arXiv subjects

L. Armus

Publications and source records attributed to L. Armus.

At least 235 records · Page 13Linked to original sources

arXiv2000

K band Spectroscopy of Ultraluminous Infrared Galaxies: The 2 Jy Sample

We present near-infrared spectroscopy for a complete sample of 33 ultraluminous infrared galaxies at a resolution of R\approx 1000. Most of the wavelength range from 1.80-2.20 microns in the rest frame is covered, including the Pa-alpha and Br-gamma hydrogen recombination lines, and the molecular hydrogen vibration-rotation 1-0 S(1) and S(3) lines. Other species, such as He I, [Fe II], and [Si VI] appear in the spectra as well, in addition to a number of weaker molecular hydrogen lines. Nuclear extractions for each of the individual galaxies are presented here, along with spectra of secondary nuclei, where available. The Pa-alpha emission is seen to be highly concentrated on the nuclei, typically with very little emision extending beyond a radius of 1 kpc. Signatures of active nuclei are rare in the present sample, occurring in only two of the 33 galaxies. It is found that visual extinctions to the nuclei via the Pa-alpha/Br-gamma line ratio in excess of 10 magnitudes are relatively common among ULIRGs, and that visual extinctions greater than 25 mag are necessary to conceal a QSO emitting half the total bolometric luminosity. The vibration-rotation lines of molecular hydrogen appear to be predominantly thermal in origin, with effective temperatures generally around 2200 K. The relative nuclear velocities between double nucleus ULIRGs are investigated, through which it is inferred that the maximum deprojected velocity difference is about 200 km/s. This figure is lower than the velocities predicted by physical models of strong interactions/mergers of large, gas-rich galaxies.

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arXiv2000

The Active Nucleus in the Ultraluminous Infrared Galaxy IRAS 08311-2459

Near-infrared spectroscopy using the new Palomar Integral Field Spectrograph indicates the presence of an AGN (active galactic nucleus) in the ultraluminous infrared galaxy IRAS 08311-2459. The high-velocity wings of the Paschen-alpha hydrogen recombination line are seen to be spatially unresolved, and with no positional offset between red and blue high velocity emission. The [Si VI] coronal line, with a 167 eV excitation potential, is also spatially unresolved with a velocity width comparable to that of the broad component of the Pa-alpha emission. The low velocity component of the Pa-alpha emission is seen to be rotating, and is extended over about 2 kpc. Molecular hydrogen is also extended, and elongated along the maximum velocity gradient, sharing the same rotation profile as the narrow Pa-alpha emission. The simple picture in agreement with the observations is that of an AGN surrounded by a rotating disk of star formation. These observations lend strength to the usage of [Si VI] as a diagnostic of AGN activity in ultraluminous infrared galaxies, and also highlight the utility of integral field spectroscopy in elucidating subtle morphological differences in line emitting regions.

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arXiv1999

NICMOS Imaging of Infrared-Luminous Galaxies

We present near-infrared images obtained with the HST NICMOS camera for a sample of 9 luminous (LIGs: L_IR (8-1000 microns) >~ 10^11 L_sun) and 15 ultra-luminous (ULIGS: L_IR >~ 10^12 L_sun) infrared galaxies. The sample includes representative systems classified as warm (f_25/f_60 > 0.2) and cold (f_25/f_60 <~ 0.2) based on the mid-infrared colors and systems with nuclear emission lines classified as HII (i.e. starburst), QSO, Seyfert and LINER. The morphologies of the sample galaxies are diverse and provide further support for the idea that they are created by the collision or interactions of spiral galaxies. Although no new nuclei are seen in the NICMOS images, the NICMOS images do reveal new spiral structures, bridges, and circumnuclear star clusters...

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arXiv1999

The Dust Content and Opacity of Actively Star-Forming Galaxies

(Abridged) We present far-infrared (FIR) photometry at 150 micron and 205 micron of eight low-redshift starburst galaxies obtained with the ISO Photometer. Five of the eight galaxies are detected in both wavebands and these data are used, in conjunction with IRAS archival photometry, to model the dust emission at lambda>40 micron. The FIR spectral energy distributions (SEDs) are best fitted by a combination of two modified Planck functions, with T~40-55 K (warm dust) and T~20-23 K (cool dust), and with a dust emissivity index epsilon=2. The cool dust can be a major contributor to the FIR emission of starburst galaxies, representing up to 60% of the total flux. This component is heated not only by the general interstellar radiation field, but also by the starburst itself. The cool dust mass is up to ~150 times larger than the warm dust mass, bringing the gas-to-dust ratios of the starbursts in our sample close to Milky Way values, once rescaled for the appropriate metallicity. The ratio between the total dust FIR emission in the range 1-1000 micron and the IRAS FIR emission in the range 40-120 micron is ~1.75, with small variations from galaxy to galaxy. The FIR emission predicted by the dust reddening of the UV-to-nearIR stellar emission is within a factor ~2 of the observed value in individual galaxies and within 20% when averaged over a large sample. If our sample of local starbursts is representative of high-redshift (z>1), UV-bright, star-forming galaxies, these galaxies' FIR emission will be generally undetected in sub-mm surveys, unless (1) their bolometric luminosity is comparable to or larger than that of ultraluminous FIR galaxies and (2) their FIR SED contains a cool dust component.

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arXiv1999

The Nature of the Compact/Symmetric Near-IR Continuum Source in 4C 40.36

Using NICMOS on HST, we have imaged the emission-line nebulae and the line-free continuum in 4C 40.36, a ultra-steep spectrum FR II radio galaxy at z=2.269. The line-free continuum was found to be extremely compact and symmetric while the emission-line nebulae seen in H-alpha+[N II] show very clumpy structures spreading almost linearly over 16 kpc. However, this linear structure is clearly misaligned from the radio axis. The SED of the line-free continuum is very flat, suggesting that if the continuum emission is produced by a single source, it is likely to be a young bursting stellar population or scattered AGN light. However, because of the lack of a line-free optical image with a comparable spatial resolution, we cannot exclude the possibility that the observed SED is a composite of a young blue population and an old red population.

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arXiv1999

Near-Infrared Spectra of Ultraluminous Infrared Galaxies

Near infrared spectra with resolution R ~ 1100 in the rest wavelength range 1.8-2.2 microns have been obtained for a complete sample of 33 ultraluminous infrared galaxies. Of the 33 objects observed, 2 show evidence of a central AGN through either a broad Paschen-alpha line or emission in the 1.963 micron fine structure of [Si VI]. In the median spectrum of the remaining 31 objects, the lines present are recombination lines of Hydrogen, neutral Helium, vibration-rotation lines of H_2, and [Fe II]. There is no indication of AGN activity in the median spectrum, either through broad atomic recombination lines or high ionization lines. No trends in luminosity are apparent when subsets of the 31 non-AGN ULIRGs are binned by luminosity and median combined. When secondary nuclei exist in ULIRGs, they typically have spectra very much like those seen in the primary nuclei.

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arXiv1999

Near Infrared Observations of the Extremely Red Object CL0939+4713B: An Old Galaxy at z~1.58?

Near infrared imaging and spectroscopic observations of the extremely red object (R-K~ 7 mag) CL 0939+4713 B have been obtained with the Near Infrared Camera on the Keck I Telescope of the W. M. Keck Observatory. The imaging shows a slightly elongated structure, while the spectroscopy shows a continuum break that allows us to determine the redshift of z = 1.58 + 0.01/-0.03 for this system. The fits of a range of models to the infrared spectrum suggests that it is predominantly an old (> 10^9 yrs) stellar system that suffers little extinction, while the measurerd R and I magnitudes suggests an age of ~ 3 x 10^8 years. The limit on the equivalent width of any emission line in the infrared spectrum argues that CL 0939+4713 B is not an actively star forming galaxy. This system, though similar in R-K color to HR 10 [also known as J1645+46] (Dey et al. 1999), is much different in morphology and emission line strengths, demonstrating the heterogeneity of extremely red extragalactic objects (EROs) selected on the basis of large values of R-K.

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arXiv1999

Evidence for a supernova in reanalyzed optical and near-infrared images of GRB970228

We present B-, V-, R_c-, I_c-, J-, H-, K- and K'-band observations of the optical transient (OT) associated with GRB970228, based on a reanalysis of previously used images and unpublished data. In order to minimize calibration differences we have collected and analyzed most of the photometry and consistently determined the magnitude of the OT relative to a set of secondary field stars. We confirm our earlier finding that the early decay of the light curves (before March 6, 1997) was faster than that at intermediate times (between March 6 and April 7, 1997). At late times the light curves resume a fast decay (after April 7, 1997). The early-time observations of GRB970228 are consistent with relativistic blast-wave models but the intermediate- and late-time observations are hard to understand in this framework. The observations are well explained by an initial power law decay with index -1.73 +0.09 -0.12 modified at later times by a type-I_c supernova light curve. Together with the evidence for GRB980326 and GRB980425 this gives further support for the idea that at least some GRBs are associated with a possibly rare type of supernova.

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arXiv1999

Submillimeter Imaging of the Luminous Infrared Galaxy Pair VV 114

We report on 450- and 850-micron observations of the interacting galaxy pair, VV114E+W (IC1623), taken with the SCUBA camera on the James Clerk Maxwell Telescope, and near-infrared observations taken with UFTI on the UK Infrared Telescope. The system VV114 is in an early stage of a gas-rich merger. We detect submillimeter (sub-mm) emission extended over 30 arcsec (12 kpc) and find a good correlation between the spatial distribution of the sub-mm and CO emission. Both the CO and sub-mm emission peak near the reddest region of VV114E and extend toward VV114W. The bulk of the sub-mm emission resides in the central region showing the largest CO velocity gradients, which is thought to mark the kinematic centroid of the merger remnant. We derived a total dust mass of 1.2x10^{8} M_sun, assuming a power-law distribution of dust temperatures. The sub-mm observations suggest that the majority of the dust is relatively cool (T_d ~20--25 K), and the total dust mass is about 4 times higher than that inferred from the IRAS data alone. The system will likely evolve into a compact starburst similar to Arp220.

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arXiv1999

High Resolution WFPC2 Imaging of IRAS 09104+4109

With a infrared luminosity of nearly 10^13 Lsuns, IRAS 09104+4109 is the most luminous galaxy with z<0.5 in the IRAS All Sky Survey. A radio-loud Seyfert 2 type optical spectrum, a cD host galaxy in a rich cluster, and a massive cooling flow make IRAS 09104+4109 unique among ultraluminous infrared galaxies. Cannibalized cluster members and the cooling intercluster medium may contribute both the fuel and the dust needed to re-radiate the power of IRAS 09104+4109 into the far-infrared. We have imaged IRAS 09104+4109 in the WFPC2 F622W, F814W, and FR680N filters on the HST to obtain rest frame 4300A, 5700A, and [OIII] emission line images on sub-kpc scales. IRAS 09104+4109 displays a complex morphology on the smallest scales, with radiating filaments, an asymmetric [OIII] nebula, and a number of very faint, irregular blue objects surrounding the cD galaxy. We discuss the nature and possible interplay between the enshrouded QSO nucleus, the cD host galaxy and the irregular cluster.

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arXiv1999

The afterglow, the redshift, and the extreme energetics of the gamma-ray burst 990123

Afterglow, or long-lived emission, has now been detected from about a dozen well-positioned gamma-ray bursts. Distance determinations made by measuring optical emission lines from the host galaxy, or absorption lines in the afterglow spectrum, place the burst sources at significant cosmological distances, with redshifts ranging from ~1--3. The energy required to produce the bright gamma-ray flashes is enormous: up to ~10^{53} erg or 10 percent of the rest mass energy of a neutron star, if the emission is isotropic. Here we present the discovery of the optical afterglow and the redshift of GRB 990123, the brightest well-localized GRB to date. With our measured redshift of >1.6, the inferred isotropic energy release exceeds the rest mass of a neutron star thereby challenging current theoretical models for the origin of GRBs. We argue that the optical and IR afterglow measurements reported here may provide the first observational evidence of beaming in a GRB, thereby reducing the required energetics to a level where stellar death models are still tenable.

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arXiv1999

The Host Galaxy of GRB 990123

We present deep images of the field of gamma-ray burst (GRB) 990123 obtained in a broad-band UV/visible bandpass with the Hubble Space Telescope, and deep near-infrared images obtained with the Keck-I 10-m telescope. Both the HST and Keck images show that the optical transient (OT) is clearly offset by 0.6 arcsec from an extended object, presumably the host galaxy. This galaxy is the most likely source of the metallic-line absorption at z = 1.6004 seen in the spectrum of the OT. With magnitudes V_{C} ~ 24.6 +/- 0.2 and K = 21.65 +/- 0.30 mag this corresponds to an L ~ 0.7 L_* galaxy, assuming that it is located at z = 1.6. The estimated unobscured star formation rate is SFR ~ 6 M_sun/yr, which is not unusually high for normal galaxies at comparable redshifts. The strength of the observed metallic absorption lines is suggestive of a relatively high metallicity of the gas, and thus of a chemically evolved system which may be associated with a massive galaxy. It is also indicative of a high column density of the gas, typical of damped Ly-alpha systems at high redshifts. We conclude that this is the host galaxy of GRB 990123. No other obvious galaxies are detected within the same projected radius from the OT. There is thus no evidence for strong gravitational lensing magnification of this burst, and some alternative explanation for its remarkable energetics may be required. The observed offset of the OT from the center of its apparent host galaxy, 5.5 +/- 0.9 proper kpc (projected) in the galaxy's rest-frame, both refutes the possibility that GRBs are related to galactic nuclear activity and supports models of GRBs which involve the death and/or merger of massive stars. Further, the HST image suggests an intimate connection of GRB 990123 and a star-forming region.

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arXiv1998

An X-ray and Optical Investigation of the Starburst-driven Superwind in the Galaxy Merger Arp 299

We present a detailed investigation of the X-ray and optical properties of the starburst-merger system Arp299 (NGC 3690, Mrk 171), with an emphasis on its spectacular gaseous nebula. We analyse \rosat and \asca X-ray data and optical spectra and narrow-band images. We suggest that the on-going galaxy collision has tidally-redistributed the ISM of the merging galaxies. The optical emission-line nebula results as this gas is photoionized by radiation that escapes from the starburst, and is shock-heated, accelerated, and pressurized by a `superwind' driven by the collective effect of the starburst supernovae and stellar winds. The X-ray nebula in Arp 299 is is plausibly a mass-loaded flow of adiabatically-cooling gas that carries out a substantial fraction of the energy and metals injected by the starburst at close to the escape velocity from Arp 299. The mass outflow rate likely exceeds the star-formation rate in this system. We conclude that powerful starbursts are able to heat (and possibly eject) a significant fraction of the ISM in merging galaxies.

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arXiv1998

HST NICMOS Observations of FSC10214+4724

High-resolution, 1.10, 2.05, 2.12, and 2.15 micron imaging of the gravitationally lensed system FSC10214+4724 are presented. These data extend HST observations of the lens system to redder wavelengths, thus providing the highest resolution images to date of the rest-frame optical and narrow-line (i.e., H-alpha+[N II]) regions of the background quasar. The length of the arc in the wide-band continuum images increases with increasing wavelength, and the H-alpha+[N II] emission has a length in between that of the 1.10 and 2.05 microns. The structure of the arc changes from having an eastern and western peak at 1.10 microns to a single peak in the center at 2.05 microns. The changing structure and length of the arc can be understood in terms of a model where the background quasar consists of a region of scattered AGN light that dominates at 1.10 microns (rest-frame 3300 angstroms), surrounded by a more extended narrow-line region. An even more extended red stellar population would thus contribute light at 2.05 microns...

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arXiv1998

Near Infrared Observations of a Redshift 5.34 Galaxy: Further Evidence for Dust Absorption in the Early Universe

Imaging at 1.25 and 2.20 microns has been obtained of the field containing the galaxy (RD1) found at redshift 5.34 by Dey et al.(1998). This galaxy has been detected at 1.25 microns, while the lower redshift (z=4.02) galaxy also found in the same field by Dey et al. was detected at both 1.25 and 2.20 microns. Comparison to stellar population synthesis models indicates that if RD1 is a young (< 100 Myr old) galaxy, significant reddening (A(V) > 0.5 mag) is indicated. Combined with observations of other high redshift systems, these data show that dust is likely to be an important component of young galaxies even at redshifts of z > 5. The extinction-corrected monochromatic luminosity of RD1 at 1500 angstroms is then a factor of about three larger than L(1500)* as determined by Dickinson (1998) for z ~ 3 starburst galaxies. The implied star formation rate in RD1, corrected for extinction, is ~ 50-100 solar masses per year.

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arXiv1997

The [OIII] Emission-line Nebula of the z=3.594 Radio Galaxy 4C +19.71

We have imaged the z=3.594 radio galaxy 4C +19.71 in the light of the redshifted [OIII] 5007 angstrom emission line, using a narrow-band filter centered at 2.3 microns with the Near Infrared Camera on the Keck Telescope. The [OIII] nebula of 4C +19.71 has a size of 74 x 9 kpc, and a luminosity of 3 x 10^37 W. The rest frame equivalent width of the 5007 line, averaged over the entire nebula, is 560 angstroms. The length of the major axis of the [OIII] emission is nearly identical to the separation of the radio lobes seen at 1465 MHz (Rottgering, et al. 1994), and the position angle of the nebula is the same as that of the two radio lobes. In addition, 4C +19.71 follows the optical emission line vs. radio power correlation seen in other powerful radio galaxies. The [OIII] and Lyman alpha emission line luminosities suggest that the ionized gas mass lies in the range of 2 x 10^8 - 10^9 solar masses. The O/H ratio in the nebula is at least a few tenths solar, and may be as high as a factor of three above solar, indicating a previous phase of star formation in 4C +19.71. Thirty five percent of the total K-band flux is contributed by the 5007 angstrom emission line, and the continuum of 4C +19.71 has a K=19.6 mag. This places 4C +19.71 along the K-z relation found for other radio galaxies and radio loud quasars. If the continuum is dominated by starlight, the host galaxy has a rest frame visual luminosity of about 40L*. There are no candidate emission-line objects at the redshift of 4C +19.71 having [OIII] rest frame equivalent widths of more than about 2% that of the radio galaxy itself within a volume of 212 cubic Mpc.

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arXiv1997

A Near Infrared Spectroscopic Survey of LINER Galaxies

This paper reports the results of a near infrared spectroscopic survey of LINER galaxies undertaken with a new infrared spectrograph at the 5 m Hale telescope. The galaxy sample includes 11 LINERs with spectra covering the [FeII] (1.2567 microns), PaBeta (1.2818 microns), H2 (1-0 S(1), 2.1218 microns) and BrGamma (2.1655 microns) near infrared emission lines, and one additional galaxy with only [FeII] and Pa$β$ line coverage. All of the LINERs with infrared line detections have strong [FeII] and/or H2 emission, with about half (4 out of 9) having extremely high ratios (>2) of [FeII] to PaBeta. The strength of the H2 and [FeII] lines is well correlated with the optical [OI] line, with many LINERs having higher ratios of [FeII]/PaBeta, H2/BrGamma and [OI]/H-Alpha than other galaxy types. The LINERs with the highest [FeII]/PaBeta ratios (termed ``strong'' [FeII] LINERs) show evidence for recent star formation. Shocks from compact supernova remnants may enhance the [FeII] emission in these ``strong'' [FeII] LINERs. The LINERs with lower [FeII]/PaBeta ratios (termed ``weak'' [FeII] LINERs) are more consistent with Seyfert-like activity, including higher ionization states, some strong x-ray sources and some broad H-Alpha detections. The [FeII] luminosity and the [FeII]/PaBeta ratio in these objects are more easily explained by hard x-ray excitation than in the ``strong'' [FeII] LINERs. These ``weak'' [FeII] LINERs are considered prime candidates for being low luminosity Seyfert nuclei.

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arXiv1997

CO (J=3-2) Emission in the Radio Galaxy 53W002 at z=2.394

We report a sensitive search for redshifted CO (3-2) emission from the weak radio galaxy 53W002 at z=2.394. Maps at resolutions of 3 arcsec and 235km/s show a significant emission peak within 0.5 arcsec of the optical and radio continuum peaks. The measured narrow band flux is approximately ten times the extrapolated cm-wavelength non-thermal radio continuum expected at 101.9 GHz and exhibits a spectral profile implying a 540 km/s width (FWHM) at a systemic redshift z=2.394 for CO (3-2). This emission has a total integrated flux of 1.51(+/-0.2) Jy km/s, approximately four times weaker than that previously seen in the lensed systems FSC10214+4724 and the Cloverleaf QSO. For a Galactic CO-to-H2 conversion ratio, the implied molecular gas mass is 7.4x10^10 solar masses (H=75 km/s/Mpc and q=0.5). The CO emission is elongated at P.A.=120 degrees with a deconvolved major axis radius of 15 kpc (2.8 arcsec). This extension is along a similar direction to that seen in the cm-wave radio continuum and the optical but approximately three times larger. A velocity gradient is seen along the major axis, and if this structure is a (forming) disk, the implied dynamical mass is 9-22x10^10 solar masses at radii less than or equal to 15 kpc, assuming inclination i=0 degrees (edge-on). The magnitude of these masses and the similarity of the high gas-mass fraction are consistent with the host galaxy of 53W002 being a young galactic system, but the metallicity (probably greater than or equal to 0.1 solar in order to produce the CO lines) implies significant heavy element production prior to z=2.4. This constitutes the first high redshift molecular gas which is detected in emission where there is probably no gravitational magnification.

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