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Register a new userThe nebular emission of star-forming galaxies in a hierarchical universe
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Galaxy surveys targeting emission lines are characterising the evolution of star-forming galaxies, but there is still little theoretical progress in modelling their physical properties. We predict nebular emission from star-forming galaxies within a cosmological galaxy formation model. Emission lines are computed by combining the semi-analytical model sag with the photoionisation code mapp. We characterise the interstellar medium (ISM) of galaxies by relating the ionisation parameter of gas in galaxies to their cold gas metallicity, obtaining a reasonable agreement with the observed ha, oii, oiii luminosity functions, and the the BPT diagram for local star-forming galaxies. The average ionisation parameter is found to increase towards low star-formation rates and high redshifts, consistent with recent observational results. The predicted link between different emission lines and their associated star-formation rates is studied by presenting scaling relations to relate them. Our model predicts that emission line galaxies have modest clustering bias, and thus reside in dark matter haloes of masses below $M_{rm halo} lesssim 10^{12} {[rm h^{-1} M_{odot}]}$. Finally, we exploit our modelling technique to predict galaxy number counts up to $zsim 10$ by targeting far-infrared (FIR) emission lines detectable with submillimetre facilities
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We highlight and discuss the importance of accounting for nebular emission in the SEDs of high redshift galaxies, as lines and continuum emission can contribute significantly or subtly to broad-band photometry. Physical parameters such as the galaxy age, mass, star-formation rate, dust attenuation and others inferred from SED fits can be affected to different extent by the treatment of nebular emission. We analyse a large sample of Lyman break galaxies from z~3-6, and show some main results illustrating e.g. the importance of nebular emission for determinations of the mass-SFR relation, attenuation and age. We suggest that a fairly large scatter in such relations could be intrinsic. We find that the majority of objects (~60-70%) is better fit with SEDs accounting for nebular emission; the remaining galaxies are found to show relatively weak or no emission lines. Our modeling, and supporting empirical evidence, suggests the existence of two categories of galaxies, starbursts and post-starbursts (lower SFR and older galaxies) among the LBG population, and relatively short star-formation timescales.
To investigate the ingredients, which allow star-forming galaxies to present Lyalpha line in emission, we studied the kinematics and gas phase metallicity (Z) of the interstellar medium. We used multi-object NIR spectroscopy with Magellan/MMIRS to study nebular emission from z=2-3 star-forming galaxies discovered in 3 MUSYC fields. We detected emission lines from four active galactic nuclei and 13 high-z star-forming galaxies, including Halpha lines down to a flux of 4.E-17 erg/sec/cm^2. This yielded 7 new redshifts. The most common emission line detected is [OIII]5007, which is sensitive to Z. We were able to measure Z for 2 galaxies and to set upper(lower) limits for another 2(2). The Z values are consistent with 0.3<Z/Zsun<1.2. Comparing the Lyalpha central wavelength with the systemic redshift, we find Delta_v(Lyalpha-[OIII])=70-270 km/sec. High-redshift star-forming galaxies, Lyalpha emitting (LAE) galaxies, and Halpha emitters appear to be located in the low mass, high star-formation rate (SFR) region of the SFR versus stellar mass diagram, confirming that they are experiencing burst episodes of star formation, which are building up their stellar mass. Their Zs are consistent with the relation found for z<2.2 galaxies in the Z versus stellar mass plane. The measured Delta_v(Lyalpha-[OIII]) values imply that outflows of material, driven by star formation, could be present in the z=2-3 LAEs of our sample. Comparing with the literature, we note that galaxies with lower Z than ours are also characterized by similar Delta_v(Lyalpha-[OIII]) velocity offsets. Strong [OIII] is detected in many Lyalpha emitters. Therefore, we propose the Lyalpha/[OIII] flux ratio as a tool for the study of high-z galaxies; while influenced by Z, ionization, and Lyalpha radiative transfer in the ISM, it may be possible to calibrate this ratio to primarily trace one of these effects.
Star forming galaxies exhibit a variety of physical conditions, from quiescent normal spirals to the most powerful dusty starbursts. In order to study these complex systems, we need a suitable tool to analyze the information coming from observations at all wavelengths. We present a new spectro-photometric model which considers in a consistent way starlight as reprocessed by gas and dust. We discuss preliminary results to interpret some observed properties of VLIRGs.
We have determined the O/H and N/O of a sample of 122751 SFGs from the DR7 of the SDSS. For all these galaxies we have also determined their morphology and their SFH using the code STARLIGHT. The comparison of the chemical abundance with the SFH allows us to describe the chemical evolution in the nearby universe (z < 0.25) in a manner which is consistent with the formation of their stellar populations and morphologies. A 45% of the SFGs in our sample show an excess of abundance in nitrogen relative to their metallicity. We also find this excess to be accompanied by a deficiency of oxygen, which suggests that this could be the result of effective starburst winds. However, we find no difference in the mode of star formation of the nitrogen rich and nitrogen poor SFGs. Our analysis suggests they all form their stars through a succession of bursts of star formation extended over a few Gyr period. What produces the chemical differences between these galaxies seems therefore to be the intensity of the bursts: the galaxies with an excess of nitrogen are those that are presently experiencing more intense bursts, or have experienced more intense bursts in their past. We also find evidence relating the chemical evolution process to the formation of the galaxies: the galaxies with an excess of nitrogen are more massive, have more massive bulges and earlier morphologies than those showing no excess. As a possible explanation we propose that the lost of metals consistent with starburst winds took place during the formation of the galaxies, when their potential wells were still building up, and consequently were weaker than today, making starburst winds more efficient and independent of the final mass of the galaxies. In good agreement with this interpretation, we also find evidence consistent with downsizing, according to which the more massive SFGs formed before the less massive ones.
The origin of nebular HeII emission, which is frequently observed in low-metallicity (O/H) star-forming galaxies, remains largely an unsolved question. Using the observed anticorrelation of the integrated X-ray luminosity per unit of star formation rate ($L_X/{rm SFR}$) of an X-ray binary population with metallicity and other empirical data from the well-studied galaxy I Zw 18, we show that the observed HeII 4686 intensity and its trend with metallicity is naturally reproduced if the bulk of He$^+$ ionizing photons are emitted by the X-ray sources. We also show that a combination of X-ray binary population models with normal single and/or binary stellar models reproduces the observed $I(4686)/I(Hbeta)$ intensities and its dependency on metallicity and age. We conclude that both empirical data and theoretical models suggest that high-mass X-ray binaries are the main source of nebular HeII emission in low-metallicity star-forming galaxies.
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