arXiv subjects

M. A. Dopita

Publications and source records attributed to M. A. Dopita.

68 records · Page 4Linked to original sources

arXiv2002

Internal Dust Correction Factors for Star Formation Rates Derived for Dusty \HII Regions and Starburst Galaxies

Star formation rates in galaxies are frequently estimated using the Balmer line fluxes. However, these can be systematically underestimated because dust competes for the absorption of Lyman continuum photons in the ionized gas. Here we present theoretical correction factors in a simple analytic form. T These factors scale as the product of the ionization parameter, ${\cal U}$, and the nebular O/H abundance ratio, both of which can now be derived from the observation of bright nebular line ratios. The correction factors are only somewhat dependent upon the photoelectron production by grains, but are very sensitive to the presence of complex PAH-like carbonaceous molecules in the ionized gas, providing that these can survive in such an environment.

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arXiv2002

COLA II - Radio and Spectroscopic Diagnostics of Nuclear Activity in Galaxies

We present optical spectroscopic observations of 93 galaxies taken from the infra-red selected COLA (Compact Objects in Low Power AGN) sample. The sample spans the range of far-IR luminosities from normal galaxies to LIRGs. Of the galaxies observed, 78 (84%) exhibit emission lines. Using a theoretically-based optical emission-line scheme we classify 15% of the emission-line galaxies as Seyferts, 77% as starbursts, and the rest are either borderline AGN/starburst or show ambiguous characteristics. We find little evidence for an increase in the fraction of AGN in the sample as a function of far-IR luminosity but our sample covers only a small range in infrared luminosity and thus a weak trend may be masked. As a whole the Seyfert galaxies exhibit a small, but significant, radio excess on the radio-FIR correlation compared to the galaxies classified as starbursts. Compact (<0.05'') radio cores are detected in 55% of the Seyfert galaxies, and these galaxies exhibit a significantly larger radio excess than the Seyfert galaxies in which cores were not detected. Our results indicate that there may be two distinct populations of Seyferts, ``radio-excess'' Seyferts, which exhibit extended radio structures and compact radio cores, and ``radio-quiet'' Seyferts, in which the majority of the radio emission can be attributed to star-formation in the host galaxy. No significant difference is seen between the IR and optical spectroscopic properties of Seyferts with and without radio cores. (Abridged)

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arXiv2002

The H-alpha and Infrared Star Formation Rates for the Nearby Field Galaxy Survey

We investigate the H-alpha and infrared star formation rate (SFR) diagnostics for galaxies in the Nearby Field Galaxy Survey (NFGS). For the 81 galaxies in our sample, we derive H-alpha fluxes (included here) from integrated spectra. There is a strong correlation between the ratio of far-infrared to optical luminosities L(FIR)/L(H-alpha) and the extinction E(B-V) measured with the Balmer decrement. Before reddening correction, the SFR(IR) and SFR(H-alpha) are related to each other by a power-law. Correction of the SFR(H-alpha) for extinction using the Balmer decrement and a classical reddening curve both reduces the scatter in the SFR(IR)-SFR(H-alpha) correlation and results in a much closer agreement (within ~10%) between the two SFR indicators. This SFR relationship spans 4 orders of magnitude and holds for all Hubble types with IRAS detections in the NFGS. A constant ratio between the SFR(IR) and SFR(H-alpha) for all Hubble types, including early types (S0-Sab), suggests that the IR emission in all of these objects results from a young stellar population.

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arXiv2002

The Dynamical Interaction of AGN with their Galaxian Environments

Jet-driven shocks are responsible for an important fraction of the emission of the narrow-line regions (NLRs) in many classes of AGN. However, this cannot explain all observations. It is clear that the remaining sources are photoionised by the active nucleus. The 2-d hydrodynamic models from the RSAA group support an evolutionary scenario whereby the shock-excited NLRs are initially jet-driven but later, ionizing photons from the central engine replace shocks as the main excitation mechanism and shock induced star formation may also become important. In their photoionized phase, dusty and radiation-pressure dominated evolution produces a self-regulated NLR spectrum. This model aso explains the coronal emission lines and fast (3000 km s$^{-1}$) outflows seen in some Seyferts.

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arXiv2002

Using Strong Lines to Estimate Abundances in Extragalactic HII Regions and Starburst Galaxies

We have used a combination of stellar population synthesis and photoionization models to develop a set of ionization parameter and abundance diagnostics based only on the use of the strong optical emission lines. These models are applicable to both extragalactic HII regions and star-forming galaxies. We show that, because our techniques solve explicitly for both the ionization parameter and the chemical abundance, the diagnostics presented here are an improvement on earlier techniques based on strong emission-line ratios. Our techniques are applicable at all metallicities. In particular, for metallicities above half solar, the ratio [NII]/[OII] provides a very reliable diagnostic since it is ionization parameter independant and does not have a local maximum. This ratio has not been used historically because of worries about reddening corrections. However, we show that the use of classical reddening curves is quite sufficient to allow this [NII]/[OII] diagnostic to be used with confidence as a reliable abundance indicator. The cause of systematic and random errors inherent in previous techniques are discussed, and we present a new `optimal' abundance diagnostic method based on the use of line ratios involving [NII], [OII], [OIII], [SII] and the Balmer lines. This combined diagnostic appears to suffer no apparent systematic errors, can be used over the entire abundance range and significantly reduces the random error inherent in previous techniques. Finally, we give a recommended procedure for the derivation of abundances in the case that only spectra of limited wavelength coverage are available so that the optimal method can no longer be used.

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arXiv2002

Are the Narrow Line Regions in Active Galaxies Dusty and Radiation Pressure Dominated?

The remarkable similarity between emission spectra of narrow line regions (NLR) in Seyfert Galaxies has long presented a mystery. In photoionization models, this similarity implies that the ionization parameter is nearly always the same, about U ~ 0.01. Here we present dusty, radiation-pressure dominated photoionization models that can provide natural physical insight into this problem. In these models, dust and the radiation pressure acting on it provide the controlling factor in moderating the density, excitation and surface brightness of photoionized NLR structures. Additionally, photoelectric heating by the dust is important in determining the temperature structure of the models. These models can also explain the coexistence of the low-, intermediate- and coronal ionization zones within a single self-consistent physical structure. The radiation pressure acting on dust may also be capable of driving the fast (~3000 km/s) outflows such as are seen in the HST observations of NGC 1068.

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arXiv2001

First Results from the COLA Project- the Radio-FIR Correlation and Compact Radio Cores in Southern COLA Galaxies

We present the first results from the COLA (Compact Objects in Low-power AGN) project which aims to determine the relationship between one facet of AGN activity, the compact radio core, with star formation in the circumnuclear region of the host galaxy. This will be accomplished by the comparison of the multi-wavelength properties of a sample of AGN with compact radio cores to those of a sample of AGN without compact cores and a matched sample of galaxies without AGN. In this paper we discuss the selection criteria for our galaxy samples and present the initial radio observations of the 107 Southern galaxies in our sample. Low-resolution ATCA observations at 4.8, 2.5 and 1.4 GHz and high resolution, single baseline snapshots at 2.3 GHz with the Australian LBA are presented. We find that for the majority of the galaxies in our sample, the radio luminosity is correlated with the FIR luminosity. Compact radio cores are detected in 9 galaxies. The majority (8/9) of these galaxies exhibit a significant radio excess and 50% (7/14) of the galaxies which lie above the radio-FIR correlation by more than 1 sigma have compact radio cores. The emission from the cores is too weak to account for this radio excess and there is no evidence that the radio luminosity of the compact cores is correlated with the FIR galaxy luminosity. The galaxies with compact cores tend to be classified optically as AGN, with two thirds (6/9) exhibiting Seyfert-like optical emission line ratios, and the remaining galaxies classified either as composite objects (2/9) or starburst (1/9). (Abridged)

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arXiv2001

Theoretical Modeling of Starburst Galaxies

We have modeled a large sample of infrared starburst galaxies using both the PEGASE v2.0 and STARBURST99 codes to generate the spectral energy distribution of the young star clusters. PEGASE utilizes the Padova group tracks while STARBURST99 uses the Geneva group tracks, allowing comparison between the two. We used our MAPPINGS III code to compute photoionization models which include a self-consistent treatment of dust physics and chemical depletion. We use the standard optical diagnostic diagrams as indicators of the hardness of the EUV radiation field in these galaxies. These diagnostic diagrams are most sensitive to the spectral index of the ionizing radiation field in the 1-4 Rydberg region. We find that warm infrared starburst galaxies contain a relatively hard EUV field in this region. The PEGASE ionizing stellar continuum is harder in the 1-4 Rydberg range than that of STARBURST99. As the spectrum in this regime is dominated by emission from Wolf-Rayet (W-R) stars, this difference is most likely due to the differences in stellar atmosphere models used for the W-R stars. We believe that the stellar atmospheres in STARBURST99 are more applicable to the starburst galaxies in our sample, however they do not produce the hard EUV field in the 1-4 Rydberg region required by our observations. The inclusion of continuum metal blanketing in the models may be one solution. Supernova remnant (SNR) shock modeling shows that the contribution by mechanical energy from SNRs to the photoionization models is << 20%. The models presented here are used to derive a new theoretical classification scheme for starbursts and AGN galaxies based on the optical diagnostic diagrams.

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arXiv2001

Calibrating nebular diagnostics of T_* and abundance

We obtained nebular spectroscopy of LMC HII regions having classified stellar populations, thereby strongly constraining the ionization input parameters. Using photoionization models, we then evaluate the performance of nebular diagnostics of T_* and abundance. We introduce [Ne III]/H-beta as a nebular diagnostic of the ionizing stellar T_*. In contrast to the widely-used η-prime parameter, [Ne III]/H-beta has greater sensitivity to mid and early O-stars, and is robust to nebular morphology and the presence of shocks. We present a preliminary calibration of both T_* diagnostics for LMC metallicity. We also introduce S234 = ([S II] + [S III] + [S IV])/H-beta as a diagnostic of S abundance. S234 is much less sensitive to the nebular ionization parameter than is S23 or R23. The intensity of [S IV]10.5 microns is easily estimated from the optical and near-IR line ratios. We present calibrations of S23 and S234 that are reliable at metallicities Z < 0.5 Z_sol.

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arXiv1999

Calibration of Nebular Emission-Line Diagnostics: I. Stellar Effective Temperatures

We present a detailed comparison of optical H II region spectra to photoionization models based on modern stellar atmosphere models. We examine both spatially resolved and integrated emission-line spectra of the HII regions DEM L323, DEM L243, DEM L199, and DEM L301 in the Large Magellanic Cloud. The published spectral classifications of the dominant stars range from O7 to WN3, and morphologies range from Stromgren sphere to shell structure. Two of the objects include SNR contamination. The overall agreement with the predictions is generally within 0.2 dex for major diagnostic line ratios. An apparent pattern in the remaining discrepancies is that the predicted T_e is ~1000 K hotter than observed. (Abridged) Our analysis of the complex DEM L199 allows a nebular emission-line test of unprecedented detail for WR atmospheres. Surprisingly, we find no nebular He II 4686 emission, despite the fact that both of the dominant WN3 stars should be hot enough to fully ionize He I in their atmospheres. We confirm that the η-prime emission-line parameter is not as useful as hoped for determining the ionizing stellar effective temperature, T*. Both empirically and theoretically, we find that it is insensitive for T* >40 kK, and that it also varies spatially. The shock-contaminated objects show that η-prime will also yield a spuriously high T* in the presence of shocks. It is furthermore sensitive to shell morphology. We suggest [Ne III]/Hb as an additional probe of T*. Although it is abundance-dependent, [Ne III]/Hb has higher sensitivity to T*, is independent of morphology, and is insensitive to shocks in our objects. We attempt a first empirical calibration of these nebular diagnostics of T*.

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arXiv1998

The Nuclear Spectrum of M87

The nuclear spectrum of M87 covering the Ly_a-H_a wavelength range was obtained with the HST Faint Object Spectrograph (FOS) trough a 0.21 arcsec aperture. Contrary to some previous claims, a single power law (F(nu)~nu^(-a)) can not reproduce the observed continuum shape and at least a broken power law is require for a good fit (a = 1.75 and 1.41 shortward and longward of the break at ~4500 A). We detect a set of broad (FWHM ~ 400 km/s) absorption lines arising in the gas associated with M87. These are only lines from neutral and very low ionization species blueshifted by ~150 km/s relative to the M87 systemic velocity, indicating a net gas outflow and turbulence. The excitation sensitive emission line ratios suggest that shocks may be the dominant energy supplier. The nuclear source in M87 is significantly variable. From the FOS target acquisition data, we have established that the flux from the optical nucleus varies by a factor ~2 on time scales of ~2.5 months and by as much as 25% over 3 weeks, and remains unchanged (<2.5%) on time scales of ~1 day. These timescales limit the physical size of the emitting region to a few hundred gravitational radii. The variability, combined with other observed spectral properties, strongly suggest that M87 is intrinsically of BL Lac type but is viewed at an angle too large to reveal the classical BL Lac properties.

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arXiv1998

Ultraviolet Diagnostics for the Emission Line Gas in Active Galaxies

Optical diagnostic diagrams are frequently ambiguous as a test of the photoionization or fast shock models of the narrow line regions of active galaxies. Here, we present a set of UV line ratio diagrams which can discriminate between pure shock and photoionization modes of excitation, and to some extent, also discriminate shocks with ionized precursors from photoionization. These diagrams use relatively bright emission lines and reddening insensitive ratios and provide a practical observational test for separating the excitation mechanisms of the narrow line regions of active galaxies. The most useful diagrams are those involving the various ionization stages of Carbon, [OIII]5007/H-beta vs. CIV 1550/ HeII 1640 and the purely UV ratio pair CII] 2326 / CIII] 1909 vs. CIV 1550 / CIII]909. Temperature sensitive FUV lines CIII 977 and NIII 991 also provide good discriminants. The models are compared to observations of nearby AGN, and also to high redshift objects where the UV lines are shifted into the optical.

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arXiv1997

Induced Compton Scattering in Gigahertz Peak Spectrum Sources

We revisit the shocked shell model for the class of Active Galactic Nuclei known as Gigahertz Peak Spectrum sources, incorporating new observational data on the radiation brightness temperatures. We argue that in addition to free-free absorption, induced Compton scattering will also have an important effect in forming the ~GHz peak and in shaping the radio spectra that characterize these sources. Indeed, our arguments suggest that GPS sources may provide the first real evidence for the role of induced Compton scattering in extragalactic radio sources.

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arXiv1997

Are Seyfert Narrow Line Regions Powered by Radio Jets?

We argue that the narrow line regions of Seyfert galaxies are powered by the transport of energy and momentum by the radio-emitting jets and consequently that the ratio of the radio power to jet energy flux is much smaller than is usually assumed for radio galaxies. This can be partially attributed to the smaller ages ($\sim 10^6 yrs$) of Seyferts compared to radio galaxies but one also requires that either the magnetic energy density is more than an order of magnitude below the equipartition value, or more likely, that the internal energy densities of Seyfert jets are dominated by thermal plasma. If Seyfert jets are initially dominated by relativistic plasma, then an analysis of the data on jets in five Seyfert galaxies shows that all but one of these would have mildly relativistic jet velocities near 100 pc in order to power the respective narrow-line regions. However, observations of jet-cloud interactions in the NLR provide additional information on jet velocities and composition via the momentum budget. Our analysis of a jet-cloud interaction in NGC 1068, implies a shocked jet pressure much larger than the minimum pressure of the radio knot, a velocity $\sim 0.06 c$ and a jet temperature $\sim 10^9 K$ implying mildly relativistic electrons but thermal protons. The jet mass flux at this point $\sim 0.5 M_\odot yr^{-1}$, is an order of magnitude higher than the mass accretion rate into the black hole, strongly indicating entrainment. The initial jet mass flux $\sim 0.02 M_\odot yr^{-1}$, comparable to the mass accretion rate and is consistent with the densities inferred for accretion disc coronae from high energy observations, together with an initially mildly relativistic velocity and an initial jet radius of order 10 gravitational radii.

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