Amorphous Galaxy Research Articles

A large population of red supergiants in the super star cluster NGC 1705-1

We present near-infrared integral field observations of the super star cluster in the amorphous galaxy NGC1705. Data have been collected with SINFONI mounted on the VLT. Adaptive optics was used under good seeing conditions. Mosaics of the cluster and its immediate surrounding have been constructed. The cluster is not spatially resolved. Its radius is smaller than 2.85 ± 0.50pc. The K-band spectrum of the cluster is dominated by strong CO absorption bandheads. It is typical of a Galactic K 4–5 supergiant. Its age is estimated to be 12 ± 6Myr. The large error bar is rooted in the uncertainties of the input physics and ingredients of different evolutionary models.

Observation of the Halo of NGC 3077 near the “Garland” Region Using theHubble Space Telescope

We report the detection of upper main-sequence stars and red giant branch stars in the halo of an amorphous galaxy, NGC 3077. The observations were made using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The red giant branch luminosity function in the I band shows a sudden discontinuity at I = 24.0 ± 0.1 mag. Identifying this with the tip of the red giant branch (TRGB) and adopting the calibration provided by Lee, Freedman, & Madore and a foreground extinction of AB = 0.21 mag, we obtain a distance modulus of (m - M)0 = 27.93 ± 0.14rand ± 0.16sys. This value agrees well with the distance estimates of four other galaxies in the M81 Group. In addition to the RGB stars, we observe a concentration of upper main-sequence stars in the halo of NGC 3077, which coincides partially with a feature known as the "Garland." Using Padua isochrones, these stars are estimated to be less than 150 Myr old. Assuming that the nearest encounter between NGC 3077 and M81 occurred 280 Myr ago, as has been predicted by the numerical simulations of Yun, the observed upper main-sequence stars are likely the result of star formation triggered by the M81+NGC 3077 tidal interaction.

The Taxonomy of Blue Amorphous Galaxies. II. Structure and Evolution

Dwarf galaxies play an important role in our understanding of galaxy formation and evolution, and starbursts are believed to aUect the structure and evolution of dwarf galaxies strongly. We have therefore embarked on a systematic study of 12 of the nearest dwarf galaxies thought to be undergoing bursts of star formation. These were selected primarily by their morphological type (blue ii amorphous ˇˇ galaxies). We show that these blue amorphous galaxies are not physically distinguishable from dwarfs selected as starbursting by other methods, such as blue compact dwarfs (BCDs) and H II galaxies. All these classes exhibit surface brightness pro—les that are exponential in the outer regions but often have a (r Z 1.5r e ) predominantly central blue excess, suggesting a young burst in an older, redder galaxy. Typically, the starbursting ii cores ˇˇ are young (D107¨108 yr) events compared to the older (D109¨1010 yr) underlying galaxy (the ii envelope ˇˇ). The ratio of the core to envelope in blue light ranges from essentially zero to about 2. These starbursts are therefore modest events involving only a few percent of the stellar mass. The envelopes have surface brightnesses that are much higher than typical dwarf irregular (dI) galaxies, so it is unlikely that there is a straightforward evolutionary relation between typical dIs and dwarf star- burst galaxies. Instead we suggest that amorphous galaxies may repeatedly cycle through starburst and quiescent phases, corresponding to the galaxies with strong and weak/absent cores, respectively. Once amorphous galaxies use up the available gas (either through star formation or galactic winds) so that star formation is shut oU, the faded remnants would strongly resemble dwarf elliptical galaxies. However, in the current cosmological epoch, this is evidently a slow process that is the aftermath of a series of many weak, recurring bursts. Present-day dEs must have experienced more rapid and intense evolution than this in the distant past. Subject headings: galaxies: evolutiongalaxies: photometrygalaxies: stellar content ¨ galaxies: starburstgalaxies: structure

The Amorphous Galaxy NGC 2777: H [CSC]i[/CSC] Evidence for Tidal Interaction with a Faint Companion

NGC 2777 is an amorphous galaxy possessing the classic signature of an A-type spectrum with superposed emission lines. There is no optical evidence for an interaction, but observations of the neutral hydrogen in this system reveal an H I bridge between NGC 2777 and U3, a companion that is 3 mag fainter. Far-infrared observations indicate that the current rate of massive star formation in NGC 2777 is low, but the optical spectra indicate that it must have been significantly higher in the recent past. We were unable to detect CO in either galaxy. We also consider a sample of 10 amorphous galaxies, all showing A-type absorption spectra: nine from the Revised Shapley-Ames Catalog, plus NGC 2777. All show optical/H I interaction and/or peculiar velocity fields, supporting the view that amorphous characteristics are the result of interaction. For some of the galaxies, the interacting companion is much fainter than the amorphous system, which indicates that previous concerns raised by the existence of isolated amorphous galaxies may have been premature.

The Taxonomy of Blue Amorphous Galaxies. I. Hα and UBVI Data

Dwarf galaxies play an important role in our understanding of galaxy formation and evolution. We have embarked on a systematic study of 12 nearby dwarf galaxies (most of which have been classified as amorphous) selected preferentially by their blue colors. The properties of the galaxies in the sample suggest that they are in a burst or postburst state. It seems likely that these amorphous galaxies are closely related to other starburst dwarfs such as blue compact dwarfs (BCDs) and H II galaxies but are considerably closer and therefore easier to study. If so, these galaxies may offer important insights into dwarf galaxy evolution. In an effort to clarify the role of starbursts in evolutionary scenarios for dwarf galaxies, we present Hα and UBVI data for our sample. Blue amorphous galaxies, like BCDs and H II galaxies, have surface brightness profiles that are exponential in the outer regions (r 1.5re) but have a predominantly blue central excess, which suggests a young burst in an older, redder galaxy. Seven of the galaxies have the bubble or filamentary Hα morphology and double-peaked emission lines that are the signature of superbubbles or superwind activity. These galaxies are typically the ones with the strongest central excesses. The underlying exponential galaxies are very similar to those found in BCDs and H II galaxies. How amorphous galaxies fit into the dwarf irregular-starburst dwarf-dwarf elliptical evolutionary debate is less clear. In this paper, we present our data and make some preliminary comparisons between amorphous galaxies and other classes of dwarf galaxies. In a future companion paper, we will compare this sample more quantitatively with other dwarf galaxy samples in an effort to determine if amorphous galaxies are a physically different class of object from other starburst dwarfs such as BCDs and H II galaxies and also investigate their place in dwarf galaxy evolution scenarios.

The Ionized Gas in the Aftermath of a Starburst: The Case of NGC 1569

Results from multislit optical spectroscopy of 16 H II regions and deep Hα imaging of the amorphous galaxy NGC 1569 are presented. The extinction across the main body of the galaxy, as derived from the Balmer Hα/Hβ line ratio, indicates that most of the observed extinction is taking place in our own Galaxy; the latter amounts to (AV)local = 1.61 ± 0.09, while the extinction due to NGC 1569 is (AV)intrinsic = 0.65 ± 0.04. The electron temperature was measured in three H II regions using the [O III] λ4363 line. The O/H distribution shows no gradient along the main axis of the galaxy, which is consistent with the behavior observed in other low-mass galaxies. The average metal abundance is 12 + log O/H = 8.26, with little scatter, suggesting, on one hand, that mixing mechanisms are very efficient throughout the main body of the galaxy, or, on the other, that the most recent nucleosynthetic products are hiding in a hot coronal gas phase. Up to 20% ± 4% of the global Hα emission originates from the faint diffuse halo component surrounding the main body of NGC 1569. We explore the possibility that runaway OB stars that are due to cluster ejection and associated with the burst of supernovae which ended ~5 × 106 yr ago could be responsible for most of the ionization of the halo gas.

Neutral Hydrogen Observations of the Amorphous Galaxy NGC 4670 at Moderate Spatial Resolution

We present a moderate resolution H I map of the amorphous galaxy NGC 4670. Our previous lower resolution data had shown a symmetric H I distribution centered on the single supergiant H II region and highly concentrated to the center of the galaxy. We now resolve the central H I distribution into three clouds, with the eastern cloud located on the central H II region and 1.1 kpc from the westernmost cloud. Masses of the clouds are of order 10(8) M. and are associated with the intense central star formation that has produced a luminous, compact star cluster. The central gas surface density is very high at the new resolution, and the surface density drops off rapidly with radius. We determine a rotation curve for the galaxy; however, the low and uncertain value of the inclination (i similar to 28 degrees) and the possible presence of a warp make the derived rotation speed (V-R similar to 110 km s(-1)) quite uncertain. There is some correspondence between features in the H I distribution and locations of Ha loops and filaments that emanate from the central H II region.

High‐Dispersion Spectroscopy of a Luminous, Young Star Cluster in NGC 1705: Further Evidence for Present‐Day Formation of Globular Clusters

We present evidence that some of the compact, luminous, young star clusters discovered recently through images taken with the Hubble Space Telescope (HST) have masses comparable to those of old Galactic globular clusters. The super star in the center of the nearby amorphous galaxy NGC 1705 has been observed with high dispersion at optical wavelengths using the HIRES echelle spectro-graph on the Keck 10 m telescope. Numerous weak metal lines arising from the atmospheres of cool supergiants have been detected in the integrated spectrum, permitting a direct measurement of the line-of-sight stellar velocity dispersion through cross-correlation with a template star; the result is σ* 11.4 ± 1.5 km s–1. Assuming that the system is gravitationally bound and using a cluster size measured from HST images, we apply the virial theorem to obtain the dynamical mass. Its derived mass [(8.2 ± 2.1) × 104 M☉], mass density (2.7 ± 104 pc–3), and predicted mass-to-light ratio after aging by 10-15 Gyr [0.6-1.6(M/LV)☉] closely resemble those of the majority of evolved Galactic globular clusters. The central cluster in NGC 1705 appears to be very similar in nature to one of the bright clusters in NGC 1569, which was discussed earlier this year by Ho & Filippenko. We also observed the brightest cluster in the Magellanic irregular galaxy NGC 4214, but no lines suitable for measuring its velocity dispersion were detected, most likely because of the very young age of the cluster. Although these observations need to be extended to a much larger sample of objects before generalizations can be made concerning the nature of similar clusters observed in other galaxies, our preliminary results are tantalizing and strongly suggest that, at least in two cases, we are witnessing globular clusters in their extreme youth.

The Dwarf Galaxy NGC 1705-A Highly Composite Stellar Population

We present visible and near-infrared images of the amorphous dwarf galaxy NGC 1705 in the $B$, $V$, $R$, $J$, $H$, and $K$ bands. Optical and near infrared colors in the galaxy are consistent with a composite population with colors similar to those of the bluest Sc nuclei. The contribution of young stars to the disk population of the galaxy decreases outwards from the center. In the near infrared images we detect three unresolved point sources, two of which have colors and luminosities comparable to intermediate age LMC clusters.

Discovery of Cepheids in NGC 5253: Absolute peak brightness of SN IA 1895B and SN IA 1972E and the value of H[SUB]0[/SUB

Observations of the Hubble Space Telescope (HST) between 1993 May 31 and 1993 July 19 in 20 epochs in the F555W passband and five epochs in the F785LP passband have led to the discovery of 14 Cepheids in the Amorphous galaxy NGC 5253. The apparent V distance modulus is (m-M)(sub AV) = 28.08 +/- 0.10 determined from the 12 Cepheids with normal amplitudes. The distance modulus using the F785LP data is consistent with the V value to within the errors. Five methods used to determine the internal reddening are consistent with zero differential reddening, accurate to a level of E(B-V) less than 0.05 mag, over the region occupied by Cepheids and the two supernovae (SNe) produced by NGC 5253. The apparent magnitudes at maximum for the two SNe in NGC 5253 are adopted as B(sub max) = 8.33 +/- 0.2 mag for SN 1895B, and B(sub max) = 8.56 +/- 0.1 and V(sub max) = 8.60 +/- 0.1 for SN 1972E which is a prototype SN of Type Ia. The apparent magnitude system used by Walker (1923) for SN 1859B has been corrected to the modern B scale and zero point to determine its adopted B(sub max) value.

Observations of the impact of starbursts on the interstellar medium in dwarf galaxies

Dwarf galaxies play a crucial role in our understanding of the formation and evolution of galaxies, and the concept of supernova-driven mass outflows is a vital ingredient in theories of the structure and evolution of dwarf galaxies. Despite the theoretical importance of these outflows, there is a very limited amount of direct observational evidence for their existence. We have therefore begun a detailed multi-wave-band search for outflows in dwarf (M(sub B) greater than or = -18) galaxies with extensive recent or ongoing centrally concentrated star formation. We report the first results of this search in the present paper. Observations of the ionized gas in dwarf amorphous galaxies with centrally concentrated populations of massive stars provide evidence for the large-scale expansion of their expansion of their ionized interstellar media. Fabry-Perot H alpha images reveal the presence of kiloparsec-scale 'superbubbles' and filaments which tend to be oriented along the galaxy minor axis. These structures are comparable in size to the chracteristic optical sizes of the galaxies, and dominate the morphology of the galaxies at low surface brightness in H alpha. Since expanding structure of this size and velocity are not observed in all low-mass galaxies with recent or ongoing star formation, we suggest that we are witnessing transient events that likely have a relatively low 'duty cycle' in such galaxies. That is, we argue that the particular galaxies in the present paper have had significantly elevated star formation rates over the past 10(exp 7)-10(exp 8) yr (i.e., these are starburst or young poststarburst systems). This interpretation is consistent with the optical colors and emission-line properties of these galaxies.

Hubble Space Telescope imaging of the central star forming region in NGC 1140 [SUP]1[/SUP

We present broadband images taken with the Hubble Space Telescope's Planetary Camera of the central supergiant H II region in the amorphous galaxy NGC 1140. These images allow observations to a resolution of about 13 pc at the galaxy, and they reveal that its central 1/2 kpc contains 6-7 blue, luminous, compact super star clusters, many of which would be comparable in luminosity to globular clusters at the same age. A blue arc-shaped structure near the center may be a grouping of less luminous, R136/NGC 2070-sized clusters or a sheet of OB stars. Additional somewhat less luminous and redder clusters are also found farther out from the center. If these clusters are older, they too could have had luminosities comparable to those of the central six clusters at a comparable age. Thus, we find that NGC 1140 is remarkable in the number of extreme clusters that it has formed recently in a relatively small area of the galaxy. Since NGC 1140 exhibits global characteristics that are consistent with a recent merger, these clusters are likely to be a product of that event. This galaxy adds to the number of cases where rapid star formation has evidently produced super star clusters.

Neutral hydrogen and optical properties of three amorphous galaxies

We present new interferometric H I and optical observations of three amorphous galaxies, systems with a smooth, high surface brightness but an asymmetrical distribution of light. All three galaxies are forming stars and have LMC-like emission-line ratios, low dust content, and high H I velocity dispersions. NGC 1140 has a boxy inner morphology with a hook off one corner. At low light levels unusual extensions of starlight are seen curving to the northwest and southeast. The galaxy contains a very luminous central star-forming region and a small chain of H II regions that coincide with the hook. The central H II region has broad H(alpha) velocity profiles full width at half maximum (FWHM) less than or equal to 140 km/s, and it is a radio continuum source. There is a rotating H I gas disk, 40 kpc in radius, at a position angle 51 deg from the optical major axis. The central gas ridge follows the chain of H II regions, and the H I peak is on the hook. The outer gas on the southeast side curves away from the H I major axis. The central gas density is high, and the surface density declines very slowly with radius. The rotation velocity yields a mass of 1 x 10(exp 11) solar mass at 3.3 Holmberg radii (R(sub H)). NGC 1800 has a hook that coincides with a large H II region, and an r(exp 1/4) luminosity distribution. There are numerous H II regions along the major axis and extraordinary filaments of ionized gas. Emanating from the major axis on either side of the galaxy are H(alpha) fingers approximately 750 pc long. About 2.3 kpc to the north is a web of filaments approximately 3 kpc in extent. H(alpha) profiles of H II regions and filaments are narrow. The H I gas disk has a position angle that is approximately 13 deg different from that of the optical axis. There are two peaks near the center, one of which is near the largest H II region. Beyond the Holmberg radius to the west is a 6 x 10(exp 6) solar mass H I cloud. Its velocity indicates a mass of approximately 6 x 10(exp 9) solar mass for NGC 1800 at 1.5 R(sub H). At approximately R(sub 25) to the east there is a large H I shell. Also at approximately R(sub 25) on both sides the velocity gradient switches by 90 deg, and in the interior the rotation is about the major axis. The central gas density is low and falls off slowly. In the inner regions NGC 4670 resembles an S0/a galaxy seen rather edge-on. It contains a central supergiant H II region with very high velocity widths (FWHM less than or equal to 180 km/s) and complex velocity structures. It is a radio continuum source as well. The H I gas is a single spherical cloud or a disk at low inclination centered on the galaxy with a slight elongation along the optical major axis and rotation about the minor axis. The central gas density is high, and there is a high degree of concentration. The rotation speed indicates a total mass of 5 x 10(exp 10) solar mass at 1.1 R(sub H). We compare these characteristics with properties of gas in the presence of stellar bar potentials, gas warps, and interacting and merging galaxy models. Although there are inconsistencies and uncertainties, we conclude that NGC 1140 is a spiral of low surface brightness that has undergone a merger, while NGC 1800 and NGC 4670 are, respectively, probably an Im system and a spiral that had an encounter of the Noguchi (1988a) kind.

A survey for extra-H II region ionized gas structures in irregular galaxies

We have conducted a deep Hα imaging survey of 51 irregular and amorphous galaxies with the objective of searching for large ionized gas structures outside of normal H II regions. In this sample 12% of the galaxies contain at least one ionized supershell (radius > 300 pc), 24% contain supergiant ionized filaments which are not obviously connected with a particular star forming region, and 27% contain one or more of these types of structures. For the most part, large ionized gas structures are found in galaxies that are engaging in intense star formation or that contain at least one unusually large concentration of massive stars. Thus, in most galaxies these structures are likely to have been produced by massive stars. However, there is a large range in the properties of the host galaxies, and not all galaxies that are actively forming stars or that contain luminous H II regions also contain supershells or giant filaments of ionized gas. As giant H II regions appear to be more common than large scale ionized features, we statistically find either that not all giant H II regions will produce supershells or giant filaments or that the extra-H II region structures have a shorter lifetime than the giant H II regions themselves. Two galaxies are particularly noteworthy. DDO 75 is a small galaxy with a low level of star formation activity but which nevertheless hosts several remarkably large ionized supershells. We cannot explain the origin and ionization of these structures. By contrast, DDO 50 has a neutral interstellar medium which contains numerous holes but which has no extraordinary ionized gas structures. The most likely explanations are either that the H I structures in DDO 50 are older than the lives of the massive stars that produced them or that the H I holes were formed by a process not directly associated with concentrations of massive stars. Only 2 out of 15 normal irregulars viewed at inclination angles greater than 60^deg^, plus two galaxies that are clearly in an abnormal state, show evidence for structures that are or could be extending out of the disk of the galaxy. In three of these systems the filaments which are farthest from the disks are roughly parallel to the galactic planes, and therefore have the morphologies expected of stalled supershells. Thus, galactic disk breakouts or chimneys containing optically visible ionized gas cannot be a frequent phenomenon in irregular galaxies and, even when they do occur, we cannot assume that a galactic wind is also present.

Abundances in the starburst galaxy II Zw 40

Low-dispersion optical imaging spectroscopy has been performed of the starburst galaxy II Zw 40, employing the area spectroscopy system at the Anglo-Australian Telescope. A procedure was applied to correct for the effects of atmospheric differential refraction arising from the large zenith distance of the observations. Maps of interstellar reddening, electron temperature and abundances of elements (O/H, N/H, He/H and Ne/H) were constructed. Comparisons are made with similar maps of the amorphous galaxy NGC 5253. II Zw 40 suffers from large intrinsic absorption due to dust, with the greater values of A v being found in the northern half of the galaxy. Electron temperatures were measured at 24 points over the bright core of the galaxy from the [O III] 5007/4363-A line ratio

CO OBSERVATIONS OF SEVERAL AMORPHOUS AND MAGELLANIC IRREGULAR GALAXIES

ABSTRACT We report upper limits on the 12CO J=1-0 and 2-1 line emission for several positions in 5 Magellanic irregular and amorphous galaxies. The positions in three of the galaxies were chosen because they were peaks in H I intererometer maps. The galactic centers were observed for the other two. The upper limits on the emission are below (sometimes far below) what was expected based upon the empirical relationship between CO and IRAS 100 micron flux, suggesting that one or more of the following is (are) true: 1) much of the molecular gas has been used up during the star formation process, 2) the H2 has been dissociated by the intense ultraviolet radiation produced by the massive stars 3) that the CO emitting surfaces of the molecular clouds are smaller due to the effects of dissociation due to the intense ultraviolet radiation, or 4) that CO is not tracing the H2. The results reported here, when combined with other recent work, suggest that molecular gas is a relatively transient phenomenon in dwarf galaxies, unlike spirals.

NGC 4286 and NGC 3377A - Galaxies with mixed morphologies between the dwarf Im and dE/dSO types

view Abstract Citations (41) References (38) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS NGC 4286 and NGC 3377A: Galaxies with Mixed Morphologies between the Dwarf Im and dE/dSO Types Sandage, Allan ; Hoffman, G. L. Abstract Optical and radio data for the nucleated dwarf galaxy NGC 4286 show a mixed morphology between a star-forming Im type and a gas-poor dS0, N early-type dwarf. The Im dwarf NGC 3377A shows no gas depletion, but its morphology and size are similar to the "huge, very low surface brightness" (VLSB) gas-poor galaxies recently identified in the Virgo cluster. We interpret the morphology of NGC 4286 as an original Im system that has transformed itself (speciation along the dwarf sequence) into a gas-poor dS0, N by losing most (but not all) of its original gas through an internal supergalactic wind that was generated earlier by a superluminous star cluster. The remnant of this cluster has now sunk to the center by dynamical friction to form the observed unresolved bright nucleus. Such star clusters and superwinds are known to exist in many present-day starburst Sm and amorphous galaxies such as NGC 1569 and NGC 1705. Publication: The Astrophysical Journal Pub Date: October 1991 DOI: 10.1086/186150 Bibcode: 1991ApJ...379L..45S Keywords: Dwarf Galaxies; Galactic Evolution; Galactic Structure; Star Clusters; Starburst Galaxies; Virgo Galactic Cluster; Astrophysics; GALAXIES: EVOLUTION; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 3377A; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4286; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (7) NED (2)

A search for quasars in a field around NGC 520

Summary We have performed an objective-prism search for quasars in a field around the galaxy NGC 520. The AUTOMATED QUASAR DETECTION (AQD) software has been applied and a sample of 175 highly rated quasar candidates has been selected and is presented here. The 2D and 3D distributions of the candidates have been analysed. The distribution on the celestial sphere turns out to be non-random. The deviations from randomness are both a tendency to exhibit anticorrelation at the scale of a few arcmin and a significant excess of objects at separations of about 20 arcmin. A very weak tendency towards spatial clustering is also detected (significance level 0.04) at a typical scale of 10 h−1 Mpc, but it concerns an extremely small number of candidates and a definitive conclusion is delayed until all the candidates have been observed spectroscopically. Finally, in order to test the claims by Arp & Duhalde that an excess of quasars exists near NGC 520 and that there is a preferential NE-SW alignment of bright quasars across this amorphous galaxy, we have also performed various statistical analyses of the distribution of the high-grade quasar candidates with respect to NGC 520: no sign of any possible relation is present in our data.

Imaging and spectrophotometry of Virgo cluster irregular galaxies

view Abstract Citations (68) References (74) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Imaging and Spectrophotometry of Virgo Cluster Irregular Galaxies Gallagher, John S., III ; Hunter, Deidre A. Abstract Large-aperture spectrophotometric observations and CCD images in the Hα and R bands are presented for Virgo Cluster Magellanic, dwarf and amorphous irregular galaxies. These data provide a basis for comparisons between star-forming properties, morphologies, and emission-line characteristics in field and cluster irregular galaxies. Most Virgo Cluster Magellanic and dwarf irregular galaxies are optically similar to field galaxies. Modest cluster-field differences may exist in H I gas content (cluster lower than field) and in star-formation rates per unit area (cluster lower than field). A minority of Virgo Cluster irregulars have optical and H I properties consistent with recent gas loss, but the rate of production of gas-poor irregulars is too low to account for the large population of Virgo Cluster dE galaxies or to explain the deficiency in the number of irregulars relative to spirals. Also puzzling are the properties of red amorphous or peculiar E/S0 galaxies in our sample. Radial brightness profiles have been measured for these galaxies, and only NGC 4641 follows the r^1/4^ law of classical ellipticals, four galaxies have exponential brightness distributions, and one system has an indeterminant profile form. The physical nature of the red amorphous galaxies thus remains unclear. We also suggest that the properties of Virgo Cluster irregular galaxies are best understood in terms of environmental modifications of normal systems in the circumcluster environment and in the cluster core. However, these processes do not appear to account for the large core region population of dE galaxies, and thus initial conditions may also have played a role in shaping the Virgo Cluster dwarf galaxy population. Publication: The Astronomical Journal Pub Date: September 1989 DOI: 10.1086/115180 Bibcode: 1989AJ.....98..806G Keywords: Astronomical Photometry; Dwarf Galaxies; Irregular Galaxies; Spectrophotometry; Virgo Galactic Cluster; Astronomical Catalogs; Brightness Distribution; Density Distribution; H I Regions; Magellanic Clouds; Visual Photometry; Astrophysics; GALAXIES: GENERAL full text sources ADS | data products SIMBAD (43) NED (41)

The structure of giant extragalactic H II regions in NGC 1365, 1566, and 2997

view Abstract Citations (10) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Structure of Giant Extragalactic H II Regions in NGC 1365, 1566, and 2997 Roy, Jean-Rene ; Belley, Julien ; Walsh, J. R. Abstract Optical spectroscopic mapping has been performed of chains of H II regions and complexes in the barred spiral galaxy NGC 1365, the normal spirals 1566, 2997, and the amorphous galaxy NGC 5253. A comparison of the spatial behavior of the line ratios between the galaxies is presented. The line ratios [N II]/[O II], [S II]/[O II], and ([O II] + [O III])/Hβ vary significantly from pixel to pixel. The amplitude of pixel-to-pixel line fluctuations is highest for line ratios involving [O II] and [0 III] and is smallest for the absorption Anu_ derived from Hα/Hβ, for [N II]/Hα, [S II]/Hα, and [N II]/[S II]. The amplitudes of line ratio fluctuations increase with galactocentric distances in NGC 2997; this trend is marginal in the weakly barred galaxy NGC 1566 and non-existent in the barred spiral NGC 1365. The line ratio fluctuations are ascribed to varying density distributions for electrons and the various ions producing different line ratios especially for lines originating from different nebular regions. The radial increase of the amplitudes of line ratio fluctuations in NGC 2997 is suggested as due to the changing density distribution within nebulae with galactocentric radius. Publication: The Astronomical Journal Pub Date: April 1989 DOI: 10.1086/115045 Bibcode: 1989AJ.....97.1010R Keywords: H Ii Regions; Intergalactic Media; Nebulae; Nitrogen; Silicon; Spiral Galaxies; Barred Galaxies; Electron Density (Concentration); Forbidden Transitions; H Lines; Oxygen Spectra; Astrophysics; GALAXIES: ABUNDANCES; GALAXIES: STRUCTURE; NEBULAE: H II REGIONS full text sources ADS | data products SIMBAD (6) NED (4)