AGN and Starburst Galaxies

Active Galactic Nuclei

AGN are very luminous and compact objects presenting strong emission lines on top of a nearly flat SED(see Figure 1.1). Dramatic luminosity variations are often observed in X-rays indicating very high brightness temperatures confined into very small regions. In order to explain these facts a standard model built around a SMB accreting matter as a central source, has been put togheter in the last decades (see Rees et al. [1982], Antonucci [1993] for a review and the books by Robson [1996] and Peterson [1997]). This model succesfully accounts for the energy budget required by the observed luminosities. For instance, a $10^{ 8 }$$M_\odot$ black hole spherically accreting at a steady rate of 1$M_\odot$yr$^{-1}$ can easily produce $10^{ 12 }$$L_\odot$ if radiating at the Eddington limit. The accretion occurs on an equatorial disk around the black hole. If the inner radius of this disk is comparable to the Schwarzschild radius then thermal radiation from this region will peak at a wavelength of $\sim$100Å explaining the X-ray emission and the observed time scale of the luminosity variations. The lack of features in the SED can be explained by the superposition of Planck spectra associated to consecutively decreasing temperatures from the inner to the outer edge of the disk. The model also includes a region of surounding obscuring material or TORUS. Associated clouds responsable for the line emission are located in two well defined regions: The BLR is directly heated by the central source producing line emission with typical FWHM of $\sim$5000km$s^{-1}$. Beyond the BLR, the less dense and cooler NLR emits narrower lines with typical linedwidths of FWHM $\sim$500km$s^{-1}$. The model may also be able to explain the relativistc outflow which will form the jets observed in some sources.

Figure 1.1: Top: SED of the Seyfert 1 NGC 5548 (Data from NED). Bottom: Optical spectrum indicating the prominent broad and narrow emission lines. The FWHM of the broad components is $\sim$5900km$s^{-1}$, and the width of the narrow components is $\sim$400km$s^{-1}$. Note the prominent [O $_{\rm III}$] $\lambda 5007$ and [N$_{\rm II}$] $\lambda 6583$ lines used in optical classification schemes (see section 1.4).

Direct imaging of the central source is impossible with the available resolution however, the effects it produces on its suroundings can still be observed. One of the best pieces of evidence supporting the existence of SMBs are the observations of water maser dynamics in the nucleus of NGC4258 (see Figure 1.2). These observations imply an enclosed mass of 3.6$\times$$10^{ 7 }$$M_\odot$ within a region of less than 9$\times$$10^{ -3 }$ pc$^{3}$ [Greenhill et al., 1995] indicating the existence of a very massive and compact object.

Figure 1.2: Left: Position-velocity diagram for the water masers observed towards NGC4258. Squares indicate the observations and the line is the $1/\sqrt {r}$ signature of Keplerian rotation (From Moran et al. [1995]). Right: Proposed maser geometry including a central SMB (From Greenhill et al. [1995]).

Starburst Galaxies

Starburst galaxies emit the bulk of their radiation in the FIR part of the spectrum and are among the most luminous objects in the local universe. These galaxies exhibit strong emission lines resembling those observed towards HII regions and do not show evidence of an AGN as their primary energy source (see section 1.1). In contrast, the emission lines arise from massive star formation occuring in their nuclei [Terlevich et al., 1987] over a region of $\sim$1kpc in size. The starburst activity is assumed to be triggered by dinamical interaction with another galaxy. The resulting tidal forces will push the gas clouds togheter inducing their gravitational collapse and the consequent formation of stars. Once the starburst has been initiated, the more massive stars will evolve quickly and explode as supernovas compressing the surounding gas inducing more star formation until most of the available gas has been transformed into stars or blown away by the explosions. This mechanism explains the observed FIR emission as dust reradiation of starlight. It also explains the radio emission as a combination of free-free emission, arising from the HII regions around young stars, and synchrotron emission from relativistic electrons generated in supernova events. Furthermore, it offers an explanation for the well known FIR-radio correlation (see Section 1.3) and for the observed conical outflows or superwinds [Heckman, 2003].

Figure: Top: SED of the nearby starburst (LINER!!) galaxy ARP 220. Note that most of the energy is radiated in the FIR (Data from NED). Bottom: Optical spectrum showing the HII like emission lines. Compare to those shown in Figure 1.2

Lying between Starburst galaxies and the weaker AGNs are the LINER galaxies. These galaxies present a scaled down version of the emission lines observed in AGN. Being a common phenomenon, it is important to discover whether is due to the presence of a mini-AGN or some other mechanism. LINERs are perhaps transition objects between two different types of activity. It now seems certain that most of the galaxies contain black holes in their centres, consequently the existence of a link between AGN and starburst activity seems plausible [Scoville, 2003].

The FIR-Radio correlation

A correlation between the $\lambda$10 $\mu$m and $\lambda$21 cm luminosities associated with Seyfert nuclei was discovered in the seventies (Rieke & Low [1972], van der Kruit [1973]). At first both, the infrared and radio emissions, were thought to be of synchrotron origin. Later, it was proposed that the infrared was actually thermal reradiation from dusty HII regions, while the radio was dominated by synchrotron emission from SNR originated by the same population of stars ionizing the HII regions [Harwit & Pacini, 1975]. The infrared to radio flux ratio was found to be correlated with the classification of luminous nuclei as starbursts or AGN according to their radio morphologies [Condon et al., 1982]. The advent of the large IRAS survey in addition to optical classification using the observed line ratios [Veilleux & Osterbrock, 1987] contributed to confirm and quantify the proportionality between infrared and radio emission. The cause of this effect can be illustrated using a simplified model. Let $L_{\rm T}$, $L_{\rm NT}$ and $L_{\rm FIR}$ be the total radio thermal, radio NT and infrared luminosities respectively. In addition, let $\mathscr{X}$ be the SFR of stars having masses $>5$$M_\odot$. The thermal (free-free) radio luminosity estimated from stellar models is:

$$\left(\frac{L_{\rm T}}{{\rm WHz^{-1}}}\right) \sim 5.5\times\hbox{$10^{ 20 }$}\left(\frac{\nu}{\rm GHz}\right)^{-0.1} \hbox{$\mathscr{X}$}$$ (1.1)

The NT synchrotron component produced by relativistic electrons originated by a sustained super nova rate $\nu_{\rm SN}$ is given by the empirical relation [Condon & Yin, 1990]:

$$\left(\frac{L_{\rm NT}}{\rm WHz^{-1}}\right)\sim 13\times\hbox{$1... ...\frac{\nu}{\rm GHz}\right)^{-0.8} \left(\frac{\nu_{\rm SN}}{\rm yr^{-1}}\right)$$ (1.2)

If the IMF $\psi(M)\propto M^{-2.5}$ is time independent and assuming that all stars more massive than $M_{\rm SN}>8$$M_\odot$ will become supernovas, then the radio SNR and $\mathscr{X}$are linearly proportional. Hence, $L_{\rm NT}$ and consequently $L_{\rm T}+L_{\rm NT}$ are linearly proportional to $\mathscr{X}$.

Figure 1.4: FIR-Radio correlation for a 60$\mu$ flux limited sample of 258 galaxies without known AGN. (From Condon et al. [1991])

Regions of massive star fomration are generally highly obscured by dust. Thus, about 2/3 of the luminosity produced by stars emerge in the FIR band between 40 and 120 $\mu$m [Helou et al., 1988]. if $\tau(M)$ is the lifetime of a star with mass $M$ and $L(M)$ its averge bolometric luminosity, then the total FIR luminosity $L_{\rm FIR}$ can be expressed as:

$$L_{\rm FIR}=\frac{2}{3}\int\psi(M)L(M)\tau(M)dM$$ (1.3)

considering that $L(M)\tau(M)\sim\hbox{$10^{ 9.6 }$}(M/\hbox{$M_\odot$})^{3/2}\hbox{$L_\odot$}$ [Maeder, 1987] the previous expression becomes:

$$\left(\frac{L_{\rm FIR}}{\hbox{$L_\odot$}}\right) \sim 1.1\times\hbox{$10^{ 10 }$}\hbox{$\mathscr{X}$}$$ (1.4)

Both $L_{\rm T}+L_{\rm NT}$ and $L_{\rm FIR}$ are linearly proprtional to $\mathscr{X}$, therefore this model implies a linear FIR/radio correlation with a logarithmic slope of $q_{\rm model}\sim 2.4$. Figure 1.4 shows a plot of the observed 1.49 GHz flux densitiy [Condon et al., 1987] for galaxies in the revised IRAS BGS [Soifer et al., 1989] versus the total far infrared luminosity derived via [Helou et al., 1988]:

$$\left(\frac{\rm FIR}{\rm W m^{-2}}\right)=1.26\hbox{$\times$}\hbox{$10^{ -14 }$} \left(\frac{2.58S_{60}+S_{100}}{\rm Jy}\right)$$ (1.5)

where $S_{60}$ and $S_{100}$ are the IRAS 60 and 100 $\mu$m observed flux densities. Least squares fitting results in a line with a slope of $\langle q_{\rm obs}\rangle\sim 2.4$ which is in perfect agreement with the previously derived $q_{\rm model}$. Despite its simplicity and accuracy, the described model is only a simplified version of reality and there are still many loose ends in the understanding of the FIR-Radio correlation. Nowdays, there is an extenseive research modelling the SED of starburst galaxies in order to achieve a grasp of its nature (see Dopita et al. [2005] and references therein). Of special interest is the work by Bressan et al. [2002] in which the origin of the FIR-Radio correlation is revisited using recent models of star forming galaxies [Silva et al., 1998] focusing on the case of obscured starbursts. They find that since the NT radio emission is delayed respect to the FIR, deviations from the correlation are expected at the early stages of a starburst, when the radio thermal emission dominates, and in the post-starburst phase, when the bulk of the NT originates from less massive stars.

Starburst galaxies tightly follow the FIR-Radio correlation for at least three orders of magnitude in FIR luminosities from dust rich dwarfs to ULIRGs. The assumption that the correlation holds at all scales allows the use of radio interferometry observations to probe the FIR emission at high resolutions . Finally, the correlation is known to apply at cosmological distances [Appleton et al., 2004] opening the possibility to study the SFR as a function of redshift which is a fundamental issue in understanding galaxy formation and evolution [Madau et al., 1996].

Emission line diagnostics

Galaxies are classified as AGN or Starbursts using the relative strength of their emission lines as indicators of the underlaying excitation mechanism [Baldwin et al., 1981]. If the excitation is due to photoionization by O and B stars then the galaxy is likely to have an HII or Starburst nucleus. In contrast, the presence of lines excited by a power-law continuum source is the clear signature of Seyferts.

The choice of the intensity ratios on which base the clasification is restricted by several issues. Veilleux & Osterbrock [1987] put forward a semi empirical method based on the reddening-insensitive line ratios [OIII] $\lambda 5007$/H$\beta$ , [NII] $\lambda 6583$/H$\alpha$ and [OI] $\lambda 6300$/H$\alpha$. Using numerical models of starbursts, Kewley et al. [2001] developed a theoretical classification scheme based on the same ratios. An empirical revision to this method supported by a massive sample of galaxies from the SDSS is proposed by Kauffmann et al. [2003]: a galaxy is classified as AGN if

$$\log\left(\frac{\rm [\hbox{O\small {III}}]}{\rm\hbox{H\small {$\b... ...\hbox{N\small {II}}]}{\rm\hbox{H\small {$\alpha$}}}\right)-0.05\right]^{-1}+1.3$$ (1.6)

otherwise it is classified as Starburst. Aditionally, for [NII]/ $\hbox{H\small {$\alpha$}}>0.6$ Seyfert and LINER galaxies are often defined to have [OIII]/ $\hbox{H\small {$\beta$}}>3$ and $<3$ respectively (see Figure 1.5).

Figure 1.5: BPT diagram for 55757 objects from the Sloan Digital Sky Survey (SDSS). The dotted curve is the demarcation between starburst and AGN defined by Kewley et al. [2001]. The dashed curve is a revised empirical demarcation defined in Kauffmann et al. [2003](From Kauffmann et al. [2003])

Circumnuclear OH megamasers

OH megamasers are associated with LIRG showing evidence of starburst activity and a high S$_{60\mu}$/S$_{100\mu}$ ratio. The analysis of the strength of the different maser transitions in addition to the observation of OH absorption in the infrared suggest that these masers are radiatively pumped by infrared photons. This may explain the $L_{\rm OH}\propto L_{\rm FIR}^{2}$ [Martin et al., 1988] proportionality between the OH and FIR luminosities (see Figure 1.6). High resolution observations show a spatial coincidence of the maser emission and radio continuum at $\sim$100pc scales indicating that both are originated within the same region of star formation activity. Hence, the study of maser emission allows to probe the physics of the dense concentrations of gas in the highly obscured nuclear environments of starbursts .

Figure 1.6: $L_{\rm OH}$ vs $L_{\rm FIR}$. The locations of well studied sources are indicated in the figure. Note that in this plot the FIR is calculated using a different formula than equation 1.5. (Adapted from Martin et al. [1988])