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Ongoing Research of Tapasi Ghosh and Future Directions
Here, I summarize my current research pro jects and indicate future directions that some of these would take in relation to the planned or existing radio telescopes in Australia. (1) Atomic and Molecular Gas in AGN Hosts: Interactions between AGNs and their hosts play an important role in determining many of the physical properties observed for AGNs. In recent years, observations of neutral hydrogen ( 21cm), OH and water-maser transitions, and rotational transitions of CO molecules in AGN hosts have provided important insights into these enigmatic ob jects. Together with a number collaborators (both within and outside NAIC), I am involved in a number of observational pro jects, looking for HI and OH in emission & absorption against radio-loud, and radio-weak AGNs of different classification. These include GPS/CSS sources, Seyfert I/II, Startburst galaxies & ULIRGs, and Type-2 AGNs. I am also involved in a search for water maser emission from high-z AGNs. Highlights of these studies are: · 3C258: The first detection of a multi-component HI absorption line within the CSS galaxy 3C258, revealing a complex interstellar medium at a redshift of 0.165. A dense ISM has often been invoked to explain the relatively small dimensions of these powerful radio sources. · PKS 1413+135: The two 18-cm OH satellite lines in the BLLac source, PKS1413+135 at z 0.247 were first detected in 2002 using the WSRT. The 1720-MHz line is seen in emission while the 1612-MHz line is in absorption. The profiles of the two satellite lines are conjugate, implying that they arise in the same gas. This allows us to test for any changes in the values of fundamental constants, without being affected by systematic uncertainties arising from relative motions between the gas clouds in which the different lines may arise. Our data constrain changes in the fine-structure constant, / = 0.6 ± 1.0 в 10-5 , over the range 0 < z < 0.247. · ARP220: We have found completely different behaviour to that and Haschick, 1987) for the OH satellite lines in the prototype These observations were made as a summer-student observing Telescope. Detailed global VLBI mapping of the ob ject in the 17 currently underway. previously published (Baan starburst galaxy, Arp 220. pro ject using the Arecibo 20- and 1612-MHz lines are

· A New OH-megamaser in the ULIRG IRAS 23327+2913: In a survey of 85 galaxies from the 2 Jy IRAS-NVSS Sample with far-IR luminosities greater than 109 L , this new OH megamaser was detected at Arecibo. This ULIRG is a system of two galaxies separated by about 20 kpc and at the early stages of interaction. Details of the line characteristics show that IRAS 23327+2913 does not fit into the commonly accepted scenario for ULIRGS of a merger between two disk galaxies, where the two progenitors of the merger are strongly disturbed during the interacting phase, and the final product of the merger resembles an elliptical galaxy. · Search for Water-Vap our Masers in High-redshift AGNs: Strong 22-GHz water-vapor masers have been previously detected in about 20 nearby AGNs. These "megamaser" sources are mostly associated with Seyfert 2 and LINER galaxies whose active nuclei are hidden by large column densities of optically obscuring, X-ray absorbing gas. Detailed studies of H2 O masers with VLBI imaging and spectral-line monitoring have become important tools for probing dense gas near the central engines of these sources, investigating the properties of their accretion disks and measuring the masses of their (supermassive) black holes. Such


studies have also been employed to estimate geometrical distances completely independent of the rungs of the extragalactic distance ladder. However, to date most of the detections of extragalactic H2 O masers have been in relatively nearby ob jects (with systemic velocities less than 7500 k ms-1 ), and (hence) in low-luminosity AGNs. We have undertaken a H2 O-maser search in the redshift ranges 1.22 ­ 1.78 and 2.70 ­ 4.55 using the X- and C-band receivers of the 305-m Arecibo telescope in 35 AGNs with either soft-X-ray absorption or other indirect evidence of an obscuring torus such as compact steep spectrum double radio structures. Future Direction: So far, my studies of the host galaxies of AGN via HI absorption had been limited to the fairly nearby universe due to the constraints imposed by the severe RFI environment at the telescope sites available to me. The RFI-quiet site of the xNTD and MWA makes these the most attractive instruments for extending these studies to the epochs dominated by violent galaxy interactions, when the AGN phenomenon was common-place. (2) The fate of Co oling Flow Gas: An Arecib o 21-cm Absorption Survey Recent Chandra and XMM-Newton observations (Peterson et al., 2003, ApJ, 590, 207) have cast serious doubts over the classic cooling flow (CF) models for rich, X-ray bright clusters of galaxies. Earlier generation X-ray satellites (EXOSAT and the Einstein Observatory) had established that X-ray surface brightness distributions in many X-ray bright clusters are centrally peaked, with temperature profiles that decrease (typically by an order of magnitude) towards the cluster center (e.g. Jones & Forman, 1984, ApJ 276, 38). Traditionally, these observations were explained by assuming that the hot intra-cluster medium loses most of its energy via thermal bremsstrahlung at X-rays. In order to maintain hydrostatic equilibrium in the presence of the gravitational potential of the cluster, the cooled gas then flows towards the center, providing the necessary pressure to support the weight of the overlaying gas (Fabian, 1994, ARAA, 32, 277 and references therein). However, direct detection of the cooling flow (CF) gas has only been partially successful. The detection of HI absorption against the radio continuum emission from galaxy clusters has been successful only for a a small number clusters, (e.g. Abell 2125, Dwarakanath & Owen 2001; Abell 780, Dwarakanath et al. 1994, ApJ, 432, 469; the Perseus cluster, Crane et al, 1982, IAUS, 97, 307). While this HI may represent cold gas in a CF, there is also evidence for galaxy cannibalism in the central regions of clusters (e.g. Perseus; Momjian et al. 2003, ApJ, 566, 195). Both may contribute to the formation of a central cD galaxy. Clearly, each new case is the sub ject of interest, especially when in a CFC. Using the Arecibo 305-m telescope, we have recently detected strong HI 21-cm absorption against the complex continuum emission from the rich galaxy cluster, Abell 2390. This cluster contains one of the strongest known cooling flows. Given this discovery we are now searching for cold neutral hydrogen in absorption against strong radio emitters within 95 rich, X-ray bright clusters of galaxies. The results will be used to constrain the various cooling flow models put forth in recent years, especially following recent X-ray observations which failed to detect gradually cooling gas towards cluster centers in many cD-dominated clusters. Future Direction: Follow up mapping with connected-element or VLBI interferometers (depending on the resolution requirement for a particular cluster) will be pursued. (3) Damp ed Lyman- Absorb ers (DLAs) Even at redshifts as low as z 0.2, it is a severe with current instrumentation. As a result, abso (viz. QSOs or radio galaxies) provide the sole the high-redshift universe. The large absorption challenge to detect neutral atomic gas in emission rption spectra against bright background sources source of information on neutral atomic gas in cross section of the Lyman- transition, and the


dominance of hydrogen in baryonic material, makes the Lyman- line one of the easiest to detect in QSO spectra. In fact, absorption lines arising from gas with column densities as low as 1013 cm-2 can be routinely detected optically. Although these lines (the Lyman- "forest") are by far the most numerous, they contain only a small fraction of the total neutral gas at high redshifts. Most of the neutral gas is contained in relatively rare, high column density (NH I > 1020 cm-2 ) systems, the so-called DLAs. As the ma jor repository of neutral gas at high z, these systems are natural candidates for being the precursors of today's galaxies. However, despite their importance in the context of galaxy evolution, the nature of high redshift DLAs is presently quite controversial, with models ranging from large rapidly rotating disks (e.g. Prochaska & Wolfe 1997, ApJ, 487, 73), through merging sub-galactic blobs (e.g. Haehnelt et al. 1998, ApJ, 495, 64), to outflows from dwarf galaxies (Schaye, 2000, ApJ, 559, L1). A successful model for DLAs needs to reproduce not only the observed cosmological evolution of gas-fraction (g (z )) and column density distribution, but must also describe the observed physical conditions in the absorbing gas. Conversely, knowledge of the physical conditions is useful in modeling the further temporal evolution of DLAs. HI 21-cm observations (possible only for those DLAs for which the background quasar is radio loud) are particularly useful because they allow the derivation of the average spin temperature (Ts ) of the absorbing gas. Two pro jects in which I am involved in this area are: · HI in High-z DLAs : Using the low-frequency capability of the Arecibo 305-m telescope, we are searching for HI absorption in a high redshift (z 3.38) DLA toward PKS 0201+113, in and other candidate DLAs at z 2.4 seen toward two radio loud quasars. Our observations are sensitive to 104 K gas in the z = 3.38 absorber providing the first possibility of detecting the warm neutral medium at high redshifts. For the two z 2.4 DLAs, the observations, when complete, will be sensitive to spin temperatures of 1000 K, a value typical of most damped systems. The observations will test an emerging trend that high-z DLAs predominantly contain the warm phase of HI, while both cold and warm HI are seen in low-z systems. The search also included OH absorption in two lower redshift absorbers at z 0.24 to study the chemical evolution of DLAs. · An unbiased search for low-redshift DLAs at radio wavelengths: We have taken 400-MHz wide spectra towards 200 compact radio sources looking for DLAs between 0 < z < 0.3. This radio search is unaffected by dust obscuration. The data acquisition and analysis process includes a rigorous RFI-detection/flagging/excision method to identify the real signature of DLA systems in these spectra. Five, yet-to-be-confirmed candidates have been found so far. When complete, this survey will contribute both to DLA statistics for 0 < z < 0.3, and to our knowledge of the HI mass fraction at z 0. Future Direction: Conducting similar surveys at all (available) frequencies below 21-cm is a natural extension of these pro jects. Evolution of neutral hydrogen through cosmic epochs is one of the key areas of SKA science drivers. (4) VLBI survey of faint FIRST sources: A VLBI visibility survey of around 1000 faint (down to 1.4-GHz flux density of 1 mJy) sources selected from the VLA-FIRST survey is being conducted making use of the ultra-high sensitivity achievable with the Arecibo and Effelsberg telescopes at 21 cm using VLBA4/MK4 recording enabling 512 Mbps data rate. The selected sources lie within the area of the Sloan Digital Sky Survey, and it is expected that 50% of FIRST sources will be identified with SDSS ob jects down to m(V) = 24 mag. Thus, redshifts for many ob jects in the sample are now becoming available.


Amongst other uses, our survey will help identify the fraction of AGNs (as opposed to starburst galaxies) in the faint source population, and the number of faint targets existing for such future instruments as the high-resolution options for the SKA. Future Direction: As a technique, the Garrett et al. (astro-ph/0205456) opens radio sky with milliarcsec resolution. I am of similar techniques in the planned SKA recent development of the wide-field VLBI mapping by up exciting possibilities for surveys of the extragalctic extremely interested in working towards the application precursor telescopes.

(5) A Continuum Survey with ALFA: I am an active member of the GALFA Continuum Transit Survey (GALFACTS) consortium which aims to make a full-Stokes, "all-sky" continuum survey using the 7-beam Arecibo L-band Feed Array (ALFA). The survey is expected to begin in the summer of 2007. One of the ma jor goals of this survey is to understand the 3-d microstructure of the magneto-ionic interstellar medium of our Galaxy via rotation-measure tomography. My own area of interest in this survey is largely in the extragalactic opportunities that it opens up. · Extragalactic radio sources: Covering the declination range, 37.8 > > -1.2 , the GALFACTS survey will span a celestial area of 4.0 Sr, with 300-MHz bandwidth, and with an rms noise of about 100 µJy/beam (on cold sky). However, for Stokes-I this is well below the rms confusion level due to the point source background, (confusion) 2 mJy/beam, implying weakest believable detections at S(min) 10 mJy/beam. Standard 1.4-GHz source counts (e.g. Condon, 1984, ApJ 287, 461) lead us to expect that 230,000 discrete extragalactic sources with flux densities greater than 10 mJ y /beam will be detected by GALFACTS. At this flux density level, the extragalactic source population will still be dominated by powerful AGNs. However, about a quarter of the sources are expected to be normal and starburst galaxies, low luminosity AGNs (such as LINERs), and Seyfert galaxies. Most of these will have been detected earlier by NVSS, although GALFACTS, with its 3-arcmin beam, will be more sensitive to low surface brightness features. Hence, we hope to detect extended halos of nearby galaxies and clusters > of galaxies, as well as identifying a number of giant radio sources (size 1M pc) that could have been considered to be a pair of unconnected sources in NVSS. · Variability: With the planned observing strategy, each point in the sky will be visited twice, with the interval between visits being uniformly distributed between 1 and 28 days. Thus, given the time interval uniformity, and the large number of (extragalactic) sources that will be measured, extremely interesting statistics on the variability properties of sources at different timescales will be obtained. · Linear p olarization of extragalctic sources: The confusion level in linear polarization will be well below the total intensity value (in fact about two orders of magnitude less, 20 µJ y /beam). Hence, we can certainly make a very sensitive, unconfused survey of the polarization properties of radio sources. For unresolved sources, polarization variability is expected to be detected on both sub-monthly and 10 year (in comparison with the NVSS) time scales, the extremely active blazar (BL Lac and quasar) population being known to show both percentage polarization and position-angle variations on such time scales. GALFACTS will enhance the number of individual sources known to show such characteristics, and allow detailed, unbiased statistical studies of this phenomenon. · Rotation Measure survey of extragalctic sources: The sensitivity of Arecibo, combined with the multiple channels across the 300-MHz bandwidth of ALFA, will permit a complete


rotation measure (RM) survey of sources with detected linear polarization. To emphasize the power of having a 300-MHz bandwidth available for polarization measurements, the change of position angle across the band is =1.22*RM deg , (or one full 180 deg turn for an RM of 150 rad/m2 ). At low latitudes, a combination of GALFACTS RM estimates for SNRs and extragalactic sources seen through the galactic plane with those measured for pulsars will allow modeling of the magnetic field distribution through the whole depth of the galactic disk (center and anticenter quadrants) visible from Arecibo. · Circular p olarization? We will also search the Stokes-V channel for evidence of significatly circularly polarized sources such as pulsars, and flares stars. This serendipitous search for unexpected circularly polarized sources will be a first. Future Direction: The enormous processing needs for the GALFACTS survey will benefit greatly from the resources that might be available via the supercomputers of the Australian Institutes. In addition, the pro ject offers great scope for the inclusion of graduate students.