Massive stars (more than 10 solar masses) dominate the light emitted by star-forming regions. Their UV photons excite and ionize the surrounding gas, which re-emits the light as bright optical emission-lines. These stars are short-lived and explode as supernovae as their nuclei are run out of light elements, polluting the interstellar medium of galaxies with heavy fresh chemical species.
Then, the star-forming complexes can be used as tracers of the properties of the galaxies where they are by means of the spectroscopic study of the emission lines coming from the ionized gas. The relative intensities of these lines give us information about the physical properties and the chemical abundances of these regions from very close diffuse nebulae in our own Galaxy up to very bright starburst galaxies in a very young star-factory Universe.
The main challenge for the study of the star-formation activity in the Local Universe is the spatial deblending between the different compounds in these complexes, including stars, gas, and dust by means of different observational techniques (e.g. integral field spectroscopy) and in different wavelength ranges. For instance the ultra-violet traces the position of the stars, the visible traces the gas and the infra-red the emission from the dust. In the case of the distant galaxies, our approach is statistical, collecting information from great surveys at different cosmological epochs taking benefit from the advent of the large telescopes in the last years.
Nearby metal-poor starburst systems, like blue compact dwarf galaxies (BCDs), represent a key piece to study processes of massive star-formation under similar conditions to those in high-z objects. In particular, investigating the origin of high-ionization lines (e.g. HeII) - still an open question - in BCDs can give important clues on the sources of UV-flux that contribute to the Universe' s reionization. In this regard, we initiated a project aiming to investigate the massive stellar population and associated feedback processes, and the ionized gas properties in BCDs using IFS. Several articles have stemmed from this project (e.g. Kehrig et al. 2013; Pérez-Montero et al. 2013). This study will be extended to the high-z Universe using the MEGARA/10.4m GTC IFU spectrograph, taking advantage of the GTO obtained by the IAA-CSIC as partner of the MEGARA consortium.
The CALIFA project aims at observing 600 galaxies from the local Universe with the Integral Field Spectrograph PPAK at Observatorio de Calar Alto. Among the several projects already ongoing based on the CALIFA database, the members of Estallidos-GR are involved in those regarding the properties of the ionized gas in spiral galaxies:
The wide field of view covered by the PPAK spectrograph yields a full spatial coverage of the CALIFA galaxies, and thus allows a detailed study on the effects of apertures in the derivation of galaxy properties. This effects might affect mostly to properties derived from fixed aperture surveys (e.g. single fiber spectroscopy) and could induce biases due to the fact that the projected size of the fiber on the sky covers different regions of galaxies depending on their size and redshift. A preliminary study on these aperture effects on quantities regarding the Balmer lines (dust attenuation, Star Formation Rates, Halpha equivalent width) of spiral galaxies was presented in Iglesias-Páramo et al. (2013). A further study regarding the influence of aperture effects on the derivation of oxygen abundances is ongoing and will soon be published.
Another aspect linked to the ionized gas is the study of the HII regions in spiral galaxies. With this aim, a complete database of HII regions is being built from the CALIFA frames, together with their integrated spectra (Sánchez et al. 2013). The first result of this work is the claim of a Universal abundance gradient when the galactocentric distance is normalized to the disk effective radius, except for interacting and/or merging galaxies which show a flatter slope.
Decades ago, early-type galaxies (ETGs) were thought to contain very little, if any, gas (e.g. Mathews & Baker 1971; White & Chevalier 1983). Nowadays, we know that the frequency of ETGs with a detectable warm ionized component in their ISM is significant, ranging from 60% to 80%, despite the differences in sample selection criteria and sample sizes (e.g. Kim 1989; Macchetto et al. 1996; Finkelman et al. 2010). Understanding the sources required to ionize the gas is needed to investigate fundamental questions of the origin and the nature of the gas in ETGs.
Most ETG nuclei show faint nebular emission with an Hα equivalent width of typically EW(Hα) < ∼10 A (e.g. Annibali et al., 2010) and are classified as low-ionization nuclear emission-line regions (LINERs; Heckman, 1980) on the basis of optical diagnostic line ratios (Baldwin et al. 1981). One widely favored mechanism for the gas excitation in ETG involves "weak", low-luminosity active galactic nuclei (LLAGN) (cf e.g., Ho, 1999). Other hypotheses are fast shocks (e.g., Dopita & Sutherland, 1995), photoionization by ongoing low-level star-formation (e.g. Vílchez & Iglesias-Páramo 1998; Schawinski et al., 2007) and evolved post-asymptotic giant branch (pAGB) stars (e.g. Binette et al., 1994; Cid Fernandes et al., 2011; Sarzi et al., 2010) which was proposed as the sole ionization mechanism of the ionized gas in ETG/LINERs. However, despite many studies, the question on the nature of the gas in ETGs is far to be answered, and we are still faced with a puzzling situation where all the processes mentioned above may contribute to the excitation of the ionized medium in these galaxies. This motivated us to embark on a project to conduct a thorough analysis of the ionized gas in ETGs aiming to shed light on the sources of ionization that excite line emission in ETGs.
At present, we are leading, together with collaborators from the Universidade do Porto (Portugal), the investigation of the ionized gas within the CALIFA survey (Sanchez et al. 2012). In the framework of the CALIFA collaboration, Kehrig et al. (2012) presented a bidimensional analysis of the ionized gas in two prototypical E/S0 galaxies. We concluded that nebular emission in ETGs is extended and typically characterized by LINER BPT ratios several kpc away from the galaxy nuclei. Kehrig et al. (2012) also support, based on different arguments, that pAGB photoionization is an important ingredient of the gas excitation on galactic scales. Papaderos et al. (2013) have confirmed and strengthened this conclusion. In Papaderos et al. (2013) we studied the radial distribution of the EW(Hα) over the entire optical extent of 32 ETGs from which we classify the sample galaxies in two groups: type i and type ii ETGs. In type i galaxies, roughly 40% of the 32 ETGs analysed, the Lyman continuum (Lyc) photon rate from pAGB stars is capable of sustaining the observed ionized gas emission. On the other hand, in the type ii ETGs (60% of our sample), the bulk of Lyc photons produced by pAGB stars escapes into the halo without being reprocessed into nebular emission. The nature of type ii ETGs is enigmatic. An important implication of the high Lyc escape fraction in type ii ETGs is that these systems could host significant nuclear AGN activity that escapes detection in optical wavelengths. At the moment we are performing a systematic investigation of the physical and kinematical properties of the ionized gas in tens of nearby (<150 Mpc) ETGs from the CALIFA survey. We hope this work to be submitted soon to Astronomy & Astrophysics. In addition, we recently obtained spectroscopic data of few ETGs with INTEGRAL/WHT. - stay tuned!
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| Left: Halpha emission from a dwarf galaxy in the cluster A426, from the INT2.5m observations. Right: SDSS r'-band of this galaxy. |
The main result of this work was the publication for the first time of the Halpha luminosity function of cluster galaxies (Iglesias-Páramo et al. 2002), showing that the slope towards low luminosities was flatter than for field galaxies in the local Universe. This means a decrease in the density of star-forming dwarf galaxies in clusters with respect to the field.
Another interesting result was the discovery of two ultra-compact (almost point-like) starbursts in the clusters A539 and A634 (Reverte et al. 2007). These galaxies are usually missed in most imaging surveys because of their stellar-like profile and their faint optical luminosities. Both galaxies were found to have oxygen abundances consistent with their luminosities and could be associated with interactions between dwarf galaxies or gas clouds being compressed by the intergalactic medium of the cluster.
A further interesting result related to this topic is the finding that in the cluster A2151 (Hercules supercluster), most galaxies with Halpha emission are located close to the region of highest galaxy density, which turns out to be shifted from the X-ray peak (Cedrés et al. 2009). As this cluster is in the merging process with A2152 and A2147, this result suggests that not only the local or global cluster properties could drive the star formation properties of galaxies, but also the cluster merging history.
We are currently developing an automatic pipe-line to produce an on-line database of the Halpha emitters detected in our frames that will soon be available to the community.
As the cluster environment leave an imprint on the star formation properties of galaxies, it must also influence their chemical enrichment, since both properties are related to each other. For this reason, our group started a study on the chemical enrichment of cluster galaxies.
A first study on the chemical abundances of star-forming galaxies (with detected Halpha emission) in the Hercules clusters was carried out with long-slit optical spectra (Petropoulou et al. 2011). The analysis of the O/H and N/O abundances suggests that Hercules star-forming galaxies are divided into three main subgroups: (1) chemically evolved spirals with truncated ionized-gas disks and nearly flat oxygen gradients, demonstrating the effect of ram-pressure stripping; (2) chemically evolved dwarfs/irregulars populating the highest local densities, possible products of tidal interactions in preprocessing events; and (3) less metallic dwarf galaxies that appear to be "newcomers" to the cluster and are experiencing pressure-triggered star formation. Most Hercules SF galaxies follow well-defined MZ and LZ sequences (for both O/H and N/O), though the dwarf/irregular galaxies located at the densest regions appear to be outliers to these global relations, suggesting a physical reason for the dispersion in these fundamental relations.
A further work on the chemical properties of star-forming dwarf galaxies in four nearby clusters (A634, A779, A1367 and A1656) was performed using SDSS optical spectra (Petropoulou et al. 2012). A flattening of the slope of the Mass-Metallicity relation has been observed for galaxies located in the core of the two more massive clusters of the sample, principally in A1656, suggesting that the imprint of the cluster environment on the chemical evolution of star-forming galaxies should be sensitive to both the galaxy mass and the host cluster mass. The H I gas content of A1656 and A1367 galaxies indicates that low-mass star-forming galaxies located at the core of these clusters have been severely affected by ram-pressure stripping (RPS). The observed mass-dependent enhancement of the metal content of low-mass galaxies in dense environments seems plausible, according to hydrodynamic simulations. This enhanced metal enrichment could be produced by the combination of effects such as wind reaccretion, due to pressure confinement by the intracluster medium (ICM), and the truncation of gas infall, as a result of the RPS. Thus, the properties of the ICM should play an important role in the chemical evolution of low-mass galaxies in clusters.