Extended UV Emission in Nearby Spiral
Galaxies
Armando Gil de Paz1, Barry F. Madore2 , David Thilker3 , Samuel Boissier4 ,
Luciana Bianchi3 , and the GALEX Team
1
2
3
4
Dept. de Astrofı́sica, Universidad Complutense de Madrid, Avda. de la
Complutense s/n, Madrid 28040, Spain; agpaz@astrax.fis.ucm.es
The Observatories, Carnegie Institution of Washington, 813 Santa Barbara
Street, Pasadena, CA 91101, USA; barry@ipac.caltech.edu
Center for Astrophysical Sciences, The Johns Hopkins University, 3400 N.
Charles St., Baltimore, MD 21218, USA; dthilker, bianchi@pha.jhu.edu
Laboratoire d’Astrophysique de Marseille, BP 8, Traverse du Siphon, 13376
Marseille Cedex 12, France; samuel.boissier@oamp.fr
Summary. We review the main properties of the extended UV (XUV) disks recently discovered by the Galaxy Evolution Explorer (GALEX) satellite in a number
of nearby spiral galaxies. Their morphological, photometric and spectroscopic properties are described, as well as their environment. We focus on the two XUV-disk
galaxies best analyzed to date: M 83 (NGC 5236) and NGC 4625. The relevance
of such extreme outer-disk star formation on the early formation and evolution of
these and other disk galaxies is also discussed.
1 Introduction
Deep, wide-field observations of a sample of nearby spiral galaxies at UV
wavelengths carried out by the Galaxy Evolution Explorer (GALEX; see [9])
satellite have revealed the presence of UV-bright complexes (XUV complexes
hereafter) in the outermost parts of their disks (see Figure 1). These galaxies
show UV emission well beyond (∼2-4×) their optical (D25) radii. To date,
the two best-studied XUV disks are those of M 83 ([13]) and NGC 4625 ([6]).
Recent studies of the photometric and spectroscopic properties of these
galaxies indicate that their XUV emission is attributable to recent star formation ([6][7]). The properties of these young stellar associations should provide
fundamental clues to understand the mechanism(s) that led to the formation
of stars in these outermost regions of the disks. We should be able to establish
also whether (1) this star formation is continuously happening in the outer
disks of a sub-population of spiral galaxies, (2) it is a transient but recurrent
phenomenon that has taken place in these and other (perhaps all) spiral galax-
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Gil de Paz et al.
ies in the past, or, alternatively, (3) it is a one-time phenomenon and these
are the first generation of stars to form in the outer parts of these galaxies.
In this contribution we summarize the properties of these newly-discovered
XUV disks and shed some light on the possible role played by these structures
on the formation and evolution of disk galaxies in general.
2 XUV disks: The case of M 83
The GALEX observations reported by [13] reveal the presence of >100 UVbright regions in the outer disk of M 83. These XUV complexes seem to be
associated with relative maxima in the neutral-gas density distribution. These
authors also reported a sharp cutoff in the Hα brightness at a galactocentric
distance of ∼5 kpc with no correspondence in the UV light profile, which
extends out more than twice that distance.
Analysis of the optical spectra of 19 Hα-selected regions in the XUV disk
of M 83 obtained with the Magellan II telescope gives oxygen abundances that
range between ≤0.1 Z in the outermost regions of the XUV disk to nearly
solar abundances in the optical disk. Line fluxes and their measured ratios
are best reproduced by photoionization models powered by single stars with
masses in the range 20-40 M ([7]). Obviously, the UV emission detected in
these XUV disks has also contribution from stars of lower masses than those
responsible for the ionization of the gas.
The metallicity gradient derived is ∆[O/H]/∆R=−0.10±0.02 dex kpc−1 ,
which expressed in units of the D25 radius is −1.0 dex/r25 , similar to the
“universal” gradient of −0.8 dex/r25 proposed by [11]. Although there is some
support for a sharp decrease in the oxygen abundance of the XUV disk at a
galactocentric distance of 10 kpc, the uncertainties in the calibration of the
R23 parameter used prevent more definitive conclusions from being drawn.
3 XUV disks: The case of NGC 4625
NGC 4625 was observed in the far-UV (FUV) and near-UV (NUV) bands
by GALEX on 2004 April 5. These observations revealed the presence of UV
emission extending up to four times the optical radius of the galaxy ([6]). Its
UV emission covers a significant fraction of the area detected in 21 cm, with
some correspondence between the position of the brightest UV complexes
and peaks in the neutral-gas distribution (see Figure 2). The XUV disk of
NGC 4625 is made up of several fragmented spiral arms in the inner regions
and possibly a large faint arm in the outermost regions.
The analysis of the UV-optical surface photometry of NGC 4625 yields very
blue UV colors for its XUV disk that suggest the presence of a young stellar
population (<1 Gyr) dominating its UV and optical emission (and probably
also its mass). A study of the photometric properties of a total of 74 individual
XUV emission in nearby spiral galaxies
3
XUV complexes shows that their UV luminosities are in the range 1023−24.5
erg s−1 Hz−1 , and their masses are between 103 and ∼104 M . Only some of
these regions show line emission and their Hα luminosities are typically below
that expected from an HII region ionized by a single O5 star ([6]).
The spectroscopic observations of a dozen XUV-disk regions in NGC 4625
carried out by [7] yielded oxygen abundances of the order of 0.1 Z with no
obvious gradient and relatively small scatter across the disk. As in the case
of M 83, the line ratios (and fluxes) are better reproduced by photoionization
models ionized by a single star than by those powered by stellar clusters.
This confirms earlier results by [6] based on the analysis of Hα narrow-band
imaging data on NGC 4625. In Figure 3 we show the diagnostic diagrams for
the line ratios measured in NGC 4625 compared those of a sample of local
star-forming galaxies ([5]) and the predictions of photoionization models using
both a single massive star or a stellar cluster as ionizing sources.
4 Discussion
4.1 Nature of the XUV disks
The line ratios measured in those XUV complexes showing line emission ([7])
indicate the presence of UV ionizing radiation being emitted locally in these
regions and, therefore, exclude the possibility that this is simply light being
scattered by dust. They also exclude post-AGB stars (whose line emission
is commonly dominated by shock-excited gas) and planetary nebulae (with
highly-excited gas) as being the agents responsible for the observed XUV
emission. Finally, a significant contribution by blue-HB stars is unlikely considering, for example, the blue NUV−optical colors of the XUV disks. These
colors are unusual in objects whose UV emission is believed to be due to
evolved stars (e.g. elliptical galaxies) but common in galaxies forming stars
in recent epoch. Moreover, the spatial distribution of UV light is clumped in
spiral fragments, indicating a recent origin.
The results of [6][7] also show that the majority of the otherwise scarce
emission-line sources found in these XUV disks seem to be powered by single
stars, with masses between 20-40 M . Consequently, a stochastic treatment of
the IMF is necessary in order to properly analyze the properties of the stellar
populations in these XUV disks and perhaps in the outer edges of all spiral
galaxies. In particular, knowledge on the fraction of low-mass stellar complexes
that, once stochastic effects are properly accounted for, would form at least
one ionizing star is key to understanding the observed differences between the
UV and Hα light profiles of XUV disks.
In this sense, one of the explanations for the reported lack of Hα emission
in the XUV disks analyzed to date is the fact that these regions have very
low levels of (current) star formation rate. This results in a low probability
for finding even one single ionizing star at a given epoch, which leads to an
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Gil de Paz et al.
effective lack of HII regions and a drop in the azimuthally-averaged Hα light
profiles compared to the time-integrated star formation which is detected in the
UV. Other scenarios brought forward to explain this paucity of Hα emission
include a truncated IMF, leakage of ionizing photons through a porous ISM,
undetected Hα emission in the form of diffuse HII regions, etc. ([10]).
4.2 Past and future evolution of the XUV disks
The very blue colors of XUV disks led to the suggestion that these might
be the first generations of stars formed in these regions ([8][6]). However,
the oxygen abundances later derived for the emission-line regions of M 83
and NGC 4625 (Z∼0.1 Z ) indicate that although these outermost regions
are relatively unevolved chemically they are not forming their first stars for
the first time now. Assuming that the gas comprising the outer disk is not
dominated by debris of prior galaxy collisions, some star formation activity
must have taken place in these regions in the past in order to enrich the ISM
to its present levels. The models for the evolution of galactic disks ([1]) predict
that an oxygen abundance Z∼0.1 Z could be reached in a pristine disk only
∼1 Gyr after its first stars had formed. Noteworthy, a disk of this age would
also show colors compatible with those measured in the XUV disks ([7]). Thus,
in principle, these XUV disks could be only 1-2 Gyr old.
Such enrichment of the ISM can be produced either by a low-level continuous star formation or through an episodic star formation history. In the
former case the amount of gas in these disks would be large enough to maintain the current level of star formation for at least several Gyr. In the latter
case, it could be argued that other (now quiescent) galaxies might have gone
through one or several of these episodes during their lives.
Perhaps the most intriguing question on the XUV phenomenon regards the
mechanism(s) responsible for the activation of the star formation in the highly
rarefied ISM found in these outer disks. Although internal mechanisms might
be partly responsible in some cases, it is clear that some XUV-disk galaxies
(e.g. NGC 4625 & NGC 1512) have companions with which they are clearly
interacting (NGC 4618 & NGC 1510, respectively). In the case of NGC 4625,
[3] argued that NGC 4618 and NGC 4625 have only had one close passage
and that the current interaction has been ongoing for ∼0.5 Gyr. This is of the
order of the timescale for the UV emission. The situation for other XUV disks
(M 83, NGC 5055, or NGC 2841) is not that clear. These galaxies do not
belong to tight binary systems, so the role of interactions in XUV-disks star
formation is uncertain. A possible explanation is that the instability in these
cases is due to the interaction with low-mass (yet undetected) companions.
GALEX is a NASA Small Explorer, launched in April 2003. We gratefully acknowledge NASA’s support for construction, operation, and science analysis for
the GALEX mission, developed in cooperation with the Centre National d’Etudes
Spatiales of France and the Korean Ministry of Science and Technology.
XUV emission in nearby spiral galaxies
5
Fig. 1. Upper panels: The face-on spiral galaxy M 83 (NGC 5236) as seen in UV
light by GALEX (left), in the optical (Digitized Sky Survey, DSS, blue plate; center),
and in the 21 cm line of neutral hygrogen (right). The GALEX image shown here is a
composite of the far-UV (FUV; in blue) and the near-UV (NUV; in yellow) imaging
data. This galaxy was the first object where an extended UV disk was discovered
([13]). Lower panels: The same for NGC 1512. The interacting dwarf companion
galaxy NGC 1510 is visible to the South-West of the nucleus of NGC 1512.
20:00.0
20:00.0
NGC 4625
18:00.0
18:00.0
16:00.0
14:00.0
DEC (J2000)
DEC (J2000)
16:00.0
12:00.0
12:00.0
41:10:00.0
41:10:00.0
08:00.0
08:00.0
06:00.0
14:00.0
NGC 4618
a)
20.0
06:00.0
10.0
12:42:00.0
50.0
40.0
RA (J2000)
30.0
20.0
41:10.0
b)
20.0
10.0
12:42:00.0
50.0
40.0
30.0
20.0
41:10.0
RA (J2000)
Fig. 2. a) GALEX false-color RGB composite image of the NGC 4618/NGC 4625
system. b) DSS image of the same region with the 21 cm Hi contours overimposed
in red (1 2 4 6 8 11 15 20×1020 cm−2 contours are shown).
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Gil de Paz et al.
Fig. 3. Diagnostic diagrams for the emission-line regions in the XUV disk of
NGC 4625. a) [OIII]λ5007Å/Hβ versus [NII]λ6584Å/Hα extinction-corrected line
ratios. For comparison we show the location of the UCM-Survey galaxies, a complete
sample of local star-forming galaxies (SFG) and AGN selected by their emission in
Hα ([5]), coded by spectroscopic type. The lines are the predictions of CLOUDY
single-star photoionization models for an age of the star of 1 Myr. Solid lines correspond to models with fixed mass for the ionizing star and metallicities ranging
between 0.1 Z and Z while dot-dashed lines correspond to models with fixed
metallicity and different masses between 20 and 85 M . Typical errors on the line
ratios are shown at the top right corner of the diagram. b) [OIII]λ5007Å/Hβ versus [OII]λλ3726,3729ÅÅ/[OIII]λ5007Å extinction-corrected line ratios. Here we also
show the predictions of photoionization models for evolving starbursts ([12]) with
Z=0.1 Z and two different densities, 10 and 104 cm−3 .
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