arXiv subjects

Enrique Pérez-Montero

Publications and source records attributed to Enrique Pérez-Montero.

25 records · Page 2Linked to original sources

arXiv2017

Using photo-ionisation models to derive carbon and oxygen gas-phase abundances in the rest UV

We present a new method to derive oxygen and carbon abundances using the ultraviolet (UV) lines emitted by the gas-phase ionised by massive stars. The method is based on the comparison of the nebular emission-line ratios with those predicted by a large grid of photo-ionisation models. Given the large dispersion in the O/H - C/O plane, our method firstly fixes C/O using ratios of appropriate emission lines and, in a second step, calculates O/H and the ionisation parameter from carbon lines in the UV. We find abundances totally consistent with those provided by the direct method when we apply this method to a sample of objects with an empirical determination of the electron temperature using optical emission lines. The proposed methodology appears as a powerful tool for systematic studies of nebular abundances in star-forming galaxies at high redshift.

astro-ph.GA↗

arXiv2016

Chemical distribution of HII regions towards the Galactic anticentre

We study the physical and chemical properties of a sample of HII regions located at RG >11 kpc and present the radial distribution of abundances towards the Galaxy anticentre. We carried out optical spectroscopic observations of nine HII regions with the WHT. The sample was increased by searching the literature for optical observations of regions towards the Galactic anticentre, re-analysing them to obtain a single sample of 23 objects covering the Galactocentric radius from 11 kpc to 18 kpc to be processed in a homogeneous and consistent manner. Accurate electron densities and temperatures of several ionic species were derived in 13 HII regions. These physical parameters were applied to the spectra to determine direct total chemical abundances. For those regions without direct estimations of temperature, chemical abundances were derived by performing tailor-made photoionisation models and/or by using an empirical relation obtained from radio recombination and optical temperatures. We performed weighted least-squares fits to the distribution of the derived abundances along the Galactocentric distances to study the radial gradients of metallicity. The distributions O/H, N/H, S/H, and Ar/H towards the anticentre can be represented by decreasing linear radial gradients, while in the case of N/O abundances the radial distribution is better fitted with a two-zone model. The He/H radial gradient is presented here for the first time; we find a slope that is not significantly different from zero. The derived gradient for oxygen shows a clear decrease with distance with a slope of -0.053dex/kpc. Although a shallower slope at large Galactocentric distances is suggested by our data, the flattening of the distribution cannot be confirmed and more objects towards the anticentre need to be studied in order to establish the true form of the metallicity gradient.

astro-ph.GA↗

arXiv2014

Deriving model-based T$_e$-consistent chemical abundances in ionised gaseous nebulae

The derivation of abundances in gaseous nebulae ionised by massive stars using optical collisionally excited emission lines is studied in this work comparing the direct or $T_e$ method with updated grids of photoionisation models covering a wide range of input conditions of O/H and N/O abundances and ionisation parameter. The abundances in a large sample of compiled objects with at least one auroral line are re-derived and later compared with the $χ^2$ weighted-mean abundances from the models. The agreement between the abundances using the two methods both for O/H and N/O is excellent with no additional assumptions about the geometry or physics governing the HII regions. Although very inaccurate model-based O/H are obtained when no auroral lines are considered, this can be overcome assuming empirical laws between O/H, log $U$, and N/O to constrain the considered models. In this way, for 12+log(O/H) $>$ 8.0, a precision better than 0.1dex consistent with the direct method is attained. For very low-$Z$, models give higher O/H values and a high dispersion, possibly owing to the contamination of the low-excitation emission-lines. However, in this regime, the auroral lines are usually well-detected. The use of this procedure, in a publicly available script, HII-CHI-mistry}, leads to the derivation of abundances in faint/high redshift objects consistent with the direct method based on CELs.

astro-ph.GA↗

arXiv2012

Complex gas kinematics in compact, rapidly assembling star-forming galaxies

Deep, high resolution spectroscopic observations have been obtained for six compact, strongly star-forming galaxies at redshift z~0.1-0.3, most of them also known as Green Peas. Remarkably, these galaxies show complex emission-line profiles in the spectral region including Hα, [NII]$λλ6548, 6584$ and [SII]$λλ6717, 6731$, consisting of the superposition of different kinematical components on a spatial extent of few kpc: a very broad line emission underlying more than one narrower component. For at least two of the observed galaxies some of these multiple components are resolved spatially in their 2D-spectra, whereas for another one a faint detached Hα blob lacking stellar continuum is detected at the same recessional velocity ~7 kpc away from the galaxy. The individual narrower Hα components show high intrinsic velocity dispersion (σ~30-80 km s$^{-1}$), suggesting together with unsharped masking HST images that star formation proceeds in an ensemble of several compact and turbulent clumps, with relative velocities of up to ~500 km s$^{-1}$. The broad underlying Hα components indicate in all cases large expansion velocities (full width zero intensity FWZI $\ge$ 1000 km s$^{-1}$) and very high luminosities (up to ~10$^{42}$ erg s$^{-1}$), probably showing the imprint of energetic outflows from SNe. These intriguing results underline the importance of Green Peas for studying the assembly of low-mass galaxies near and far.

astro-ph.CO↗

arXiv2012

Auto-consistent metallicity and star formation history of the nearest blue compact dwarf galaxy NGC 6789

We present a detailed auto-consistent study of the nearest blue compact dwarf galaxy NGC 6789 by means of optical and UV archive photometry data and optical long-slit ISIS-WHT spectroscopy observations of the five brightest star-forming knots. The analysis of the spectra in all knots allowed the derivation of ionic chemical abundances of oxygen, nitrogen, sulphur, argon and neon using measures of both the high- and low-excitation electron temperatures, leading to the conclusion that NGC 6789 is chemically homogeneous with low values of the abundance of oxygen in the range 12+log(O/H) = 7.80-7.93, but presenting at the same time higher values of the nitrogen-to-oxygen ratio than expected for its metal regime. We used archival HST/WFPC2 F555W and F814W observations of NGC 6789 to perform a photometric study of the colour-magnitude diagram (CMD) of the resolved stellar populations and derive its star formation history (SFH), which is compatible with the presence of different young and old stellar populations whose metallicities do not necessarily increase with age. We fit the observed optical spectrum in all the five knots using the STARLIGHT code and a combination of single stellar populations following the SFH obtained from the CMD. We compare the resulting stellar masses and the relative fractions of the ionising populations with a non-constrained SFH case. The properties of the younger populations were obtained using CLOUDY photoionisation models, giving similar ages in all the knots in the range 3-6 Myr and the estimation of the dust absorption factor, which correlates with the observed GALEX FUV-NUV colour indices. The total photometric extinction and dust-absorption corrected Hα fluxes were finally used to derive the star formation rates.

astro-ph.CO↗

arXiv2011

On the equivalent effective temperature of massive young star clusters: The case of NGC 595

The softness parameter is based on the relative intensity of several optical emission lines emitted by the gas ionized by young massive star clusters and can be used to derive the equivalent effective temperature (T*) in those objects whose stellar population cannot be resolved. This method has several uncertainties due to the disagreement between different synthesis model atmospheres but it is robust to study the relative variations between objects. Following the 2D photoionization models of the giant HII region NGC 595 (Pérez-Montero et al. 2011) we show that the determination of T* with the softness parameter is also robust in different regions of a same object with large variations in the geometry of the gas and in the dust-to-gas ratio.

astro-ph.CO↗

arXiv2011

Unveiling the nature of the "Green Pea" galaxies

We review recent results on the oxygen and nitrogen chemical abundances in extremely compact, low-mass starburst galaxies at redshifts between 0.1-0.3 recently named to as "Green Pea" galaxies. These galaxies are genuine metal-poor galaxies ($\sim$ one fifth solar) with N/O ratios unusually high for galaxies of the same metallicity. In combination with their known general properties, i.e., size, stellar mass and star-formation rate, these findings suggest that these objects could be experiencing a short and extreme phase in their evolution. The possible action of both recent and massive inflow of gas, as well as stellar feedback mechanisms are discussed here as main drivers of the starburst activity and their oxygen and nitrogen abundances.

astro-ph.CO↗