arXiv subjects

D. F. de Mello

Publications and source records attributed to D. F. de Mello.

15 recordsLinked to original sources

arXiv2015

NGC 6845: metallicity gradients and star formation in a complex compact group

We have obtained Gemini/GMOS spectra of 28 regions located across the interacting group NGC 6845, spanning from the inner regions of the four major galaxies (NGC 6845A, B, C, D) to the tidal tails of NGC 6845A. All regions in the tails are star-forming objects with ages younger than 10 Myr. We derived the gas-phase metallicity gradients across NGC 6845A and its two tails and we find that these are shallower than those for isolated galaxies. NGC 6845A has a gas-phase oxygen central metallicity of \mbox{12+log(O/H)$\sim$8.5} and a flat gas-phase metallicity gradient ($β$=0.002$\pm$0.004 dex kpc$^{-1}$) out to $\sim$4 $\times$ R$_{25}$ (to the end of the longest tidal tail). Considering the mass-metallicity relation, the central region of NGC 6845A displays a lower oxygen abundance than the expected for its mass. Taking into account this fact and considering the flat oxygen distribution measured along the eastern tidal tail, we suggest that an interaction event has produced a dilution in the central metallicity of this galaxy and the observed flattening in its metal distribution. We found that the star formation process along the eastern tidal structure has not been efficient enough to increase the oxygen abundances in this place, suggesting that this structure was formed from enriched material.

astro-ph.GA↗

arXiv2015

Probing the nature of the pre-merging system Hickson Compact Group 31 through IFU data

We present a study of the kinematics and the physical properties of the central region of the Hickson Compact Group 31 (HCG 31), focusing on the HCG 31A+C system, using integral field spectroscopy data taken with the Gemini-south telescope. The main players in the merging event (galaxies A and C) are two dwarf galaxies that in the past have already had one close encounter, given the observed tidal tails, and may now be in their second approach, possibly about to merge. We present new velocity fields and Hα emission, stellar continuum, velocity dispersion, electron density, Hα equivalent width and age maps. Considering the high spatial resolution of the IFU data, we were able to measure various components and estimate their physical parameters, spatially resolving the different structures in this region. Our main findings are the following: (1) we report for the first time the presence of a super stellar cluster next to the burst associated to the HCG 31C central blob, related to the high values of velocity dispersion observed in this region as well as to the highest value of stellar continuum emission. This may suggest that this system is cleaning its environment through strong stellar winds that may then trigger a strong star formation event in its neighborhood, (2) among other physical parameters, we estimate an L(Hα)~14x10^{41} erg/s and a SFR~11 Msol/yr for the central merging region of HCG 31 A+C. These values indicate a high star formation density, suggesting that the system is part of a merging object, supporting previous scenarios proposed for this system.

astro-ph.GA↗

arXiv2014

Witnessing gas mixing in the metal distribution of the Hickson Compact Group HCG 31

We present, for the first time, direct evidence that, in a merger of disk galaxies, the preexisting central metallicities will mix as a result of gas being transported in the merger interface region, along the line that joins the two coalescing nuclei. This is shown using detailed 2D kinematics as well as metallicity measurements for the nearby ongoing merger in the center of the compact group HCG 31. We focus on the emission line gas, which is extensive in the system. The two coalescing cores display similar oxygen abundances while in between the two nuclei the metallicity changes smoothly from one nucleus to the other, indicating a mix of metals in this region, which is confirmed by the high resolution H$α$ kinematics (R=45900). This nearby system is especially important because it involves the merging of two fairly low mass and clumpy galaxies (LMC-like galaxies), making it an important system for comparison with high redshift galaxies.

astro-ph.GA↗

arXiv2012

Star formation in HI tails: HCG 92, HCG 100 and 6 interacting systems

We present new Gemini spectra of 14 new objects found within the HI tails of Hickson Compact Groups 92 and 100. Nine of them are GALEX Far-UV (FUV) and Near-UV (NUV) sources. The spectra confirm that these objects are members of the compact groups and have metallicities close to solar, with an average value of 12+log(O/H)~8.5. They have average FUV luminosities 7 x 10^40 erg/s, very young ages (< 100 Myr) and two of them resemble tidal dwarf galaxies (TDGs) candidates. We suggest that they were created within gas clouds that were ejected during galaxy-galaxy interactions into the intergalactic medium, which would explain the high metallicities of the objects, inherited from the parent galaxies from which the gas originated. We conduct a search for similar objects in 6 interacting systems with extended HI tails, NGC 2623, NGC 3079, NGC 3359, NGC 3627, NGC 3718, NGC 4656. We found 35 UV sources with ages < 100 Myr, however most of them are on average less luminous/massive than the UV sources found around HCG 92 and 100. We speculate that this might be an environmental effect and that compact groups of galaxies are more favorable to TDG formation than other interacting systems.

astro-ph.CO↗

arXiv2012

NGC 2782: a merger remnant with young stars in its gaseous tidal tail

We have searched for young star-forming regions around the merger remnant NGC 2782. By using GALEX FUV and NUV imaging and HI data we found seven UV sources, located at distances greater than 26 kpc from the center of NGC 2782, and coinciding with its western HI tidal tail. These regions were resolved in several smaller systems when Gemini/GMOS r-band images were used. We compared the observed colors to stellar population synthesis models and we found that these objects have ages of ~1 to 11 Myr and masses ranging from 10^3.9 to 10^4.6 Msun. By using Gemini/GMOS spectroscopic data we confirm memberships and derive high metallicities for three of the young regions in the tail (12+log(O/H)=8.74\pm0.20, 8.81\pm0.20 and 8.78\pm0.20). These metallicities are similar to the value presented by the nuclear region of NGC 2782 and also similar to the value presented for an object located close to the main body of NGC 2782. The high metallicities measured for the star-forming regions in the gaseous tidal tail of NGC 2782 could be explained if they were formed out of highly enriched gas which was once expelled from the center of the merging galaxies when the system collided. An additional possibility is that the tail has been a nursery of a few generations of young stellar systems which ultimately polluted this medium with metals, further enriching the already pre-enriched gas ejected to the tail when the galaxies collided.

astro-ph.CO↗

arXiv2011

Extreme Emission Line Galaxies in CANDELS: Broad-Band Selected, Star-Bursting Dwarf Galaxies at z>1

We identify an abundant population of extreme emission line galaxies (EELGs) at redshift z~1.7 in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) imaging from Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3). 69 EELG candidates are selected by the large contribution of exceptionally bright emission lines to their near-infrared broad-band magnitudes. Supported by spectroscopic confirmation of strong [OIII] emission lines -- with rest-frame equivalent widths ~1000Å-- in the four candidates that have HST/WFC3 grism observations, we conclude that these objects are galaxies with 10^8 Msol in stellar mass, undergoing an enormous starburst phase with M_*/(dM_*/dt) of only ~15 Myr. These bursts may cause outflows that are strong enough to produce cored dark matter profiles in low-mass galaxies. The individual star formation rates and the co-moving number density (3.7x10^-4 Mpc^-3) can produce in ~4 Gyr much of the stellar mass density that is presently contained in 10^8-10^9 Msol dwarf galaxies. Therefore, our observations provide a strong indication that many or even most of the stars in present-day dwarf galaxies formed in strong, short-lived bursts, mostly at z>1.

astro-ph.CO↗

arXiv2010

A Multiwavelength Study on the Fate of Ionizing Radiation in Local Starbursts

The fate of ionizing radiation is vital for understanding cosmic ionization, energy budgets in the interstellar and intergalactic medium, and star formation rate indicators. The low observed escape fractions of ionizing radiation have not been adequately explained, and there is evidence that some starbursts have high escape fractions. We examine the spectral energy distributions of a sample of local star-forming galaxies, containing thirteen local starburst galaxies and ten of their ordinary star-forming counterparts, to determine if there exist significant differences in the fate of ionizing radiation in these galaxies. We find that the galaxy-to-galaxy variations in the SEDs is much larger than any systematic differences between starbursts and non-starbursts. For example, we find no significant differences in the total absorption of ionizing radiation by dust, traced by the 24um, 70um, and 160um MIPS bands of the Spitzer Space Telescope, although the dust in starburst galaxies appears to be hotter than that of non-starburst galaxies. We also observe no excess ultraviolet flux in the GALEX bands that could indicate a high escape fraction of ionizing photons in starburst galaxies. The small H-alpha fractions of the diffuse, warm ionized medium in starburst galaxies are apparently due to temporarily boosted H-alpha luminosity within the star-forming regions themselves, with an independent, constant WIM luminosity. This independence of the WIM and starburst luminosities contrasts with WIM behavior in non-starburst galaxies and underscores our poor understanding of radiation transfer in both ordinary and starburst galaxies.

astro-ph.CO↗

arXiv2009

Star-forming regions in the intragroup medium of compact groups of galaxies

We present the results of a multiwavelength campaign searching for young objects in the intragroup medium of seven compact groups of galaxies: HCG 2, 7, 22, 23, 92, 100 and NGC 92. We used Fabry-Perot velocity fields and rotation curves together with GALEX NUV and FUV images, optical R-band and HI maps to evaluate the stage of interaction of each group. We conclude that groups (i) HCG 7 and HCG 23 are in an early stage of interaction, (ii) HCG 2 and HCG 22 are mildly interacting, and (iii) HCG 92, HCG 100 and NGC 92 are in a late stage of evolution. Evolved groups have a population of young objects in their intragroup medium while no such population is found within the less evolved groups. We also report the discovery of a tidal dwarf galaxy candidate in the tail of NGC 92. These three groups, besides containing galaxies which have peculiar velocity fields, also show extended HI tails. Our results indicate that the advanced stage of evolution of a group together with the presence of intragroup HI clouds may lead to star formation in the intragroup medium.

astro-ph.CO↗

arXiv2009

Star formation in the intragroup medium and other diagnostics of the evolutionary stages of compact groups of galaxies

Context: Compact groups of galaxies are entities that have high densities of galaxies and serve as laboratories to study galaxy interactions, intergalactic star formation and galaxy evolution. Aims: The main goal of this study is to search for young objects in the intragroup medium of seven compact groups of galaxies: HCG 2, 7, 22, 23, 92, 100 and NGC 92 as well as to evaluate the stage of interaction of each group. Methods: We used Fabry-Perot velocity fields and rotation curves together with GALEX NUV and FUV images and optical R-band and HI maps. Results: (i) HCG 7 and HCG 23 are in early stages of interaction, (ii) HCG 2 and HCG 22 are mildly interacting, and (iii) HCG 92, HCG 100 and NGC 92 are in late stages of evolution. We find that all three evolved groups contain populations of young blue objects in the intragroup medium, consistent with ages < 100 Myr, of which several are younger than < 10 Myr. We also report the discovery of a tidal dwarf galaxy candidate in the tail of NGC 92. These three groups, besides containing galaxies that have peculiar velocity fields, also show extended HI tails. Conclusions: Our results indicate that the advanced stage of evolution of a group, together with the presence of intragroup HI clouds, may lead to star formation in the intragroup medium. A table containing all intergalactic HII regions and tidal dwarf galaxies confirmed to date is appended.

astro-ph.CO↗

arXiv2008

Holmberg Ix: The Nearest Young Galaxy

Deep images taken with the Wide Field Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope provide the basis for study the resolved stellar population of the M81 companion dwarf irregular galaxy Holmberg IX. Based on color-magnitude diagrams the stellar population toward Holmberg IX contains numerous stars with ages of <~200 Myr as well as older red giant stars. By charting the spatial distribution of the red giant stars and considering their inferred metallicities, we concluded that most of these older stars are associated with M81 or its tidal debris. At least 20% of the stellar mass in Holmberg IX was produced in the last ~200 Myr, giving it the youngest stellar populations if any nearby galaxy. The location of Holmberg IX, its high gas content, and its youthful stellar population suggests that it is a tidal dwarf galaxy, perhaps formed during the last close passage of M82 around M81.

astro-ph↗

arXiv2006

Far-Ultraviolet Imaging of the Hubble Deep Field North: Star Formation in Normal Galaxies at z<1

We present far-ultraviolet (FUV) imaging of the Hubble Deep Field North (HDF-N) taken with the Solar Blind Channel of the Advanced Camera for Surveys (ACS/SBC) and the FUV MAMA detector of the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope. The full WFPC2 deep field has been observed at 1600 Angstroms. We detect 134 galaxies and one star down to a limit of FUV_{AB} ~ 29. All sources have counterparts in the WFPC2 image. Redshifts (spectroscopic or photometric) for the detected sources are in the range 0<z<1. We find that the FUV galaxy number counts are higher than those reported by GALEX, which we attribute at least in part to cosmic variance in the small HDF-N field of view. Six of the 13 Chandra sources at z<0.85 in the HDF-N are detected in the FUV, and those are consistent with starbursts rather than AGN. Cross-correlating with Spitzer sources in the field, we find that the FUV detections show general agreement with the expected L_IR/L_UV vs. Beta relationship. We infer star formation rates (SFRs), corrected for extinction using the UV slope, and find a median value of 0.3 Msun/yr for FUV-detected galaxies, with 75% of detected sources have SFR<1 Msun/yr. Examining the morphological distribution of sources, we find that about half of all FUV-detected sources are identied as spiral galaxies. Half of morphologically-selected spheroids at z<0.85 are detected in the FUV, suggesting that such sources have significant ongoing star-formation in the epoch since z=1.

astro-ph↗

arXiv1999

Two Galaxy Clusters: A3565 and A3560

We report 102 new redshifts and magnitudes for a sample of galaxies to RF ~ 15.5 mag in a 2.17 deg x 2.17 deg region centered on the galaxy IC 4296, the most luminous member of the A3565 cluster. Up to the limiting magnitude we find 29 cluster members, and measure a velocity dispersion of 228 km/s. The estimated total mass for this system is ~ 3.0 x h**-1 10**13 Msun (where h = H0/100 km/s/Mpc), and its dynamical properties are quite typical of poor clusters presenting X-ray emission. We also find that galaxies with absorption lines are more concentrated towards the center of the cluster, while systems with emission lines are mainly located in the outer parts. The small velocity dispersion of the cluster, coupled to the known presence of an interacting pair of galaxies, and the large extent of the brightest cluster galaxy, could indicate that galaxy formation through mergers may still be underway in this system. The surveyed region also contains galaxies belonging to the Shapley Concentration cluster A3560. Within 30 arc min of the cluster center, we detect 32 galaxies, for which we measure a velocity dispersion of 588 km/s and a mass of ~2 x h**-1 10**14 Msun. However, because our sample is restricted to galaxies brighter than M*, these values should be considered only as rough estimates.

astro-ph↗

arXiv1998

Local order and magnetic field effects on the electronic properties of disordered binary alloys in the Quantum Site Percolation limit

Electronic properties of disordered binary alloys are studied via the calculation of the average Density of States (DOS) in two and three dimensions. We propose a new approximate scheme that allows for the inclusion of local order effects in finite geometries and extrapolates the behavior of infinite systems following `finite-size scaling' ideas. We particularly investigate the limit of the Quantum Site Percolation regime described by a tight-binding Hamiltonian. This limit was chosen to probe the role of short range order (SRO) properties under extreme conditions. The method is numerically highly efficient and asymptotically exact in important limits, predicting the correct DOS structure as a function of the SRO parameters. Magnetic field effects can also be included in our model to study the interplay of local order and the shifted quantum interference driven by the field. The average DOS is highly sensitive to changes in the SRO properties, and striking effects are observed when a magnetic field is applied near the segregated regime. The new effects observed are twofold: there is a reduction of the band width and the formation of a gap in the middle of the band, both as a consequence of destructive interference of electronic paths and the loss of coherence for particular values of the magnetic field. The above phenomena are periodic in the magnetic flux. For other limits that imply strong localization, the magnetic field produces minor changes in the structure of the average DOS.

cond-mat.dis-nn↗

arXiv1998

The Peculiar Type II Supernova 1997 D: A Case for a Very Low 56Ni Mass

SN 1997D in NGC 1536 is possibly the least luminous and energetic Type II supernova discovered to date. The entire light curve is subluminous, never reaching $M_V = -14.65$. The radioactive tail follows the 56Co decay slope. In the case of nearly complete trapping of the $γ$-rays, the 56Ni mass derived from the tail brightness is extremely small, $\sim 0.002$ \M. At discovery the spectra showed a red continuum and line velocities of the order of 1000 \kms. The luminosity and the photospheric expansion velocity suggest that the explosion occurred about 50 days before discovery, and that a plateau probably followed. Model light curves and spectra of the explosion of a 26 \M\ star successfully fit the observations. Low mass models are inconsistent with the observations. The radius of the progenitor, constrained by the prediscovery upper limits, is \r0 \ltsim 300 \R. A low explosion energy of $\sim 4 \times 10^{50}$ ergs is then required in the modeling. The strong \ion{Ba}{2} lines in the photospheric spectra are reproduced with a solar abundance and low $T_{eff}$. A scenario in which the low 56Ni mass observed in SN 1997D is due to fall--back of material onto the collapsed remnant of the explosion of a 25--40 \M star appears to be favored over the case of the explosion of an 8--10 \M\ star with low 56Ni production.

astro-ph↗

arXiv1996

Strong Clustering of Faint Galaxies at Small Angular Scales

The 2-point angular correlation function of galaxies, \wt, has been computed on equatorial fields observed with the CTIO 4m prime focus, within a total area of 2.31 deg$^2$. In the magnitude range $19\le m_R \le 21.5$, corresponding to $ \approx 0.35$, we find an excess of power in \wt at scales $2''\leθ\le6''$ over what would be expected from an extrapolation of \wt measured at larger $θ$. The significance of this excess is $\approx 5σ$. At larger scales, $6''< θ\le 24''$, the amplitude of \wt is 1.6 times smaller than the standard no evolutionary model. At these scales there is remarkable agreement between the present data and Infante \& Pritchet (1995). At large angular scales ($6''< θ\le 24''$) the data is best described by a model where clustering evolution in $ξ(r,z)$ has taken place. Strong luminosity evolution cannot be ruled out with the present data. At smaller scales, $2''\le θ\le 6''$, our data are formally fit by models where $ε=-2.4 (Ω=0.2, r_o=5.1h^{-1}$Mpc) or $r_o = 7.3h^{-1}$Mpc $(Ω=0.2, ε=0)$. If the mean redshift of our sample is 0.35 then our data show a clear detection of the scale ($\approx 19h^{-1}kpc$) where the clustering evolution approaches a highly non linear regime, i.e., $ε\le 0$. The rate at which galaxies merge has been computed. If this rate is proportional to $(1+z)^m$, then $m=2.2 \pm 0.5$.

astro-ph↗