[Paper Review] GMASS ultradeep spectroscopy of galaxies at z ~ 2 - VII. Star formation, extinction, and gas outflows from UV spectra

M. Talia, M. Mignoli+14 more|arXiv (Cornell University)|Nov 18, 2011

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy21 citations

TL;DR

This study uses rest-frame UV spectroscopy from the GMASS survey to analyze star formation, dust extinction, and gas outflows in 74 z ~ 2 star-forming galaxies. It finds a mean SFR of 52 ± 48 M☉ yr⁻¹, blueshifted low-ionization absorption lines indicating ~100 km/s outflows, and correlations between interstellar absorption line equivalent widths and dust extinction, SFR, and stellar mass, suggesting a link between gas kinematics and galaxy properties at high redshift.

ABSTRACT

We use rest-frame UV spectroscopy to investigate the properties related to large-scale gas outflows, and to the dust extinction and star-formation rates of a sample of z ~ 2 star-forming galaxies from the Galaxy Mass Assembly ultradeep Spectroscopic Survey (GMASS). Dust extinction is estimated from the rest-frame UV continuum slope and used to obtain dust-corrected star-formation rates for the galaxies of the sample. For the entire sample, a mean value of the continuum slope = -1.11 \pm 0.44 (r.m.s.) was derived, while the average SFR was found to be = 52 \pm 48 M_sun/yr (r.m.s.). A positive correlation between SFR and stellar mass was observed, in agreement with other works, the logarithmic slope of the relation being 1.10 \pm 0.10. Low-ionization absorption lines, associated with the interstellar medium, were found to be blueshifted, with respect to the rest frame of the system, which indicates that there is outflowing gas with typical velocities of the order of ~ 100 km/s. Finally, investigating correlations between galaxy UV spectral characteristics and galaxy general properties, we report a possible correlation between the equivalent width of the interstellar absorption lines and SFR, stellar mass, and colour excess similar to that seen to hold at different redshifts.

Motivation & Objective

• To measure dust extinction and correct UV-derived star-formation rates (SFR) in high-redshift galaxies using rest-frame UV continuum slope.
• To investigate large-scale gas outflows via blueshifted interstellar absorption lines in the UV spectra of z ~ 2 star-forming galaxies.
• To explore correlations between UV spectral features (e.g., absorption line equivalent widths, velocity offsets) and galaxy properties such as stellar mass, SFR, and dust extinction.
• To assess the role of dust and stellar populations in shaping UV spectral features, particularly the He II λ1640 emission line.
• To compare results with existing models and surveys to understand the evolution of star formation and outflows in early galaxies.

Proposed method

• Calculated dust extinction from the rest-frame UV continuum slope (β), using the relation A(1500 Å) = 1.42 × (β + 4.05) for a Calzetti extinction law.
• Derived dust-corrected SFRs from the 1500 Å flux using the Kennicutt (1998) calibration with a Kroupa initial mass function.
• Created a composite spectrum by averaging high signal-to-noise UV spectra of 74 z ~ 2 star-forming galaxies to study average interstellar medium (ISM) properties.
• Measured velocity offsets of low-ionization ISM absorption lines (e.g., Si II, C II) relative to systemic redshifts derived from stellar and nebular lines.
• Divided the sample into two equal-mass bins based on stellar mass, SFR, E(B-V), and morphology to create subsample composites and compare ISM line properties.
• Analyzed the Lyα profile and He II λ1640 emission line strength in composites of Lyα-emitting vs. absorbing galaxies to probe dust and wind effects.

Experimental results

Research questions

• RQ1 What is the average dust extinction and dust-corrected star-formation rate in z ~ 2 star-forming galaxies, as derived from UV continuum slope?
• RQ2 Are interstellar absorption lines in the UV spectra of z ~ 2 galaxies systematically blueshifted, indicating large-scale gas outflows?
• RQ3 How do the equivalent widths and velocity offsets of ISM absorption lines correlate with galaxy properties such as stellar mass, SFR, and E(B-V) dust extinction?
• RQ4 Is there a relationship between the Lyα profile (emission vs. absorption) and the UV continuum slope or dust content?
• RQ5 How well do current stellar population synthesis models reproduce the observed He II λ1640 emission line strength in high-redshift galaxies?

Key findings

• The sample has a mean UV continuum slope of <β> = -1.11 ± 0.44, indicating moderate dust extinction, with dust-corrected SFRs averaging 52 ± 48 M☉ yr⁻¹.
• Low-ionization ISM absorption lines are blueshifted by ~100 km/s on average, indicating widespread large-scale gas outflows in z ~ 2 star-forming galaxies.
• A positive correlation is found between the equivalent width of ISM absorption lines and E(B-V), SFR, and stellar mass, with the E(B-V) correlation being the strongest.
• The composite spectrum of Lyα-emitting galaxies has a bluer UV slope (β = -1.40 ± 0.06) than Lyα-absorbing galaxies (β = -1.04 ± 0.03), confirming a link between dust extinction and Lyα profile morphology.
• The He II λ1640 emission line is detected in all composites and shows stronger equivalent widths in less massive, more star-forming galaxies, though this is inconsistent with expectations from metallicity trends.
• No significant correlation was found between ISM absorption line velocity offsets and galaxy properties, though uncertainties may mask underlying trends.