We have analyzedHSTbroad-band (F606W) images of a sample of 91 Seyfert 1 galaxies to study their nuclear morphology. We employed structure maps to enhance fine dust structure in the nuclear regions. Accompanying images in Figure B.1 provide a good repository to study nuclear dust structures on scales <
∼1 kpc in Seyfert 1 galaxies.
Our sample contains 12 NLS1 hosts and 75 BLS1 host galaxies. This allowed us to compare the nuclear morphology of NLS1 host galaxies to the BLS1 host galaxies. In Crenshaw et al. (2003a) (CKG03), we had noticed that NLS1s showed more large-scale bars as compared to BLS1s. We have revised the main morphological classification from CKG03 with the aid of structure maps, and conclude that 76% of NLS1 host galaxies are barred as compared to 40% for BLS1 hosts. Overall, almost half (47%) of the spiral Seyfert 1 galaxies from CKG03 are barred.
Here, we find that nuclear dust spirals are the most common kind of morphology in the central kiloparsec of Seyfert 1 galaxies. We find that 80% of the NLS1s have grand-design type nuclear spirals as compared to 32% for BLS1s. Further we see that 69% of barred galaxies have grand-design morphology as compared to 7% for unbarred ones. This is in agreement with the trend noted by Martini et al. (2003b). This is also indicative of the
fact that these nuclear spirals are being driven and maintained by large-scale stellar bars. We also find that most BLS1 host galaxies have multi-arm, flocculent or chaotic nuclear dust spirals.
We find that 42% of the NLS1s have nuclear star formation in the form of nuclear rings as compared to 11% of BLS1s. A similar distribution is seen when one compares barred galaxies with the unbarred ones. This again indicates that the large-scale bar is the main driver of these differences. This strengthens the idea that large-scale bars are important to support high fueling rates in NLS1s. Thus our results here support the fueling scenario for barred vs. unbarred galaxies via nuclear dust spirals. Recent simulations of gas and dust inflow in barred galaxies give support to the idea that the galactic disk gas loses its angular momentum via nuclear spiral shocks which are being driven by the main bar potential within the central kiloparsec. Further, numerical simulations have shown that when sufficient mass has been accreted into the inner bulge via the primary stellar bar, that bar will be disrupted on time scales ∼ 108 Myr. It has also been claimed that the
SMBH’s in NLS1s are under-massive for their bulges (Mathur 2000). Thus, a likely evolutionary scenario is that NLS1s are relatively young AGN accreting at high rates, and that they eventually become BLS1s as they accrete enough matter to disrupt the bar and increase the mass of the SMBH.
Figure 3.5: Structure Maps of Seyfert 1s: NGC 6212, a flocculent dust spiral (Category: FL).
Figure 3.5: Structure Maps of Seyfert 1s: TOL 2327-027, a strongly barred galaxy with a tightly-wound nuclear spiral (Category: GD/TW).
Figure 3.5: Structure Maps of Seyfert 1s: NGC 7469 hosting a multi-arm nuclear spiral with a starburst ring (Category: FL + NR).
Figure 3.6: Structure Maps of Seyfert 1s: shown above is IR 1249-131; it has a prominent nuclear ring (Category: NR).
Figure 3.6: Structure Maps of Seyfert 1s: shown above is Mrk 1126 a grand design nuclear spiral with a nuclear ring (Category: GD + NR).
Figure 3.6: Structure Maps of Seyfert 1s: Mrk 915 shows a large-scale dust lane (Category: DL) passing through the nucleus. An emission bicone is clearly visible in this image.
Figure 3.6: Structure Maps of Seyfert 1s: Mrk 10 does not show much dust structure in the nuclear regions (Category: ND).
Figure 3.7: Frequency of Nuclear Dust Structures: Figure shows bar plots for various morphological classes shown in Table 3.3. See § 3.4.1 in text for description of morphology classes. Here we compare the NLS1s against BLS1s in their primary nuclear dust morphology. As can be seen dust spirals (DS) is the most common type of morphology.
Figure 3.8: Frequency of nuclear dust spirals: Here we compare the NLS1s with BLS1s in the nature of their nuclear dust spirals (secondary nuclear morphology of the DS category in Figure 3.7). NLS1s show mainly GD-type spirals, while BLS1s show mainly FL-type spirals, although BLS1s are more than twice in number than NLS1s.
Figure 3.9: Primary Nuclear Dust Structure: barred vs. unbarred spirals. Barred galaxies have a slightly higher fraction of grand-design dust spirals.
Figure 3.10: Frequency of nuclear dust spirals: barred vs. unbarred spirals. This graph clearly shows that the grand-design spiral is driven mainly by large-scale bars, while flocculent spirals exist in unbarred galaxies.
Mid-IR Spectroscopy of Seyfert
Galaxies
As we saw in §1.2.3, we will concentrate on mid-infrared spectroscopy of Seyfert galaxies in this part of the dissertation. In this chapter, we will look at early (1960s–1990s) infrared studies of Seyfert galaxies. We will pay special attention to the spectroscopic results from the Infrared Space Observatory(ISO). In Chapter 5, we will describe theSpitzer Space Telescope
and the Infrared Spectrograph (IRS). In the same chapter we will also cover data reduction procedures for mid-IR spectra from IRS. In Chapter 6, we will look at mid-IR spectra of Seyfert 1.8/1.9s in comparison to spectra of Seyfert 1s and Seyfert 2s. We will describe the morphological characteristics of the spectra. In Chapter 7, we will look at the spectral diagnostics derived from our sample and the archival data.
4.1
Early Infrared Studies
Oke & Sargent (1968) showed that Seyfert galaxies have a power-law spec- trum similar to quasars. It was also shown (Neugebauer et al. 1976) that a larger fraction of the stellar radiation contributes to the spectrum in Seyfert
2s than in Seyfert 1s. Rieke & Low (1972) suggested that the bulk of the bolometric luminosity is contained in the infrared. NGC 1068 was recog- nized as a strong infrared source as early as 1966. Pacholczyk & Wisniewski (1967) studied the broad-band near-IR (up-to 0.22 µm) characteristics for NGC 1068. Further, far-infrared observations of NGC 1068 by Telesco et al. (1976) showed that the spectrum of NGC 1068 shows a turnover (see Fig- ure 4.1) around 100 µm.
Figure 4.1: Far-IR Turnover in NGC 1068 from Telesco et al. (1976) Initially, there was uncertainty as to what generated the infrared emission. Is it generated by non-thermal processes like synchrotron radiation, or is it re-radiation from dust in the galaxy? Angel et al. (1976) found that the continuum in NGC 1068 was circularly polarized, thus favoring the idea that scattering and re-radiation from dust is involved in generating the observed strong infrared continuum. This also implied that Seyfert 2 nuclei like NGC 1068 are embedded in envelopes of dust. Further, it was shown that Seyfert galaxies in general are not strong radio sources (Kellermann 1972) and thus should have a weak contribution from the synchrotron process. A number of early infrared observations of Seyfert galaxies also noted the presence of the
10µm silicate absorption feature (e.g., in Mrk 231, Allen 1976; Gillett et al. 1975; Rieke & Low 1975), indicating that the nuclei of Seyfert galaxies had large quantities of dust. A detailed review of early infrared work on AGN is given in Rieke & Lebofsky (1979).