accurate spectral measurements including a significantly better match of the derived Balmer line profiles (H$β$, H$γ$, H$δ$), in contrast with results obtained using the other templates. Our comparison tests show that the choice of iron template can introduce a systematic bias in measurements of the H$β$ line width, which consequently impacts single-epoch black hole mass estimates by $\sim0.1$ dex on average and possibly up to $\sim0.3-0.5$ dex individually.

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arXiv2021

Optical Flux and Colour Variability of Blazars in the ZTF Survey

We investigate the temporal and colour variability of 897 blazars, comprising 455 BL Lacertae objects (BL Lacs) and 442 Flat Spectrum Radio Quasars (FSRQs), selected from the Roma-BZCAT catalogue, using the multi-band light curves from the Zwicky Transient Facility (ZTF DR6) survey. Assessing the colour variability characteristics over ~2 year timescales, we found that 18.5 per cent (84 out of 455) BL Lacs showed a stronger bluer when brighter (BWB) trend, whereas 9.0 per cent (41 out of 455) showed a redder when brighter (RWB) trend. The majority (70 per cent) of the BL Lacs showing RWB are host galaxy dominated. For the FSRQ subclass, 10.2 per cent (45 out of 442) objects showed a strong BWB trend and 17.6 per cent (78 out of 442) showed a strong RWB trend. Hence we find that BL Lacs more commonly follow a BWB trend than do FSRQs. This can be attributed to the more dominant jet emission in the case of BL Lacs and the contribution of thermal emission from the accretion disc for FSRQs. In analysing the colour behaviour on shorter time windows, we find many blazars evince shorter partial trends of BWB or RWB nature (or occasionally both). Some of such complex colour behaviours observed in the colour-magnitude diagrams of the blazars may result from transitions between the jet-dominated state to the disc-dominated state and vice versa.

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arXiv2021

Detection of a Parsec-Scale Jet in a Radio-Quiet Narrow-Line Seyfert 1 Galaxy with Highly Accreting Supermassive Black Hole

The jet in active galactic nuclei (AGN) is a key ingredient in understanding the co-evolution of galaxies and their central supermassive black holes (SMBHs). Unfortunately, the mechanism of jet launching and collimation is still elusive. The observational evidence of decreasing radio loudness with increasing Eddington ratio implies that the jet should be coupled with the accretion process. To further explore the relationship between the jet and accretion, it is necessary to extend our knowledge of the jet to an extreme end of the Eddington ratio distribution of AGN. Using Very Long Baseline Array (VLBA), we report the detection of the parsec-scale radio structure in Mrk 335, a radio-quiet narrow-line Seyfert 1 galaxy with an Eddington ratio close to/above unity. The VLBA image at 1.5 GHz reveals an elongated structure extending $\sim20$ parsec in north-south direction with a peak flux density of $1.98\pm0.05$ mJy/beam and radio brightness temperatures as high as $6\times10^{7}$ K. This feature provides a strong evidence of a parsec-scale (bipolar) jet launched from a highly accreting SMBH. We discuss the result by comparing Mrk 335 with other highly accreting systems, e.g. Galactic black holes and tidal disruption events, and recall the discovery of collimated corona in the vicinity of SMBH in Mrk 335 by previous X-ray observations, whose relation to the parsec-scale radio jet should be explored by future simultaneous X-ray spectroscopy and high resolution radio observations.

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arXiv2021

SAGAN -- III: New insights into giant radio quasars

Giant radio quasars (GRQs) are radio-loud active galactic nuclei (AGNs), propelling megaparsec-scale jets. In order to understand GRQs and their properties, we have compiled all known GRQs ("the GRQ catalogue"), and a subset of small (size <700 kpc) radio quasars (SRQs) from the literature. In this process, we have found 10 new FR-II GRQs, in the redshift range of 0.66 < z < 1.72, which we include in the GRQ catalogue. Using the above samples, we have carried out a systematic comparative study of GRQs and SRQs, using optical and radio data. Our results show that the GRQs and SRQs statistically have similar spectral index and black hole mass distributions. However, SRQs have higher radio core power, core dominance factor, total radio power, jet kinetic power and Eddington ratio compared to GRQs. On the other hand, when compared to giant radio galaxies (GRGs), GRQs have higher black hole mass and Eddington ratio. The high core dominance factor of SRQs is an indicator of them lying closer to the line of sight than GRQs. We also find a correlation of the accretion disc luminosity with the radio core and jet power of GRQs, which provides evidence for disc-jet coupling. Lastly, we find the distributions of Eddington ratios of GRGs and GRQs to be bi-modal, similar to that found in small radio galaxies (SRGs) and SRQs, which indicate that size is not strongly dependent on the accretion state. Using all of these, we provide a basic model for the growth of SRQs to GRQs.

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arXiv2021

The Relation between Morphological Asymmetry and Nuclear Activity in Low-redshift Galaxies

The morphology of galaxies reflects their assembly history and ongoing dynamical perturbations from the environment. Analyzing i-band images from the Pan-STARRS1 Survey, we study the optical morphological asymmetry of the host galaxies of a large, well-defined sample of nearby active galactic nuclei (AGNs) to investigate the role of mergers and interactions in triggering nuclear activity. The AGNs, comprising 245 type 1 and 4514 type 2 objects, are compared with 4537 star-forming galaxies matched in redshift and stellar mass. We develop a comprehensive masking strategy to isolate the emission of the target from foreground stars and other contaminating sources, all the while retaining projected companions of comparable brightness that may be major mergers. Among three variants of nonparametric indices, both the popular CAS asymmetry parameter and the outer asymmetry parameter (A_outer) yield robust measures of morphological distortion for star-forming galaxies and type 2 AGNs, while only A_outer is effective for type 1 AGNs. The shape asymmetry, by comparison, is affected more adversely by background noise. Asymmetry indices > 0.4 effectively trace systems that are candidate ongoing mergers. Contrary to theoretical expectations, galaxy interactions and mergers are not the main drivers of nuclear activity, at least not in our sample of low-redshift, relatively low-luminosity AGNs, whose host galaxies are significantly less asymmetric than the control sample of star-forming galaxies. Moreover, type 2 AGNs are morphologically indistinguishable from their type 1 counterparts. The level of AGN activity does not correlate with asymmetry, not even among the major merger candidates. As a by-product, we find, consistent with previous studies, that the average asymmetry of star-forming galaxies increases above the main sequence, although not all major mergers exhibit enhanced star formation.

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arXiv2021

The Variability of the Black-Hole Image in M87 at the Dynamical Time Scale

The black-hole images obtained with the Event Horizon Telescope (EHT) are expected to be variable at the dynamical timescale near their horizons. For the black hole at the center of the M87 galaxy, this timescale (5-61 days) is comparable to the 6-day extent of the 2017 EHT observations. Closure phases along baseline triangles are robust interferometric observables that are sensitive to the expected structural changes of the images but are free of station-based atmospheric and instrumental errors. We explored the day-to-day variability in closure phase measurements on all six linearly independent non-trivial baseline triangles that can be formed from the 2017 observations. We showed that three triangles exhibit very low day-to-day variability, with a dispersion of $\sim3-5^\circ$. The only triangles that exhibit substantially higher variability ($\sim90-180^\circ$) are the ones with baselines that cross visibility amplitude minima on the $u-v$ plane, as expected from theoretical modeling. We used two sets of General Relativistic magnetohydrodynamic simulations to explore the dependence of the predicted variability on various black-hole and accretion-flow parameters. We found that changing the magnetic field configuration, electron temperature model, or black-hole spin has a marginal effect on the model consistency with the observed level of variability. On the other hand, the most discriminating image characteristic of models is the fractional width of the bright ring of emission. Models that best reproduce the observed small level of variability are characterized by thin ring-like images with structures dominated by gravitational lensing effects and thus least affected by turbulence in the accreting plasmas.

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arXiv2021

The extreme properties of the nearby hyper-Eddington accreting Active Galactic Nucleus in IRAS 04416+1215

The physical properties of the accretion flow and of the X-ray emitting plasma, in supermassive black holes accreting at extreme Eddington rates, are still very unclear. Here we present the analysis of simultaneous XMM-Newton and NuSTAR observations of the hyper-Eddington Seyfert 1 galaxy IRAS 04416+1215, carried out in 2020. The main goal of these observations is to investigate the properties of the X-ray corona, as well as the structure of the accretion flow and of the circumnuclear environment, in this regime of extreme accretion. IRAS 04416+1215 has one of the highest Eddington ratio ($λ_{\rm Edd}\simeq 472$) in the local Universe. It shows an interesting spectral shape, very similar to the standard Narrow Line Seyfert 1 galaxy's spectra, with the presence of multi-phase absorption structure composed of three phases, whose estimate of the minimum and maximum distances suggests two different interpretations, one consistent with the three X-ray winds being co-spatial, and possibly driven by magnetohydrodynamical processes, the other consistent with the multi-phase winds being also multi-scale. The X-ray spectrum of IRAS 04416+1215 also has a prominent soft excess component and a hard X-ray emission dominated by a reflection component. Moreover, our detailed spectral analysis shows that IRAS 04416+1215 has the lowest coronal temperature measured so far by NuSTAR ($kT_e=3-22$ keV, depending on the model). This is consistent with a hybrid coronal plasma, in which the primary continuum emission is driven by pair production due to high-energy tail of the energy distribution of non-thermal electrons.

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arXiv2021

On the Mass Loading of AGN-Driven Outflows in Elliptical Galaxies and Clusters

Outflows driven by active galactic nuclei (AGN) are an important channel for accreting supermassive black holes (SMBHs) to interact with their host galaxies and clusters. Properties of the outflows are however poorly constrained due to the lack of kinetically resolved data of the hot plasma that permeates the circumgalactic and intracluster space. In this work, we use a single parameter, outflow-to-accretion mass-loading factor $m=\dot{M}_{\rm out}/\dot{M}_{\rm BH}$, to characterize the outflows that mediate the interaction between SMBHs and their hosts. By modeling both M87 and Perseus, and comparing the simulated thermal profiles with the X-ray observations of these two systems, we demonstrate that $m$ can be constrained between $200-500$. This parameter corresponds to a bulk flow speed between $4,000-7,000\,{\rm km\,s}^{-1}$ at around 1 kpc, and a thermalized outflow temperature between $10^{8.7}-10^{9}\,{\rm K}$. Our results indicate that the dominant outflow speeds in giant elliptical galaxies and clusters are much lower than in the close vicinity of the SMBH, signaling an efficient coupling with and deceleration by the surrounding medium on length scales below 1 kpc. Consequently, AGNs may be efficient at launching outflows $\sim10$ times more massive than previously uncovered by measurements of cold, obscuring material. We also examine the mass and velocity distribution of the cold gas, which ultimately forms a rotationally supported disk in simulated clusters. The rarity of such disks in observations indicates that further investigations are needed to understand the evolution of the cold gas after it forms.

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arXiv2021

The Ionization and Destruction of Polycyclic Aromatic Hydrocarbons in Powerful Quasars

We reanalyze the mid-infrared (5-40 $μ$m) Spitzer spectra of 86 low-redshift ($z < 0.5$) Palomar-Green quasars to investigate the nature of polycyclic aromatic hydrocarbon (PAH) emission and its utility as a star formation rate (SFR) indicator for the host galaxies of luminous active galactic nuclei (AGNs). We decompose the spectra with our recently developed template-fitting technique to measure PAH fluxes and upper limits, which we interpret using mock spectra that simulate the effects of AGN dilution. While luminous quasars can severely dilute and affect the detectability of emission lines, PAHs are intrinsically weak in some sources that are otherwise gas-rich and vigorously forming stars, conclusively demonstrating that powerful AGNs destroy PAH molecules. Comparing PAH-based SFRs with independent SFRs derived from the mid-infrared fine-structure neon lines and the total infrared luminosity reveals that PAHs can trace star formation activity in quasars with bolometric luminosities $\lesssim 10^{46}\, \rm erg\,s^{-1}$, but increasingly underestimate the SFR for more powerful quasars, typically by $\sim 0.5$ dex. Relative to star-forming galaxies and low-luminosity AGNs, quasars have a comparable PAH 11.3 $μ$m/7.7 $μ$m ratio but characteristically lower ratios of 6.2 $μ$m/7.7 $μ$m, 8.6 $μ$m/7.7 $μ$m, and 11.3 $μ$m/17.0 $μ$m. We suggest that these trends indicate that powerful AGNs preferentially destroy small grains and enhance the PAH ionization fraction.

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arXiv2021

A Hubble Space Telescope Imaging Survey of Low-Redshift Swift-BAT Active Galaxies

We present initial results from a Hubble Space Telescope snapshot imaging survey of the host galaxies of Swift-BAT active galactic nuclei (AGN) at z<0.1. The hard X-ray selection makes this sample sample relatively unbiased in terms of obscuration compared to optical AGN selection methods. The high-resolution images of 154 target AGN enable us to investigate the detailed photometric structure of the host galaxies, such as the Hubble type and merging features. We find that 48% and 44% of the sample is hosted by early-type and late-type galaxies, respectively. The host galaxies of the remaining 8% of the sample are classified as peculiar galaxies because they are heavily disturbed. Only a minor fraction of host galaxies (18%-25%) exhibit merging features (e.g., tidal tails, shells, or major disturbance). The merging fraction increases strongly as a function of bolometric AGN luminosity, revealing that merging plays an important role in triggering luminous AGN in this sample. However, the merging fraction is weakly correlated with the Eddington ratio, suggesting that merging does not necessarily lead to an enhanced Eddington ratio. Type 1 and type 2 AGN are almost indistinguishable in terms of their Hubble type distribution and merging fraction. However, the merging fraction of type 2 AGN peaks at a lower bolometric luminosity compared with those of type 1 AGN. This result may imply that the triggering mechanism and evolutionary stages of type 1 and type 2 AGN are not identical.

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arXiv2021

The HASHTAG project: The First Submillimeter Images of the Andromeda Galaxy from the Ground

Observing nearby galaxies with submillimeter telescopes on the ground has two major challenges. First, the brightness is significantly reduced at long submillimeter wavelengths compared to the brightness at the peak of the dust emission. Second, it is necessary to use a high-pass spatial filter to remove atmospheric noise on large angular scales, which has the unwelcome by-product of also removing the galaxy's large-scale structure. We have developed a technique for producing high-resolution submillimeter images of galaxies of large angular size by using the telescope on the ground to determine the small-scale structure (the large Fourier components) and a space telescope (Herschel or Planck) to determine the large-scale structure (the small Fourier components). Using this technique, we are carrying out the HARP and SCUBA-2 High Resolution Terahertz Andromeda Galaxy Survey (HASHTAG), an international Large Program on the James Clerk Maxwell Telescope, with one aim being to produce the first high-fidelity high-resolution submillimeter images of Andromeda. In this paper, we describe the survey, the method we have developed for combining the space-based and ground-based data, and present the first HASHTAG images of Andromeda at 450 and 850um. We also have created a method to predict the CO(J=3-2) line flux across M31, which contaminates the 850um band. We find that while normally the contamination is below our sensitivity limit, the contamination can be significant (up to 28%) in a few of the brightest regions of the 10 kpc ring. We therefore also provide images with the predicted line emission removed.

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arXiv2021

What is Important? Morphological Asymmetries are Useful Predictors of Star Formation Rates of Star-forming Galaxies in SDSS Stripe 82

Morphology and structure of galaxies reflect their star formation and assembly histories. We use the framework of mutual information ($\mathrm{MI}$) to quantify interdependence among several structural variables and to rank them according to their relevance for predicting specific star formation rate (SSFR) by comparing the $\mathrm{MI}$ of the predictor variables with SSFR and penalizing variables that are redundant. We apply this framework to study $\sim 3,700$ face-on star-forming galaxies (SFGs) with varying degrees of bulge dominance and central concentration and with stellar mass $M_\star \approx 10^9\,M_\odot - 5\times 10^{11}\,M_\odot$ at redshift $z = 0.02-0.12$. We use the Sloan Digital Sky Survey (SDSS) Stripe 82 deep $i$-band imaging data, which improve measurements of asymmetry and bulge dominance indicators. We find that star-forming galaxies are a multi-parameter family. In addition to $M_\star$, asymmetry emerges as the most powerful predictor of SSFR residuals of SFGs, followed by bulge prominence/concentration. Star-forming galaxies with higher asymmetry and stronger bulges have higher SSFR at a given $M_\star$. The asymmetry reflects both irregular spiral arms and lopsidedness in seemingly isolated SFGs and structural perturbations by galaxy interactions or mergers.

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arXiv2021

Dynamics and Morphology of Cold Gas in Fast, Radiatively Cooling Outflows: Constraining AGN Energetics with Horseshoes

Warm ionized and cold neutral outflows with velocities exceeding $100\,{\rm km\,s}^{-1}$ are commonly observed in galaxies and clusters. Theoretical studies however indicate that ram pressure from a hot wind, driven either by the central active galactic nucleus (AGN) or a starburst, cannot accelerate existing cold gas to such high speeds without destroying it. In this work we explore a different scenario, where cold gas forms in a fast, radiatively cooling outflow with temperature $T\lesssim 10^7\,{\rm K}$. Using 3D hydrodynamic simulations, we demonstrate that cold gas continuously fragments out of the cooling outflow, forming elongated filamentary structures extending tens of kiloparsecs. For a range of physically relevant temperature and velocity configurations, a ring of cold gas perpendicular to the direction of motion forms in the outflow. This naturally explains the formation of transverse cold gas filaments such as the blue loop and the horseshoe filament in the Perseus cluster. Based on our results, we estimate that the AGN outburst responsible for the formation of these two features drove bipolar outflows with velocity $>2,000\,{\rm km\,s}^{-1}$ and total kinetic energy $>8\times10^{57}\,{\rm erg}$ about $\sim10$ Myr ago. We also examine the continuous cooling in the mixing layer between hot and cold gas, and find that radiative cooling only accounts for $\sim10\%$ of the total mass cooling rate, indicating that observations of soft X-ray and FUV emission may significantly underestimate the growth of cold gas in the cooling flow of galaxy clusters.

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arXiv2021

An ACA Survey of [CI] $^3P_1-^3P_0$, CO $J=4-3$, and Dust Continuum in Nearby U/LIRG

We present the results of surveying [CI] $^3P_1-^3P_0$, $^{12}$CO $J=4-3$, and 630 $μ$m dust continuum emission for 36 nearby ultra/luminous infrared galaxies (U/LIRGs) using the Band 8 receiver mounted on the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array. We describe the survey, observations, data reduction, and results; the main results are as follows. (i) We confirmed that [CI] $^3P_1-^3P_0$ has a linear relationship with both the $^{12}$CO $J=4-3$and 630 $μ$m continuum. (ii) In NGC 6052 and NGC 7679, $^{12}$CO $J=4-3$ was detected but [CI] $^3P_1-^3P_0$ was not detected with a [CI] $^3P_1-^3P_0$/ $^{12}$CO $J=4-3$ ratio of $\lesssim0.08$. Two possible scenarios of weak [CI] $^3P_1-^3P_0$ emission are C$^0$-poor/CO-rich environments or an environment with an extremely large [CI] $^3P_1-^3P_0$ missing flux. (iii) There is no clear evidence showing that galaxy mergers, AGNs, and dust temperatures control the ratios of [CI] $^3P_1-^3P_0$/ $^{12}$CO $J=4-3$ and $L'_{\rm [CI](1-0)}/L_{\rm 630μm}$. (iv) We compare our nearby U/LIRGs with high-z galaxies, such as galaxies on the star formation main sequence (MS) at z$\sim1$ and submillimeter galaxies (SMGs) at $z=2-4$. We found that the mean value for the [CII] $^3P_1$--$^3P_0$/ $^{12}$CO $J=4-3$ ratio of U/LIRGs is similar to that of SMGs but smaller than that of galaxies on the MS.

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arXiv2021

Simulations of Torus Reverberation Mapping Experiments with SPHEREx

Reverberation mapping (RM) is an efficient method to investigate the physical sizes of the broad line region (BLR) and dusty torus in an active galactic nucleus (AGN). The Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) mission will provide multi-epoch spectroscopic data at optical and near-infrared wavelengths. These data can be used for RM experiments for bright AGNs. We present results of a feasibility test using SPHEREx data in the SPHEREx deep regions for the torus RM measurements. We investigate the physical properties of bright AGNs in the SPHEREx deep field. Based on this information, we compute the efficiency of detecting torus time lags in simulated light curves. We demonstrate that, in combination with the complementary optical data with a depth of $\sim20$ mag in $B-$band, lags of $\le 750$ days for tori can be measured for more than $\sim200$ bright AGNs. If high signal-to-noise ratio photometric data with a depth of $\sim21-22$ mag are available, RM measurements can be applied for up to $\sim$900 objects. When complemented by well-designed early optical observations, SPHEREx can provide a unique dataset for studies of the physical properties of dusty tori in bright AGNs.

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arXiv2021

KMTNet Nearby Galaxy Survey III. Deficient H$α$ flux in the Extended Disks of Spiral Galaxies

We perform a deep wide-field imaging survey of nearby galaxies using H$α$ and broadband filters to investigate the characteristics of star formation in galaxies. Motivated by the finding that star formation rates (SFRs) derived from H$α$ fluxes in dwarf galaxies are systematically lower than those inferred from far-ultraviolet (FUV) fluxes, we attempt to determine whether the same trend exists in the extended disks of two star-forming galaxies. We perform spatially resolved photometry using grid-shaped apertures to measure the FUV and H$α$ fluxes of star-forming regions. We also perform spectral energy distribution (SED) fittings using 11 photometric data (FUV-to-MIR) including data from the literature to estimate the local properties such as internal attenuation of individual star-forming clumps. Comparing SFR$_\mathrm{FUV}$ and SFR$_\mathrm{Hα}$, which are converted from the H$α$ and FUV fluxes corrected for the local properties, we find that SFR$_\mathrm{Hα}$/SFR$_\mathrm{FUV}$ tends to decrease as the SFR decreases. We evaluate possible causes of this discrepancy between the two SFRs by restricting parameters in the SED fitting and conclude that deficient H$α$ fluxes in the extended disks of galaxies are tightly correlated with recent starbursts. The strong and short starburst which is being rapidly suppressed over the last 10 Myr seems to induce a significant discrepancy between the SFR$_\mathrm{Hα}$ and SFR$_\mathrm{FUV}$. In addition, the recent bursts in the extended disks of galaxies appear to have occurred azimuth-symmetrically, implying that these were likely triggered by gas accretion or internal processes rather than external perturbation.

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