arXiv subjects

A. Alonso-Herrero

Publications and source records attributed to A. Alonso-Herrero.

207 records · Page 12Linked to original sources

arXiv2000

The Multitude of Unresolved Continuum Sources at 1.6 microns in Hubble Space Telescope images of Seyfert Galaxies

We examine 112 Seyfert galaxies observed by the Hubble Space Telescope (HST) at 1.6 microns. We find that ~50% of the Seyfert 2.0 galaxies which are part of the Revised Shapeley-Ames (RSA) Catalog or the CfA redshift sample contain unresolved continuum sources at 1.6 microns. All but a couple of the Seyfert 1.0-1.9 galaxies display unresolved continuum sources. The unresolved sources have fluxes of order a mJy, near-infrared luminosities of order 10^41 erg/s and absolute magnitudes M_H ~-16. Comparison non-Seyfert galaxies from the RSA Catalog display significantly fewer (~20%), somewhat lower luminosity nuclear sources, which could be due to compact star clusters. We find that the luminosities of the unresolved Seyfert 1.0-1.9 sources at 1.6 microns are correlated with [OIII] 5007A and hard X-ray luminosities, implying that these sources are non-stellar. Assuming a spectral energy distribution similar to that of a Seyfert 2 galaxy, we estimate that a few percent of local spiral galaxies contain black holes emitting as Seyferts at a moderate fraction, 10^-1 to 10^-4, of their Eddington luminosities. With increasing Seyfert type the fraction of unresolved sources detected at 1.6 microns and the ratio of 1.6 microns to [OIII] fluxes tend to decrease. These trends are consistent with the unification model for Seyfert 1 and 2 galaxies.

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arXiv2000

Star formation properties of UCM galaxies

We present new near-infrared J and K imaging data for 67 galaxies from the Universidad Complutense de Madrid survey used to determine the SFR density of the local universe by Gallego et al. (1995). This is a sample of local star-forming galaxies with redshift lower than 0.045, and they constitute a representative subsample of the galaxies in the complete UCM survey. From the new data, complemented with our own Gunn-r images and long-slit optical spectroscopy, we have measured integrated K-band luminosities, r-J and J-K colours, and Halpha luminosities and equivalent widths. Using a maximum likelihood estimator and a complete set of evolutionary synthesis models, these observations have allowed us to estimate the strength of the current (or most recent) burst of star formation, its age, the star-formation rate and the total stellar mass of the galaxies. An average galaxy in the sample has a stellar mass of 5E10 Msun and is undergoing (or recently completed) a burst of star formation involving about 2 per cent of its total stellar mass. We have identified two separate classes of star-forming galaxies in the UCM sample: low luminosity, high excitation galaxies (HII-like) and relatively luminous spirals galaxies (starburst disk-like). The former show higher specific star formation rates (SFR per unit mass) and burst strengths, and lower stellar masses than the latter. With regard to their specific star formation rates, the UCM galaxies are intermediate objects between normal quiescent spirals and the most extreme HII galaxies.

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arXiv2000

The Variability of Seyfert 1.8 and 1.9 Galaxies at 1.6 microns

We present a study of Seyfert 1.5-2.0 galaxies observed at two epochs with the Hubble Space Telescope (HST) at 1.6 microns. We find that unresolved nuclear emission from 9 of 14 nuclei varies at the level of 10-40% on timescales of 0.7-14 months, depending upon the galaxy. A control sample of Seyfert galaxies lacking unresolved sources and galaxies lacking Seyfert nuclei show less than 3% instrumental variation in equivalent aperture measurements. This proves that the unresolved sources are non-stellar and associated with the central pc of active galactic nuclei. Unresolved sources in Seyfert 1.8 and 1.9 galaxies are not usually detected in HST optical surveys, however high angular resolution infrared observations will provide a way to measure time delays in these galaxies.

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arXiv1999

Extreme Star Formation in the Interacting Galaxy Arp299 (IC694 + NGC3690)

We present a comprehensive study of the star-formation properties of the infrared luminous galaxy Arp299 (IC694 + NGC3690). The observations include HST/NICMOS imaging and MMT optical and near-infrared spectroscopy together with HST archival data. We correct the galaxy parameters for extinction and use the results as boundary conditions for evolutionary starburst models. These models and other arguments show that Arp 299 has been going through a broad variety of interaction-induced star formation for the last ~15 Myr. In addition to nuclear starbursts in the two colliding galaxies, there are nearby luminous star forming regions that may be the result of molecular clouds breaking up and starting to form massive stars as they approach the nuclear potential. Two regions near the interaction region have very recently formed massive stars (~4 Myr old). One of these regions in particular appears to have undergone a starburst of very short duration, in contrast with the 5-10 Myr durations that are typical of nuclear events. These regions will probably form gravitationally bound new dwarf galaxies, although they are close enough to the more massive original galaxies that they will probably eventually be subsumed into them. In addition, we find 19 HII regions at least as energetic as 30 Doradus, and 21 older star clusters that likely are the products of similar HII regions formed in the past. This population of supergiant HII regions is unprecedented in normal galaxies and emphasizes that the effects of the interaction propagate through the entire pair of galaxies.

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arXiv1999

Discovery of a nuclear gas bar feeding the active nucleus in Circinus

We report the discovery of gas inflow motions towards the active nucleus of the Circinus galaxy caused by the non-axisymmetric potential of a nuclear gas bar. Evidence for dust associated with the bar comes from the HST/NICMOS H-K color map, whereas the streaming motions along the gas bar are seen in the velocity field of the H2 S(1)(1-0) emission line. The gas bar is about 100 pc long with a visual extinction in excess of 10 mag. Indication for the gaseous nature of this bar comes from the lack of a stellar counterpart even in the K band where the extinction is greatly reduced. We also use the NICMOS emission line images (Pa-alpha, [SiVI], and [FeII]) to study the innermost region of the ionization cones and the nuclear star forming activity. We discuss the possible relationship of these components with the gaseous bar.

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arXiv1999

Searching for X-ray Luminous Starburst Galaxies

The existence or otherwise of X-ray luminous star-forming galaxies has been an open question since the era of the Einstein satellite. Various authors have claimed the discovery of X-ray luminous star-forming galaxies but in many cases more careful spectroscopic studies of these objects have shown that many of them are in fact obscured AGN. In order to investigate the possibility that such a class of galaxies do exist, we have carried out a cross-correlation between optical and IRAS samples of galaxies which are known to contain large numbers of star-forming galaxies and catalogs of sources detected in X-ray surveys. The selection criteria for the optical follow-up observations was based on their X-ray and infrared (IRAS) colors and their X-ray luminosities. We note that this sample is by no means complete or uniformly selected and hence cannot be used for statistical studies, but nevertheless confirmation of the existence of such a class of objects would be a important step, and would require us to understand the physical process responsible for such powerful X-ray emission. We have initiated an optical spectroscopic survey in order to obtain accurate spectroscopic classifications for all the objects which are claimed to be starburst galaxies. Here we present preliminary results from this survey. We have discovered a number of starburst galaxies with X-ray luminosities above ~10^(41)erg s^(-1) (for Ho=50km s^(-1)Mpc^(-1)). We investigate possible origins for the X-ray emission in individual cases.

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arXiv1999

NICMOS Imaging of Molecular Hydrogen Emission in Seyfert Galaxies

We present NICMOS imaging of broad band and molecular hydrogen emission in Seyfert galaxies. In 6 of 10 Seyferts we detect resolved or extended emission in the 1-0 S(1) 2.121 or 1-0 S(3) 1.9570 micron molecular hydrogen lines. We did not detect emission in the most distant galaxy or in the 2 Seyfert 1 galaxies in our sample because of the luminosity of the nuclear point sources. In NGC 5643, NGC 2110 and MKN 1066, molecular hydrogen emission is detected in the extended narrow line region on scales of a few hundred pc from the nucleus. Emission is coincident with [OIII] and H alpha+[NII] line emission. This emission is also near dust lanes observed in the visible to near-infrared color maps suggesting that a multiphase medium exists near the ionization cones and that the morphology of the line emission is dependent on the density of the ambient media. The high 1-0 S(1) or S(3) H2 to H alpha flux ratio suggests that shock excitation of molecular hydrogen (rather than UV fluorescence) is the dominant excitation process in these extended features. In NGC 2992 and NGC 3227 the molecular hydrogen emission is from 800 and 100 pc diameter `disks' (respectively) which are not directly associated with [OIII] emission and are near high levels of extinction (AV > 10). In NGC 4945 the molecular hydrogen emission appears to be from the edge of a 100 pc superbubble. In these 3 galaxies the molecular gas could be excited by processes associated with local star formation. We confirm previous spectroscopic studies finding that no single mechanism is likely to be responsible for the molecular hydrogen excitation in Seyfert galaxies.

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arXiv1999

Infrared 3D Observations of Nearby Active Galaxies

We present multi-wavelength imaging observations of three nearby and famous active galaxies obtained with NICMOS, ISOCAM and the MPE near-IR integral field spectrometer. The data reveal a variety of features and properties that are missed in optical studies and in traditional IR monodimensional spectroscopy.

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arXiv1999

Dust lanes causing structure in the extended narrow line region of early-type Seyfert galaxies

We construct near-infrared to visible broad band NICMOS/WFPC color maps for 4 early-type Seyfert galaxies with S-shaped or one-sided ionization cones. We find that dust lanes are near or connected to many of the features seen in the [OIII] and H$α$+[NII] line emission maps. This suggests that much of the structure of line emission in these ionization cones is determined by the distribution of ambient dense galactic gas. Spiral arms, dust lanes caused by bars, or gaseous warps provide dense gas which when illuminated by a conical beam of ultraviolet photons can result in the complicated line emission morphologies observed.

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