arXiv subjects

Noah Brosch

Publications and source records attributed to Noah Brosch.

72 records · Page 4Linked to original sources

arXiv2004

Principal component analysis of IUE galaxy spectra

We analyse the UV spectral energy distribution of a sample of normal galaxies listed in the IUE-INES Guide No. 2-Normal Galaxies (Formiggini & Brosch, 2000) using a Principal Component Analysis. The sample consists of the IUE-SW spectra of the central regions of 118 galaxies, where the IUE aperture included more than 1 per cent of the galaxy size. The principal components are associated with the main components observed in the UV spectra of galaxies. The first component, accounting for the largest source of diversity, can be associated with the UV continuum emission. The second component represents the UV contribution of an underlying evolved stellar population. The third component is sensitive to the amount of activity in the central regions of galaxies and measures the strength of star formation events. In all the samples analysed here the principal component representative of star-forming activity accounts for a significant percentage of the variance. The fractional contribution to the SED by the evolved stars and by the young population are similar. Projecting the SEDs onto their eigenspectra, we find that none of the coefficients of the principal components can outline an internal correlation or can correlate with the optical morphological types. In a sub-sample of 43 galaxies, consisting of almost only compact and BCD galaxies, the third principal component defines a sequence related to the degree of starburst activity of the galaxy.

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arXiv2003

Are interactions the primary triggers of star formation in dwarf galaxies?

We investigate the assumption that the trigger of star formation in dwarf galaxies are interactions with other galaxies, in the context of a search for a "primary" trigger of a first generation of stars. This is cosmologically relevant because the galaxy formation process consists not only of the accumulation of gas in a gravitational potential well, but also of the triggering of star formation in this gas mass and also because some high-z potentially primeval galaxy blocks look like nearby star-forming dwarf galaxies. We review theoretical ideas proposed to account for the tidal interaction triggering mechanism and present a series of observational tests of this assumption using published data. We show also results of a search in the vicinity of a composite sample of 96 dwarf late-type galaxies for interaction candidates showing star formation. The small number of possible perturbing galaxies indentified in the neighborhood of our sample galaxies, along with similar findings from other studies, supports the view that tidal interactions may not be relevant as primary triggers of star formation. We conclude that interactions between galaxies may explain some forms of star formation triggering, perhaps in central regions of large galaxies, but they do not seem to be significant for dwarf galaxies and, by inference, for first-time galaxies forming at high redshifts. Intuitive reasoning, based on an analogy with stellar dynamics, shows that conditions for primary star formation triggering may occur in gas masses oscillating in a dark matter potential. We propose this mechanism as a plausible primary trigger scenario, worth investigating theoretically.

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arXiv2002

The dwarf galaxy population of the Virgo Cluster

I review the status of knowledge about the dwarf galaxies in the Virgo Cluster (VC) concentrating on work published after 1995. I explain the present ideas about the nature and history of the dwarf elliptical galaxies and emphasize the major increments in understanding achieved in the study of the dwarf irregular population. Special attention is given to the samples of late-type dwarf galaxies in the VC studied at the Wise Observatory(BCDs and dIs), and to a comparison of their properties.

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arXiv2000

Saving Astronomical Treasures

The time domain is under-exploited in an astrophysical context, because the large photographic plate collections are almost never consulted by young astronomers. This is a result of lack of education of the more recent generations of astronomers, and of the deluge of terabyte-size datasets produced by new observations. Yet astronomical plates represent a unique set of resources and their quality does NOT improve with time. We point out valuable scientific projects that can be conducted with archival images and describe a first step to realize this at the Uccle Observatory, in Belgium.

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arXiv2000

Stromgren Photometry from z=0 to z~1. The Method

We use rest-frame Stromgren photometry to observe clusters of galaxies in a self-consistent manner from z=0 to z=0.8. Stromgren photometry of galaxies is an efficient compromise between standard broad-band photometry and spectroscopy, in the sense that it is more sensitive to subtle variations in spectral energy distributions than the former, yet much less time-consuming than the latter. Principal Component Analysis (PCA) is used to extract maximum information from the Stromgren data. By calibrating the Principal Components using well-studied galaxies (and stellar population models), we develop a purely empirical method to detect, and subsequently classify, cluster galaxies at all redshifts smaller than 0.8. Interlopers are discarded with unprecedented efficiency (up to 100%). The first Principal Component essentially reproduces the Hubble Sequence, and can thus be used to determine the global star formation history of cluster members. The (PC2, PC3) plane allows us to identify Seyfert galaxies (and distinguish them from starbursts) based on photometric colors alone. In the case of E/S0 galaxies with known redshift, we are able to resolve the age-dust- metallicity degeneracy, albeit at the accuracy limit of our present observations. This technique will allow us to probe galaxy clusters well beyond their cores and to fainter magnitudes than spectroscopy can achieve. We are able to directly compare these data over the entire redshift range without a priori assumptions because our observations do not require k-corrections. The compilation of such data for different cluster types over a wide redshift range is likely to set important constraints on the evolution of galaxies and on the clustering process.

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arXiv2000

Lopsidedness in dwarf irregular galaxies

We quantify the amplitude of the lopsidedness, the azimuthal angular asymmetry index, and the concentration of star forming regions, as represented by the distribution of the H$α$ emission, in a sample of 78 late-type irregular galaxies. We bin the observed galaxies in two groups representing blue compact galaxies (BCDs) and low surface brightness dwarf galaxies (LSBs). The light distribution is analysed with a novel algorithm, which allows detection of details in the light distribution pattern. We find that while the asymmetry of the underlying continuum light, representing the older stellar generations, is relatively small, the H$α$ emission is very asymmetric and is correlated in position angle with the continuum light. We test a model of random star formation over the extent of a galaxy by simulating HII regions in artificial dwarf galaxies. The implication is that random star formation over the full extent of a galaxy may be generated in LSB dwarf-irregular galaxies but not in BCD galaxies.

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arXiv1998

Late-type dwarf irregular galaxies in the Virgo cluster.I. H-alpha and red continuum data

We present H-alpha and red continuum observations for a sample of late-type low surface brightness (LSB) dwarf irregular galaxies(DIGs), consisting of all the m_B<17.2 ImIV and V galaxies in the Virgo cluster (VC), and compare them with a representative sample of VC high surface brightness (HSB) DIGs. Line fluxes and equivalent widths are listed for individual HII regions and total H-alpha emission is measured for the entire DIG. The HII regions we identified do not produce all the line emission. For LSBs with line emission we find star formation rates (SFR) of 6.9-43.0 10^{-3} M_sol/yr, ~10x weaker than for HSB objects. About 2 HII regions are detected per LSB galaxy; they are smaller and fainter than in HSB galaxies in the VC, have line equivalent widths ~50% of those in HSBs, and cover similar fractions of the galaxies. The line-emitting regions of LSB galaxies are preferentially located at the periphery of a galaxy, while in HSBs they tend to be central. The line strength of an HII region is correlated with the red continuum light underneath the region. No fundamental differences were identified in the star formation properties of the LSB and HSB dwarf galaxies we studied and infer that these galaxies must be similar, with the difference being the intensity of the present star-formation burst.

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arXiv1998

Dwarf Irregular Galaxies and the Intergalactic Medium

Dwarf galaxies (DGs) are more numerous than large galaxies. Most dwarfs in clusters are dEs, but in the field they belong mostly to late types. The importance of late-type DGs in the context of the intergalactic medium (IGM) lies in the fact that (at least some) are probably ``young'' galaxies forming stars for the first time. Many DGs have significant amounts of interstellar gas, they may form out of intergalactic gas clouds, and matter ejected from such galaxies as a result of star formation processes may enrich the IGM with metals. The physical mechanism responsible for triggering the star formation process has not yet been identified. The extended halos of DGs may provide (at least some of) the QSO absorption line systems. Recent observations also show that the LSB dwarf galaxies may be good tests of MOND.

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arXiv1998

Morphological aspects of star formation in dwarf galaxies

We studied the morphology of star formation in dwarf irregular galaxies and found that, in general, this takes place on one side of a galaxy and far from the center. This is mainly true for low surface brightness galaxies; in high surface brightness dwarf irregulars the star formation tends to be more centrally concentrated, as well as being more intense. We discuss possible star formation triggers in dwarf irregular galaxies, and evaluate the reasons for the peculiar distribution of star forming regions of these galaxies. Stochastic star formation, interactions with external gas, and tidal interactions appear to be ruled out as responsible for the asymmetric pattern of star formation. We conclude that asymmetry of a dark matter halo or of an asymmetric underlying stellar distribution may trigger an asymmetric pattern of star formation.

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arXiv1998

The shape of the LoTr 5 planetary nebula

We observed the large and faint planetary nebula (PN) around IN Com in H-alpha and [NII] light with a coronagraphic CCD device on the Wise Observatory reflector blocking the light from the central star. Our goal was to provide a second image of the object with which to confirm the features seen in the only published photograph from the paper reporting the discovery of this object. The nebula is extremely faint, but a combination of images totalling ~one and a half hours of exposure shows it fairly well. A novel image processing algorithm has been applied to the noisy image in order to reveal faint extended details of the images. The algorithm is based on a non-linear self-adaptive filter applied to the wavelet transform of the image. The nebula is not round or elliptical, but shows a two-lobed and possibly three-lobed morphology, as well as a peculiar hole-like feature East of the central star. There is definite East-West and slightly less definite North-South asymmetry.

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arXiv1998

Far-Ultraviolet Imaging of the Field Star Population in the Large Magellanic Cloud with HST

We present an analysis of the deepest pure-UV observations with the highest angular resolution ever performed, a set of 12 exposures with the HST WFPC2 and F160BW filter obtained in parallel observing mode, which cover $\sim$12 square arcminutes in the LMC, North of the bar and in the ``general field'' regime of the LMC. The 341 independent measurements of 198 objects represent an accumulated exposure of $\geq 2 10^4$ sec and reveal stars as faint as m$_{UV}\simeq$22 mag. The observations show that $\sim 2/3$ of the UV emission from the LMC is emitted by our HST-detected UV stars in the field, i.e., not in clusters or associations. We identified optical counterparts in the ROE/NRL photometric catalog for $\sim 1/3$ of the objects. The results are used to discuss the nature of these UV sources, to estimate the diffuse UV emission from the LMC as a prototype of dwarf galaxies, and to evaluate the contamination by field stars of UV observations of globular and open clusters in the LMC. We find that the projected density of UV stars in the general field of the LMC is a few times higher than in the Galactic disk close to the Sun. Combining our data with observations by UIT allows us to define the stellar UV luminosity function from $m_{UV}=8$ to 18 mag, and to confirm that the field regions in the LMC have been forming stars at a steady rate during the last 1 Gyr, with an IMF close to the Salpeter law.

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arXiv1998

UV sky surveys

I review the development of UV and EUV astronomy, covering the spectral range from 5 to 300 nm, with emphasis on sky surveys for discrete sources. I discuss studies which resulted in lists of sources observed by imaging and deliberately omit most spectroscopic studies. Technical issues, such as detector and telescope developments, are treated separately from descriptions of specific missions and their results, which contributed to the understanding of the UV sky. The missions are compared in terms of their ``survey power'', a variable which combines sky coverage and survey depth. I use the existing knowledge of UV sources to predict views of the UV sky, which I then compare with those actually detected. Finally, UV missions which will detect fainter sources and will fly in the near future are described, and a wish list for low-cost ventures, which could advance considerably our knowledge of the UV sky is presented.

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arXiv1998

Morphology of Star Formation Regions in Irregular Galaxies

The location of HII regions, which indicates the locus of present star formation in galaxies, is analyzed for a large collection of 110 irregular galaxies (Irr) imaged in H$α$ and nearby continuum. The analysis is primarily by visual inspection, although a two-dimensional quantitative measure is also employed. The two different analyses yield essentially identical results. HII regions appear preferentially at the edges of the light distribution, predominantly on one side of the galaxy, contrary to what is expected from stochastic self-propagating star formation scenarios. This peculiar distribution of star forming regions cannot be explained by a scenario of star formation triggered by an interaction with extragalactic gas, or by a strong one-armed spiral pattern.

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arXiv1998

Late-Type Dwarf Galaxies in the Virgo Cluster: I. The Samples

We selected samples of late-type dwarf galaxies in the Virgo cluster with HI information. The galaxies were observed at the Wise-Observatory using several broad-band and H$α$ bandpasses. UV measurements were carried out with the IUE Observatory from VILSPA, and with the FAUST shuttle-borne UV telescope. We describe our observations in detail, paying particular attention to the determination of measurement errors, and present the observational results together with published data and far-infrared information from IRAS. The sample will be analyzed in subsequent papers, in order to study star formation mechanisms.

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arXiv1998

Late-Type Dwarf Galaxies in the Virgo Cluster: II. Star Formation Properties

We study star-formation-inducing mechanisms in galaxies through multi- wavelength measurements of a sample of dwarf galaxies in the Virgo cluster described in paper I. Our main goal is to test how star formation inducing mechanisms depend on several parameters of the galaxies, such as morphological type and hydrogen content. We derive the star formation rate and star formation histories of the galaxies, and check their dependence on other parameters. Comparison of the sample galaxies with population synthesis models shows that these objects have significantly lower metallicity than the Solar value. The colors can generally be explained as a combination of two different stellar populations: a young (3--20 Myr) population which represents the stars currently forming, and an older (0.1--1 Gyr) population of previous stellar generations. This is consistent with the explanation that star formation in this type of objects takes place in short bursts followed by long quiescent periods. No significant relation is found between the star formation properties of the sample galaxies and their hydrogen content. Apparently, when star formation occurs in bursts, other parameters influence the star formation properties more significantly than the amount of hydrogen. No correlation is found between the projected Virgocentric distance and the rate of star formation in the galaxies.

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arXiv1998

Star Formation in Dwarf Galaxies

We explore mechanisms for the regulation of star formation in dwarf galaxies. We concentrate primarily on a sample in the Virgo cluster, which has HI and blue total photometry, for which we collected H$α$ data at the Wise Observatory. We find that dwarf galaxies do not show the tight correlation of the surface brightness of H$α$ (a star formation indicator) with the HI surface density, or with the ratio of this density to a dynamical timescale, as found for large disk or starburst galaxies. On the other hand, we find the strongest correlation to be with the average blue surface brightness, indicating the presence of a mechanism regulating the star formation by the older (up to 1 Gyr) stellar population if present, or by the stellar population already formed in the present burst.

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arXiv1996

Ultraviolet Sky Surveys

I review the development of UV and EUV astronomy, covering the spectral range from 5 to 300 nm, with emphasis on sky surveys for discrete sources. I discuss studies which resulted in lists of sources observed by imaging. The missions are compared in terms of their "survey power", a variable which combines sky coverage and survey depth. I describe UV missions which will detect faint sources and will fly in the near future, and present a wish list for low-cost ventures, which could advance considerably the knowledge of the UV sky.

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arXiv1996

The TAUVEX UV Imager

I present an up-to-date description of the TAUVEX UV imager, constructed to operate on the Spectrum X-Gamma international space observatory. The projected performance and the expected scientific yield of TAUVEX are described.

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