Circumnuclear Star-forming Regions Research Articles

Revisiting the Integrated Star Formation Law. II. Starbursts and the Combined Global Schmidt Law

Abstract We compile observations of molecular gas contents and infrared-based star formation rates (SFRs) for 112 circumnuclear star-forming regions, in order to reinvestigate the form of the disk-averaged Schmidt surface density star-formation law in starbursts. We then combine these results with total gas and SFR surface densities for 153 nearby nonstarbursting disk galaxies from de los Reyes & Kennicutt (2019), to investigate the properties of the combined star formation law, following Kennicutt (1998). We confirm that the combined Schmidt law can be fitted with a single power law with slope n = 1.5 ± 0.05 (including fitting method uncertainties), somewhat steeper than the value n = 1.4 ± 0.15 found by Kennicutt. Fitting separate power laws to the nonstarbursting and starburst galaxies, however, produces very different slopes (n = 1.34 ± 0.07 and 0.98 ± 0.07, respectively), with a pronounced offset in the zero-point (∼0.6 dex) of the starburst relation to higher SFR surface densities. This offset is seen even when a common conversion factor between CO intensity and molecular hydrogen surface density is applied, and it is confirmed when disk surface densities of interstellar dust are used as proxies for gas measurements. Tests for possible systematic biases in the starburst data fail to uncover any spurious sources for such a large offset. We tentatively conclude that the global Schmidt law in galaxies, at least as it is conventionally measured, is bimodal or possibly multimodal. Possible causes may include changes in the small-scale structure of the molecular interstellar medium or the stellar initial mass function. A single n ∼ 1.5 power law still remains as a credible approximation or “recipe” for analytical or numerical models of galaxy formation and evolution.

The Nature of the Double Nuclei in the Barred S0 Galaxy IC 676

Abstract The lenticular galaxy IC 676 is a barred galaxy with double nuclei and active star formation in the central region. In this work we present the long-slit spectroscopy and archival multiwavelength images to investigate the nature and origin of the double nuclei in IC 676. Through photometric 1D brightness profiles and 2D image decomposition, we show that this galaxy consists of a stellar bar with the length of ∼2.5 kpc and two Sérsic disks both of which with Sérsic index n ∼ 1.3. There is probably little or no bulge component assembled in IC 676. The luminosities of the double nuclei are primarily dominated by young stellar populations within the ages of 1–10 Myr. The northern nucleus has stronger star formation activity than the southern one. The surface densities of the star formation rate in the double nuclei are similar to those in starburst galaxies or the circumnuclear star-forming regions in spiral galaxies. Each of the double nuclei in IC 676 likely consists of young massive star clusters, which can be resolved as bright knots in the Hubble Space Telescope high-resolution image. Our results suggest that IC 676 likely has a complex formation and evolutionary history. The secular processes driven by the stellar bar and external accretion may dominate the formation and evolution of its double nuclei. This indicates that the secular evolution involving the internal and external drivers may have an important contribution for the evolution of lenticular galaxies.

Characterizing circumnuclear starbursts in the local universe with the VLA

AbstractNuclear rings are excellent laboratories to study star formation (SF) under extreme conditions. We compiled a sample of 9 galaxies that exhibit bright nuclear rings at 3-33 GHz radio continuum observed with the Jansky Very Large Array, of which 5 are normal star-forming galaxies and 4 are Luminous Infrared Galaxies (LIRGs). Using high frequency radio continuum as an extinction-free tracer of SF, we estimated the size and star formation rate of each nuclear ring and a total of 37 individual circumnuclear star-forming regions. Our results show that majority of the SF in the sample LIRGs take place in their nuclear rings, and circumnuclear SF in local LIRGs are much more spatially concentrated compared to those in the local normal galaxies and previously studied nuclear and extra-nuclear SF in normal galaxies at both low and high redshifts.

Circumnuclear star formation in Mrk 42 mapped with Gemini Near-infrared Integral Field Spectrograph

We present Gemini Near-infrared Integral Field Spectrograph (NIFS) observations of the inner $1.5\times1.5$ kpc$^2$ of the narrow-line Seyfert 1 galaxy Mrk 42 at a spatial resolution of 60 pc and spectral resolution of 40 km s$^{-1}$. The emission-line flux and equivalent width maps clearly show a ring of circumnuclear star formation regions (CNSFRs) surrounding the nucleus with radius of $\sim$500 pc. The spectra of some of these regions show molecular absorption features which are probably of CN, TiO or VO, indicating the presence of massive evolved stars in the thermally pulsing asymptotic giant branch (TP-AGB) phase. The gas kinematics of the ring is dominated by rotation in the plane of the galaxy, following the large scale disk geometry, while at the nucleus an additional outflowing component is detected blueshifted by 300-500 kms$^{-1}$, relative to the systemic velocity of the galaxy. Based on the equivalent width of Br$\gamma$, we find evidences of gradients in the age of HII regions along the ring of Mrk 42, favoring the pearls on a string scenario of star formation. The broad component of Pa$\beta$ emission line presents a Full Width at Half Maximum (FWHM) of $\sim$1480 kms$^{-1}$, implying in a mass of $\sim\,2.5\times10^{6}$~M$_{\odot}$ for the central supermassive black hole. Based on emission-line ratios we conclude that besides the active galactic nucleus, Mrk 42 presents nuclear Starburst activity.

A SINFONI view of the nuclear activity and circumnuclear star formation in NGC 4303

We present new maps of emission-line flux distributions and kinematics in both ionized (traced by HI and [FeII] lines) and molecular (H2) gas of the inner 0.7x0.7kpc2 of the galaxy NGC4303, with a spatial resolution 40-80pc and velocity resolution 90-150 km/s obtained from near-IR integral field specroscopy using the VLT instrument SINFONI. The most promiment feature is a 200-250pc ring of circum-nuclear star-forming regions. The emission from ionized and molecular gas shows distinct flux distributions: while the strongest HI and [FeII] emission comes from regions in the west side of the ring (ages~4Myr), the H2 emission is strongest at the nucleus and in the east side of the ring (ages>10Myr). We find that regions of enhanced hot H2 emission are anti-correlated with those of enhanced [FeII] and HI emission, which can be attributed to post starburst regions that do not have ionizing photons anymore but still are hot enough (~2000K) to excite the H2 molecule. The line ratios are consistent with the presence of an AGN at the nucleus. The youngest regions have stellar masses in the range 0.3-1.5E5 MSun and ionized and hot molecular gas masses of ~0.25-1.2E4 Msun and 2.5-5 Msun, respectively. The stellar and gas velocity fields show a rotation pattern, with the gas presenting larger velocity amplitudes than the stars, with a deviation observed for the H2 along the nuclear bar, where increased velocity dispersion is also observed, possibly associated with non circular motions along the bar. The stars in the ring show smaller velocity dispersion than the surroundings, that can be attributed to a cooler dynamics due to their recent formation from cool gas.

A comprehensive photometric study of circumnuclear star-forming rings - I. The sample

We present photometry in U, B, V, R and I continuum bands and in H$\alpha$ and H$\beta$ emission lines for a sample of 336 circumnuclear star forming regions (CNSFR) located in early type spiral galaxies with different levels of activity in their nuclei. They are nearby galaxies, with distances less than 100 Mpc, 60\% of which are considered as interacting objects. This survey of 20 nuclear rings aims to provide insight into their star formation properties as age, stellar population and star formation rate. Extinction corrected H$\alpha$ luminosities range from $1.3\times 10^{38}$ to $4\times 10^{41} erg s^{-1}$, with most of the regions showing values between 39.5 $\leq log L(H\alpha) \leq$ 40, which implies masses for the ionizing clusters higher than $2\times 10^{5} M_\odot $. H$\alpha$ and H$\beta$ images have allowed us to obtain an accurate measure of extinction. We have found an average value of A$_V$ = 1.85 magnitudes. (U-B) colour follows a two maximum distribution around (U-B)$ \simeq$ -0.7, and -0.3; (R-I) also presents a bimodal behaviour, with maximum values of 0.6 and 0.9. Reddest (U-B) and (R-I) regions appear in non-interacting galaxies. Reddest (R-I) regions lie in strongly barred galaxies. For a significant number of HII regions the observed colours and equivalent widths are not well reproduced by single burst evolutionary theoretical models.

THE SIZES OFz∼ 6–8 LENSED GALAXIES FROM THE HUBBLE FRONTIER FIELDS ABELL 2744 DATA

We investigate sizes of $z\sim6-8$ dropout galaxies using the complete data of the Abell 2744 cluster and parallel fields in the Hubble Frontier Fields program. By directly fitting light profiles of observed galaxies with lensing-distorted S\'ersic profiles on the image plane with the \texttt{glafic} software, we accurately measure intrinsic sizes of 31 $z\sim6-7$ and eight $z\sim8$ galaxies, including those as faint as $M_{\mathrm{UV}}\simeq-16.6$. We find that half-light radii $r_\mathrm{e}$ positively correlates with UV luminosity at each redshift, although the correlation is not very tight. Largest ($r_\mathrm{e}>0.8$ kpc) galaxies are mostly red in UV color while smallest ($r_\mathrm{e} < 0.08$ kpc) ones tend to be blue. We also find that galaxies with multiple cores tend to be brighter. Combined with previous results at $2.5\lesssim z\lesssim12$, our result confirms that the average $r_{\mathrm{e}}$ of bright ($(0.3-1)L^\star_{z=3}$) galaxies scales as $r_{\mathrm{e}}\propto(1+z)^{-m}$ with $m=1.24\pm0.1$. We find that the ratio of $r_\mathrm{e}$ to virial radius is virtually constant at $3.3\pm0.1\%$ over a wide redshift range, where the virial radii of hosting dark matter halos are derived based on the abundance matching. This constant ratio is consistent with the disk formation model by Mo et al. (1998) with $j_\mathrm{d}\sim m_\mathrm{d}$, where $j_\mathrm{d}$ and $m_\mathrm{d}$ are the fractions of the angular momentum and mass within halos confined in the disks. A comparison with various types of local galaxies indicates that our galaxies are most similar to circumnuclear star-forming regions of barred galaxies in the sense that a sizable amount of stars are forming in a very small area.

Implications of the kinematical structure of circumnuclear star-forming regions on their derived properties

[Abbreviated] We review the results of high dispersion spectroscopy of 17 circumnuclear starforming regions (CNSFRs) in 3 nearby early spiral galaxies, NGC2903, NGC3310 and NGC3351. We find that single Gaussian fitting to the H$\beta$ and [OIII]5007A line profiles results in velocity dispersions around 32km/s and 52km/s, respectively, while the IR CaII triplet cross-correlation technique provides stellar velocity dispersion values close to 50km/s. Even though multiple kinematical components are present, the relation between gas velocity dispersion and Balmer emission line luminosity (L-sigma relation) reproduces the correlation for disk giant HII regions albeit with a larger scatter. The scatter in the L-sigma relation is considerably reduced when theoretical evolutionary corrections are applied suggesting that an age range is present in the sample of CNSFRs. To analyse the observed complex profiles, we performed multiple Gaussian component fits to the Hbeta and [OIII]?5007A lines obtaining optimal fits with two Gaussians of different width. These best fits indicate that the narrower component has average velocity dispersion close to 23km/s while the broader component shows average values in the range 50-60km/s for both lines, close to the observed stellar velocity dispersions. The fluxes of the broad and narrow Hbeta components are similar. This is not the case for [OIII]5007A for which the broad components have higher fluxes than the narrow ones, thus producing a clear segregation in their [OIII]/Hbeta ratios. We suggest a possible scenario for understanding the behaviour of CNSFRs in the L-sigma and sigma_gas-sigma_star diagrams involving an inner gaseous disk responsible for the narrow component of the emission lines.

PPAK Wide-field Integral Field Spectroscopy of NGC 628 - I. The largest spectroscopic mosaic on a single galaxy★

We present a wide-field IFS survey on the nearby face-on Sbc galaxy NGC 628, comprising 11094 individual spectra, covering a nearly circular field-of-view of ~6 arcmin in diameter, with a sampling of ~2.7 arcsec per spectrum in the optical wavelength range (3700--7000 AA). This galaxy is part of the PPAK IFS Nearby Galaxies Survey, (PINGS, Rosales-Ortega et al. 2009). To our knowledge, this is the widest spectroscopic survey ever made in a single nearby galaxy. A detailed flux calibration was applied, granting a spectrophotometric accuracy of $\sim$\,0.2 mag. The age of the stellar populations shows a negative gradient from the inner (older) to the outer (younger) regions. We found an inversion of this gradient in the central ~1 kpc region, where a somewhat younger stellar population is present within a ring at this radius. This structure is associated with a circumnuclear star-forming region at ~ 500 pc, also found in similar spiral galaxies. From the study of the integrated and spatially resolved ionized gas we found a moderate SFR of ~ 2.4 Msun yr$^{-1}$. The oxygen abundance shows a a clear gradient of higher metallicity values from the inner part to the outer part of the galaxy, with a mean value of 12~+~log(O/H) ~ 8.7. At some specific regions of the galaxy, the spatially resolved distribution of the physical properties show some level of structure, suggesting real point-to-point variations within an individual \hh region. Our results are consistent with an inside-out growth scheme, with stronger star formation at the outer regions, and with evolved stellar populations in the inner ones.

Subarcsecond radio continuum mapping in and around the spiral galaxy NGC 3351 using MERLIN

We report sub-arcsecond scale radio continuum observations of a field of 35 by 22 arcmin centred in NGC3351 obtained with the Multi-Element Radio Linked Interferometer Network (MERLIN). We found 23 radio sources in this field, 6 of which are projected within the D$_{25}$ isophote of the galaxy, and 3 are located inside the central 100 arcsec in radius. Two of these three are significantly extended, while the third one is relatively compact. This one is the only source with a previously detected counterpart at other wavelengths and could constitute the radio counterpart of a young supernova remnant. The other two are probably related to jets from a background AGN. We are not able to detect individual supernovae or SNRs in the central region ($r<600$ pc) of the galaxy. This could imply that the ionising populations of the circumnuclear star-forming regions (CNSFRs) are too young (less than a few Myr) to host supernovae. Also the diffusion length of the relativistic electrons in the ISM associated with the SN from previous events of star formation seems to be larger than our maximum resolution of 50 pc in radius. Detecting the thermal bremsstrahlung emission from the circumnuclear HII regions probably requires deeper observations.

The circumnuclear environment of the peculiar galaxy NGC 3310

Gas and star velocity dispersions have been derived for eight circumnuclear star-forming regions (CNSFRs) and the nucleus of the spiral galaxy NGC3310 using high resolution spectroscopy in the blue and far red. Stellar velocity dispersions have been obtained from the CaII triplet in the near-IR, using cross-correlation techniques, while gas velocity dispersions have been measured by Gaussian fits to the Hb 4861A and [OIII]5007A emission lines. The CNSFRs stellar velocity dispersions range from 31 to 73 km/s. These values, together with the sizes measured on archival HST images, yield upper limits to the dynamical masses for the individual star clusters between 1.8 and 7.1 x 10$^6$ M$_\odot$, for the whole CNSFR between 2 x 10$^7$ and 1.4 x 10$^8$ M$_\odot$, and 5.3 x 10$^7$ M$_\odot$ for the nucleus inside the inner 14.2 pc. The masses of the ionizing stellar population responsible for the HII region gaseous emission have been derived from their published Ha luminosities and are found to be between 8.7 x 10$^5$ and 2.1 x 10$^6$ M$_\odot$ for the star-forming regions, and 2.1 x 10$^5$ M$_\odot$ for the galaxy nucleus; they therefore constitute between 1 and 7 per cent of the total dynamical mass. The ionized gas kinematics is complex; two different kinematical components seem to be present as evidenced by different line widths and Doppler shifts.

Kinematics of gas and stars in circumnuclear star-forming regions of early-type spirals

We present high-resolution (R∼20,000) spectra in the blue and the far red of circumnuclear star-forming regions (CNSFRs) in three early-type spirals (NGC3351, NGC2903 and NGC3310), which have allowed the study of the kinematics of the stars and the ionized gas in these structures and, for the first time, the derivation of their dynamical masses for the first two. In some cases, these regions, about 100 to 150 pc in size, are composed of several individual star clusters with sizes between 1.5 and 4.9 pc, estimated from Hubble Space Telescope images. The stellar dispersions have been obtained from the Calcium triplet (CaT) lines at λ λ 8494, 8542, 8662 Å, while the gas velocity dispersions have been measured by means of Gaussian fits to the Hβ and [Oiii]λ 5007 Å lines in the high-dispersion spectra. Values of the stellar velocity dispersions are between 30 and 68 km s−1. We apply the virial theorem to estimate dynamical masses of the clusters, assuming that systems are gravitationally bounded and spherically symmetric, and using previously measured sizes. The measured values of the stellar velocity dispersions yield dynamical masses of the order of 107 to 108 M⊙ for the entire CNSFRs. Stellar and gas velocity dispersions are found to differ by about 20 to 30 km s−1, with the Hβ emission lines being narrower than both the stellar lines and the [Oiii]λ 5007 Å lines. The douby-ionized oxygen, on the other hand, exhibits velocity dispersions comparable to those of the stars or, in some cases, even larger. We have found indications of the presence of two different kinematical components in the ionized gas of the regions. We have mapped the velocity field in the central kpc of the spiral galaxies NGC3351 and NGC2903. For the first object, the radial velocity curve shows deviations from circular motions for the ionized hydrogen consistent with its infall towards the central regions of the galaxy, at a velocity of about 25 km s−1. For NGC3310, we present preliminary results for the velocity dispersions for one of the two observed slit position angles, two CNSFRs and the nucleus.

Properties of the ionised gas of circumnuclear star-forming regions in early-type spirals

(Abbr.) A study of cicumnuclear star-forming regions (CNSFRs) in several early type spirals has been made in order to investigate their main properties: stellar and gas kinematics, dynamical masses, ionising stellar masses, chemical abundances and other properties of the ionised gas. Both high resolution (R$ \sim $20000) and moderate resolution (R ~ 5000) have been used. In some cases these regions, about 100 to 150 pc in size, are seen to be composed of several individual star clusters with sizes between 1.5 and 4.9 pc estimated from Hubble Space Telescope (HST) images. Stellar and gas velocity dispersions are found to differ by about 20 to 30 km/s with the H$\beta$ emission lines being narrower than both the stellar lines and the [OIII] $\lambda$ 5007 \AA lines. The twice ionized oxygen, on the other hand, shows velocity dispersions comparable to those shown by stars. We have applied the virial theorem to estimate dynamical masses of the clusters, assuming that systems are gravitationally bounded and spherically symmetric, and using previously measured sizes. The measured values of the stellar velocity dispersions yield dynamical masses of the order of 10$^7$ to 10$^8$ solar masses for the whole CNSFRs. ...

On the derivation of dynamical masses of the stellar clusters in the circumnuclear region of NGC 2903

(Abridged) Gas and star velocity dispersions have been derived for four circumnuclear star-forming regions (CNSFRs) and the nucleus of the spiral galaxy NGC2903 using high resolution spectroscopy in the blue and far red. Stellar velocity dispersions have been obtained from the CaII triplet (CaT) lines at 8494, 8542, 8662A, using cross-correlation techniques while gas velocity dispersions have been measured by Gaussian fits to the Hbeta line. The CNSFRs, with sizes of about 100 to 150pc in diameter, show a complex structure at the Hubble Space Telescope resolution, with a good number of subclusters with linear diameters between 3 and 8pc. Their stellar velocity dispersions range from 39 to 67 km/s. These values, together with the sizes measured on archival HST images yield upper limits to the dynamical masses for the individual star clusters between 1.8 and 8.7 x 10$^6$ M$_\odot$ and upper limits to the masses for the whole CNSFR between 4.9 x 10$^6$ and 4.3 x 10$^7$ M$_\odot$. ...

The nucleus of NGC 253 and its massive stellar clusters at parsec scales

Abstract Very Large Telescope adaptive optics images of NGC 253 with resolutions down to 200 mas resolve the central 300 pc of this galaxy in ∼37 infrared (IR) bright knots, a factor of 3 larger than previously reported and extended diffuse emission. The angular resolution, comparable to that of available Very Large Array 2 cm maps, permits us a very accurate IR-radio registration. Eight radio sources are found to have an IR counterpart. The knots have Hα equivalent width of about 80 Å, sizes of ∼3 pc, magnitudes in L band of about 12 mag and relatively high extinction, AV∼ 7 mag. Their spectral energy distributions (SEDs) look very similar, characterized by a maximum at 20 μm and a gentle bump in the 1–2 μm range. These features can be well reproduced by considering an important contribution of very young stellar objects to the IR, efficiently heating their dust envelope. The evidence indicates that these are young massive clusters bursting from their dust cocoons. A median SED of the knots is provided, which may represent one of the most genuine templates of an extragalactic circumnuclear star-forming region. The lack of any optical or IR counterpart for the previously identified radio core calls into question its supposed active nucleus nature. This source may instead represent a scaled up version of Sgr A* at the Galactic Centre.

Circumnuclear Star Forming Activity in NGC 3982

We present a study of the nearby Seyfert galaxy NGC 3982 using optical, infrared and X-ray data acquired by SDSS, Spitzer and Chandra. Our main results are as follows: (1) A simple stellar population synthesis on the nuclear and circumnuclear SDSS spectra gives unambiguous evidence of young stellar components in both the nuclear and circumnuclear regions. (2) The Spitzer Infrared Spectrograph (IRS) spectrum of the central region (~3'') shows a power-law continuum, a silicate emission feature at 9.7 μm, and significant PAH emission features at 7.7, 8.6, 11.3 and 12.7 μm, suggesting the coexistence of AGN and starburst activities in the central region of NGC 3982. (3) We estimate the star formation rate (SFR) of the circumnuclear (~5''−20'') region from the Hα luminosity to be SFRHα = 0.52 M⊙ yr−1, which is consistent with the result from the Spitzer IRAC 8 μm luminosity, SFR8.0 μm = 0.57 M⊙ yr−1. (4) We measure the spectral energy distribution for the active nucleus of NGC 3982 from radio to X-ray, and obtain a bolometric luminosity of Lbol = 4.5×1042 erg s−1, corresponding to an Eddington ratio (Lbol/LEdd) of 0.014. The HST image of NGC 3982 shows a nuclear mini-spiral between the circumnuclear star-forming region and the nucleus, which could be the channel through which gas is transported to the supermassive black hole from the circumnuclear star-forming region.

Ages and metallicities of circumnuclear star formation regions from Gemini IFU observations

Aims: We derive the age and metallicity of circumnuclear star formation regions (CNSFRs) located in the spiral galaxies NGC6951 and NGC1097, and investigate the cause of the very low equivalent widths of emission lines found for these regions. Methods: We used optical two-dimensional spectroscopic data obtained with Gemini GMOS-IFUs and a grid of photoionization models to derive the the metallicities and ages of CNSFRs. Results: We find star formation rates in the range 0.002-0.14 Msun/yr and oxygen abundance of 12+log(O/H)~8.8 dex, similar to those of most metal-rich nebulae located in the inner region of galactic disks. Conclusions: We conclude that the very low emission-line equivalent widths observed in CNSFRs are caused by the ``contamination'' of the continuum by (1) contribution of the underlying bulge continuum combined with (2) contribution from previous episodes of star formation at the CNSFRs.

The metal abundance of circumnuclear star-forming regions in early-type spirals. Spectrophotometric observations

We have obtained long-slit observations in the optical and near infrared of 12 circumnuclear HII regions (CNSFR) in the early type spiral galaxies NGC 2903, NGC 3351 and NGC 3504 with the aim of deriving their chemical abundances. Only for one of the regions, the [SIII] 6312 A was detected providing, together with the nebular [SIII] lines at 9069, 9532 A, a value of the electron temperature of Te([SIII])= 8400 +4650 1250K. A semi-empirical method for the derivation of abundances in

Kinematics of gas and stars in the circumnuclear star-forming ring of NGC 3351

We have measured gas and stellar velocity dispersions in five circumnuclear star-forming regions (CNSFRs) and the nucleus of the barred spiral galaxy NGC 3351. The stellar dispersions have been obtained from high-resolution spectra of the Ca II triplet (CaT) lines at λλ8494, 8542, 8662 A, while the gas velocity dispersions have been measured by Gaussian fits to the Hβ λ4861 A line on high-dispersion spectra. The CNSFRs, with sizes of about 100 to 150pc in diameter, are seen to be composed of several individual star clusters with sizes between 1.7 and 4.9 pc on a Hubble Space Telescope (HST) image. Using the stellar velocity dispersions, we have derived dynamical masses for the entire star-forming complexes and for the individual star clusters. Values of the stellar velocity dispersions are between 39 and 67 km s -1 . Dynamical masses for the whole CNSFRs are between 4.9 x 106 and 4.3 x 10 7 M ⊙ and between 1.8 and 8.7 x 10 6 M ⊙ for the individual star clusters. Stellar and gas velocity dispersions are found to differ by about 20kms -1 with the Hβ lines being narrower than both the stellar lines and the [O III]λ5007 A lines. We have found indications for the presence of two different kinematical components in the ionized gas of the regions. The radial velocity curve shows deviation from circular motions for the ionized hydrogen consistent with its infall towards the central regions of the galaxy at a velocity of about 25 km s -1 . To disentangle the origin of these two components it will be necessary to map these regions with high spectral and spatial resolution and much better signal-to-noise ratio in particular for the O 2+ lines.

The Metal Abundances of Circumnuclear Star Forming Regions in Early Type Spirals

AbstractWe present a spectrophotometric study of circumnuclear star forming regions (CNSFR) in the galaxies: NGC 2903, NGC 3351 and NGC 3504, all of them of over solar metallicity according to standard empirical calibrations. A detailed determination of their abundances is made after careful subtraction of the prominent underlying stellar absorption. It is found that most regions show the highest abundances in HII region-like objects. It is also shown that CNSFR, as a class, segregate from the disk HII region family, clustering around smaller η' values, and therefore higher ionizing temperatures.From [Sii],λλ 6717,6731 Å lines, we have derived an electron density range from 180 to 650 −3, with average values of around 350 cm−3, higher than that found in Giant Extragalactic HII Regions (GEHR). The very low intensity of the [Oiii] lines (λλ 4959, 5007 Å implies high metallicity. Measurement of line temperature of [Oiii] (t[Oiii]) requires detection of auroral [Oiii]λ 4363 Å lines a few percent of [Oiii]λ 5007 Å which is impossible.Therefore, abundances for most of the regions require the use of empirical methods. CNSFRs show values of O23 (=([Oii]3727+[Oiii]4959, 5007)/Hβ; Pagel et al., 1979. MNRAS.189.95) which indicate oversolar metallicities, but the actual values are difficult to estimate since O23 levels off. At times [Oiii] lines are very difficult to measure. The combined parameter S23/O23 (S23 = ([Sii]6717, 6731+[Siii]9069, 9532)/Hβ; Vílchez &amp; Esteban, 1996. MNRAS.280.720) yields values of the oxygen abundances higher than twice solar. Also the N2 parameter (log([Nii]/Hβ; Denicoló, Terlevich &amp; Terlevich, 2002. MNRAS.330.69) renders oversolar abundances. Furthermore, the CNSFRs of our sample show the highest values of the ratio N/O known for the HII region-like objects.When we study the relation between the metallicity and the ionisation degree parametrized by the ratio [Sii]6717, 6731[Siii]9069, 9532 we find that the CNSFRs of our sample are among the HII region-like objects with the lowest ionisation parameter. Moreover, we estimate that the S23 parameter is, in this case, better than N2 to discriminate between different metallicities.The temperature of the ionising stars can be parametrized by the “softness parameter” η' (=([Oii]3727/[Oiii]4959, 5007)/([Sii]6717, 6731/[Siii]9069, 9532); Vílchez &amp; Pagel, 1988. MNRAS.231.257). This parameter can be used as an ionising temperature indicator. Analysing N2 vs. η' we find that the CNSFR of our sample have, in general, lower values of η' than those of the GEHRs, implying higher temperatures. In an Eta Prime Plot (log([Oii]3727/[Oiii]4959, 5007) vs. log([Sii]6717, 6731/[Siii]9069, 9532)) we can see that the CNSFRs follow the behaviour of HII galaxies rather than that of GEHR implying, again, higher temperatures.Some general conclusions are: CNSFR spectra are difficult to analyse; they show prominent underlying stellar populations so a careful subtraction of the absorption spectrum can be critical; abundance determination mostly comes from empirical calibrations. Furthermore, some conclusions derived from line diagnostics are that CNSFR can show very high metal abundances and N/O overabundances too; they show indications of low ionization parameter; regarding the temperature of ionizing stars, CNSFRs seem to share more properties with HII galaxies than with disk GEHR, which is puzzling.