Contents
Images
Upload your image
DSS Images Other Images
Related articles
The Opaque Nascent Starburst in NGC 1377: Spitzer SINGS Observations
We analyze extensive data on NGC 1377 from the Spitzer Infrared NearbyGalaxies Survey (SINGS). Within the category of nascent starbursts thatwe previously selected as having infrared-to-radio continuum ratios inlarge excess of the average and containing hot dust, NGC 1377 has thelargest infrared excess yet measured. Optical imaging reveals amorphological distortion suggestive of a recent accretion event.Infrared spectroscopy reveals a compact and opaque source dominated by ahot, self-absorbed continuum (τ~20 in the 10 μm silicate band).We provide physical evidence against nonstellar activity being theheating source. H II regions are detected through the single [Ne II]line, probing <1% of the ionizing radiation. Not only is the opticaldepth in different gas and dust phases very high, but >85% ofionizing photons are suppressed by dust. The only other detectedemission features are molecular hydrogen lines, arguably excited mainlyby shocks, besides photodissociation regions, and weak aromatic bands.The new observations support our interpretation in terms of an extremelyyoung starburst (<1 Myr). More generally, galaxies deficient in radiosynchrotron emission are likely observed within a few Myr of the onsetof a starburst and after a long quiescence, prior to the replenishmentof the interstellar medium with cosmic rays. The similar infrared-radioproperties of NGC 1377 and some infrared-luminous galaxies suggest thatNGC 1377 constitutes an archetype that will be useful to betterunderstand starburst evolution. Although rare locally because observedin a brief evolutionary stage, nascent starbursts may represent anonnegligible fraction of merger-induced starbursts that dominate deepinfrared counts. Since they differ dramatically from usual starbursttemplates, they have important consequences for the interpretation ofdeep surveys.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Environment and luminosity of supernova remnants
The explosion of supernovae and the evolution of their remnants (SNRs)accelerate cosmic rays over a vast range of timescales. Magnetic fieldscan be investigated indirectly through one of the observationalsignatures of this acceleration, namely radio synchrotron emission. Withthe aim of better understanding the role of the magnetic field insupernova evolution, we explore the variation of SNR radio luminositieswith physical conditions in the surrounding interstellar medium. With adata set that comprises more than 90 individual SNRs in 10 galaxies, anda range of 3000 in ISM density and 104 in radio synchrotronluminosity, we find a significant correlation between the twoquantities. The observed trends support the hypothesis that adiabaticcompression of magnetic fields by itself is insufficient to explain theradio emission of the brighter and more luminous in SNRs.
| Low-Luminosity Active Galaxies and Their Central Black Holes
Central black hole masses for 117 spiral galaxies representingmorphological stages S0/a through Sc and taken from the largespectroscopic survey of Ho et al. are derived using Ks-banddata from the Two Micron All Sky Survey. Black hole masses are foundusing a calibrated black hole-Ks bulge luminosity relation,while bulge luminosities are measured by means of a two-dimensionalbulge-disk decomposition routine. The black hole masses are correlatedagainst a variety of parameters representing properties of the nucleusand host galaxy. Nuclear properties such as line width (FWHM [N II]), aswell as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [NII]/Hα, and [S II]/Hα), show a very high degree ofcorrelation with black hole mass. The excellent correlation with linewidth supports the view that the emission-line gas is in virialequilibrium with either the black hole or bulge potential. The very goodemission-line ratio correlations may indicate a change in ionizingcontinuum shape with black hole mass in the sense that more massiveblack holes generate harder spectra. Apart from theinclination-corrected rotational velocity, no excellent correlations arefound between black hole mass and host galaxy properties. Significantdifferences are found between the distributions of black hole masses inearly-, mid-, and late-type spiral galaxies (subsamples A, B, and C) inthe sense that early-type galaxies have preferentially larger centralblack holes, consistent with observations that Seyfert galaxies arefound preferentially in early-type systems. The line width distributionsshow a marked difference among subsamples A, B, and C in the sense thatearlier type galaxies have larger line widths. There are also cleardifferences in line ratios between subsamples A+B and C that likely arerelated to the level of ionization in the gas. Finally, aKs-band Simien & de Vaucouleurs diagram shows excellentagreement with the original B-band relation, although there is a largedispersion at a given morphological stage.
| Extragalactic H_2O masers and X-ray absorbing column densities
Having conducted a search for the λ 1.3 cm (22 GHz) water vaporline towards galaxies with nuclear activity, large nuclear columndensities or high infrared luminosities, we present H2O spectra for NGC2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and400 Lȯ. The H2O maser in UGC 5101 is by far the mostluminous yet found in an ultraluminous infrared galaxy. NGC 3393 revealsthe classic spectrum of a "disk maser", represented by three distinctgroups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below1000 km s-1. Based on the literature and archive data, X-rayabsorbing column densities are compiled for the 64 galaxies withreported maser sources beyond the Magellanic Clouds. For NGC 2782 andNGC 5728, we present Chandra archive data that indicate the presence ofan active galactic nucleus in both galaxies. Modeling the hard nuclearX-ray emission, NGC 2782 is best fit by a high energy reflectionspectrum with NH 1024 cm-2. ForNGC 5728, partial absorption with a power law spectrum indicatesNH 8 × 1023 cm-2. Thecorrelation between absorbing column and H2O emission is analyzed. Thereis a striking difference between kilo- and megamasers with megamasersbeing associated with higher column densities. All kilomasers (L_H_2O< 10 Lȯ) except NGC 2273 and NGC 5194 areCompton-thin, i.e. their absorbing columns are <1024cm-2. Among the H{2}O megamasers, 50% arise fromCompton-thick and 85% from heavily obscured (>1023cm-2) active galactic nuclei. These values are not larger butconsistent with those from samples of Seyfert 2 galaxies not selected onthe basis of maser emission. The similarity in column densities can beexplained by small deviations in position between maser spots andnuclear X-ray source and a high degree of clumpiness in thecircumnuclear interstellar medium.
| A Search for Candidate Radio Supernova Remnants in the Nearby Irregular Starburst Galaxies NGC 4214 and NGC 4395
We present the results of a search for new candidate radio supernovaremnants (SNRs) in the nearby starburst irregular galaxies NGC 4214 andNGC 4395 using archived radio observations made with the Very LargeArray (VLA) at the wavelengths of 3.5 cm, 6 cm and 20 cm for NGC 4214and 6 cm and 20 cm for NGC 4395. These observations were analyzed aspart of our ongoing search for candidate radio SNRs in nearby galaxies:the goal of this search is to prepare a large sample of candidate radioSNRs for the purpose of a robust statistical study of the properties ofthese sources. Based on our analysis, we have confirmed the non-thermalnature of the discrete radio sources alpha and beta in NGC 4214 andclassify these sources as candidate radio SNRs based on their positionalcoincidences with HII regions in that galaxy. We have measured the fluxdensities of the two candidate radio SNRs at each wavelength andcalculated corresponding spectral indices: we have also measured fluxdensities of two other discrete radio sources in these galaxies - rho inNGC 4214 and #3 in NGC 4395 - which we suspect to be additionalcandidate radio SNRs based on their positional coincidences with otherHII regions in these galaxies. However, the radio data presentlyavailable for these sources cannot confirm such a classification andadditional observations are needed. We have also calculated the radioluminosities Lradio at the wavelength of 20 cm for these twocandidate radio SNRs as well as the corresponding values for the minimumtotal energy Emin required to power these radio sources viasynchrotron emission and the corresponding magnetic field strengthBmin. We have compared our mean calculated values for theseproperties with the mean values for populations of candidate radio SNRsin other starburst galaxies: while the values for Lradio andBmin are roughly comparable to the values seen in otherstarburst galaxies, the mean value for Emin is higher thanthe mean value of any other starburst galaxy. Finally, we include thesetwo candidate radio SNRs in a discussion of the Sigma-D relation forextragalactic candidate radio SNRs and find that these sources arelocated on the shallower end of the master Sigma-D relation for allextragalactic SNRs as derived by Urosevic et al. (2005).
| Chandra Observation of the Starburst Galaxy NGC 2146
We present six monitoring observations of the starburst galaxy NGC 2146using the Chandra X-ray Observatory. We detected 67 point sources in thefield of view of the ACIS-S detector. Six of these sources wereUltra-Luminous X-ray Sources, the brightest of which had a luminosity of5 × 1039 erg s-1. One of them, with a luminosity of 1× 1039 erg s-1, is coincident with the dynamical center location,which may be a low-luminosity active galactic nucleus. We have produceda table where the positions and main characteristics of the detectedsources are reported. A comparison between the positions of the X-raysources and those detected in NIR or radio indicates no definitecounterpart. We have derived a log N ‑ log S relation and aluminosity function. The luminosity function has a slope of 0.71 above adetection limit, which is similar to those found in other starburstgalaxies. Diffuse emissions were detected in both soft (0.5-2.0keV) andhard (2.0-10.0keV) energy bands. The spectra of the diffuse componentwere fitted with two (hard and soft) components. The hard power-lawcomponent, with a luminosity of 4 × 1039 erg s-1, is likelyto have originated by unresolved point sources.
| How large are the bars in barred galaxies?
I present a study of the sizes (semimajor axes) of bars in discgalaxies, combining a detailed R-band study of 65 S0-Sb galaxies withthe B-band measurements of 70 Sb-Sd galaxies from Martin (1995). As hasbeen noted before with smaller samples, bars in early-type (S0-Sb)galaxies are clearly larger than bars in late-type (Sc-Sd) galaxies;this is true both for relative sizes (bar length as fraction ofisophotal radius R25 or exponential disc scalelength h) andabsolute sizes (kpc). S0-Sab bars extend to ~1-10 kpc (mean ~ 3.3 kpc),~0.2-0.8R25 (mean ~ 0.38R25) and ~0.5-2.5h (mean ~1.4h). Late-type bars extend to only ~0.5-3.5 kpc,~0.05-0.35R25 and 0.2-1.5h their mean sizes are ~1.5 kpc, ~0.14R25 and ~0.6h. Sb galaxies resemble earlier-type galaxiesin terms of bar size relative to h; their smallerR25-relative sizes may be a side effect of higher starformation, which increases R25 but not h. Sbc galaxies form atransition between the early- and late-type regimes. For S0-Sbcgalaxies, bar size correlates well with disc size (both R25and h); these correlations are stronger than the known correlation withMB. All correlations appear to be weaker or absent forlate-type galaxies; in particular, there seems to be no correlationbetween bar size and either h or MB for Sc-Sd galaxies.Because bar size scales with disc size and galaxy magnitude for mostHubble types, studies of bar evolution with redshift should selectsamples with similar distributions of disc size or magnitude(extrapolated to present-day values); otherwise, bar frequencies andsizes could be mis-estimated. Because early-type galaxies tend to havelarger bars, resolution-limited studies will preferentially find bars inearly-type galaxies (assuming no significant differential evolution inbar sizes). I show that the bars detected in Hubble Space Telescope(HST) near-infrared(IR) images at z~ 1 by Sheth et al. have absolutesizes consistent with those in bright, nearby S0-Sb galaxies. I alsocompare the sizes of real bars with those produced in simulations anddiscuss some possible implications for scenarios of secular evolutionalong the Hubble sequence. Simulations often produce bars as large as(or larger than) those seen in S0-Sb galaxies, but rarely any as smallas those in Sc-Sd galaxies.
| GHASP: an Hα kinematic survey of spiral and irregular galaxies - IV. 44 new velocity fields. Extension, shape and asymmetry of Hα rotation curves
We present Fabry-Perot observations obtained in the frame of the GHASPsurvey (Gassendi HAlpha survey of SPirals). We have derived the Hαmap, the velocity field and the rotation curve for a new set of 44galaxies. The data presented in this paper are combined with the datapublished in the three previous papers providing a total number of 85 ofthe 96 galaxies observed up to now. This sample of kinematical data hasbeen divided into two groups: isolated (ISO) and softly interacting(SOFT) galaxies. In this paper, the extension of the Hα discs, theshape of the rotation curves, the kinematical asymmetry and theTully-Fisher relation have been investigated for both ISO and SOFTgalaxies. The Hα extension is roughly proportional toR25 for ISO as well as for SOFT galaxies. The smallestextensions of the ionized disc are found for ISO galaxies. The innerslope of the rotation curves is found to be correlated with the centralconcentration of light more clearly than with the type or thekinematical asymmetry, for ISO as well as for SOFT galaxies. The outerslope of the rotation curves increases with the type and with thekinematical asymmetry for ISO galaxies but shows no special trend forSOFT galaxies. No decreasing rotation curve is found for SOFT galaxies.The asymmetry of the rotation curves is correlated with themorphological type, the luminosity, the (B-V) colour and the maximalrotational velocity of galaxies. Our results show that the brightest,the most massive and the reddest galaxies, which are fast rotators, arethe least asymmetric, meaning that they are the most efficient withwhich to average the mass distribution on the whole disc. Asymmetry inthe rotation curves seems to be linked with local star formation,betraying disturbances of the gravitational potential. The Tully-Fisherrelation has a smaller slope for ISO than for SOFT galaxies.
| Supernovae 2002hh and 2005V
IAUC 8572 available at Central Bureau for Astronomical Telegrams.
| Supernova 2005V in NGC 2146
IAUC 8474 available at Central Bureau for Astronomical Telegrams.
| Supernova 2005V in NGC 2146
IAUC 8474 available at Central Bureau for Astronomical Telegrams.
| Supernova 2005V in NGC 2146
IAUC 8474 available at Central Bureau for Astronomical Telegrams.
| An Analytic Model of Galactic Winds and Mass Outflows
Galactic winds and mass outflows are observed both in nearby starburstgalaxies and in high-redshift star-forming galaxies. We develop a simpleanalytic model to understand the observed superwind phenomenon with adiscussion of the model uncertainties. Our model is built upon the modelof McKee & Ostriker for the interstellar medium. It allows one topredict how properties of a superwind, such as wind velocity and massoutflow rate, are related to properties of its star-forming host galaxy,such as size, gas density and star formation rate. The model predicts athreshold of star formation rate density for the generation ofobservable galactic winds. Galaxies with more concentrated starformation activities produce superwinds with higher velocities. Thepredicted mass outflow rates are comparable to (or slightly larger than)the corresponding star formation rates. We apply our model to both localstarburst galaxies and high-redshift Lyman break galaxies, and find itspredictions to be in good agreement with current observations. Our modelis simple and so can be easily incorporated into numerical simulationsand semi-analytical models of galaxy formation.
| Mega-Masers and Galaxies
In the Galaxy, microwave radiation can be amplified in the interstellarmedium in the immediate neighborhood of young stellar objects, orcircumstellar envelopes around evolved stars, resulting in cosmic maseremission. Cosmic masers exist because, in contrast to terrestrialconditions, the interstellar gas density is very low so that levelpopulation in molecules is typically not in thermal equilibrium, andsometimes inverted. In the nuclear regions of external galaxies, thereexist very powerful OH ( 18 cm) and H2O ( 1.35 cm) cosmicmasers with line luminosities of 102 104Lȯ, 106 times more luminous than typicalGalactic maser sources. These are the "mega-masers," found inhigh-density molecular gas located within parsecs of active galacticnuclei in the case of H2O mega-masers, or within the central100 pc of nuclear star-burst regions in the case of OH mega-masers.H2O mega-masers are most frequently found in galactic nucleiwith Seyfert2 or LINER spectral characteristics, in spiral and someelliptical galaxies. OH mega-masers are found in ultra-luminous IRgalaxies (ULIRG) with the warmest IR colors, and importantly, the OHluminosity is observed to increase with the IR luminosity:LOH L1.2IR. Because of the extremelyhigh-surface brightness, H2O mega-maser emission can bemapped at sub-milli-arc-second resolution by Very Long BaselineInterferometry (VLBI), providing a powerful tool to probe spatial andkinematic distributions of molecular gas in distant galactic nuclei atscales below one parsec. An excellent example is the active galaxy, NGC4258, in which mapping of the H2O mega-maser emission hasprovided the first direct evidence in an active galactic nucleus for theexistence of a thin Keplerian accretion disk with turbulence, as well ashighly compelling evidence for the existence of a massive black hole.The NGC 4258 mega-maser has also provided a geometric distancedetermination of extremely high precision. H2O mega-maseremission is also found to arise from postshocked gas from the impact ofnuclear jets or outflows on the surrounding molecular clouds.High-resolution observations have shown that OH mega-masers originatefrom the molecular gas medium in 100-pc scale nuclear star-burstregions. It is proposed that such extreme star-burst regions, withextensive high-density gas bathed in a very high far-IR radiation field,are conducive to the formation of a very large number of OH masersources that collectively produce the OH mega-maser emission. In theearly Universe, galaxies or mergers could go through a very luminousphase, powered by intensive star-bursts and AGN formation, and couldhave extremely large OH and H2O maser luminosities, possiblyproducing giga-masers. With the increasing sensitivity of new telescopesand receivers, surveys and high-resolution studies of mega-masers andgiga-masers will be very important tracers and high-resolution probes ofactive galactic nuclei, dust embedded star-bursts in the earliestgalaxies and galaxy mergers in the epoch of very active star formationat z 2 and beyond. Distance determination of giga-masers at z 1 2can provide on independent measure of how fast the universe isexpanding.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| Mid-Infrared Spectra of Classical AGNs Observed with the Spitzer Space Telescope
Full low-resolution (65
| EGRET Upper Limits and Stacking Searches of Gamma-Ray Observations of Luminous and Ultraluminous Infrared Galaxies
We present a stacking analysis of EGRET γ-ray observations at thepositions of luminous and ultraluminous infrared galaxies. The latterwere selected from the recently presented HCN survey, which is thoughtto contain the most active star-forming regions of the universe.Different sorting criteria are used, and since there is no positivecollective detection of γ-ray emission from these objects, wedetermined both collective and individual upper limits. The uppermostexcess we find appears in the case of ULIRGs ordered by redshift, at avalue of 1.8 σ.
| On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback from Momentum-driven Winds
We investigate large-scale galactic winds driven by momentum deposition.Momentum injection is provided by (1) radiation pressure produced by thecontinuum absorption and scattering of photons on dust grains and (2)supernovae (momentum injection by supernovae is important even if thesupernova energy is radiated away). Radiation can be produced by astarburst or active galactic nucleus (AGN) activity. We argue thatmomentum-driven winds are an efficient mechanism for feedback during theformation of galaxies. We show that above a limiting luminosity,momentum deposition from star formation can expel a significant fractionof the gas in a galaxy. The limiting, Eddington-like luminosity isLM~=(4fgc/G)σ4, where σ isthe galaxy velocity dispersion and fg is the gas fraction;the subscript M refers to momentum driving. A starburst that attainsLM moderates its star formation rate and its luminosity doesnot increase significantly further. We argue that elliptical galaxiesattain this limit during their growth at z>~1 and that this is theorigin of the Faber-Jackson relation. We show that Lyman break galaxiesand ultraluminous infrared galaxies have luminosities nearLM. Since these starbursting galaxies account for asignificant fraction of the star formation at z>~1, this supports ourhypothesis that much of the observed stellar mass in early-type galaxieswas formed during Eddington-limited star formation. Star formation isunlikely to efficiently remove gas from very small scales in galacticnuclei, i.e., scales much smaller than that of a nuclear starburst. Thisgas is available to fuel a central black hole (BH). We argue that a BHclears gas out of its galactic nucleus when the luminosity of the BHitself reaches ~LM. This shuts off the fuel supply to the BHand may also terminate star formation in the surrounding galaxy. As aresult, the BH mass is fixed to beMBH~=(fgκes/πG2)σ4,where κes is the electron scattering opacity. Thislimit is in accord with the observed MBH-σ relation.
| Hα Imaging of Early-Type Sa-Sab Spiral Galaxies. II. Global Properties
New results, based on one of the most comprehensive Hα imagingsurveys of nearby Sa-Sab spirals completed to date, reveals early-typespirals to be a diverse group of galaxies that span a wide range inmassive star formation rates. While the majority of Sa-Sab galaxies inour sample are forming stars at a modest rate, a significant fraction(~29%) exhibit star formation rates greater than 1 Msolaryr-1, rivaling the most prolifically star-forming late-typespirals. A similar diversity is apparent in the star formation historyof Sa-Sab spirals as measured by their Hα equivalent widths.Consistent with our preliminary results presented in the first paper inthis series, we find giant H II regions [L(Hα)>=1039ergs s-1] in the disks of ~37% of early-type spirals. Wesuspect that recent minor mergers or past interactions are responsiblefor the elevated levels of Hα emission and, perhaps, for thepresence of giant H II regions in these galaxies. Our results, however,are not in total agreement with the Hα study of Kennicutt &Kent, who did not find any early-type spirals with Hα equivalentwidths >14 Å. A close examination of the morphologicalclassification of galaxies, however, suggests that systematicdifferences between the Revised Shapley-Ames Catalog and the SecondReference Catalogue may be responsible for the contrasting results.Based on observations obtained with the 3.5 m telescope at Apache PointObservatory (APO) and the 0.9 m telescope at Kitt Peak NationalObservatory (KPNO). The APO 3.5 m telescope is owned and operated by theAstrophysical Research Consortium.
| Can rotation curves reveal the opacity of spiral galaxies?
The observed anti-correlation between the inclination and the slope ofthe innermost part of the rotation curve can be attributed to dustextinction. However, the implied central face-on opacity of τ=2{-}4for the most luminous galaxies is embarrassingly high compared withstudies of edge-on galaxies, which yield central face-on opacities equalto at most τ=1 in the optical band. I show that the controversy canbe resolved by adopting a model that includes the velocity dispersion ofthe emitters. Through the use of such a model, I reproduce thecorrelation between galaxy inclination and the slope of its innerrotation curve without having to resort to excessively high opticaldepth.
| New H2O masers in Seyfert and FIR bright galaxies
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
| Radio recombination lines from the starburst galaxy NGC 3256
We have detected the radio recombination lines H91α and H92αwith rest frequencies of 8.6 GHz and 8.3 GHz from the starburst nucleusNGC 3256 at an angular resolution of 16.4'' × 9.6'' using theAustralia Telescope Compact Array and at an angular resolution of 12.0''× 2.9'' using the VLA. The line was detected at ~1 mJybeam-1 peak with a width of 160 km s-1 with theATCA and at ~0.5 mJy beam-1 peak with a width of 114 kms-1 with the VLA. Modelling the line emitting region as acollection of H II regions, we derive constraints on the required numberof H II regions, their temperature, density, and distribution. We findthat a collection of 10 to 300 H II regions with temperatures of 5000 K,densities of 1000 cm-3 to 5000 cm-3 and diametersof 15 pc produced good matches to the line and continuum emmission. TheLyman continuum production rate required to maintain the ionization is 2× 1052~s-1 to 6 ×1053~s-1, which requires 600 to 17 000 O5 stars tobe produced in the starburst.
| The Σ - D relation for supernova remnants in nearby galaxies
This paper examines relations between the radio surface brightnessΣ and the diameter D (also known as Σ-D relations) for asample of extragalactic supernova remnants (SNRs) as constructed from acombination of published data and data from our own surveys. Our sampleof extragalactic SNRs is the largest ever devised for the purpose ofanalyzing Σ-D relations. The main results of this paper may besummarized as follows: (i) the empirical relations for SNRs in 10 of the11 nearby galaxies studied have the approximately trivial Σ∝D-2 form, therefore limiting their interpretation asphysically meaningful relations. In addition, these relations aresubject to selection effects rendering them even less useful. FurtherMonte Carlo simulations suggest that the effect of survey sensitivityhas the opposite effect of volume selection (e.g. Malmquist bias, avolume selection effect that shapes the Galactic sample) by tending toflatten the slopes toward a trivial relation. In this case, the trueslopes may be steeper than the observed slopes; (ii) compact M 82 SNRsappear to follow a uniquely different Σ-D relation in comparisonto the larger, older SNRs in the other 10 galaxies. Monte Carlosimulations suggest that the probability of this difference arising bychance is ≈1% to 10%, depending on what is assumed regarding theunderlying SNR population; (iii) three candidate hypernova remnants wereidentified in our sample of 11 nearby galaxies.
| Star formation, massive stars, and super star clusters in nearby galaxies
The Square Kilometer Array (SKA) will enable studies of star formationin nearby galaxies with a level of detail never before possible outsideof the Milky Way. Because the earliest stages of stellar evolution areoften inaccessible at optical and near-infrared wavelengths, highspatial resolution radio observations are necessary to exploreextragalactic star formation. The SKA will have the sensitivity todetect individual ultracompact HII regions out to the distance of nearly50 Mpc, allowing us to study their spatial distributions, morphologies,and populations statistics in a wide range of environments. Radioobservations of Wolf-Rayet stars outside of the Milky Way will also bepossible for the first time, greatly expanding the range of conditionsin which their mass loss rates can be determined from free-freeemission. On a vastly larger scale, natal of super star clusters will beaccessible to the SKA out to redshifts of nearly z 0.1. Theunprecedented sensitivity of radio observations with the SKA will alsoplace tight constraints on the star formation rates as low as1Mȯ yr‑1 in galaxies out to a redshiftof z 1 by directly measuring the thermal radio flux densitywithout assumptions about a galaxy’s magnetic field strength,cosmic ray production rate, or extinction.
| Neutral hydrogen absorption at the centre of NGC 2146
We present 1.4-GHz HI absorption line observations towards thestarburst in NGC 2146, made with the Very Large Array and theMulti-Element Radio-Linked Interferometer Network. The HI gas has arotating disc/ring structure with column densities between 6 and 18× 1021 atom cm-2. The HI absorption has auniform spatial and velocity distribution, and does not reveal anyanomalous material concentration or velocity in the central region ofthe galaxy which might indicate an encounter with another galaxy or afar-evolved merger. We conclude that the signs of an encounter causingthe starburst should be searched for in the outer regions of the galaxy.
| Structural parameters of nearby emission-line galaxies
We present the results of an investigation on the main structuralproperties derived from VRI and Hα surface photometry of galaxieshosting nuclear emission-line regions [including Seyfert 1, Seyfert 2,low-ionization nuclear emission region (LINER) and starburst galaxies]as compared with normal galaxies. Our original sample comprises 22active galaxies, four starbursts and one normal galaxy and has beenextended with several samples obtained from the literature. Bulge anddisc parameters, along with the bulge-to-disc luminosity ratio, havebeen derived applying an iterative procedure. The resulting parametershave been combined with additional data in order to reach astatistically significant sample. We find some differences in the bulgedistribution across the different nuclear types that could implyfamilies of bulges with different physical properties. Bulge and disccharacteristic colours have been defined and derived for our sample andcompared with a control sample of early-type objects. The resultssuggest that bulge and disc stellar populations are comparable in normaland active galaxies.
| Multifrequency radio-continuum observations of NGC 1569: evidence for a convective wind
We present high-sensitivity radio-continuum observations with the VeryLarge Array (VLA) and Ryle Telescope at 1.5, 4.9, 8.4 and 15.4 GHz ofthe dwarf irregular galaxy NGC 1569. The radio data show an extended,irregularly shaped halo with filamentary structure around the galaxy.The spectral index maps reveal an unusually patchy distribution withregions of flat spectral index extending into the halo. The data allowus to perform a spatially resolved spectral-fitting analysis of thecontinuum emission from which we derive maps of the thermal andsynchrotron emission. The thermal radio emission is concentrated towardsthe brightest H II region west of the super star clusters A and B,whereas the distribution of the synchrotron emission peaks in a bar-likestructure in the disc extending between the two clusters. The total fluxdensity of the thermal radio emission allows us to derive the integratedsynchrotron spectrum and we confirm the break in the spectrum that wasfound by Israel & de Bruyn. We discuss various possibilities thatcould produce such a break and conclude that the only mechanism able tofit the radio data and remain consistent with data at other wavelengthsis a convective wind allowing cosmic ray electrons to escape from thehalo.
| HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies
We report systematic HCN J=1-0 (and CO) observations of a sample of 53infrared (IR) and/or CO-bright and/or luminous galaxies, including sevenultraluminous infrared galaxies, nearly 20 luminous infrared galaxies,and more than a dozen of the nearest normal spiral galaxies. This is thelargest and most sensitive HCN survey of galaxies to date. All galaxiesobserved so far follow the tight correlation between the IR luminosityLIR and the HCN luminosity LHCN initially proposedby Solomon, Downes, & Radford, which is detailed in a companionpaper. We also address here the issue of HCN excitation. There is noparticularly strong correlation between LHCN and the 12 μmluminosity; in fact, of all the four IRAS bands, the 12 μm luminosityhas the weakest correlation with the HCN luminosity. There is also noevidence of stronger HCN emission or a higher ratio of HCN and COluminosities LHCN/LCO for galaxies with excess 12μm emission. This result implies that mid-IR radiative pumping, orpopulating, of the J=1 level of HCN by a mid-IR vibrational transitionis not important compared with the collisional excitation by densemolecular hydrogen. Furthermore, large velocity gradient calculationsjustify the use of HCN J=1-0 emission as a tracer of high-densitymolecular gas (>~3×104/τcm-3) andgive an estimate of the mass of dense molecular gas from HCNobservations. Therefore, LHCN may be used as a measure of thetotal mass of dense molecular gas, and the luminosity ratioLHCN/LCO may indicate the fraction of moleculargas that is dense.
| Very Large Array H92α and H53α Radio Recombination Line Observations of M82
We present high angular resolution (0.6") observations made with theVery Large Array of the radio continuum at 8.3 and 43 GHz, as well asH92α and H53α radio recombination lines from the nearby (~3Mpc) starburst galaxy M82. In the continuum we identify 58 sources at8.3 GHz, of which 19 have no counterparts in catalogs published at otherfrequencies. At 43 GHz we identify 18 sources, unresolved at 0.6"resolution, of which five were unknown previously. The spatialdistribution of the H92α line is inhomogeneous; we identify 27features; about half of them are associated with continuum emissionsources. Their sizes are typically in the range 2-10 pc. Althoughobserved with poorer signal-to-noise ratio, the H53α line isdetected. The line and continuum emission are modeled using a collectionof H II regions at different distances from the nucleus. Theobservations can be interpreted assuming a single-density component, butequally well with two components if constraints originating fromprevious high-resolution continuum observations are used. Thehigh-density component has a density of ~4×104cm-3. However, the bulk of the ionization is in regions withdensities that are typically a factor of 10 lower. The gas kinematics,using the H92α line, confirms the presence of steep velocitygradient (26 km s-1 arcsec-1) in the nuclearregion, as previously reported, in particular from observations of the[Ne II] line at 12 μm. This gradient has about the same amplitude onboth sides of the nucleus. Since this steep gradient is observed notonly on the major axis but also at large distances along a band atP.A.~150deg, the interpretation in terms of x2orbits elongated along the minor axis of the bar, which would beobserved at an angle close to the inclination of the main disk, seemsinadequate. The observed kinematics cannot be modeled using a simplemodel that consists of a set of circular orbits observed at differenttilt angles. Ad hoc radial motions must be introduced to reproduce thepattern of the velocity field. Different families of orbits areindicated since we detect a signature in the kinematics at thetransition between the two plateaus observed in the NIR lightdistribution. These H92α data also reveal the base of the outflowwhere the injection toward the halo on the northern side occurs. Theoutflow has a major effect on the observed kinematics, present even inthe disk at distances close to the nucleus. The kinematic patternsuggests a connection between the gas flowing in the plane of M82 towardthe center; this behavior most likely is due to the presence of a barand the outflow out of the plane.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
INHABITED SKY
