Star Formation In Central Regions Research Articles

Extending the Dynamic Range of Galaxy Outflow Scaling Relations: Massive Compact Galaxies with Extreme Outflows

We investigate galactic winds in the HizEA galaxies, a collection of 46 late-stage galaxy mergers at z = 0.4–0.8, with stellar masses of , star formation rates (SFRs) of 20–500 M ⊙ yr−1, and ultra-compact (a few 100 pc) central star-forming regions. We measure their gas kinematics using the Mg ii λ λ 2796,2803 absorption lines in optical spectra from MMT, Magellan, and Keck. We find evidence of outflows in 90% of targets, with maximum outflow velocities of 550–3200 km s−1. We combine these data with ten samples from the literature to construct scaling relations for outflow velocity versus SFR, star formation surface density (ΣSFR), M *, and SFR/M *. The HizEA galaxies extend the dynamic range of the scaling relations by a factor of ∼2–4 in outflow velocity and an order of magnitude in SFR and ΣSFR. The ensemble scaling relations exhibit strong correlations between outflow velocity, SFR, SFR/R, and ΣSFR, and weaker correlations with M * and SFR/M *. The HizEA galaxies are mild outliers on the SFR and M * scaling relations, but they connect smoothly with more typical star-forming galaxies on plots of outflow velocity versus SFR/R and ΣSFR. These results provide further evidence that the HizEA galaxies’ exceptional outflow velocities are a consequence of their extreme star formation conditions rather than hidden black hole activity, and they strengthen previous claims that ΣSFR is one of the most important properties governing the velocities of galactic winds.

Kiloparsec view of a typical star-forming galaxy when the Universe was ∼1 Gyr old

We present new Atacama Large Millimeter/Submillimeter Array observations of the [C II] 158 μm transition and the dust continuum in HZ4, a typical star-forming galaxy when the Universe was only ∼1 Gyr old (z ≈ 5.5). Our high ≈0.3″ spatial resolution allows us to study the relationships between [C II] line emission, star formation rate, and far-infrared emission on spatial scales of ∼2 kpc. In the central ∼4 kpc of HZ4, the [C II]/FIR is ∼3 × 10−3 on global scales as well as on spatially resolved scales of ∼2 kpc, comparable to the ratio observed in local moderate starburst galaxies such as M 82 or M 83. For the first time in an individual normal galaxy at this redshift, we find evidence for outflowing gas from the central star-forming region in the direction of the minor axis of the galaxy. The projected velocity of the outflow is ∼400 km s−1, and the neutral gas-mass outflow rate is ∼3 − 6 times higher than the star formation rate in the central region. Finally, we detect a diffuse component of [C II] emission, or [C II] halo, that extends beyond the star-forming disk and has a diameter of ∼12 kpc. The outflow, which has a velocity approximately half of the escape velocity of the system, most likely partly fuels the [C II] extended emission. Together with the kinematic analysis of HZ4 (presented in a forthcoming paper), the analysis supports the hypothesis that HZ4 is a typical star-forming disk at z ∼ 5 with interstellar medium conditions similar to present-day galaxies forming stars at a similar level, driving a galactic outflow that may already play a role in its evolution.

A Method to Extract Spatially Resolved Polycyclic Aromatic Hydrocarbon Emission from Spitzer Spectra: Application to M51

Abstract The mid-infrared spectrum contains rich diagnostics to probe the physical properties of galaxies, among which the pervasive emission features from polycyclic aromatic hydrocarbons (PAHs) offer promising means of estimating the star formation rate (SFR) relatively immune from dust extinction. This paper investigates the effectiveness of PAH emission as a SFR indicator on subkiloparsec scales by studying the Spitzer/IRS mapping-mode observations of the nearby grand-design spiral galaxy M51. We present a new approach of analyzing the spatial elements of the spectral data cube that simultaneously maximizes spatial resolution and spatial coverage, while yielding reliable measurements of the total, integrated 5–20 μm PAH emission. We devise a strategy of extracting robust PAH emission using spectra with only partial spectral coverage, complementing missing spectral regions with properly combined mid-infrared photometry. We find that in M51 the PAH emission correlates tightly with the extinction-corrected far-ultraviolet, near-ultraviolet, and Hα emission, from scales of ∼0.4 kpc close to the nucleus to 6 kpc out in the disk of the galaxy, indicating that PAH serves as an excellent tracer of SFR over a wide range of galactic environments. But regional differences exist. Close to the active nucleus of M51 the 6.2 μm feature is weaker, and the overall level of PAH emission is suppressed. The spiral arms and the central star-forming region of the galaxy emit stronger 7.7 and 8.6 μm PAH features than the inter-arm regions.

Photoionisation modelling of the H II regions surrounding star-forming regions within the metallicity range Z=0.003-0.012

We present the new approach to the ionisation structure modelling for the high-metallicity H II regions. The method is based on the multicomponent photoionisation modelling (MPhM) of these objects that takes into account their complicate structure due to superwind from the central star-forming region. The complex structure of H II region has been divided into internal and external components. Internal components correspond to the region of free expanding superwind and the cavity of superwind, respectively, while the external ones — to a thick layer of gas compressed by a superwind shock, and hydrodynamically undisturbed outer part of H II region, where the most of observed strong emission lines are formed. The components of the model were calculated within the assumption of spherical symmetry. The gas photoinisation was caused by the ionising quanta of both direct and diffuse ionising radiation. The fluxes of this radiation were calculated during the simulation using the radiative transfer equations which account for all important processes in the H II region causing this transfer. The diffuse ionising radiation was calculated in the Outward Only approach. In the region of free expansion of the superwind the chemical abundances were determined using the evolutionary population synthesis models of a star-forming region. The distributions of the electron temperature and density in the external components were obtained in such modelling as the solutions of the photoionisation energy balance equation. The stop criterion for evolutionary modelling corresponding to the condition of equilibrium of pressure on the boundary between the third and fourth components was adopted. The evolutionary grid of multicomponent high-metallicity models of the H II regions was calculated. It was shown that the internal structure of a H II region under certain conditions can cause the lack of quanta in the spectrum of ionising radiation.

The Type I Superluminous Supernova PS16aqv: Lightcurve Complexity and Deep Limits on Radioactive Ejecta in a Fast Event

Abstract We present UV/optical observations of PS16aqv (SN 2016ard), a fast-evolving Type I superluminous supernova (SLSN-I) that reached a peak absolute magnitude of M r ≈ −22.1. The lightcurves exhibit a significant undulation at 30 rest-frame days after peak, with a behavior similar to undulations seen in the slowly fading SLSN-I SN 2015bn. This similarity strengthens the case that fast and slow SLSNe-I form a continuum with a common origin. At ≈80 days after peak, the lightcurves exhibit a transition to a slow decline, followed by significant steepening, indicative of a plateau phase or a second significant undulation. Deep limits at ≈280 days after peak imply a tight constraint on the nickel mass, M Ni ≲ 0.35 M ⊙ (lower than for previous SLSNe-I), and indicate that some SLSNe-I do not produce significantly more nickel than normal Type Ic SNe. Using MOSFiT, we model the lightcurve with a magnetar central engine model and find P spin ≈ 0.9 ms, B ≈ 1.5 × 1014 G, and M ej ≈ 16 M ⊙. The implied rapid spin-down time and large reservoir of available energy coupled with the high ejecta mass may account for the fast lightcurve and slow spectroscopic evolution. We also study PS16aqv’s location within its host galaxy and find that it occurred at an offset of 2.46 ± 0.21 kpc from the central star-forming region. Aside from high extinction, the host properties are similar to most other SLSN-I host galaxies. The complexity in the lightcurves of PS16aqv and other events highlights the importance of obtaining well-sampled lightcurves for exploring deviations from a uniform decline.

THE MOLECULAR CLOUDS FUELING A 1/5 SOLAR METALLICITY STARBURST

ABSTRACT Using the Atacama Large Millimeter/submillimeter Array, we have made the first high spatial and spectral resolution observations of the molecular gas and dust in the prototypical blue compact dwarf galaxy II Zw 40. The and emission is clumpy and distributed throughout the central star-forming region. Only one of eight molecular clouds has associated star formation. The continuum spectral energy distribution is dominated by free–free and synchrotron; at 870 μm, only 50% of the emission is from dust. We derive a CO-to-H2 conversion factor using several methods, including a new method that uses simple photodissocation models and resolved CO line intensity measurements to derive a relationship that uniquely predicts for a given metallicity. We find that the CO-to-H2 conversion factor is 4–35 times that of the Milky Way (18.1–150.5 ). The star formation efficiency of the molecular gas is at least 10 times higher than that found in normal spiral galaxies, which is likely due to the burst-dominated star formation history of II Zw 40 rather than an intrinsically higher efficiency. The molecular clouds within II Zw 40 resemble those in other strongly interacting systems like the Antennae: overall they have high size–linewidth coefficients and molecular gas surface densities. These properties appear to be due to the high molecular gas surface densities produced in this merging system rather than to increased external pressure. Overall, these results paint a picture of II Zw 40 as a complex, rapidly evolving system whose molecular gas properties are dominated by the large-scale gas shocks from its ongoing merger.

DIFFUSE X-RAY EMISSION FROM STAR-FORMING GALAXIES

ABSTRACT We study the diffuse X-ray luminosity (L X) of star-forming galaxies using two-dimensional axisymmetric hydrodynamical simulations and analytical considerations of supernovae-(SNe-)driven galactic outflows. We find that the mass loading of the outflows, a crucial parameter for determining the X-ray luminosity, is constrained by the availability of gas in the central star-forming region, and a competition between cooling and expansion. We show that the allowed range of the mass loading factor can explain the observed scaling of L X with star formation rate (SFR) as L X ∝ ?> SFR2 for SFR ≳ 1 ?> M ⊙ ?> yr−1, and a flatter relation at low SFRs. We also show that the emission from the hot circumgalactic medium (CGM) in the halo of massive galaxies can explain the large scatter in the L X – SFR ?> relation for low SFRs (≲few M ⊙ ?> yr−1). Our results suggest that galaxies with small SFRs and large diffuse X-ray luminosities are excellent candidates for the detection of the elusive CGM.

High-resolution ALMA observations of SDP.81. II. Molecular clump properties of a lensed submillimeter galaxy at z = 3.042

Abstract We present spatially resolved properties of molecular gas and dust in a gravitationally lensed submillimeter galaxy H-ATLAS J090311.6+003906 (SDP.81) at z = 3.042 revealed by the Atacama Large Millimeter/submillimeter Array (ALMA). We identified 14 molecular clumps in the CO(5–4) line data. The surface density of molecular gas ($\Sigma _{\rm H_2}$) and star-formation rate (ΣSFR) of the clumps are more than three orders of magnitude higher than those found in local spiral galaxies. The clumps are placed in the “burst” sequence in the $\Sigma _{\rm H_2}$–ΣSFR plane, suggesting that z ∼ 3 molecular clumps follow the star-formation law derived for local starburst galaxies. With our gravitational lens model, the positions in the source plane are derived for the molecular clumps, dust clumps, and stellar components identified in the Hubble Space Telescope image. The molecular and dust clumps are confined within a ∼ 2 kpc region, while the spatial extent of the stellar components is as large as ∼ 6 kpc and offset toward the west. The molecular clumps have a systematic velocity gradient in the north–south direction, which may indicate a rotating gas disk. One possible scenario is that the components of molecular gas, dust, and stars are distributed in a several-kpc-scale rotating disk, and the stellar emission is heavily obscured by dust in the central star-forming region. Alternatively, SDP.81 can be explained by a merging system, where dusty starbursts occur in the region where the two galaxies collide, surrounded by tidal features traced in the stellar components.

ATOMIC HYDROGEN IN A GALACTIC CENTER OUTFLOW

We describe a population of small, high velocity, atomic hydrogen clouds, loops, and filaments found above and below the disk near the Galactic Center. The objects have a mean radius of 15 pc, velocity widths of $\sim 14$ km/s and are observed at $|z|$ heights up to 700 pc. The velocity distribution of the clouds shows no signature of Galactic rotation. We propose a scenario where the clouds are associated with an outflow from a central star-forming region at the Galactic Center. We discuss the clouds as entrained material traveling at $\sim 200$ km/s in a Galactic wind.

Properties of free–free, dust and CO emissions in the starbursts of blue compact dwarf galaxies

The central star-forming regions in three blue compact dwarf galaxies (He 2-10, NGC 5253, and II Zw 40) were observed in the 340 GHz (880 micron) band at 5 arcsec resolution with the Submillimetre Array (SMA). Continuum emission associated with the central star-forming complex was detected in all these galaxies. The SMA 880 micron flux is decomposed into free-free emission and dust emission by using centimetre-wavelength data in the literature. We find that free-free emission contributes half or more of the SMA 880 micron flux in the central starbursts in those three galaxies. In spite of the dominance of free-free emission at 880 micron, the radio-to-far infrared (FIR) ratios in the central star-forming regions are not significantly higher than those of the entire systems, showing the robustness of radio-FIR relation. Based on the robustness of the radio-FIR relation, we argue that the free--free fraction in the 880 micron emission is regulated by the dust temperature. We also analyze the CO (J = 3--2) emission data. We find that CO is a good tracer of the total gas mass in solar-metallicity object He 2-10. Low-metallicity objects, NGC 5253 and II Zw 40, have apparently high star formation efficiencies; however, this may be an artifact of significant dissociation of CO in the low-metallicity environments. We also point out a potential underestimate of dust mass, since the dust traced by emission is biased to the most luminous high-temperature regions, particularly when a system hosts a compact star-forming region where the dust temperature is high.

Central free-free dominated 880-μm emission in II Zw 40

The central star-forming region in a blue compact dwarf galaxy, II Zw 40, was observed in the 340 GHz ($880 \micron$) band at $\sim 5$ arcsec (250 pc) resolution with the Submillimetre Array (SMA). A source associated with the central star-forming complex was detected with a flux of $13.6\pm 2.0$ mJy. The structure is more extended than the beam in the east-west direction. The SMA 880 $\micron$ flux is analyzed by using theoretical models of radio spectral energy distribution along with centimetre interferometric measurements in the literature. We find (i) that the SMA 880 $\micron$ flux is dominated ($\sim 75$ per cent) by free-free emission from the central compact star-forming region, and (ii) that the contribution from dust emission to the SMA 880 $\micron$ flux is at most $4\pm 2.5$ mJy. We also utilize our models to derive the radio--FIR relation of the II Zw 40 centre, suggesting that free-free absorption at low frequencies ($\nu\la$ several GHz; $\lambda\ga$ several cm) and spatial extent of dust affect the radio-FIR relation.

H i in isolated extremely metal-deficient galaxies

We present Giant Metrewave Radio Telescope H i observations of two extremely metal-deficient (XMD) galaxies, UM 133 and SDSS J011914.27−093546.4, with no known nearby companions. Earlier H i observations of XMD galaxies have shown that disturbed H i morphologies and kinematics, generally related to tidal interaction with a companion, are common in them. However, some of these galaxies were already known to be in pairs before the H i observations. The two galaxies studied here were specifically selected because they do not have any known nearby companions. None the less, we find that both the galaxies have highly disturbed H i morphologies and velocity fields. These could have arisen from interactions with more distant companions than considered in our isolation criteria or merger with extremely gas-rich dwarf companions. It is also possible that distorted H i distributions of these XMD galaxies are the result of cold gas accretion from the intergalactic medium. Our observations provide further support to the idea that inflow of metal-poor gas from the outskirts of a galaxy to the central star-forming regions (in the case of interaction), or cold gas accretion is the probable cause for the observed low emission-line metallicities of XMD galaxies.

H i and star formation in the most metal-deficient galaxies

We present Giant Metrewave Radio Telescope (GMRT) observations for three (viz., DDO 68, SDSS J2104−0035 and UGC 772) of the six most metal-deficient actively star-forming galaxies known. Although there is a debate as to whether these galaxies are undergoing their first episode of star formation or not, they are ‘young’ in the sense that their interstellar medium is chemically unevolved. In this regard, they are the nearest equivalents of young galaxies in the early Universe. All three galaxies, that we have observed, have irregular H i morphologies and kinematics, which we interpret as either due to tidal interaction with neighbouring galaxies, or the consequences of a recent merger. The remaining three of the six most metal-deficient galaxies are also known to have highly disturbed H i distributions and are interacting. It is interesting because these galaxies were chosen solely on the basis of their metallicity and not for any particular signs of interaction. In this sense (i.e. their gas has not yet had time to settle into a regular disc), one could regard these extremely metal deficient (XMD) galaxies as ‘young’. The current star-formation episode is likely to have been triggered by interaction/merger. It is also possible that the tidal interaction has lead to enhanced mixing with metal-poor gas in outer disc, and hence to a low gas-phase metallicity in the central star-forming regions. We also try to determine the threshold gas density for star-formation in our sample of galaxies, and find that in general these galaxies do not show a one-to-one correspondence between regions of high H i column density and regions with current star formation. However, to the extent that one can define a threshold density, its value (∼1021 atoms cm−2) is similar to that in galaxies with much higher metallicity. The highest column densities that we detect in regions far outside star-forming regions (i.e. a lower limit to the star-formation threshold) are ∼2 × 1021 atoms cm−2.

AND THE REST: THE STELLAR ARCHEOLOGICAL RECORD OF M82 OUTSIDE THE CENTRAL STARBURST

Deep images obtained with MegaCam and WIRCam on the Canada-France-Hawaii Telescope are used to probe the stellar content outside of the central star-forming regions of M82. Stars evolving on the asymptotic giant branch (AGB) are traced along the major axis out to projected distances of 12 kpc, which corresponds to 13 disk scale lengths. The numbers of red supergiants (RSGs) and AGB stars normalized to local surface brightness (the specific frequency or SF) is constant when R GC > 4 kpc, indicating that RSGs and AGB stars are well mixed throughout the disk. Moreover, the SF of bright AGB stars in the outer disks of M82 and the Sc galaxy NGC 2403 are identical, suggesting that the specific star-formation rates (SFRs) in these galaxies during intermediate epochs were similar. This similarity in stellar content, coupled with the presence of an extended stellar disk, is consistent with M82 having been a late-type disk galaxy prior to interacting with M81. Still, there is a paucity of RSGs in the outer disk of M82 when compared with NGC 2403, indicating that the SFR in the outer regions of M82 during the past ~0.1 Gyr has declined dramatically with respect to that in isolated late-type galaxies. The stellar content of the M82 disk plane is also investigated. A mixture of bright main-sequence stars, RSGs, and AGB stars is detected out to minor-axis distances of 7 kpc. These stars, which span a range of ages, are concentrated along the outflow. The brightest extraplanar AGB stars define a system with an exponential scale height of 1.8 ± 0.2 kpc, as measured along the minor axis. It is suggested that the young- and intermediate-aged stars in the extraplanar regions formed in structures similar to M82 South, and that these were subsequently disrupted by the tidal action of M82.

Embedded clusters in NGC 1808 central starburst

In the course of a mid-infrared imaging campaign of close-by active galaxies, we discovered the mid-infrared counterparts of bright compact radio sources in the central star-forming region of NGC1808. We aim at confirming that these sources are deeply embedded, young star clusters and at deriving some of their intrinsic properties. To complement the mid-infrared data, we have collected a set of near-infrared data with ISAAC at the VLT: J, Ks, and L' images, as well as low-resolution, long-slit spectra for three of the sources. Surprisingly, the new images unveil a near-infrared counterpart for only one of the mid-infrared/radio sources, namely M8 in the L' band. All the other sources are so deeply embedded that their emission does not pop out above an extended diffuse near-infrared emission. The near-infrared spectra of the sources look alike, with intense, ionised hydrogen lines. This supports the interpretation of these sources in terms of embedded young clusters. We derive extinctions and ionising photon production rates for two of the clusters.

Cluster and nebular properties of the central star-forming region of NGC 1140

We present new high spatial resolution Hubble Space Telescope/Advanced Camera for Surveys (ACS) imaging of NGC 1140 and high spectral resolution Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph spectroscopy of its central star-forming region. The central region contains several clusters, the two brightest of which are clusters 1 and 6 from Hunter, O’Connell & Gallagher, located within star-forming knots A and B, respectively. A nebular analysis indicates that the knots have a Large Magellanic Cloud-like metallicity of 12 + log O/H = 8.29 ± 0.09. According to continuum-subtracted Hα ACS imaging, cluster 1 dominates the nebular emission of the brighter knot A. Conversely, negligible nebular emission in knot B originates from cluster 6. Evolutionary synthesis modelling implies an age of 5 ± 1 Myr for

X-Ray Study of Discrete Sources in NGC 1808 with Chandra

Abstract We analyzed discrete sources in a typical starburst galaxy, NGC 1808, with Chandra in order to study the relation between X-ray and star-forming activities. We detected thirty-eight X-ray sources with an X-ray luminosity ($L_{\rm X}$) of $\gt$ 10$^{37}$ erg s$^{-1}$ in the central 5$^{\prime}\times$5$^{\prime}$ (13.4 kpc $\times$ 13.4 kpc) region. About 90% of the hard X-rays previously detected with ASCA came from these discrete sources. Among the detected sources, sixteen sources had an $L_{\rm X}$ of $\gt$ 10$^{38}$ erg s$^{-1}$. The number of luminous sources is nearly equal to that of M 82, although their concentration into the central star-forming region is lower than that of M 82. There are several soft X-ray sources with a temperature of 0.6 keV in the central star-forming region. The X-ray properties of the soft sources are similar to those of a H II region, or a supernova remnant. The existence of both luminous sources and soft X-ray sources indicates the star-forming activity of NGC 1808. However, we detected only a few candidates of high-mass X-ray binaries that would be produced by enhanced star-forming activity. This may be due to the starburst age of NGC 1808.

Starburst Models for Far‐Infrared/Submillimeter/Millimeter Line Emission. I. An Expanding Supershell Surrounding a Massive Star Cluster

The effect of a newly born star cluster inside a giant molecular cloud (GMC) is to produce a hot bubble and a thin, dense shell of interstellar gas and dust swept up by the H II expansion, strong stellar winds, and repeated supernova explosions. Lying at the inner side of the shell is the photodissociation region (PDR), the origin of much of the far-infrared/submillimeter/millimeter (FIR/submm/mm) radiation from the interstellar medium (ISM). We present a model for the expanding shell at different stages of its expansion that predict mm/submm and far-IR emission line intensities from a series of key molecular and atomic constituents in the shell. The kinematic properties of the swept-up shell predicted by our model are in very good agreement with the measurements of the supershell detected in the nearby starburst galaxy M82. We compare the modeling results with the ratio-ratio plots of the FIR/submm/mm line emission in the central 1.0 kpc region to investigate the mechanism of star-forming activity in M82. Our model has yielded appropriate gas densities, temperatures, and structure scales compared to those measured in M82, and the total H2 content is compatible with the observations. This implies that the neutral ISM of the central star-forming region is a product of fragments of the evolving shells.

P Cygni type Ly from starburst galaxies

P Cygni type Lya emission is exhibited in the spectra of nearly half the starburst galaxies in either the nearby Universe or the high-z Universe. Such emission is presumably formed by an expanding supershell surrounding star-forming regions. We assume that the Lya photons from the central star-forming regions transfer in a spherical galactic supershell of uniform neutral hydrogen that is expanding radially in a bulk flow. We apply the Monte Carlo code developed previously for static and plane-parallel media. We consider typical cases in which the supershell has Lya line-centre optical depth τ 0 = 10 5 -10 7 , a radial expansion velocity V e x p 300 km s - 1 and turbulence b ≃ 40 km s - 1 . We find that a series of emission peaks appear redward of the systemic redshift, their frequencies being given by (2N - 1)V e x p , where the number of back-scatterings N is greater than zero. We also investigate how the emergent profiles are changed in accordance with the expansion velocity and the column density of the supershell. We find that the number and the flux ratios of emission peaks are determined by interplay of the two parameters. It is very important that the kinematics of the supershell is imprinted on both the widths of the peaks and the velocity differences between peaks. We discuss the effects of dust extinction and the implication of our work in relation to recent spectroscopic observations of starburst galaxies.

Spectroscopic Observations of the Star Formation Activities in the Central Regions of Early-Type Spiral Galaxies

We study the characteristics of central star-forming regions of early-type spiral galaxies by optical spectroscopic observations. The sample consists of 13 galaxies which have ratios of far-infrared (FIR) to optical B-band luminosity, log(LFIR/LB), larger than the average of this type, -0.5. Strong line emissions are detected around the nucleus, and the line ratios of most regions are H II region-like, except for a few shocked regions. There is no low-ionization nuclear emission-line region (LINER). The [N II] and [S II] lines are somewhat enhanced, compared to the disk H II regions, but the enhancements cannot be attributed to hidden active galactic nuclei (AGNs) and the shock excitation. The median extinction derived from the Hα/Hβ intensity ratio is 1.0 mag in AV, and the median SFR of our sample galaxies, corrected for extinction, is estimated to be 2 M⊙ yr-1. Both values are comparable to those of late-type spiral galaxies. We find a correlation that the radii of the central star-forming regions tend to become larger with the turn-over radii of the rotation curves.