arXiv subjects

A. Gil de Paz

Publications and source records attributed to A. Gil de Paz.

At least 109 records · Page 6Linked to original sources

arXiv2005

GALEX UV observations of the interacting galaxy NGC 4438 in the Virgo cluster

We present GALEX NUV (2310 A) and FUV (1530 A) images of the interacting galaxy NGC 4438 (Arp 120) in the center of the Virgo cluster. These images show an extended (20 kpc) tidal tail at the north-west edge of the galaxy previously undetected at other wavelengths, at 15-25 kpc from its nucleus. Except in the nucleus, the UV morphology of NGC 4438 is totally different from the Halpha+[NII] one, more similar to the X-ray emission, confirming its gas cooling origin. We study the star formation history of NGC 4438 combining spectro-photometric data in the UV-visible-near-IR wavelength range with population synthesis and galaxy evolution models. The data are consistent with a recent (~ 10 Myr), instantaneous burst of star formation in the newly discovered UV north-western tail which is significantly younger than the age of the tidal interaction with NGC 4435, dated by dynamical models at ~ 100 Myr ago. Recent star formation events are also present at the edge of the northern arm and in the southern tail, while totally lacking in the other regions, which are dominated by the old stellar population perturbed during the dynamical interaction with NGC 4435. The contribution of this recent starburst to the total galaxy stellar mass is lower than 0.1%, an extremely low value for such a violent interaction. High-velocity, off-center tidal encounters such as that observed in Arp 120 are thus not sufficient to significantly increase the star formation activity of cluster galaxies.

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arXiv2004

Recent star formation in nearby galaxies from GALEX imaging:M101 and M51

The GALEX (Galaxy Evolution Explorer) Nearby Galaxies Survey is providing deep far-UV and near-UV imaging for a representative sample of galaxies in the local universe. We present early results for M51 and M101, from GALEX UV imaging and SDSS optical data in five bands. The multi-band photometry of compact stellar complexes in M101 is compared to population synthesis models, to derive ages, reddening, reddening-corrected luminosities and current/initial masses. The GALEX UV photometry provides a complete census of young compact complexes on a approximately 160pc scale. A galactocentric gradient of the far-UV - near-UV color indicates younger stellar populations towards the outer parts of the galaxy disks, the effect being more pronounced in M101 than in M51.

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arXiv2004

Extinction radial profiles of M83 from GALEX UV imaging

We use the far-UV (FUV) and near-UV (NUV) images of M83 obtained by GALEX to compute the radial profile of the UV spectral slope in the star forming disk. We briefly present a model of its chemical evolution which allows us to obtain realistic intrinsic properties of the stellar populations. Using corollary data, we also compute the profiles of H-alpha/H-beta and of the total IR (TIR) to FUV ratio. Both data and model are used to estimate and compare the extinction gradients at the FUV wavelength obtained from these various indicators. We discuss the implications for the determination of the star formation rate.

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arXiv2004

UV Emission from Stellar Populations within Tidal Tails: Catching the Youngest Galaxies in Formation?

New GALEX observations have detected significant FUV (1530 Angstroms) and NUV (2310 Angstroms) emission from stellar substructures within the tidal tails of four ongoing galaxy mergers. The UV-bright regions are optically faint and are coincident with HI density enhancements. FUV emission is detected at any location where the HI surface density exceeds ~2 M_sun pc^{-2}, and is often detected in the absence of visible wavelength emission. UV luminosities of the brighter regions of the tidal tails imply masses of 10^6 M_sun to ~10^9 M_sun in young stars in the tails, and HI luminosities imply similar HI masses. UV-optical colors of the tidal tails indicate stellar populations as young as a few Myr, and in all cases ages < 400Myr. Most of the young stars in the tails formed in single bursts rather than resulting from continuous star formation, and they formed *in situ* as the tails evolved. Star formation appears to be older near the parent galaxies and younger at increasing distances from the parent galaxy. This could be because the star formation occurs progressively along the tails, or because the star formation has been inhibited near the galaxy/tail interface. The youngest stellar concentrations, usually near the ends of long tidal tails, have masses comparable to confirmed tidal dwarf galaxies and may be newly forming galaxies undergoing their first burst of star formation.

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arXiv2004

A comparative study of the spatial distribution of ultraviolet and far-infrared fluxes from M 101

The total ultraviolet (UV) flux (from 1412 to 2718 AA) of M 101 is compared on a pixel-to-pixel basis with the total far-infrared (FIR) flux (from 60 to 170 micron), using the maps of the galaxy taken by GALEX in the near-UV and far-UV and by ISOPHOT at 60, 100 and 170 micron. The main result of this investigation is the discovery of a tight dependence of the FIR/UV ratio on radius, with values monotonically decreasing from around 4 in the nuclear region to nearly zero towards the edge of the optical disk. Although the tightness of this dependence is in part attributable to resolution effects, the result is consistent with the presence of a large-scale distribution of diffuse dust having a face-on optical depth which decreases with radius and which dominates over the more localized variations in opacity between the arm and interarm regions. We also find a trend for the FIR/UV ratio to take on higher values in the regions of diffuse interarm emission than in the spiral-arm regions, at a given radius. This is interpreted quantitatively in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.

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arXiv2004

GALEX observations of the UV surface brightness and color profiles of the Local Group elliptical galaxy M32 (NGC221)

M32, the compact elliptical-galaxy companion to the Andromeda spiral galaxy has been imaged by the Galaxy Evolution Explorer (GALEX) in two ultraviolet bands, centered at ~1500 (FUV) and 2300 Angstroms (NUV). The imaging data have been carefully decomposed so as to properly account for the complicated background contamination from the disk of M31. We have derived the surface brightness and color profiles finding a slightly positive color gradient of Delta(FUV-B)/Delta log(r)=+0.15+/-0.03 within one effective radius. Earlier data from the Ultraviolet Imaging Telescope suggested that M32 had an extremely large (negative) FUV-optical color gradient (Delta(FUV-B)/Delta log(r)<-2), inverted with respect to the majority of gradients seen in giant elliptical galaxies. Our new results show that, despite of its very low UV-upturn, M32 has similar UV properties to those observed in luminous elliptical galaxies.

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arXiv2004

GALEX Observations of the Ultraviolet Halos of NGC 253 and M82

We present Galaxy Evolution Explorer (GALEX) images of the prototypical edge-on starburst galaxies M82 and NGC253. Our initial analysis is restricted to the complex of ultraviolet (UV) filaments in the starburst-driven outflows in the galaxy halos. The UV luminosities in the halo are too high to be provided by shock-heated or photoionized gas except perhaps in the brightest filaments in M82, suggesting that most of the UV light is the stellar continuum of the starburst scattered into our line of sight by dust in the outflow. This interpretation agrees with previous results from optical imaging polarimetry in M82. The morphology of the UV filaments in both galaxies shows a high degree of spatial correlation with H-alpha and X-ray emission. This indicates that these outflows contain cold gas and dust, some of which may be vented into the intergalactic medium (IGM). UV light is seen in the ``H-alpha cap'' 11 kpc North of M82. If this cap is a result of the wind fluid running into a pre-existing gas cloud, the gas cloud contains dust and is not primordial in nature but was probably stripped from M82 or M81. If starburst winds efficiently expel dust into the IGM, this could have significant consequences for the observation of cosmologically distant objects.

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arXiv2004

Recent Star Formation in the Extreme Outer Disk of M83

Ultraviolet imaging with the Galaxy Evolution Explorer (GALEX) has revealed an extensive sample of UV-bright stellar complexes in the extreme outer disk of M83, extending to about four times the radius where the majority of HII regions are detected (R_HII = 5.1' or 6.6 kpc). These sources are typically associated with large-scale filamentary HI structures in the warped outer disk of M83, and are distributed beyond the galactocentric radii at which molecular ISM has yet been detected. We present measured properties of these stellar complexes, including FUV and NUV magnitudes and local gas surface density. Only a subset of the outer disk UV sources have corresponding HII regions detected in H-alpha imaging, consistent with a sample of mixed age in which some sources are a few Myr old and others are much more evolved (~ 10^8 yr).

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arXiv2004

Palomar/Las Campanas Imaging Atlas of Blue Compact Dwarf Galaxies: II. Surface Photometry and the Properties of the Underlying Stellar Population

We present the results from an analysis of surface photometry of B, R, and Halpha images of a total of 114 nearby galaxies drawn from the Palomar/Las Campanas Imaging Atlas of Blue Compact Dwarf galaxies. Surface brightness and color profiles for the complete sample have been obtained. We determine the exponential and Sersic profiles that best fit the surface brightness distribution of the underlying stellar population detected in these galaxies. We also compute the (B-R) color and total absolute magnitude of the underlying stellar population and compared them to the integrated properties of the galaxies in the sample. Our analysis shows that the (B-R) color of the underlying population is systematically redder than the integrated color, except in those galaxies where the integrated colors are strongly contaminated by line and nebular-continuum emission. We also find that galaxies with relatively red underlying stellar populations (typically (B-R)>~1mag) show structural properties compatible with those of dwarf elliptical galaxies (i.e. a smooth light distribution, fainter extrapolated central surface brightness and larger scale lengths than BCD galaxies with blue underlying stellar populations). At least ~15% of the galaxies in the sample are compatible with being dwarf elliptical (dE) galaxies experiencing a burst of star formation. For the remaining BCD galaxies in the sample we do not find any correlation between the recent star formation activity and their structural differences with respect to other types of dwarf galaxies.

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arXiv2003

Discovery of a double ring in the dwarf galaxy Mrk409

We report the discovery of a double ring of emission-line regions around the nucleus of the Blue Compact Dwarf (BCD) galaxy Mrk409 as seen by deep, ground-based Halpha images. Echelle spectroscopy obtained at Magellan-I with MIKE shows the presence of ionized gas flowing out of the galaxy from a very massive (>7E6 Msun) nuclear starburst with projected expansion velocities of \~50 km/s. Different scenarios for the formation of these rings are discussed. While the innermost, nuclear ring is most probably formed by the interaction of a starburst-driven shock with the surrounding interstellar medium, the origin of the outer ring is less clear.

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arXiv2003

Luminosity and Stellar Mass Functions of Local Star-Forming Galaxies

We present the optical and near-infrared luminosity and mass functions of the local star-forming galaxies in the Universidad Complutense de Madrid Survey. A bivariate method which explicitly deals with the Halpha selection of the survey is used when estimating these functions. Total stellar masses have been calculated on a galaxy-by-galaxy basis taking into account differences in star formation histories. The main difference between the luminosity distributions of the UCM sample and the luminosity functions of the local galaxy population is a lower normalization (phi^*), indicating a lower global volume density of UCM galaxies. The typical near-infrared luminosity (L^*) of local star-forming galaxies is fainter than that of normal galaxies. This is a direct consequence of the lower stellar masses of our objects. However, at optical wavelengths (B and r) the luminosity enhancement arising from the young stars leads to M^* values that are similar to those of normal galaxies. The fraction of the total optical and near infrared luminosity density in the local Universe associated with star-forming galaxies is 10-20%. Fitting the total stellar mass function using a Schechter parametrization we obtain alpha=-1.15+/-0.15, log({M}^*)=10.82+/-0.17 Msun and log(phi^*)=-3.04+/-0.20 Mpc^{-3}. This gives an integrated total stellar mass density of 10^{7.83+/-0.07} Msun Mpc^{-3} in local star-forming galaxies (H_0=70 km s^{-1} Mpc^{-1}, Omega_M=0.3, Lambda=0.7). The volume-averaged burst strength of the UCM galaxies is b=0.04+/-0.01, defined as the ratio of the mass density of stars formed in recent bursts (age<10 Myr) to the total stellar mass density in UCM galaxies. Finally, we derive that, in the local Universe, (13+/-3)% of the total baryon mass density in the form of stars is associated with star-forming galaxies.

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arXiv2003

Spatial analysis of the Halpha emission in the local star-forming UCM galaxies

We present a photometric study of the Halpha emission in the Universidad Complutense de Madrid (UCM) Survey galaxies. This work complements our previously-published spectroscopic data. We study the location of the star-forming knots, their intensity, concentration, and the relationship of these properties with those of the host galaxy. We also estimate that the amount of Halpha emission that arises from the diffuse ionized gas is about 15-30% of the total Halpha flux for a typical UCM galaxy. This percentage seems to be independent of the Hubble type. Conversely, we found that an `average' UCM galaxy harbours a star formation event with 30% of its Halpha luminosity arising from a nuclear component. The implications of these results for higher-redshift studies are discussed, including the effects of galaxy size and the depth of the observations. A correlation between the SFR and the Balmer decrement is observed, but such correlation breaks down for large values of the extinction. Finally, we recalculate the Halpha luminosity function and star formation rate density of the local Universe using the new imaging data. Our results point out that, on average, spectroscopic observations detected about one third of the total emission-line flux of a typical UCM galaxy. The new values obtained for the Halpha luminosity density and the star formation rate density of the local Universe are 10^(39.3+/-0.2) erg s-1 Mpc-3, and rho_SFR=0.016^(+0.007)_(-0.004) Mass_sun yr-1 Mpc-3 (H_0=50 km s-1 Mpc-1, Omega_M=1.0, Lambda=0). The corresponding values for the `concordance cosmology' (H_0=70 km s-1 Mpc-1, Omega_M=0.3, Lambda=0.7) are 10^(39.5+/-0.2) erg s-1 Mpc-3 rho_SFR=0.029^(+0.008)_(-0.005) Mass_sun yr-1 Mpc-3.

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arXiv2003

Palomar/Las Campanas Imaging Atlas of Blue Compact Dwarf galaxies: I. Images and integrated photometry

We present B, R, and Halpha images for a total of 114 nearby galaxies (v_helio<4000km/s) that, with exception of 9 objects, are classified as Blue Compact Dwarfs (BCDs). BR integrated magnitudes, Halpha fluxes and Halpha equivalent widths for all the objects in the sample are presented. A new set of quantitative, observational criteria for a galaxy to be classified as a BCD is proposed. These criteria include a limit on the K-band luminosity (i.e. stellar mass; M_K>-21mag), peak surface brightness (mu_B,peak<22mag/arcsec^2), and color at the peak surface brightness (mu_B,peak-mu_R,peak<~1). Halpha emission is detected in all but three sample galaxies. Typical color, absolute magnitude, and Halpha luminosity are (B-R)=0.7+/-0.3mag, M_B=-16.1+/-1.4mag, and log(L_Halpha)=40.0+/-0.6(erg/s). Galaxies morphologically classified as nE and iE BCDs within our sample show lower Halpha equivalent widths and redder colors, on average, than the iI and i0-type BCDs. For most of the galaxies the presence of an evolved stellar population is required to explain their observed properties; only the most metal-poor BCDs (e.g. IZw18, Tol65) are still compatible with a pure, young burst. The flux-calibrated and WCS-compliant images in this Atlas are individually available through the NASA/IPAC Extragalactic Database (NED) image server and collectively through a dedicated web page at http://nedwww.ipac.caltech.edu/level5/Sept02/Palco_BCD/frames.html

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arXiv2002

Stellar populations in local star-forming galaxies. I.-Data and modelling procedure

We present an analysis of the integrated properties of the stellar populations in the Universidad Complutense de Madrid Survey of Halpha-selected galaxies. In this paper, the first of a series, we describe in detail the techniques developed to model star-forming galaxies using a mixture of stellar populations, and taking into account the observational uncertainties. We assume a recent burst of star formation superimposed on a more evolved population. The effects of the nebular continuum, line emission and dust attenuation are taken into account. We also test different model assumptions including the choice of specific evolutionary synthesis model, initial mass function, star formation scenario and the treatment of dust extinction. Quantitative tests are applied to determine how well these models fit our multi-wavelength observations for the UCM sample. Our observations span the optical and near infrared, including both photometric and spectroscopic data. Our results indicate that extinction plays a key role in this kind of studies, revealing that low- and high-extinction objects may require very different extinction laws and must be treated differently. We also demonstrate that the UCM Survey galaxies are best described by a short burst of star formation occurring within a quiescent galaxy, rather than by continuous star formation. A detailed discussion on the inferred parameters, such as the age, burst strength, metallicity, star formation rate, extinction and total stellar mass for individual objects, is presented in paper II of this series.

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arXiv2002

Stellar populations in local star-forming galaxies. II.-Recent star formation properties and stellar masses

We present the integrated properties of the stellar populations in the Universidad Complutense de Madrid Survey galaxies. Applying the techniques described in the first paper of this series, we derive ages, burst masses and metallicities of the newly-formed stars in our sample galaxies. The population of young stars is responsible for the Halpha emission used to detect the objects in the UCM Survey. We also infer total stellar masses and star formation rates in a consistent way taking into account the evolutionary history of each galaxy. We find that an average UCM galaxy has a total stellar mass of ~1E10 Msun, of which about 5% has been formed in an instantaneous burst occurred about 5 Myr ago, and sub-solar metallicity. Less than 10% of the sample shows massive starbursts involving more than half of the total mass of the galaxy. Several correlations are found among the derived properties. The burst strength is correlated with the extinction and with the integrated optical colours for galaxies with low obscuration. The current star formation rate is correlated with the gas content. A stellar mass-metallicity relation is also found. Our analysis indicates that the UCM Survey galaxies span a broad range in properties between those of galaxies completely dominated by current/recent star formation and those of normal quiescent spirals. We also find evidence indicating that star-formation in the local universe is dominated by galaxies considerably less massive than L*.

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arXiv2002

Star Formation Rate estimators: [OII]3727 vs. Halpha for local star-forming galaxies

The [OII]3727 emission line is frequently used as an indicator of the star formation rate (SFR) despite its complex dependence on metallicity and excitation conditions. We have analysed the properties of the [OII] and Halpha emission lines for a complete sample of local Halpha-selected galaxies, the Universidad Complutense de Madrid (UCM) survey. We find a large scatter in the [OII]/Halpha line ratios, although the scatter in the extinction-corrected [OII]^0/Halpha^0 ratio is considerably smaller. We also find that the [OII]/Halpha ratios are reasonably well correlated with the absolute B- and K-band magnitudes and with EW([OII]). However, the extinction-corrected [OII]^0/Halpha^0 ratio is largely independent of these quantities, indicating that extinction is the main driver of the correlations. These correlations allow us to statistically predict--with varying degrees of accuracy--the observed and extinction-corrected Halpha fluxes from the observed [OII] flux using the information contained in EW([OII]) and/or the absolute magnitudes, but extreme caution is needed to make sure that the sample selection effects are correctly taken into account.

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arXiv2002

Physical structure of the protoplanetary nebula CRL618. I. Optical long-slit spectroscopy and imaging

In this paper (paper I) we present optical long-slit spectroscopy and imaging of the protoplanetary nebula CRL618. The optical lobes of CRL618 consist of shock-excited gas, which emits many recombination and forbidden lines, and dust, which scatters light from the innermost regions. From the analysis of the scattered Halpha emission, we derive a nebular inclination of i=24+-6 deg. The spectrum of the innermost part of the east lobe (visible as a bright, compact nebulosity close to the star in the Halpha HST image) is remarkably different from that of the shocked lobes but similar to that of the inner HII region, suggesting that this region represents the outermost parts of the latter. We find a non-linear radial variation of the gas velocity along the lobes. The largest projected LSR velocities (~80 km/s) are measured at the tips of the lobes, where the direct images show the presence of compact bow-shaped structures. The velocity of the shocks in CRL618 is in the range ~75-200 km/s, as derived from diagnostic line ratios and line profiles. We report a brightening (weakening) of [OIII]5007AA ([OI]6300AA) over the last ~10 years that may indicate a recent increase in the speed of the exciting shocks. From the analysis of the spatial variation of the nebular extinction, we find a large density contrast between the material inside the lobes and beyond them: the optical lobes seem to be `cavities' excavated in the AGB envelope by interaction with a more tenuous post-AGB wind. The electron density, with a mean value n_e~5E3-1E4 cm-3, shows significant fluctuations but no systematic decrease along the lobes, in agreement with most line emission arising in a thin shell of shocked material (the lobe walls) rather than in the post-AGB wind filling the interior of the lobes. (...)

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arXiv2002

12CO mapping of the low-metallicity BCD galaxy Mrk86

We have mapped the 12CO J=1-0 and J=2-1 line emission in Mrk86, one of the most metal-deficient Blue Compact Dwarf galaxies so far detected in 12CO. The 12CO emission is distributed in a horseshoe-like structure that follows the locus of the most recent star formation regions. The minimum in molecular-line emission corresponds to the position of an older, massive nuclear starburst. The H2 mass of the galaxy (in the range 0.4-5x10^7 Msun) and its morphology have been compared with the predictions of hydrodynamic simulations of the evolution of the interstellar medium surrounding a nuclear starburst. These simulations suggest that the physical conditions in the gas swept out by the starburst could have led to the formation of the ring of molecular gas reported here. This result provides an attractive scenario for explaining the propagation (in a galactic scale) of the star formation in dwarf galaxies.

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